Regime Shifts DataBase

Large persistent changes in ecosystem services

This regime shift entails extensive change in biodiversity and soil structure in the Arctic tundra due to increased climate warming. To date shrub invasion has been the main indicator of a potential regime shift.

Arctic tundra regime

This regime is characterized by low atmospheric temperatures that enable the formation of a layer of permanently frozen subsoil (permafrost) consisting mostly of gravel and finer material. The ecosystem has a low rate of primary production due to impeded mineral nutrient cycling. The short growing season in summer influences the vegetation and its root systems by limiting vertical expansion. Only plant species (lichens, liverworts, mosses) that are adapted to sweeping winds and soil disturbance can survive in these conditions. The atmospheric cold and existing permafrost restrains the plant growth ensuring that their roots are shallow and surface parts are low growing due to the frozen ground underneath the permafrost. Snow does not accumulate keeping the soil temperature low thus not allowing for permafrost to thaw and release carbon into atmosphere keeping the atmosphere cold.

Boreal forest regime

This regime is characterized by flora that mostly consists of cold-tolerant evergreen conifers, such as the evergreen spruce (Picea), fir (Abies), and pine (Pinus), and the deciduous larch or tamarack (Larix). Before these species are established in a region, there is an increased canopy of early-successional species such as aspen and paper birch (Frelich et al. 1995). Dwarf shrubs (e.g., Vaccinium uliginosum, Ledum decumbens, Betula nana and Cassiope tetragona) and low shrubs (e.g., Betula glandulosa, Salix glauca) act as pioneers that colonize the tundra before trees are able to establish (CAVM Team 2003). The shift to boreal forest can occur over a long time period varying from several decades to centuries depending on the migration time lag for each species and the occurrence of favorable conditions for seedling establishment.

The main external driver of this regime shift is CO₂ emissions that increase climate warming (IPCC 2007), leading to changes in the composition and abundance of arctic vegetation, animals and soil structure (Olofsson et al. 2009). Anthropogenic activities that elevate atmospheric greenhouse gas concentrations are generally considered to be the primary cause for this driver. Carbon release from anthropogenic sources is projected to continue and increase during the coming decades (IPCC 2007). This global driver is well established and could be looked as irreversible in the scale of next hundred years.

The tundra regime is maintained by the low atmospheric temperatures that ensure low soil temperature and the existence of permafrost. This determines the lack of microbial activity and scarce nutrient availability therefore ensuring species composition that are only able to exist in such circumstances.

A shift from the tundra regime to boreal forest is initiated by the atmospheric warming that is caused by continuing increase of CO₂ emissions. The atmospheric temperature increases the period of time when microbial activity and thus nutrient availability is sufficient enough for shrubs and boreal forest pioneer species (birch etc.) to withstand the winter conditions.

As a result the increasing number of shrubs ensure that snow drifting creates deep drifts that surround and extend downwind from shrub patches. These drifts increase the insulation of the soil below. This insulation elevates the soil temperature thus further allowing microbial activity to continue during the frigid arctic winter. The elevated soil temperatures under the snow lead to permafrost degradation that release carbon trapped in tundra soils, thereby further contributing to climate warming.

Shift from Tundra to Boreal Forest

Tundra provides biodiversity for different animal and plant species that characterize tundra ecosystem. The shift from tundra to boreal forest is projected to occur over large geographic areas throughout the tundra zone, with substantial impacts on the landscape and biodiversity (Bonan 1992). Changes in vegetation are also likely to affect composition of foraging mammals and birds (Hinzman et al. 2005). In addition to modifying wildlife habitat, increased woody shrub abundance will make traversing tundra more difficult for caribou, subsistence hunters and communities that rely on caribou for food, as well as hikers. Herbivores such as reindeers and microtine rodents can also be influenced by the shift towards woody vegetation as they prefer lichens, dwarf shrubs graminoids and deciduous shrubs over tall woody shrubs (Sturm et al. 2005). Increased expansion of woody vegetation may therefore result in tundra species extinctions or radical decrease in numbers. The loss of the climate regulating ecosystem service is a major concern as permafrost thawing is reducing the carbon sink service.

For local populations the tundra provides traditional recreation opportunities and contains their knowledge and educational as well as spiritual and religious values, which would be lost with the regime shift.

Shift from Boreal Forest to Tundra 

The change in biodiversity could provide new ecosystem services such as timber production and different wild animals and plant foods. This would benefit loggers as they would increase their income from tree cutting and local communities from goods that come from forest.

In order to prevent or reverse this regime shift it is essential that the input of carbon to the atmosphere is reduced in order to prevent further climate warming. The suggestion of a planetary boundary of 350 ppm for CO₂ levels is a first step in this direction (Rockstrom et al. 2009). Increased understanding of the influence of climate change on the complexity of arctic systems is essential for informing the adaptation of social, economic, and cultural systems to the changes taking place in the Arctic (Hinzman et al. 2005). Technology transfer could be a good initiative from developed countries as they can provide more advanced technological solutions to developing countries to help accelerate the learning curve on GHG emissions. A global response particularly from developed nations that are using the majority of the world's resources on a per capita basis should be in place to deal with such complex systems. Nevertheless, discussions about desirable management strategies and outcomes will be essential as there will be stakeholders who stand to lose from this regime shift (e.g. caribou herders) while others may see the change as desirable and beneficial (e.g. timber production, game hunting).

This regime shift entails extensive change in biodiversity and soil structure in the Arctic tundra due to increased climate warming. To date shrub invasion has been the main indicator of a potential regime shift.

Arctic tundra regime

This regime is characterized by low atmospheric temperatures that enable the formation of a layer of permanently frozen subsoil (permafrost) consisting mostly of gravel and finer material. The ecosystem has a low rate of primary production due to impeded mineral nutrient cycling. The short growing season in summer influences the vegetation and its root systems by limiting vertical expansion. Only plant species (lichens, liverworts, mosses) that are adapted to sweeping winds and soil disturbance can survive in these conditions. The atmospheric cold and existing permafrost restrains the plant growth ensuring that their roots are shallow and surface parts are low growing due to the frozen ground underneath the permafrost. Snow does not accumulate keeping the soil temperature low thus not allowing for permafrost to thaw and release carbon into atmosphere keeping the atmosphere cold.

Boreal forest regime

This regime is characterized by flora that mostly consists of cold-tolerant evergreen conifers, such as the evergreen spruce (Picea), fir (Abies), and pine (Pinus), and the deciduous larch or tamarack (Larix). Before these species are established in a region, there is an increased canopy of early-successional species such as aspen and paper birch (Frelich et al. 1995). Dwarf shrubs (e.g., Vaccinium uliginosum, Ledum decumbens, Betula nana and Cassiope tetragona) and low shrubs (e.g., Betula glandulosa, Salix glauca) act as pioneers that colonize the tundra before trees are able to establish (CAVM Team 2003). The shift to boreal forest can occur over a long time period varying from several decades to centuries depending on the migration time lag for each species and the occurrence of favorable conditions for seedling establishment.

The main external driver of this regime shift is CO₂ emissions that increase climate warming (IPCC 2007), leading to changes in the composition and abundance of arctic vegetation, animals and soil structure (Olofsson et al. 2009). Anthropogenic activities that elevate atmospheric greenhouse gas concentrations are generally considered to be the primary cause for this driver. Carbon release from anthropogenic sources is projected to continue and increase during the coming decades (IPCC 2007). This global driver is well established and could be looked as irreversible in the scale of next hundred years.

The tundra regime is maintained by the low atmospheric temperatures that ensure low soil temperature and the existence of permafrost. This determines the lack of microbial activity and scarce nutrient availability therefore ensuring species composition that are only able to exist in such circumstances.

A shift from the tundra regime to boreal forest is initiated by the atmospheric warming that is caused by continuing increase of CO₂ emissions. The atmospheric temperature increases the period of time when microbial activity and thus nutrient availability is sufficient enough for shrubs and boreal forest pioneer species (birch etc.) to withstand the winter conditions.

As a result the increasing number of shrubs ensure that snow drifting creates deep drifts that surround and extend downwind from shrub patches. These drifts increase the insulation of the soil below. This insulation elevates the soil temperature thus further allowing microbial activity to continue during the frigid arctic winter. The elevated soil temperatures under the snow lead to permafrost degradation that release carbon trapped in tundra soils, thereby further contributing to climate warming.

Shift from Tundra to Boreal Forest

Tundra provides biodiversity for different animal and plant species that characterize tundra ecosystem. The shift from tundra to boreal forest is projected to occur over large geographic areas throughout the tundra zone, with substantial impacts on the landscape and biodiversity (Bonan 1992). Changes in vegetation are also likely to affect composition of foraging mammals and birds (Hinzman et al. 2005). In addition to modifying wildlife habitat, increased woody shrub abundance will make traversing tundra more difficult for caribou, subsistence hunters and communities that rely on caribou for food, as well as hikers. Herbivores such as reindeers and microtine rodents can also be influenced by the shift towards woody vegetation as they prefer lichens, dwarf shrubs graminoids and deciduous shrubs over tall woody shrubs (Sturm et al. 2005). Increased expansion of woody vegetation may therefore result in tundra species extinctions or radical decrease in numbers. The loss of the climate regulating ecosystem service is a major concern as permafrost thawing is reducing the carbon sink service.

For local populations the tundra provides traditional recreation opportunities and contains their knowledge and educational as well as spiritual and religious values, which would be lost with the regime shift.

Shift from Boreal Forest to Tundra 

The change in biodiversity could provide new ecosystem services such as timber production and different wild animals and plant foods. This would benefit loggers as they would increase their income from tree cutting and local communities from goods that come from forest.

In order to prevent or reverse this regime shift it is essential that the input of carbon to the atmosphere is reduced in order to prevent further climate warming. The suggestion of a planetary boundary of 350 ppm for CO₂ levels is a first step in this direction (Rockstrom et al. 2009). Increased understanding of the influence of climate change on the complexity of arctic systems is essential for informing the adaptation of social, economic, and cultural systems to the changes taking place in the Arctic (Hinzman et al. 2005). Technology transfer could be a good initiative from developed countries as they can provide more advanced technological solutions to developing countries to help accelerate the learning curve on GHG emissions. A global response particularly from developed nations that are using the majority of the world's resources on a per capita basis should be in place to deal with such complex systems. Nevertheless, discussions about desirable management strategies and outcomes will be essential as there will be stakeholders who stand to lose from this regime shift (e.g. caribou herders) while others may see the change as desirable and beneficial (e.g. timber production, game hunting).

Thermohaline circulation is complex, but can be summarised as a global scale deep overturning of water masses, with sinking motions occurring in the northern Atlantic and around Antarctica and a transport of relatively warm surface waters towards these sinking regions (Steffen et al. 2004). The term thermohaline circulation thus refers to a particular driving mechanism; it is a physical, not an observational concept.

