Strong Thermohaline circulation
- Freshwater-overturning mechanism (regional, well established) This feedback mechanism is perceived as the key for the THC to exist. In the deep ocean, the predominant driving force of movement of currents is differences in water density, caused by salinity and temperature: water is denser as salinity increase and temperature decrease. This is why deep water is presently formed in the North Atlantic, which is salty, but not in the North Pacific, which is fresher.
Part of this mechanism relies on the ice-albedo feedback mechanism. The optimal atmospheric temperatures that maintain sea surface temperature appropriate to avoid the loss of ice in Northern Hemisphere, is ensured by the ice-albedo mechanism. This regulates the amount of freshwater entering the cold and saline waters of North Atlantic in particular. As the amount of freshwater entering the system is kept in balance, the overturning is ensuring the vertical exchange of dense, sinking water with lighter water above. As a result the salinity levels increase due to the income of salt transport from the south. The cold and saline water of North Atlantic ensures that water density is increased in the deep-water formation region and vertically spread through water mass, therefore stratification doesn't occur. The formation of vertical stratification in the water column is a consequence of water masses with different densities. Water density is strongly influenced by temperature and salinity; with less dense, warmer surface waters floating on top of denser, colder waters. All this ensures that ocean waters act as CO2 sinks as the uptake of this greenhouse gas is transported and deposited in the sediments of the ocean basin. Containing this green house gas allows the system to keep the CO2 levels in atmosphere at a level that keeps the atmospheric temperature in a certain amplitude that continues to maintain the ice volume in Northern Hemisphere.
- The ice-albedo mechanism (regional, well established) This mechanism is considered to add to the freshwater-overturning mechanism as it helps to maintain the desired atmospheric temperatures for the regime to exist. The feedback mechanism can be described with low atmospheric temperatures at winter that increase the volume of ice sheet while the snow accumulates. With the expanding ice sheet the dark open surface of bedrock and sea surface decline making the albedo rates to increase (see Fig.1). That results in decreased absorption of solar radiation as most of it is being reflected back to space. This leads back to decreased atmospheric temperatures that will allow increasing the ice sheet volume, locking this reinforcing mechanism and therefore avoiding large freshwater influx in the system.
Collapse of the Thermohaline circulation
- Freshwater-overturning mechanism (regional, well established) This mechanism is closely related to both the water temperature-density mechanism and the evaporation-salinity mechanism. It is due to the fact that this mechanism has an important impact on the main variables – water salinity and density of the other two mechanisms. The precipitation-river runoff mechanism is having an impact by altering the freshwater input. This is key for the mechanism to continue functioning and maintain the new regime. The certainty regarding this feedback mechanism is considered high. It is well established and proved that the increasing amounts of freshwater affecting salinity is one of the key processes that affect the existing regime [Rahmstorf 2000]. Nevertheless it is yet to be debated if this is the dominant mechanism that maintains the new regime. This discussion is due to the important influence of water temperature-density mechanism due to the significance of water density variable for the existence of THC.
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 denser and the colder the denser). This is why deep water is presently formed in the North Atlantic, which is salty, but not in the North Pacific, which is fresher. When the surface temperatures increase, the upper ocean warms and ice growth decreases. This instead enhances the amount of warmer freshwater from melting ice entering the cold and saline waters of North Atlantic in particular [Rahmstorf 2000]. The increased freshwater at high latitudes will decrease the overturning (vertical exchange of dense, sinking water with lighter water above) and a more sluggish surface flow is exposed longer to the freshwater forcing [Otterå et al. 2004]. As a result the salinity levels decrease as the salt transport from the south depletes [Rahmstorf et al. 2000]. Warmer and less saline water results in lower water density in the upper ocean (see Fig.1). With fresher, less dense, upper water there is now increased stratification of ocean layers thus altering the CO2 uptake of ocean waters through limiting the absorption. The positive feedback mechanism continues as the CO2 concentrations continue to increase in atmosphere leading to further surface water temperature increase in oceans.
- Water temperature – density mechanism (regional, well established) This feedback mechanism is discussed as being key for the THC to switch to a regime where it has collapsed alongside the freshwater-overturning mechanism (Lenton et al. 2008). The feedback strength depends on water temperature which in turn affect its density. Water density is the key slow variable that also links to other feedback mechanisms of the THC regime (see Fig.1).