Strong Thermohaline circulation

THC regime can be characterized by the formation of deep water currents in the high latitudes of both hemispheres; the spreading of deep waters, as deep western boundary currents; upwelling (or upwards movement) of deep waters which is thought to take place mainly in the Antarctic Circumpolar Current region, possibly aided by the wind, and near-surface currents, required to close the flow (Rahmstorf et al. 2006).

Collapse of the Thermohaline circulation

This regime is characterized by weak formation of the dense and saline high latitude deep waters due to the increased influx of freshwater. The decreased salinity of the deep waters is weakening the density southwards driven water movement. The transport of less dense and saline waters from south towards north is decreased causing additional atmospheric temperature increase in tropical areas.

Increasing greenhouse gas concentration from anthropogenic sources is predicted to cause a rise in global mean temperatures (Cubasch et al. 2001). One of the most common anthropogenic greenhouse gases is carbon dioxide (CO2). It is considered to be the main driver for this regime shift and is typically operating in global scale due to the global input of CO2 in the atmosphere. Increasing CO2 levels in atmosphere increases the mean atmospheric temperature. This results in weakening ice-albedo mechanism for the initial regime as a result of increased atmospheric temperatures and changes in albedo.

The increasing freshwater influx from sea-ice loss is accelerated by freshwater-overturning and precipitation-river runoff. This leads to an in increased evaporation, river runoff, and changes to water salinity and density. The burning of fossile fuels such as coal and natural gas also drives this regime shift. This occurs in a regional scale but has global impact and is well established in the literature.

In the regime of strong THC, the most simple explanation of the main variable that maintains the regime would be the high-latitude cooling. In the deep ocean, the predominant driving force of movement of currents is differences in density, caused by salinity and temperature (the more saline the more denser and the colder the water, the more dense). The ocean's density distribution is itself affected by currents and mixing. Thermohaline and wind-driven currents therefore interact in non-linear ways and cannot be separated by oceanographic measurements. Changing the wind stress will alter the THC; altering thermohaline forcing will also change the wind driven currents.

The collapse of THC occurs due to increasing CO2 levels in atmosphere that slowly enhance the mean atmospheric temperature. As a result the high latitude cooling is weakened, increasing freshwater influx from the loss of ice, both from the Arctic, Greenland and possibly the north of Canada. This strengthens two main mechanisms for the new regime: freshwater-overturning mechanism and the precipitation-river runoff mechanism. Both increase the fresh water inflow into the ocean. Therefore new mechanism such as increased evaporation, river runoff, changes in water salinity and density are activated possibly reinforcing the new regime. Once the system has shifted, the warm conditions in Northern latitudes maintain a weak formation of the dense and saline high latitude deep waters due to the increased influx of freshwater.

Most ocean–atmosphere models predict a reduction of the THC for a warmer climate in the near future. Recent observational evidence provides some support to the model projections that the North Atlantic THC circulation could weaken or shut down later this century (Steffen et al. 2004). Lenton et al. (2008) report that the IPCC models predict the regime shift to happen after 2100, however, their models do not include freshwater runoff which is expected to accelerate the process. The models that project the collapse of THC under greater perturbations identify that the collapsed state can last for centuries and might be irreversible (Delworth et al. 2008). Nevertheless the latest IPCC report shows that it is very unlikely that the THC will undergo a large abrupt transition during the course of the 21st century (IPCC 2007). Some models show a recovery of the THC after the elimination of the freshwater perturbation whereas others do not. The mechanism for this different dynamical behavior of the THC is not completely understood (Stouffer et al. 2006).

The weakening of THC will impact global water regulation as water circulations in global scale are influenced by the changes in temperature, salinity and density. In response, climate regulation is altered and changes as the transport of heat is distributed in a different pattern. The water mass and heat circulation will no longer transport warm currents in a northly direction. This could increase the occurence of hurricanes, a southward shift of tropical rainfall belts with resulting agricultural impacts, as well as disruptions to marine ecosystems. Provisioning services such as fisheries, livestock and food crops are therefore potentially affected. Schmittner et al. (2007) show that changes in Atlantic circulation can have large effects on marine ecosystems and biogeochemical cycles, even in areas remote from the Atlantic, such as the Indian and North Pacific Oceans. Although loss of THC could affect Western Europe and result in colder climates. 

Food crisis and emigration are potential threats in the regions that are subjected to this regime shift (Barnett & Adger 2007). In addition, regional changes in sea level are predicted to rise of up to 80 cm in the North Atlantic (Knutti et al. 2002). This could affect the coastlines of the United States, Canada, and Europe causing coastal erosion and affect the security of housing and infrastructure (Delworth et al. 2008). The marine biodiversity of flora and fauna will also be affected. The change in salinity and temperature may facilitate the introduction and spread of invasive species that are able to thrive in the new conditions. This could affect the livelihoods and economic activity of local fishermen as they would have to adapt to new circumstances.  

This regime shift is mainly driven by the increase in greenhouse gases into the atmosphere at a global scale. Therefore a decrease in CO2 emissions and other greenhouse gases has to be achieved in order to prevent further climate warming leading to large influx of freshwater from the depletion of Arctic ice. In the best-case scenario, it may be possible to avert a regime change in a situation where the underlying driver(s) of change are amenable to fast manipulation through management, and where there are not substantial lags in ecosystem response. In the case of the THC neither of these factors hold: the underlying causes of global warming appear very difficult to address through rapid changes in management, and there are substantial inertias in ecosystem response to changes in the driving factors. Therefore an early warning indicators would be useful in providing advance warning of a substantial coming change in ecosystem conditions, but is unlikely to be useful in averting the regime change.

This regime shift entails extensive change in biodiversity and soil structure in the Arctic tundra due to increased climate warming. To date shrub invasion has been the main indicator of a potential regime shift.

Arctic tundra regime

This regime is characterized by low atmospheric temperatures that enable the formation of a layer of permanently frozen subsoil (permafrost) consisting mostly of gravel and finer material. The ecosystem has a low rate of primary production due to impeded mineral nutrient cycling. The short growing season in summer influences the vegetation and its root systems by limiting vertical expansion. Only plant species (lichens, liverworts, mosses) that are adapted to sweeping winds and soil disturbance can survive in these conditions. The atmospheric cold and existing permafrost restrains the plant growth ensuring that their roots are shallow and surface parts are low growing due to the frozen ground underneath the permafrost. Snow does not accumulate keeping the soil temperature low thus not allowing for permafrost to thaw and release carbon into atmosphere keeping the atmosphere cold.

Boreal forest regime

This regime is characterized by flora that mostly consists of cold-tolerant evergreen conifers, such as the evergreen spruce (Picea), fir (Abies), and pine (Pinus), and the deciduous larch or tamarack (Larix). Before these species are established in a region, there is an increased canopy of early-successional species such as aspen and paper birch (Frelich et al. 1995). Dwarf shrubs (e.g., Vaccinium uliginosum, Ledum decumbens, Betula nana and Cassiope tetragona) and low shrubs (e.g., Betula glandulosa, Salix glauca) act as pioneers that colonize the tundra before trees are able to establish (CAVM Team 2003). The shift to boreal forest can occur over a long time period varying from several decades to centuries depending on the migration time lag for each species and the occurrence of favorable conditions for seedling establishment.

The main external driver of this regime shift is CO₂ emissions that increase climate warming (IPCC 2007), leading to changes in the composition and abundance of arctic vegetation, animals and soil structure (Olofsson et al. 2009). Anthropogenic activities that elevate atmospheric greenhouse gas concentrations are generally considered to be the primary cause for this driver. Carbon release from anthropogenic sources is projected to continue and increase during the coming decades (IPCC 2007). This global driver is well established and could be looked as irreversible in the scale of next hundred years.

The tundra regime is maintained by the low atmospheric temperatures that ensure low soil temperature and the existence of permafrost. This determines the lack of microbial activity and scarce nutrient availability therefore ensuring species composition that are only able to exist in such circumstances.

A shift from the tundra regime to boreal forest is initiated by the atmospheric warming that is caused by continuing increase of CO₂ emissions. The atmospheric temperature increases the period of time when microbial activity and thus nutrient availability is sufficient enough for shrubs and boreal forest pioneer species (birch etc.) to withstand the winter conditions.

As a result the increasing number of shrubs ensure that snow drifting creates deep drifts that surround and extend downwind from shrub patches. These drifts increase the insulation of the soil below. This insulation elevates the soil temperature thus further allowing microbial activity to continue during the frigid arctic winter. The elevated soil temperatures under the snow lead to permafrost degradation that release carbon trapped in tundra soils, thereby further contributing to climate warming.

Shift from Tundra to Boreal Forest

Tundra provides biodiversity for different animal and plant species that characterize tundra ecosystem. The shift from tundra to boreal forest is projected to occur over large geographic areas throughout the tundra zone, with substantial impacts on the landscape and biodiversity (Bonan 1992). Changes in vegetation are also likely to affect composition of foraging mammals and birds (Hinzman et al. 2005). In addition to modifying wildlife habitat, increased woody shrub abundance will make traversing tundra more difficult for caribou, subsistence hunters and communities that rely on caribou for food, as well as hikers. Herbivores such as reindeers and microtine rodents can also be influenced by the shift towards woody vegetation as they prefer lichens, dwarf shrubs graminoids and deciduous shrubs over tall woody shrubs (Sturm et al. 2005). Increased expansion of woody vegetation may therefore result in tundra species extinctions or radical decrease in numbers. The loss of the climate regulating ecosystem service is a major concern as permafrost thawing is reducing the carbon sink service.

For local populations the tundra provides traditional recreation opportunities and contains their knowledge and educational as well as spiritual and religious values, which would be lost with the regime shift.

Shift from Boreal Forest to Tundra 

The change in biodiversity could provide new ecosystem services such as timber production and different wild animals and plant foods. This would benefit loggers as they would increase their income from tree cutting and local communities from goods that come from forest.

In order to prevent or reverse this regime shift it is essential that the input of carbon to the atmosphere is reduced in order to prevent further climate warming. The suggestion of a planetary boundary of 350 ppm for CO₂ levels is a first step in this direction (Rockstrom et al. 2009). Increased understanding of the influence of climate change on the complexity of arctic systems is essential for informing the adaptation of social, economic, and cultural systems to the changes taking place in the Arctic (Hinzman et al. 2005). Technology transfer could be a good initiative from developed countries as they can provide more advanced technological solutions to developing countries to help accelerate the learning curve on GHG emissions. A global response particularly from developed nations that are using the majority of the world's resources on a per capita basis should be in place to deal with such complex systems. Nevertheless, discussions about desirable management strategies and outcomes will be essential as there will be stakeholders who stand to lose from this regime shift (e.g. caribou herders) while others may see the change as desirable and beneficial (e.g. timber production, game hunting).

Thermohaline circulation is complex, but can be summarised as a global scale deep overturning of water masses, with sinking motions occurring in the northern Atlantic and around Antarctica and a transport of relatively warm surface waters towards these sinking regions (Steffen et al. 2004). The term thermohaline circulation thus refers to a particular driving mechanism; it is a physical, not an observational concept.

Strong Thermohaline circulation

THC regime can be characterized by the formation of deep water currents in the high latitudes of both hemispheres; the spreading of deep waters, as deep western boundary currents; upwelling (or upwards movement) of deep waters which is thought to take place mainly in the Antarctic Circumpolar Current region, possibly aided by the wind, and near-surface currents, required to close the flow (Rahmstorf et al. 2006).

Collapse of the Thermohaline circulation

This regime is characterized by weak formation of the dense and saline high latitude deep waters due to the increased influx of freshwater. The decreased salinity of the deep waters is weakening the density southwards driven water movement. The transport of less dense and saline waters from south towards north is decreased causing additional atmospheric temperature increase in tropical areas.

Increasing greenhouse gas concentration from anthropogenic sources is predicted to cause a rise in global mean temperatures (Cubasch et al. 2001). One of the most common anthropogenic greenhouse gases is carbon dioxide (CO2). It is considered to be the main driver for this regime shift and is typically operating in global scale due to the global input of CO2 in the atmosphere. Increasing CO2 levels in atmosphere increases the mean atmospheric temperature. This results in weakening ice-albedo mechanism for the initial regime as a result of increased atmospheric temperatures and changes in albedo.

The increasing freshwater influx from sea-ice loss is accelerated by freshwater-overturning and precipitation-river runoff. This leads to an in increased evaporation, river runoff, and changes to water salinity and density. The burning of fossile fuels such as coal and natural gas also drives this regime shift. This occurs in a regional scale but has global impact and is well established in the literature.

In the regime of strong THC, the most simple explanation of the main variable that maintains the regime would be the high-latitude cooling. In the deep ocean, the predominant driving force of movement of currents is differences in density, caused by salinity and temperature (the more saline the more denser and the colder the water, the more dense). The ocean's density distribution is itself affected by currents and mixing. Thermohaline and wind-driven currents therefore interact in non-linear ways and cannot be separated by oceanographic measurements. Changing the wind stress will alter the THC; altering thermohaline forcing will also change the wind driven currents.

The collapse of THC occurs due to increasing CO2 levels in atmosphere that slowly enhance the mean atmospheric temperature. As a result the high latitude cooling is weakened, increasing freshwater influx from the loss of ice, both from the Arctic, Greenland and possibly the north of Canada. This strengthens two main mechanisms for the new regime: freshwater-overturning mechanism and the precipitation-river runoff mechanism. Both increase the fresh water inflow into the ocean. Therefore new mechanism such as increased evaporation, river runoff, changes in water salinity and density are activated possibly reinforcing the new regime. Once the system has shifted, the warm conditions in Northern latitudes maintain a weak formation of the dense and saline high latitude deep waters due to the increased influx of freshwater.

Most ocean–atmosphere models predict a reduction of the THC for a warmer climate in the near future. Recent observational evidence provides some support to the model projections that the North Atlantic THC circulation could weaken or shut down later this century (Steffen et al. 2004). Lenton et al. (2008) report that the IPCC models predict the regime shift to happen after 2100, however, their models do not include freshwater runoff which is expected to accelerate the process. The models that project the collapse of THC under greater perturbations identify that the collapsed state can last for centuries and might be irreversible (Delworth et al. 2008). Nevertheless the latest IPCC report shows that it is very unlikely that the THC will undergo a large abrupt transition during the course of the 21st century (IPCC 2007). Some models show a recovery of the THC after the elimination of the freshwater perturbation whereas others do not. The mechanism for this different dynamical behavior of the THC is not completely understood (Stouffer et al. 2006).

The weakening of THC will impact global water regulation as water circulations in global scale are influenced by the changes in temperature, salinity and density. In response, climate regulation is altered and changes as the transport of heat is distributed in a different pattern. The water mass and heat circulation will no longer transport warm currents in a northly direction. This could increase the occurence of hurricanes, a southward shift of tropical rainfall belts with resulting agricultural impacts, as well as disruptions to marine ecosystems. Provisioning services such as fisheries, livestock and food crops are therefore potentially affected. Schmittner et al. (2007) show that changes in Atlantic circulation can have large effects on marine ecosystems and biogeochemical cycles, even in areas remote from the Atlantic, such as the Indian and North Pacific Oceans. Although loss of THC could affect Western Europe and result in colder climates. 

Food crisis and emigration are potential threats in the regions that are subjected to this regime shift (Barnett & Adger 2007). In addition, regional changes in sea level are predicted to rise of up to 80 cm in the North Atlantic (Knutti et al. 2002). This could affect the coastlines of the United States, Canada, and Europe causing coastal erosion and affect the security of housing and infrastructure (Delworth et al. 2008). The marine biodiversity of flora and fauna will also be affected. The change in salinity and temperature may facilitate the introduction and spread of invasive species that are able to thrive in the new conditions. This could affect the livelihoods and economic activity of local fishermen as they would have to adapt to new circumstances.  

This regime shift is mainly driven by the increase in greenhouse gases into the atmosphere at a global scale. Therefore a decrease in CO2 emissions and other greenhouse gases has to be achieved in order to prevent further climate warming leading to large influx of freshwater from the depletion of Arctic ice. In the best-case scenario, it may be possible to avert a regime change in a situation where the underlying driver(s) of change are amenable to fast manipulation through management, and where there are not substantial lags in ecosystem response. In the case of the THC neither of these factors hold: the underlying causes of global warming appear very difficult to address through rapid changes in management, and there are substantial inertias in ecosystem response to changes in the driving factors. Therefore an early warning indicators would be useful in providing advance warning of a substantial coming change in ecosystem conditions, but is unlikely to be useful in averting the regime change.

The Greenland Ice Sheet (GIS) is approximately 1.7 million km² in areas covering approximately 80%  of Greenland. It is grounded on bedrock that mostly rests near or above sea level thus would contribute to the globally averaged sea-level rise of 7.3 m if melted completely (Parizek et al. 2004, Lemke et al. 2007).  Anticipated future climate warming has the potential to permanently reduce large areas of GIS or even abate it completely. The evidence suggests nearly total ice-sheet loss may result from warming of more than a few degrees above mean 20th century values, but this threshold is poorly defined (perhaps as little as 2oC or more than 7^oC) (Alley et al. 2010).

Greenland with permanent ice sheet

This regime can be described as permanent ice body cover over major parts (~80%) of Greenland only exposing the bedrock in western and southern parts. The Greenland Ice Sheet has been more closely tied to temperature than to anything else. It shrinks with warming and grows with cooling, thus the volume and cover vary throughout the seasons, but in case of cold climate the relationship between the ice growth/decline would be approximately evenly balanced or with a slightly increased ice growth. In winter when the atmospheric temperatures decrease below freezing point and precipitation levels decline, the accumulation of the ice steadily increases (Bamber et al. 2007).

Greenland without permanent ice sheet

Due to the loss of the ice-sheet volume as a result of the warming, Greenland territory in future could become free from permanent ice sheet cover. This would happen as a consequence of the negative relation to ice growth/ice decline during the winter and summer where the lost ice volume in summer could not be reproduced in the following winter. Rising sea level as a result of warming tends to float marginal regions of ice sheets and force further retreat, so the generally positive relation between sea level and temperature means that typically both reduce the volume of the ice sheet (Alley et al. 2010).

Increasing greenhouse gas concentration from anthropogenic sources is predicted to cause a rise in global mean temperatures (Cubasch et al. 2001). One of the most common anthropogenic greenhouse gases is carbon dioxide (CO2). The influence of this driver is well established as confirmed by many studies. The indirect driver that is increasing anthropogenic CO2 levels in atmosphere is the burning of fossil fuels such as coal and natural gas. It is occurring regionally but has global impact and is well established in literature.

The initial regime would typically occur in cold climate conditions where the relationship between the ice growth/decline would be approximately evenly balanced or with a slightly increased ice growth. The two main mechanisms that maintain this regime are ice-albedo mechanism and meltwater-ice sliding mechanism.

Increasing CO2 levels in atmosphere - the key driver of the regime shift, initiates the increase of atmospheric temperatures and changes in albedo. As a result - increased absorption of solar energy promotes higher air, ice, water and land temperatures which leads towards degrading sea ice. Also the inland surface temperature increase can cause surface melting in the ablation zone that presently accounts for roughly half of the mass loss from the GIS (Parizek et al. 2004). Thus this driver indirectly is increasing drainage of meltwater feeding into crevasses close to the glacier margin resulting in higher calving rates (Murray et al. 2010). Furthermore, thinning and retreating of the glacier tongue due to these increased rates cause reduced effective pressures beneath the glacier, promoting faster flow that results in decrease of ice volume.  

The increase in surface air temperatures changes the ice-albedo feedback thermodynamics. This means that the heat exchange within the sea ice, as well as between the top and bottom of the ice is changed. This leads to a decrease in sea ice volume. The resulting increased amount of open land and water surface in summer decreases the albedo, as the dark ocean surface absorbs more solar radiation (Lindsay et al. 2005).  The annually integrated absorption of solar radiation is observed to increase when the surface albedo is relatively low (Rigor et al. 2002, Holland et al. 2006). This increasingly accumulated amount of heat on the surface reinforces the initial warming. Due to the loss of the ice-sheet volume as a result of the warming, Greenland territory in future could become free from permanent ice sheet cover. This would happen as a consequence of the negative relation to ice growth/ice decline during the winter and summer where the lost ice volume in summer could not be reproduced in the following winter. The Greenland without permanent ice sheet regime is characterized by other dominant feedback mechanisms. For example ice volume-wave action, the water temperature-density and meltwater-ice sliding velocity mechanisms.

The shift to the regime of Greenland without ice sheet will mainly result in loss of some desirable ecosystem services. The ecosystem service of desirable climate regulation could be lost as the change in movement of currents (change in thermohaline circulation) and air masses would alter the transport of heat. This could lead to increased hurricane activity, a southward shift of tropical rainfall belts with resulting agricultural impacts, and disruptions to marine ecosystems.

The loss of certain animal and plant food species as provisioning services is predicted in the future. These changes may have important consequences for food webs and could well be extremely significant for the Greenland economy, which is highly dependent on fisheries (AMAP 2007). Such cultural services like recreation and aesthetical values would also be affected. Each of those services attracts more people to see the Ice sheet thus also bringing in more tourists. Water regulation as regulating ecosystem services could be altered through the large input of freshwater in the water cycle.  The vast amount of "stored" water entering the water cycle within warmer climate would result in severe winter precipitation.