The climate warming caused by increased concentrations of Greenhouse gases – particularly CO2 in the atmosphere, results in warming of sea surface temperatures. The cold surface waters of the Earth's oceans are dense enough to sink down to the abyss at only a few key locations. These locations are formed at high latitudes because the density of seawater is strongly temperature dependent. Therefore the warming of the surface temperatures decreases the density of the water resulting in less dense upper water. This increase the stratification of ocean layers and slows the surface to deep transport of CO2 thus reducing the ocean CO2 uptake. The weakening of one of the main global CO2 sinks would add to the continuing increase of CO2 in the atmosphere further increasing the atmospheric warming [Zickfeld et al 2008].
- The evaporation-salinity mechanism (regional, contested) Increasing atmospheric temperatures and the following reduction of ice volume in Arctic determine that the space of ice-free water expands [Lindsay et al. 2005]. Thus the evaporation from extended open water bodies increase, leading to decrease in salinity. Less saline water results in lower water density in the upper ocean. Furthermore this mechanism has the same pattern as freshwater-overturning mechanism and water temperature-density mechanism. The decreased water density enhances stratification thus altering the CO2 uptake of ocean waters through limiting the absorption (see Fig.1). The positive feedback mechanism continues as the CO2 concentrations continue to increase in atmosphere leading to further surface water temperature increase in oceans. This mechanism is contested as its presence can be argued at different times due to two potential event scenarios that influence the salinity of water. If the evaporation dominates upon precipitation then the salinity increases as salt is not evaporated into the atmosphere. If there is more precipitation caused by the increased amount of water vapour in the atmosphere than evaporation then the salinity decreases (Broadus et al. 2011). Therefore the presence of this mechanism closely relates to the occurrence precipitation-river runoff mechanism. The strength of this mechanism has still been discussed and depends on processes of other mechanisms. For example on ice volume and ice-albedo mechanism for increased open water and evaporation to occur.
- Precipitation-river runoff mechanism (regional, contested) It is affected by ice-albedo mechanism as the increasing atmospheric temperature is initiating other processes in this mechanism to take place. There are continuing discussions about the dominance of this mechanism for this regime to exist. It is considered that this mechanism alongside freshwater-overturning mechanism are the two dominant mechanisms for the new regime. This is determined by the main slow variables – water salinity and density that are affected mostly by these two mechanisms. Increasing atmospheric temperatures and the following reduction of ice volume in Northern Hemisphere determine that the space of ice free water expands [Lindsay et al. 2005]. Thus the evaporation from extended open water bodies increase leading to precipitation increase. If precipitation exceeds evaporation it leads to increased river runoff. Evidence of increasing arctic river discharge has been reported in several recent studies. Total river inflow to the Arctic Ocean is dominated by contributions from Eurasia. The average annual discharge is now about 128 km3/year greater than it was when routine measurements of discharge from the major Eurasian arctic rivers began in the 1930s. The average annual discharge is an approximate 7% increase. Increases in discharge from other arctic rivers, net precipitation directly over the ocean, and meltwater from Greenland provide additional freshwater forcing [Peterson et al. 2002]. The increased freshwater at high latitudes will decrease the overturning and a more sluggish surface flow is exposed longer to the freshwater forcing [Otterå et al. 2004]. As a result the salinity levels decrease as the salt transport from the south depletes [Rahmstorf et al. 2000]. Warmer and less saline water results in lower water density in the upper ocean (see Fig.1). With, fresher, less dense upper water there is now increased stratification of ocean layers thus altering the CO2 uptake of ocean waters through limiting the absorption. The positive feedback mechanism continues as the CO2 concentrations continue to increase in atmosphere leading to further surface water temperature increase in oceans.
- Salinity-Convection mechanism (regional, well established) This mechanism is initiated and closely linked to evaporation-salinity mechanism as water salinity decrease is driving this mechanism. Water salinity decrease due to impact from evaporation-salinity mechanism results in water density decline. Oceanic convection is driven by density differences and is of crucial importance in global ocean circulation. Convection is more likely and/or more rapid with a greater variation in density between the two fluids [Bitz et al.2006]. The decrease of water density and balance of the differences also affects convection. In this case it is possible for relatively warm, saline water to sink, and colder, fresher water to rise, terminating the normal overturning. This result in a more sluggish surface flow that is longer exposed to the freshwater forcing [Otterå et al. 2004]. As a result the salinity levels decrease as the salt transport from the south depletes.