A new ecosystem service is possible as the thawing ice sheet will potentially form glacial freshwater lakes in Greenland. This will generate new recreation opportunities in summer – using lakes for different purposes from different social groups. Flora could expand deeper into Greenland and new species could be introduced as the climate warms giving the local population the chance to gain additional plant foods.

The potential options for preventing or reversing this potential regime shift mainly relates to the decrease of greenhouse gas input into the atmosphere at a global scale. This has to be achieved in order to prevent further climate warming leading to the loss of Greenland Ice Sheets. Options include reduction of deforestation, use of fossil fuels and charcoal as energy source, and cleaner economies. Geoengineering strategies has also been proposed, large scale experiments to decrease global temperature and CO2 concentration in the atmosphere. However, their applicability is debated and the usefulness is contested. As this system boundaries are set mainly around geophysical variables it is necessary to look at the social mechanisms involved to limit the influence on the main direct driver of greenhouse gas emissions. Nevertheless even if CO2 levels in atmosphere leads to atmospheric temperature increase, it is very hard to achieve from the local to regional management perspective. It does require global coordination and cooperation in order to achieve CO2 reduction targets.

This regime shift entails extensive change in biodiversity and soil structure in the Arctic tundra due to increased climate warming. To date shrub invasion has been the main indicator of a potential regime shift.

Arctic tundra regime

This regime is characterized by low atmospheric temperatures that enable the formation of a layer of permanently frozen subsoil (permafrost) consisting mostly of gravel and finer material. The ecosystem has a low rate of primary production due to impeded mineral nutrient cycling. The short growing season in summer influences the vegetation and its root systems by limiting vertical expansion. Only plant species (lichens, liverworts, mosses) that are adapted to sweeping winds and soil disturbance can survive in these conditions. The atmospheric cold and existing permafrost restrains the plant growth ensuring that their roots are shallow and surface parts are low growing due to the frozen ground underneath the permafrost. Snow does not accumulate keeping the soil temperature low thus not allowing for permafrost to thaw and release carbon into atmosphere keeping the atmosphere cold.

Boreal forest regime

This regime is characterized by flora that mostly consists of cold-tolerant evergreen conifers, such as the evergreen spruce (Picea), fir (Abies), and pine (Pinus), and the deciduous larch or tamarack (Larix). Before these species are established in a region, there is an increased canopy of early-successional species such as aspen and paper birch (Frelich et al. 1995). Dwarf shrubs (e.g., Vaccinium uliginosum, Ledum decumbens, Betula nana and Cassiope tetragona) and low shrubs (e.g., Betula glandulosa, Salix glauca) act as pioneers that colonize the tundra before trees are able to establish (CAVM Team 2003). The shift to boreal forest can occur over a long time period varying from several decades to centuries depending on the migration time lag for each species and the occurrence of favorable conditions for seedling establishment.

The main external driver of this regime shift is CO₂ emissions that increase climate warming (IPCC 2007), leading to changes in the composition and abundance of arctic vegetation, animals and soil structure (Olofsson et al. 2009). Anthropogenic activities that elevate atmospheric greenhouse gas concentrations are generally considered to be the primary cause for this driver. Carbon release from anthropogenic sources is projected to continue and increase during the coming decades (IPCC 2007). This global driver is well established and could be looked as irreversible in the scale of next hundred years.

The tundra regime is maintained by the low atmospheric temperatures that ensure low soil temperature and the existence of permafrost. This determines the lack of microbial activity and scarce nutrient availability therefore ensuring species composition that are only able to exist in such circumstances.

A shift from the tundra regime to boreal forest is initiated by the atmospheric warming that is caused by continuing increase of CO₂ emissions. The atmospheric temperature increases the period of time when microbial activity and thus nutrient availability is sufficient enough for shrubs and boreal forest pioneer species (birch etc.) to withstand the winter conditions.

As a result the increasing number of shrubs ensure that snow drifting creates deep drifts that surround and extend downwind from shrub patches. These drifts increase the insulation of the soil below. This insulation elevates the soil temperature thus further allowing microbial activity to continue during the frigid arctic winter. The elevated soil temperatures under the snow lead to permafrost degradation that release carbon trapped in tundra soils, thereby further contributing to climate warming.

Shift from Tundra to Boreal Forest

Tundra provides biodiversity for different animal and plant species that characterize tundra ecosystem. The shift from tundra to boreal forest is projected to occur over large geographic areas throughout the tundra zone, with substantial impacts on the landscape and biodiversity (Bonan 1992). Changes in vegetation are also likely to affect composition of foraging mammals and birds (Hinzman et al. 2005). In addition to modifying wildlife habitat, increased woody shrub abundance will make traversing tundra more difficult for caribou, subsistence hunters and communities that rely on caribou for food, as well as hikers. Herbivores such as reindeers and microtine rodents can also be influenced by the shift towards woody vegetation as they prefer lichens, dwarf shrubs graminoids and deciduous shrubs over tall woody shrubs (Sturm et al. 2005). Increased expansion of woody vegetation may therefore result in tundra species extinctions or radical decrease in numbers. The loss of the climate regulating ecosystem service is a major concern as permafrost thawing is reducing the carbon sink service.

For local populations the tundra provides traditional recreation opportunities and contains their knowledge and educational as well as spiritual and religious values, which would be lost with the regime shift.

Shift from Boreal Forest to Tundra 

The change in biodiversity could provide new ecosystem services such as timber production and different wild animals and plant foods. This would benefit loggers as they would increase their income from tree cutting and local communities from goods that come from forest.

In order to prevent or reverse this regime shift it is essential that the input of carbon to the atmosphere is reduced in order to prevent further climate warming. The suggestion of a planetary boundary of 350 ppm for CO₂ levels is a first step in this direction (Rockstrom et al. 2009). Increased understanding of the influence of climate change on the complexity of arctic systems is essential for informing the adaptation of social, economic, and cultural systems to the changes taking place in the Arctic (Hinzman et al. 2005). Technology transfer could be a good initiative from developed countries as they can provide more advanced technological solutions to developing countries to help accelerate the learning curve on GHG emissions. A global response particularly from developed nations that are using the majority of the world's resources on a per capita basis should be in place to deal with such complex systems. Nevertheless, discussions about desirable management strategies and outcomes will be essential as there will be stakeholders who stand to lose from this regime shift (e.g. caribou herders) while others may see the change as desirable and beneficial (e.g. timber production, game hunting).

Thermohaline circulation is complex, but can be summarised as a global scale deep overturning of water masses, with sinking motions occurring in the northern Atlantic and around Antarctica and a transport of relatively warm surface waters towards these sinking regions (Steffen et al. 2004). The term thermohaline circulation thus refers to a particular driving mechanism; it is a physical, not an observational concept.

Strong Thermohaline circulation

THC regime can be characterized by the formation of deep water currents in the high latitudes of both hemispheres; the spreading of deep waters, as deep western boundary currents; upwelling (or upwards movement) of deep waters which is thought to take place mainly in the Antarctic Circumpolar Current region, possibly aided by the wind, and near-surface currents, required to close the flow (Rahmstorf et al. 2006).

Collapse of the Thermohaline circulation

This regime is characterized by weak formation of the dense and saline high latitude deep waters due to the increased influx of freshwater. The decreased salinity of the deep waters is weakening the density southwards driven water movement. The transport of less dense and saline waters from south towards north is decreased causing additional atmospheric temperature increase in tropical areas.

Increasing greenhouse gas concentration from anthropogenic sources is predicted to cause a rise in global mean temperatures (Cubasch et al. 2001). One of the most common anthropogenic greenhouse gases is carbon dioxide (CO2). It is considered to be the main driver for this regime shift and is typically operating in global scale due to the global input of CO2 in the atmosphere. Increasing CO2 levels in atmosphere increases the mean atmospheric temperature. This results in weakening ice-albedo mechanism for the initial regime as a result of increased atmospheric temperatures and changes in albedo.

The increasing freshwater influx from sea-ice loss is accelerated by freshwater-overturning and precipitation-river runoff. This leads to an in increased evaporation, river runoff, and changes to water salinity and density. The burning of fossile fuels such as coal and natural gas also drives this regime shift. This occurs in a regional scale but has global impact and is well established in the literature.

In the regime of strong THC, the most simple explanation of the main variable that maintains the regime would be the high-latitude cooling. In the deep ocean, the predominant driving force of movement of currents is differences in density, caused by salinity and temperature (the more saline the more denser and the colder the water, the more dense). The ocean's density distribution is itself affected by currents and mixing. Thermohaline and wind-driven currents therefore interact in non-linear ways and cannot be separated by oceanographic measurements. Changing the wind stress will alter the THC; altering thermohaline forcing will also change the wind driven currents.

The collapse of THC occurs due to increasing CO2 levels in atmosphere that slowly enhance the mean atmospheric temperature. As a result the high latitude cooling is weakened, increasing freshwater influx from the loss of ice, both from the Arctic, Greenland and possibly the north of Canada. This strengthens two main mechanisms for the new regime: freshwater-overturning mechanism and the precipitation-river runoff mechanism. Both increase the fresh water inflow into the ocean. Therefore new mechanism such as increased evaporation, river runoff, changes in water salinity and density are activated possibly reinforcing the new regime. Once the system has shifted, the warm conditions in Northern latitudes maintain a weak formation of the dense and saline high latitude deep waters due to the increased influx of freshwater.

Most ocean–atmosphere models predict a reduction of the THC for a warmer climate in the near future. Recent observational evidence provides some support to the model projections that the North Atlantic THC circulation could weaken or shut down later this century (Steffen et al. 2004). Lenton et al. (2008) report that the IPCC models predict the regime shift to happen after 2100, however, their models do not include freshwater runoff which is expected to accelerate the process. The models that project the collapse of THC under greater perturbations identify that the collapsed state can last for centuries and might be irreversible (Delworth et al. 2008). Nevertheless the latest IPCC report shows that it is very unlikely that the THC will undergo a large abrupt transition during the course of the 21st century (IPCC 2007). Some models show a recovery of the THC after the elimination of the freshwater perturbation whereas others do not. The mechanism for this different dynamical behavior of the THC is not completely understood (Stouffer et al. 2006).

The weakening of THC will impact global water regulation as water circulations in global scale are influenced by the changes in temperature, salinity and density. In response, climate regulation is altered and changes as the transport of heat is distributed in a different pattern. The water mass and heat circulation will no longer transport warm currents in a northly direction. This could increase the occurence of hurricanes, a southward shift of tropical rainfall belts with resulting agricultural impacts, as well as disruptions to marine ecosystems. Provisioning services such as fisheries, livestock and food crops are therefore potentially affected. Schmittner et al. (2007) show that changes in Atlantic circulation can have large effects on marine ecosystems and biogeochemical cycles, even in areas remote from the Atlantic, such as the Indian and North Pacific Oceans. Although loss of THC could affect Western Europe and result in colder climates. 

Food crisis and emigration are potential threats in the regions that are subjected to this regime shift (Barnett & Adger 2007). In addition, regional changes in sea level are predicted to rise of up to 80 cm in the North Atlantic (Knutti et al. 2002). This could affect the coastlines of the United States, Canada, and Europe causing coastal erosion and affect the security of housing and infrastructure (Delworth et al. 2008). The marine biodiversity of flora and fauna will also be affected. The change in salinity and temperature may facilitate the introduction and spread of invasive species that are able to thrive in the new conditions. This could affect the livelihoods and economic activity of local fishermen as they would have to adapt to new circumstances.  

This regime shift is mainly driven by the increase in greenhouse gases into the atmosphere at a global scale. Therefore a decrease in CO2 emissions and other greenhouse gases has to be achieved in order to prevent further climate warming leading to large influx of freshwater from the depletion of Arctic ice. In the best-case scenario, it may be possible to avert a regime change in a situation where the underlying driver(s) of change are amenable to fast manipulation through management, and where there are not substantial lags in ecosystem response. In the case of the THC neither of these factors hold: the underlying causes of global warming appear very difficult to address through rapid changes in management, and there are substantial inertias in ecosystem response to changes in the driving factors. Therefore an early warning indicators would be useful in providing advance warning of a substantial coming change in ecosystem conditions, but is unlikely to be useful in averting the regime change.

The Greenland Ice Sheet (GIS) is approximately 1.7 million km² in areas covering approximately 80%  of Greenland. It is grounded on bedrock that mostly rests near or above sea level thus would contribute to the globally averaged sea-level rise of 7.3 m if melted completely (Parizek et al. 2004, Lemke et al. 2007).  Anticipated future climate warming has the potential to permanently reduce large areas of GIS or even abate it completely. The evidence suggests nearly total ice-sheet loss may result from warming of more than a few degrees above mean 20th century values, but this threshold is poorly defined (perhaps as little as 2oC or more than 7^oC) (Alley et al. 2010).

Greenland with permanent ice sheet

This regime can be described as permanent ice body cover over major parts (~80%) of Greenland only exposing the bedrock in western and southern parts. The Greenland Ice Sheet has been more closely tied to temperature than to anything else. It shrinks with warming and grows with cooling, thus the volume and cover vary throughout the seasons, but in case of cold climate the relationship between the ice growth/decline would be approximately evenly balanced or with a slightly increased ice growth. In winter when the atmospheric temperatures decrease below freezing point and precipitation levels decline, the accumulation of the ice steadily increases (Bamber et al. 2007).

Greenland without permanent ice sheet

Due to the loss of the ice-sheet volume as a result of the warming, Greenland territory in future could become free from permanent ice sheet cover. This would happen as a consequence of the negative relation to ice growth/ice decline during the winter and summer where the lost ice volume in summer could not be reproduced in the following winter. Rising sea level as a result of warming tends to float marginal regions of ice sheets and force further retreat, so the generally positive relation between sea level and temperature means that typically both reduce the volume of the ice sheet (Alley et al. 2010).

Increasing greenhouse gas concentration from anthropogenic sources is predicted to cause a rise in global mean temperatures (Cubasch et al. 2001). One of the most common anthropogenic greenhouse gases is carbon dioxide (CO2). The influence of this driver is well established as confirmed by many studies. The indirect driver that is increasing anthropogenic CO2 levels in atmosphere is the burning of fossil fuels such as coal and natural gas. It is occurring regionally but has global impact and is well established in literature.

The initial regime would typically occur in cold climate conditions where the relationship between the ice growth/decline would be approximately evenly balanced or with a slightly increased ice growth. The two main mechanisms that maintain this regime are ice-albedo mechanism and meltwater-ice sliding mechanism.

Increasing CO2 levels in atmosphere - the key driver of the regime shift, initiates the increase of atmospheric temperatures and changes in albedo. As a result - increased absorption of solar energy promotes higher air, ice, water and land temperatures which leads towards degrading sea ice. Also the inland surface temperature increase can cause surface melting in the ablation zone that presently accounts for roughly half of the mass loss from the GIS (Parizek et al. 2004). Thus this driver indirectly is increasing drainage of meltwater feeding into crevasses close to the glacier margin resulting in higher calving rates (Murray et al. 2010). Furthermore, thinning and retreating of the glacier tongue due to these increased rates cause reduced effective pressures beneath the glacier, promoting faster flow that results in decrease of ice volume.  

The increase in surface air temperatures changes the ice-albedo feedback thermodynamics. This means that the heat exchange within the sea ice, as well as between the top and bottom of the ice is changed. This leads to a decrease in sea ice volume. The resulting increased amount of open land and water surface in summer decreases the albedo, as the dark ocean surface absorbs more solar radiation (Lindsay et al. 2005).  The annually integrated absorption of solar radiation is observed to increase when the surface albedo is relatively low (Rigor et al. 2002, Holland et al. 2006). This increasingly accumulated amount of heat on the surface reinforces the initial warming. Due to the loss of the ice-sheet volume as a result of the warming, Greenland territory in future could become free from permanent ice sheet cover. This would happen as a consequence of the negative relation to ice growth/ice decline during the winter and summer where the lost ice volume in summer could not be reproduced in the following winter. The Greenland without permanent ice sheet regime is characterized by other dominant feedback mechanisms. For example ice volume-wave action, the water temperature-density and meltwater-ice sliding velocity mechanisms.

The shift to the regime of Greenland without ice sheet will mainly result in loss of some desirable ecosystem services. The ecosystem service of desirable climate regulation could be lost as the change in movement of currents (change in thermohaline circulation) and air masses would alter the transport of heat. This could lead to increased hurricane activity, a southward shift of tropical rainfall belts with resulting agricultural impacts, and disruptions to marine ecosystems.

The loss of certain animal and plant food species as provisioning services is predicted in the future. These changes may have important consequences for food webs and could well be extremely significant for the Greenland economy, which is highly dependent on fisheries (AMAP 2007). Such cultural services like recreation and aesthetical values would also be affected. Each of those services attracts more people to see the Ice sheet thus also bringing in more tourists. Water regulation as regulating ecosystem services could be altered through the large input of freshwater in the water cycle.  The vast amount of "stored" water entering the water cycle within warmer climate would result in severe winter precipitation.

A new ecosystem service is possible as the thawing ice sheet will potentially form glacial freshwater lakes in Greenland. This will generate new recreation opportunities in summer – using lakes for different purposes from different social groups. Flora could expand deeper into Greenland and new species could be introduced as the climate warms giving the local population the chance to gain additional plant foods.

The potential options for preventing or reversing this potential regime shift mainly relates to the decrease of greenhouse gas input into the atmosphere at a global scale. This has to be achieved in order to prevent further climate warming leading to the loss of Greenland Ice Sheets. Options include reduction of deforestation, use of fossil fuels and charcoal as energy source, and cleaner economies. Geoengineering strategies has also been proposed, large scale experiments to decrease global temperature and CO2 concentration in the atmosphere. However, their applicability is debated and the usefulness is contested. As this system boundaries are set mainly around geophysical variables it is necessary to look at the social mechanisms involved to limit the influence on the main direct driver of greenhouse gas emissions. Nevertheless even if CO2 levels in atmosphere leads to atmospheric temperature increase, it is very hard to achieve from the local to regional management perspective. It does require global coordination and cooperation in order to achieve CO2 reduction targets.

The system is defined by the ice volume and the territory it covers in the Arctic Ocean and the regional/global processes that ensure the existence of ice in this area. The loss of surface area and thinning of Arctic sea ice has not occurred at a linear rate which may be indicative of a systematic change towards an alternate regime.

Arctic with summer ice

Under this regime, the Arctic Ocean has an abundance of sea ice.  It is characterized by very long and cold winters, during which the ice surface area and thickness reach their maximum. The low winter temperatures and short summer help to maximize the sea ice surface area and volume over time.

Arctic without summer ice

In this regime the surface area and volume of summer sea ice in the Arctic rapidly decreases due to atmospheric warming caused by greenhouse gases. In summer when open water surface area is greater, the albedo is reduced, which causes greater absorption of solar radiation.  This raises the temperature of the water and ice, which facilitates greater losses in sea ice surface area and volume. Several models predict that ice free Arctic conditions in summer will be reached within this century (Arzel et al. 2006). Several authors have suggested that the system has already surpassed a tipping point, but convincing evidence is lacking (Lenton et al. 2008).

The main driver of this regime shift is elevated greenhouse gas concentrations in the atmosphere causing an increase in arctic air temperatures. This global driver is well established and could be looked as irreversible in the scale of next hundred years. In regards to the loss of sea ice in the Arctic, the regime shift is generally considered to be irreversible unless the main driver (increased atmospheric temperatures resulting from climate change) is changed in the near future.

Anthropogenic activities that elevate atmospheric greenhouse gas concentrations are generally considered to be the primary driver of climate change (IPCC 2007; Kinnard et. al 2011). Carbon release from anthropogenic sources is projected to continue and increase during the coming decades (IPCC 2007). This is expected to contribute to an increase in average global temperatures and more rapid decrease in sea ice cover and thickness in the Arctic. This driver initially affects the main ice-albedo mechanism thus changing the processes that characterize its initial state. Once the main mechanism has shifted the driver and the altered ice-albedo mechanism initiates change in other parts of the system.

The Arctic with summer ice regime is maintained by permanent low surface air temperatures (SAT) that maintain the thermal balance, thus ensuring balanced heat exchange between the atmosphere, sea ice, and water. The result is maintained sea ice volume, thickness and surface area. This occurs due to ensuring high albedo (a measure of reflectance) level as the dark ocean surface absorbs more solar radiation than the sea ice (Lindsay et al. 2005). This means that the high albedo reflects more radiation avoiding surface temperature increase. Avoiding increased absorption of solar energy promotes lower air, ice, water and land temperatures which lead towards maintaining sea ice. In the end, the low temperatures further promote ice maintaining arctic conditions.

There is near universal agreement that the extent of Arctic sea ice will decline through the 21st century in response to increasing atmospheric greenhouse gas (GHG) concentrations (Zhang 2006). The resulting increase in surface air temperatures (SAT) change the thermal balance which means that the heat exchange between the atmosphere, sea ice, and water is changed. The result is a decrease in sea ice volume, thickness and surface area. The increased area of open water in summer decreases the albedo as the dark ocean surface absorbs more solar radiation than the sea ice (Lindsay et al. 2005).  Increased absorption of solar energy promotes higher air, ice, water and land temperatures which lead towards degrading sea ice volume (Rigor et al. 2002; Holland et al. 2006). In the end, the increasing temperatures and accumulated heat further promote warming arctic conditions

Changes in the summer extent of Arctic sea ice are not solely forced by SATs, but could also be affected by fluctuations of atmospheric pressure at sea level that controls the strength and direction of windsin the region. More probably these changes could be driven by a combination of these (and/or other) forcing (Kinnard et al. 2011).