- The ice-albedo mechanism (regional, well established) This mechanism typically operates in regional scale and the existence of it is well established amongst researchers thus regarded with high certainty. By itself the mechanism is not strong enough to flip the system to a new regime as it has to enter other feedback mechanisms to achieve that. There is near universal agreement that Arctic sea ice extent will decline through the 21st century in response to atmospheric green house gas loading [Zhang et. al 2006]. The resulting increase of surface air temperatures changes the ice 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 water 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.
Strong Thermohaline circulation to collapse of the Thermohaline circulation
The main external direct drivers that contribute to the shift include:
- Greenhouse gas emissions (global; well established): 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. Increasing CO2 levels in atmosphere slowly enhance 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 loss of ice is initiating the two main mechanisms for the new regime – freshwater-overturning mechanism and the precipitation-river runoff mechanism. Therefore new variables (for example increased evaporation, river runoff, changes in water salinity and density) are activated and links established that cause the system to shift to a new regime.
The main external indirect drivers that contribute to the shift include:
- Burning of fossil fuels (global, well established): Fossil fuels such as coal and natural gas increase the anthropogenic CO2 levels in atmosphere leading to climate warming. This further contributes to the loss of sea ice and thus the changes in water salinity and density that are essential for thermohaline circulation.
Slow internal system changes that contribute to the regime shift include:
- Mean atmospheric temperature (global, well established): It is expected that the changes will be more pronounced in high northern latitudes than in the global mean. This is mainly due to the impact on positive feedback of reduced sea ice extent and snow cover on the ice-free landmasses, and consequently reduced albedo [Cubasch et al. 2001]. The influence of this variable is important for the ice albedo mechanism and thus the main freshwater-overturning mechanism. Slowly rising temperature cause the change in ice-albedo mechanism therefore causing change in variables that are also part of the main freshwater-overturning mechanism. Increased depletion of ice volume in Northern Hemisphere generating freshwater runoff and precipitation increase eventually shifts the existing feedback mechanism towards different state.
- Water salinity and density (regional, well established): These two variables are very closely related as change in water salinity directly impact water density. Both of these variables are exposed to slow change as freshwater input is increasing from ice melting and river runoff. These changes originate from the slow variable of atmospheric temperature that initially changes the system behaviour. These two variables depend on the region where the freshwater runoff takes place.
Collapse of the Thermohaline circulation to strong Thermohaline circulation
Important shocks includes:
- Extreme droughts (regional, contested): This event could indirectly decrease the risk of regime shift decreasing river runoff from lack of precipitation. Therefore precipitation-river runoff mechanism is altered and further having an impact on the main freshwater-overturning mechanism as freshwater volume would be lowered.
Strong Thermohaline circulation to collapse of the Thermohaline circulation
- Atmospheric temperature - threshold at which thermal balance is established for promoting ice depleting conditions that further lead to freshwater influx that affects the density driven water transport.
- Water salinity – threshold at which the density driven water transport weakens due to freshwater influx
Collapse of the Thermohaline circulation to strong Thermohaline circulation
- Atmospheric temperature - threshold at which thermal balance is established for promoting ice maintaining conditions thus avoiding freshwater influx from ice melting
- Water salinity – threshold at which the density driven water transport is present as the freshwater influx is decreased.
Atmospheric temperatures (global, well established): It is essential to alter increasing atmospheric temperatures as this event is ensuring the other processes that occur in this regime shift
CO2 concentrations in atmosphere (global, well established): In order to alter temperature increase it is essential to decrease CO2 concentrations in atmosphere that directly affect temperature oscillations causing changes in the main feedback mechanisms that result in collapse of THC.
River runoff (regional, well established): Altering river runoff would cause decrease in freshwater added from rivers that affect water salinity and density and lead to collapse of THC.
Albedo (local/regional, well established): altering low albedo would decrease the absorbtion of solar radiation thus avoiding atmospheric temperature increase that would cause ice melting and changes in water density and salinity.
Ecosystem service impacts
THC is linked with climate regulation due to the current atmospheric regulation maintained by the water circulation. It links to atmospheric temperatures as the water masses regulate the heat transported around the World Ocean thus regulating the climate. Closely linked to this regulating service are the provisioning services of food crops, livestock and fisheries. This is due to the typical climate induced atmospheric processes that ensure the provision of these services. Biodiversity of local species as an ecosystem service is also recognized as the desired conditions are maintained by THC. The biggest social groups that would benefit of maintaining this regime would be fishermen and farmers who could continue to extract resources from the particular source.