Local knowledge and spiritual values might be lost as the local communities have to adapt to the new circumstances and thus their lifestyle. In addition to concerns about the security of infrastructure and impacts on human well being, ice free Arctic summers have important impacts on ecosystems. One such impact is that loss of ice cover could affect the Arctic's freshwater system and surface energy budget, and manifest in middle latitudes as altered patterns in atmospheric circulation and precipitation (Serreze et al. 2007). This presents the way how water and atmospheric circulations could be altered as ecosystem services.

Summer sea ice concentration is important for navigation, and may have implications for the transport of sediments and pollutants across the Arctic. Most of the sea ice formed in the Arctic Ocean is exported through the Fram Strait into the Greenland Sea and to the North Atlantic where the ice may affect the global thermohaline circulation (Rigor et al. 2002). Sea ice also blocks the solar flux to the water and hence is a major control factor phytoplankton to seals, walrus, and polar bears while limiting access to the surface for seals and whales (Lindsay et al. 2005).

The rapidly melting sea ice in the Arctic Ocean has increased political and economic interest in the region's resource extraction and in the potential for more accessible shipping routes. By opening the Northwest passage, shipping route through the northern Canadian waters, could result in a positive economic impact. Although this also could potentially result in ecological disasters as the possibility of oil spills and other disasters associated with development would increase.

The options for preventing or reversing the loss of summer sea ice in the Arctic primarily relate to the decrease of greenhouse gas emissions on a global scale to reduce climatic warming. As atmospheric greenhouse gas concentrations increase, it is essential to understand local and regional actions that may influence the feedback mechanisms influencing the shift to an ice free summer Arctic. Technology transfer could be a good initiative from developed countries as they can provide more advanced technological solutions to developing countries to help accelerate the learning curve on GHG emissions. A Global response particularly from developed nations that are using the majority of the world’s resources on a per capita basis should be in place to deal with such complex system.

This regime shift entails extensive change in biodiversity and soil structure in the Arctic tundra due to increased climate warming. To date shrub invasion has been the main indicator of a potential regime shift.

Arctic tundra regime

This regime is characterized by low atmospheric temperatures that enable the formation of a layer of permanently frozen subsoil (permafrost) consisting mostly of gravel and finer material. The ecosystem has a low rate of primary production due to impeded mineral nutrient cycling. The short growing season in summer influences the vegetation and its root systems by limiting vertical expansion. Only plant species (lichens, liverworts, mosses) that are adapted to sweeping winds and soil disturbance can survive in these conditions. The atmospheric cold and existing permafrost restrains the plant growth ensuring that their roots are shallow and surface parts are low growing due to the frozen ground underneath the permafrost. Snow does not accumulate keeping the soil temperature low thus not allowing for permafrost to thaw and release carbon into atmosphere keeping the atmosphere cold.

Boreal forest regime

This regime is characterized by flora that mostly consists of cold-tolerant evergreen conifers, such as the evergreen spruce (Picea), fir (Abies), and pine (Pinus), and the deciduous larch or tamarack (Larix). Before these species are established in a region, there is an increased canopy of early-successional species such as aspen and paper birch (Frelich et al. 1995). Dwarf shrubs (e.g., Vaccinium uliginosum, Ledum decumbens, Betula nana and Cassiope tetragona) and low shrubs (e.g., Betula glandulosa, Salix glauca) act as pioneers that colonize the tundra before trees are able to establish (CAVM Team 2003). The shift to boreal forest can occur over a long time period varying from several decades to centuries depending on the migration time lag for each species and the occurrence of favorable conditions for seedling establishment.

The main external driver of this regime shift is CO₂ emissions that increase climate warming (IPCC 2007), leading to changes in the composition and abundance of arctic vegetation, animals and soil structure (Olofsson et al. 2009). Anthropogenic activities that elevate atmospheric greenhouse gas concentrations are generally considered to be the primary cause for this driver. Carbon release from anthropogenic sources is projected to continue and increase during the coming decades (IPCC 2007). This global driver is well established and could be looked as irreversible in the scale of next hundred years.

The tundra regime is maintained by the low atmospheric temperatures that ensure low soil temperature and the existence of permafrost. This determines the lack of microbial activity and scarce nutrient availability therefore ensuring species composition that are only able to exist in such circumstances.

A shift from the tundra regime to boreal forest is initiated by the atmospheric warming that is caused by continuing increase of CO₂ emissions. The atmospheric temperature increases the period of time when microbial activity and thus nutrient availability is sufficient enough for shrubs and boreal forest pioneer species (birch etc.) to withstand the winter conditions.

As a result the increasing number of shrubs ensure that snow drifting creates deep drifts that surround and extend downwind from shrub patches. These drifts increase the insulation of the soil below. This insulation elevates the soil temperature thus further allowing microbial activity to continue during the frigid arctic winter. The elevated soil temperatures under the snow lead to permafrost degradation that release carbon trapped in tundra soils, thereby further contributing to climate warming.

Shift from Tundra to Boreal Forest

Tundra provides biodiversity for different animal and plant species that characterize tundra ecosystem. The shift from tundra to boreal forest is projected to occur over large geographic areas throughout the tundra zone, with substantial impacts on the landscape and biodiversity (Bonan 1992). Changes in vegetation are also likely to affect composition of foraging mammals and birds (Hinzman et al. 2005). In addition to modifying wildlife habitat, increased woody shrub abundance will make traversing tundra more difficult for caribou, subsistence hunters and communities that rely on caribou for food, as well as hikers. Herbivores such as reindeers and microtine rodents can also be influenced by the shift towards woody vegetation as they prefer lichens, dwarf shrubs graminoids and deciduous shrubs over tall woody shrubs (Sturm et al. 2005). Increased expansion of woody vegetation may therefore result in tundra species extinctions or radical decrease in numbers. The loss of the climate regulating ecosystem service is a major concern as permafrost thawing is reducing the carbon sink service.

For local populations the tundra provides traditional recreation opportunities and contains their knowledge and educational as well as spiritual and religious values, which would be lost with the regime shift.

Shift from Boreal Forest to Tundra 

The change in biodiversity could provide new ecosystem services such as timber production and different wild animals and plant foods. This would benefit loggers as they would increase their income from tree cutting and local communities from goods that come from forest.

In order to prevent or reverse this regime shift it is essential that the input of carbon to the atmosphere is reduced in order to prevent further climate warming. The suggestion of a planetary boundary of 350 ppm for CO₂ levels is a first step in this direction (Rockstrom et al. 2009). Increased understanding of the influence of climate change on the complexity of arctic systems is essential for informing the adaptation of social, economic, and cultural systems to the changes taking place in the Arctic (Hinzman et al. 2005). Technology transfer could be a good initiative from developed countries as they can provide more advanced technological solutions to developing countries to help accelerate the learning curve on GHG emissions. A global response particularly from developed nations that are using the majority of the world's resources on a per capita basis should be in place to deal with such complex systems. Nevertheless, discussions about desirable management strategies and outcomes will be essential as there will be stakeholders who stand to lose from this regime shift (e.g. caribou herders) while others may see the change as desirable and beneficial (e.g. timber production, game hunting).

Thermohaline circulation is complex, but can be summarised as a global scale deep overturning of water masses, with sinking motions occurring in the northern Atlantic and around Antarctica and a transport of relatively warm surface waters towards these sinking regions (Steffen et al. 2004). The term thermohaline circulation thus refers to a particular driving mechanism; it is a physical, not an observational concept.

Strong Thermohaline circulation

THC regime can be characterized by the formation of deep water currents in the high latitudes of both hemispheres; the spreading of deep waters, as deep western boundary currents; upwelling (or upwards movement) of deep waters which is thought to take place mainly in the Antarctic Circumpolar Current region, possibly aided by the wind, and near-surface currents, required to close the flow (Rahmstorf et al. 2006).

Collapse of the Thermohaline circulation

This regime is characterized by weak formation of the dense and saline high latitude deep waters due to the increased influx of freshwater. The decreased salinity of the deep waters is weakening the density southwards driven water movement. The transport of less dense and saline waters from south towards north is decreased causing additional atmospheric temperature increase in tropical areas.

Increasing greenhouse gas concentration from anthropogenic sources is predicted to cause a rise in global mean temperatures (Cubasch et al. 2001). One of the most common anthropogenic greenhouse gases is carbon dioxide (CO2). It is considered to be the main driver for this regime shift and is typically operating in global scale due to the global input of CO2 in the atmosphere. Increasing CO2 levels in atmosphere increases the mean atmospheric temperature. This results in weakening ice-albedo mechanism for the initial regime as a result of increased atmospheric temperatures and changes in albedo.

The increasing freshwater influx from sea-ice loss is accelerated by freshwater-overturning and precipitation-river runoff. This leads to an in increased evaporation, river runoff, and changes to water salinity and density. The burning of fossile fuels such as coal and natural gas also drives this regime shift. This occurs in a regional scale but has global impact and is well established in the literature.

In the regime of strong THC, the most simple explanation of the main variable that maintains the regime would be the high-latitude cooling. In the deep ocean, the predominant driving force of movement of currents is differences in density, caused by salinity and temperature (the more saline the more denser and the colder the water, the more dense). The ocean's density distribution is itself affected by currents and mixing. Thermohaline and wind-driven currents therefore interact in non-linear ways and cannot be separated by oceanographic measurements. Changing the wind stress will alter the THC; altering thermohaline forcing will also change the wind driven currents.

The collapse of THC occurs due to increasing CO2 levels in atmosphere that slowly enhance the mean atmospheric temperature. As a result the high latitude cooling is weakened, increasing freshwater influx from the loss of ice, both from the Arctic, Greenland and possibly the north of Canada. This strengthens two main mechanisms for the new regime: freshwater-overturning mechanism and the precipitation-river runoff mechanism. Both increase the fresh water inflow into the ocean. Therefore new mechanism such as increased evaporation, river runoff, changes in water salinity and density are activated possibly reinforcing the new regime. Once the system has shifted, the warm conditions in Northern latitudes maintain a weak formation of the dense and saline high latitude deep waters due to the increased influx of freshwater.

Most ocean–atmosphere models predict a reduction of the THC for a warmer climate in the near future. Recent observational evidence provides some support to the model projections that the North Atlantic THC circulation could weaken or shut down later this century (Steffen et al. 2004). Lenton et al. (2008) report that the IPCC models predict the regime shift to happen after 2100, however, their models do not include freshwater runoff which is expected to accelerate the process. The models that project the collapse of THC under greater perturbations identify that the collapsed state can last for centuries and might be irreversible (Delworth et al. 2008). Nevertheless the latest IPCC report shows that it is very unlikely that the THC will undergo a large abrupt transition during the course of the 21st century (IPCC 2007). Some models show a recovery of the THC after the elimination of the freshwater perturbation whereas others do not. The mechanism for this different dynamical behavior of the THC is not completely understood (Stouffer et al. 2006).

The weakening of THC will impact global water regulation as water circulations in global scale are influenced by the changes in temperature, salinity and density. In response, climate regulation is altered and changes as the transport of heat is distributed in a different pattern. The water mass and heat circulation will no longer transport warm currents in a northly direction. This could increase the occurence of hurricanes, a southward shift of tropical rainfall belts with resulting agricultural impacts, as well as disruptions to marine ecosystems. Provisioning services such as fisheries, livestock and food crops are therefore potentially affected. Schmittner et al. (2007) show that changes in Atlantic circulation can have large effects on marine ecosystems and biogeochemical cycles, even in areas remote from the Atlantic, such as the Indian and North Pacific Oceans. Although loss of THC could affect Western Europe and result in colder climates. 

Food crisis and emigration are potential threats in the regions that are subjected to this regime shift (Barnett & Adger 2007). In addition, regional changes in sea level are predicted to rise of up to 80 cm in the North Atlantic (Knutti et al. 2002). This could affect the coastlines of the United States, Canada, and Europe causing coastal erosion and affect the security of housing and infrastructure (Delworth et al. 2008). The marine biodiversity of flora and fauna will also be affected. The change in salinity and temperature may facilitate the introduction and spread of invasive species that are able to thrive in the new conditions. This could affect the livelihoods and economic activity of local fishermen as they would have to adapt to new circumstances.  

This regime shift is mainly driven by the increase in greenhouse gases into the atmosphere at a global scale. Therefore a decrease in CO2 emissions and other greenhouse gases has to be achieved in order to prevent further climate warming leading to large influx of freshwater from the depletion of Arctic ice. In the best-case scenario, it may be possible to avert a regime change in a situation where the underlying driver(s) of change are amenable to fast manipulation through management, and where there are not substantial lags in ecosystem response. In the case of the THC neither of these factors hold: the underlying causes of global warming appear very difficult to address through rapid changes in management, and there are substantial inertias in ecosystem response to changes in the driving factors. Therefore an early warning indicators would be useful in providing advance warning of a substantial coming change in ecosystem conditions, but is unlikely to be useful in averting the regime change.

The Greenland Ice Sheet (GIS) is approximately 1.7 million km² in areas covering approximately 80%  of Greenland. It is grounded on bedrock that mostly rests near or above sea level thus would contribute to the globally averaged sea-level rise of 7.3 m if melted completely (Parizek et al. 2004, Lemke et al. 2007).  Anticipated future climate warming has the potential to permanently reduce large areas of GIS or even abate it completely. The evidence suggests nearly total ice-sheet loss may result from warming of more than a few degrees above mean 20th century values, but this threshold is poorly defined (perhaps as little as 2oC or more than 7^oC) (Alley et al. 2010).

Greenland with permanent ice sheet

This regime can be described as permanent ice body cover over major parts (~80%) of Greenland only exposing the bedrock in western and southern parts. The Greenland Ice Sheet has been more closely tied to temperature than to anything else. It shrinks with warming and grows with cooling, thus the volume and cover vary throughout the seasons, but in case of cold climate the relationship between the ice growth/decline would be approximately evenly balanced or with a slightly increased ice growth. In winter when the atmospheric temperatures decrease below freezing point and precipitation levels decline, the accumulation of the ice steadily increases (Bamber et al. 2007).

Greenland without permanent ice sheet

Due to the loss of the ice-sheet volume as a result of the warming, Greenland territory in future could become free from permanent ice sheet cover. This would happen as a consequence of the negative relation to ice growth/ice decline during the winter and summer where the lost ice volume in summer could not be reproduced in the following winter. Rising sea level as a result of warming tends to float marginal regions of ice sheets and force further retreat, so the generally positive relation between sea level and temperature means that typically both reduce the volume of the ice sheet (Alley et al. 2010).

Increasing greenhouse gas concentration from anthropogenic sources is predicted to cause a rise in global mean temperatures (Cubasch et al. 2001). One of the most common anthropogenic greenhouse gases is carbon dioxide (CO2). The influence of this driver is well established as confirmed by many studies. The indirect driver that is increasing anthropogenic CO2 levels in atmosphere is the burning of fossil fuels such as coal and natural gas. It is occurring regionally but has global impact and is well established in literature.

The initial regime would typically occur in cold climate conditions where the relationship between the ice growth/decline would be approximately evenly balanced or with a slightly increased ice growth. The two main mechanisms that maintain this regime are ice-albedo mechanism and meltwater-ice sliding mechanism.

Increasing CO2 levels in atmosphere - the key driver of the regime shift, initiates the increase of atmospheric temperatures and changes in albedo. As a result - increased absorption of solar energy promotes higher air, ice, water and land temperatures which leads towards degrading sea ice. Also the inland surface temperature increase can cause surface melting in the ablation zone that presently accounts for roughly half of the mass loss from the GIS (Parizek et al. 2004). Thus this driver indirectly is increasing drainage of meltwater feeding into crevasses close to the glacier margin resulting in higher calving rates (Murray et al. 2010). Furthermore, thinning and retreating of the glacier tongue due to these increased rates cause reduced effective pressures beneath the glacier, promoting faster flow that results in decrease of ice volume.  

The increase in surface air temperatures changes the ice-albedo feedback thermodynamics. This means that the heat exchange within the sea ice, as well as between the top and bottom of the ice is changed. This leads to a decrease in sea ice volume. The resulting increased amount of open land and water surface in summer decreases the albedo, as the dark ocean surface absorbs more solar radiation (Lindsay et al. 2005).  The annually integrated absorption of solar radiation is observed to increase when the surface albedo is relatively low (Rigor et al. 2002, Holland et al. 2006). This increasingly accumulated amount of heat on the surface reinforces the initial warming. Due to the loss of the ice-sheet volume as a result of the warming, Greenland territory in future could become free from permanent ice sheet cover. This would happen as a consequence of the negative relation to ice growth/ice decline during the winter and summer where the lost ice volume in summer could not be reproduced in the following winter. The Greenland without permanent ice sheet regime is characterized by other dominant feedback mechanisms. For example ice volume-wave action, the water temperature-density and meltwater-ice sliding velocity mechanisms.

The shift to the regime of Greenland without ice sheet will mainly result in loss of some desirable ecosystem services. The ecosystem service of desirable climate regulation could be lost as the change in movement of currents (change in thermohaline circulation) and air masses would alter the transport of heat. This could lead to increased hurricane activity, a southward shift of tropical rainfall belts with resulting agricultural impacts, and disruptions to marine ecosystems.

The loss of certain animal and plant food species as provisioning services is predicted in the future. These changes may have important consequences for food webs and could well be extremely significant for the Greenland economy, which is highly dependent on fisheries (AMAP 2007). Such cultural services like recreation and aesthetical values would also be affected. Each of those services attracts more people to see the Ice sheet thus also bringing in more tourists. Water regulation as regulating ecosystem services could be altered through the large input of freshwater in the water cycle.  The vast amount of "stored" water entering the water cycle within warmer climate would result in severe winter precipitation.

A new ecosystem service is possible as the thawing ice sheet will potentially form glacial freshwater lakes in Greenland. This will generate new recreation opportunities in summer – using lakes for different purposes from different social groups. Flora could expand deeper into Greenland and new species could be introduced as the climate warms giving the local population the chance to gain additional plant foods.

The potential options for preventing or reversing this potential regime shift mainly relates to the decrease of greenhouse gas input into the atmosphere at a global scale. This has to be achieved in order to prevent further climate warming leading to the loss of Greenland Ice Sheets. Options include reduction of deforestation, use of fossil fuels and charcoal as energy source, and cleaner economies. Geoengineering strategies has also been proposed, large scale experiments to decrease global temperature and CO2 concentration in the atmosphere. However, their applicability is debated and the usefulness is contested. As this system boundaries are set mainly around geophysical variables it is necessary to look at the social mechanisms involved to limit the influence on the main direct driver of greenhouse gas emissions. Nevertheless even if CO2 levels in atmosphere leads to atmospheric temperature increase, it is very hard to achieve from the local to regional management perspective. It does require global coordination and cooperation in order to achieve CO2 reduction targets.

The system is defined by the ice volume and the territory it covers in the Arctic Ocean and the regional/global processes that ensure the existence of ice in this area. The loss of surface area and thinning of Arctic sea ice has not occurred at a linear rate which may be indicative of a systematic change towards an alternate regime.

Arctic with summer ice

Under this regime, the Arctic Ocean has an abundance of sea ice.  It is characterized by very long and cold winters, during which the ice surface area and thickness reach their maximum. The low winter temperatures and short summer help to maximize the sea ice surface area and volume over time.

Arctic without summer ice

In this regime the surface area and volume of summer sea ice in the Arctic rapidly decreases due to atmospheric warming caused by greenhouse gases. In summer when open water surface area is greater, the albedo is reduced, which causes greater absorption of solar radiation.  This raises the temperature of the water and ice, which facilitates greater losses in sea ice surface area and volume. Several models predict that ice free Arctic conditions in summer will be reached within this century (Arzel et al. 2006). Several authors have suggested that the system has already surpassed a tipping point, but convincing evidence is lacking (Lenton et al. 2008).

The main driver of this regime shift is elevated greenhouse gas concentrations in the atmosphere causing an increase in arctic air temperatures. This global driver is well established and could be looked as irreversible in the scale of next hundred years. In regards to the loss of sea ice in the Arctic, the regime shift is generally considered to be irreversible unless the main driver (increased atmospheric temperatures resulting from climate change) is changed in the near future.

Anthropogenic activities that elevate atmospheric greenhouse gas concentrations are generally considered to be the primary driver of climate change (IPCC 2007; Kinnard et. al 2011). Carbon release from anthropogenic sources is projected to continue and increase during the coming decades (IPCC 2007). This is expected to contribute to an increase in average global temperatures and more rapid decrease in sea ice cover and thickness in the Arctic. This driver initially affects the main ice-albedo mechanism thus changing the processes that characterize its initial state. Once the main mechanism has shifted the driver and the altered ice-albedo mechanism initiates change in other parts of the system.

The Arctic with summer ice regime is maintained by permanent low surface air temperatures (SAT) that maintain the thermal balance, thus ensuring balanced heat exchange between the atmosphere, sea ice, and water. The result is maintained sea ice volume, thickness and surface area. This occurs due to ensuring high albedo (a measure of reflectance) level as the dark ocean surface absorbs more solar radiation than the sea ice (Lindsay et al. 2005). This means that the high albedo reflects more radiation avoiding surface temperature increase. Avoiding increased absorption of solar energy promotes lower air, ice, water and land temperatures which lead towards maintaining sea ice. In the end, the low temperatures further promote ice maintaining arctic conditions.

There is near universal agreement that the extent of Arctic sea ice will decline through the 21st century in response to increasing atmospheric greenhouse gas (GHG) concentrations (Zhang 2006). The resulting increase in surface air temperatures (SAT) change the thermal balance which means that the heat exchange between the atmosphere, sea ice, and water is changed. The result is a decrease in sea ice volume, thickness and surface area. The increased area of open water in summer decreases the albedo as the dark ocean surface absorbs more solar radiation than the sea ice (Lindsay et al. 2005).  Increased absorption of solar energy promotes higher air, ice, water and land temperatures which lead towards degrading sea ice volume (Rigor et al. 2002; Holland et al. 2006). In the end, the increasing temperatures and accumulated heat further promote warming arctic conditions

Changes in the summer extent of Arctic sea ice are not solely forced by SATs, but could also be affected by fluctuations of atmospheric pressure at sea level that controls the strength and direction of windsin the region. More probably these changes could be driven by a combination of these (and/or other) forcing (Kinnard et al. 2011).

Local knowledge and spiritual values might be lost as the local communities have to adapt to the new circumstances and thus their lifestyle. In addition to concerns about the security of infrastructure and impacts on human well being, ice free Arctic summers have important impacts on ecosystems. One such impact is that loss of ice cover could affect the Arctic's freshwater system and surface energy budget, and manifest in middle latitudes as altered patterns in atmospheric circulation and precipitation (Serreze et al. 2007). This presents the way how water and atmospheric circulations could be altered as ecosystem services.

Summer sea ice concentration is important for navigation, and may have implications for the transport of sediments and pollutants across the Arctic. Most of the sea ice formed in the Arctic Ocean is exported through the Fram Strait into the Greenland Sea and to the North Atlantic where the ice may affect the global thermohaline circulation (Rigor et al. 2002). Sea ice also blocks the solar flux to the water and hence is a major control factor phytoplankton to seals, walrus, and polar bears while limiting access to the surface for seals and whales (Lindsay et al. 2005).

The rapidly melting sea ice in the Arctic Ocean has increased political and economic interest in the region's resource extraction and in the potential for more accessible shipping routes. By opening the Northwest passage, shipping route through the northern Canadian waters, could result in a positive economic impact. Although this also could potentially result in ecological disasters as the possibility of oil spills and other disasters associated with development would increase.

The options for preventing or reversing the loss of summer sea ice in the Arctic primarily relate to the decrease of greenhouse gas emissions on a global scale to reduce climatic warming. As atmospheric greenhouse gas concentrations increase, it is essential to understand local and regional actions that may influence the feedback mechanisms influencing the shift to an ice free summer Arctic. Technology transfer could be a good initiative from developed countries as they can provide more advanced technological solutions to developing countries to help accelerate the learning curve on GHG emissions. A Global response particularly from developed nations that are using the majority of the world’s resources on a per capita basis should be in place to deal with such complex system.

The Indian Monsoon system is solar heating driven and starts in the southernmost point of the Indian Peninsula where it splits into two parts. One branch moves along western part of India northwards and the other flows over the Bay of Bengal heading towards North-East India and Bengal, picking up moisture from the Bay of Bengal. The winds then arrive at the Eastern Himalayas along with heavy rainfall. After the arrival at the Eastern Himalayas, the winds turn West, covering northern and eastern India, Bangladesh, parts of Pakistan, and southern Nepal pouring rain all along its way. There are several changes in monsoon intensity that have been documented on centennial to millennial time scales, when the East Asian Monsoon rapidly strengthened while the Indian Summer Monsoon rapidly weakened (Bing et al. 2006). Although several studies have indicated that the East Asian summer monsoon has also become weaker after the end of the 1970s (Wang et. al 2001). Therefore, the end of 1970s may be viewed as an abrupt and significant change point in the inter-decadal variability of the East Asian summer monsoon (Ding et al. 2008). The change of both monsoon systems weakening at the same time might therefore point to a shift in the interconnected monsoon system. 

Indian summer monsoon with mean and regular precipitation within the season

In this regime precipitation reaches its annual mean and ensures that agriculture receives enough water to sustain the production of crops and other goods. The rapid change in increase of CO² concentrations in the atmosphere has supported the existing regime increasing the sea surface temperature thus increasing the precipitation. The Indian Summer monsoon circulation is characterized by monsoon rains arriving at the start of June as warm air converges and ascends in the low pressure over the continent, leading to clouds and heavy rainfall. Monsoon rains advance northward as summer progresses, enhanced by forced ascent as the flow reaches the Himalayan foothills. In winter, this thermally direct circulation reverses as the land surface cools relative to the oceans. Rains continue till the end of September when the season ends. (Rickenbach et al. 2009).

Indian summer monsoon with weak and irregular precipitation within the season

This regime is characterized by weak precipitation causing droughts with the rain arriving much later than expected in June. The increasing emissions of fossil fuel SO² and black carbon result in large atmospheric concentrations of black carbon and other aerosols generating atmospheric Brown clouds. This induces strong negative trends in surface solar radiation, surface evaporation, and summer monsoon rainfall (Ramanathan 2005). As the monsoon season starts, precipitation can be irregular - raining in June and then having weak or no precipitation in July and returning in August.

The main direct driver of changing monsoon rains is deforestation associated with the indirect driver of increased food production. As a consequence of the vegetation loss due to deforestation the surface albedo is increased. Therefore the amount of reflected solar radiation increases due to high albedo, thus decreasing the temperature difference between ocean and land. All this leads to change in the main monsoon circulation mechanism that is responsible for precipitation in the region.

The increasing concentration of CO² in the atmosphere is a driver, although its not sure as to whether it maintains the current regime or pushes the system towards a regime with weaker precipitation. Researchers are still arguing which case is most likely and it depends on the source to present the influence of this driver. IPCC report (2007) predicts that carbon release from anthropogenic sources will continue increasing during the coming decades. There are studies indicating that emissions of greenhouse gases that alter the heat budget of the system and therefore the land-sea temperature contrast, could increase the monsoon intensity and/or variability (Knopf et al 2008; Kripalani et al. 2007). Nevertheless Palmer et al. (1992) pointed out that enhanced convection (the transfer of heat by the actual movement of the warmed matter) associated with the warm SST anomalies suppresses the monsoon rainfall.

For the monsoon precipitation to occur the main atmospheric processes have to be present (latent heat, land to ocean pressure gradient, advection etc.). The release of latent heat from precipitation over land ensures the increase in the temperature difference between land and ocean. This enhances the land to ocean pressure gradient that determine in which direction and at what rate the pressure changes around the Indian peninsula. The increased pressure gradient leads to stronger winds and pushes more moist air northward from the ocean onto the continent. The stronger flows on shore increases landward advection of moisture eventually forming rain clouds which leads to increased precipitation and associated release of latent heat.

However the increasing deforestation in regional scale reduces vegetation cover therefore decreasing rainfall and increasing surface albedo. The monsoon circulation is thereby weakened as the amount of reflected solar radiation increases due to high albedo, decreasing the temperature difference between ocean and land.

As a result weaker pressure gradient leads to weaker winds pushing less moist air onto the continent. As the flows onshore are weaker landward advection of moisture decreases, which leads to decreased precipitation. This reinforces the lack of vegetation cover as the soil moisture decreases resulting in droughts and increased biomass burning occurs which decreases vegetation cover even further.

Due to irregular precipitation one of the most essential and recognized provisioning services that could be lost is freshwater as the groundwater levels are becoming deeper thus drying up the soil. Food crops and livestock are two other provisioning services that are directly affected by this regime shift and linked with the freshwater service. Losing these services would mean poor harvests and food shortages from lack of cattle among the rural population, which constitutes two thirds of India's total population (Knopf et al. 2008). Timber production would also be affected, as barren and dried soils can no longer support the growth of trees. Also the fire frequency would increase damaging timber productivity even more. Regulating services would be lost, such as climate regulation as a weak summer monsoon can change climate variability (Gordon et al. 2008). Air quality regulation as a service is endangered because of increased aerosol/dust and Brown cloud concentrations caused from biomass burning and industrial pollution. Regulation of water and soil erosion would be lost as monsoon rains ensure that soils are moist and inhabited with flora and fauna enough not to lose the fertile topsoil due to wind or other type of erosions. Water regulation would also be in danger, as lack of precipitation would alter the water cycle changing the typical water distribution in it. Biodiversity would rapidly decrease in the case of weak precipitation in the long term.

Human wellbeing would also be affected, as monsoon rains are critical to the functioning of hydroelectric power plants. The lack of precipitation would therefore disrupt energy supply which may cause delays in productions or increase product costs. The loss of food crops may results in food crisis, leading to rapid inflation on food prices. In turn this can lead to large number of people suffering from hunger, as any adverse effect on farming will affect the purchasing power of the people as well. The hunger and rise of poverty could result in large numbers of people emigrating from the region (Barnet & Adger 2007). Lack of freshwater also could aggravate the sanitation and health issues that already haven't been completely solved. Crime levels could potentially rise as the depression among society would increase and the necessity for food would drive the people to support their families in any circumstances (Barnet & Adger 2007).

Options for preventing a weakening of the Indian summer monsoon circulation system primary relates to the sustainable management of the local and regional vegetation cover. The area of complete deforestation should be decreased and cropland area planning has to be in place. That has to be done in order to avoid rapid changes in surface albedo in large areas that can change the existing feedback mechanisms. Sustainable water management planning has to be practised to avoid significant water due to irrigation. New irrigation practises has to be considered such as the installation of drip irrigation and low pressure pivots to get more yield with less use of water. It has also been suggested that a better weather forecasting system for India would help people to better adapt strategies in times of droughts and floods induced by the monsoon variability (Nature News Blog, Aug-2011). Greenhouse gas emissions also need to be managed to reduce air pollution and the amount of black carbon in the atmosphere tht increases the formation of brown clouds that negatively influence the monsoon rainfall. 

Technology transfer could be a good initiative from developed countries as they can provide more advanced technological solutions and funding to developing countries to help accelerate reduction of greenhouse gas emissions and irrigation management. Investing in sustainable irrigation tools and supporting the industrial production industry for example with filters that decrease the amount of pollutants entering the atmosphere, techniques that are energy efficient. Overall the shift in the Indian summer monsoon circulation is considered to be irreversible if the changes in vegetation cover due to food production continue to increase.