Arctic with summer ice
- The ice-albedo mechanism (reinforcing, regional, well established): The permanent low surface air temperatures 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 level as the dark ocean surface absorbs more solar radiation than the sea ice. 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.
Arctic without summer ice
- The ice-albedo mechanism (reinforcing, regional, well established) Increased atmospheric greenhouse gas concentrations in atmosphere increase surface air temperatures that change the thermal balance. 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. Increased absorption of solar energy promotes higher air, ice, water and land temperatures which lead towards degrading sea ice. In the end, the increasing temperatures and accumulated heat further promote warming arctic conditions.
- The wind-ice circulation mechanism (reinforcing, regional/global, well established) Ice circulation patterns develop in response to wind and ocean currents. Retreating ice cover and increasing open water surface area generates a longer fetch for winds over the water surface. These altered surface winds result in morecyclonic motion of the ice and an enhanced transport of ice awayfrom the Siberian and Alaskan coasts. This change in circulation fostersopenings in the ice cover along the coasts. Although these openings quickly refreezein response to low winter surface air temperatures, coastal areas in spring are left with an anomalous coverage of young, thin ice. This thin ice then usually melts completely during the summer promoting stronger heat fluxes tothe atmosphere, which fosters higher surface air temperatures in the spring andearlier melt onset.
- The wind-CHL mechanism (reinforcing, regional, not well understood) The loss of ice volume and resulting increase in open surface water are allowing increased wind fetch. The cold halocline layer (CHL) effectively shields the surface from heat stored at intermediate depths in the Atlantic layer. A study has shown a changing trend in wind patterns that cause a different distribution pattern of river water into the ocean. The relatively low input of river water into the Arctic Ocean can result in anomalously high salinities in the water that that links with the retreat of CHL. When warm Atlantic watersenter the Arctic Ocean, they form an intermediate layer of warm water as they subduct below the colder,fresher (less dense) arctic surface waters. The CHL separatesthe Atlantic and surface waters and largely insulates the ice fromthe heat of the Atlantic layer. The retreat of CHL is proposed toincrease Atlantic layer heat loss and ice-ocean heat exchange affecting the surface energy and mass balance of sea ice in Arctic.
- The ice-currents mechanism (reinforcing, regional, speculative) A relationship between ice lossand oceanic warm water flux through the Bering Strait has been proposed. Delayed winter ice formation allows for more efficient coupling between theocean and wind forcing - a process that drives general circulation in Oceans. This redirects warm surface water from the Pacific Ocean from the shelf slopealong Alaska into the Arctic Ocean, where it may retard winter ice formation. In contrast, studies of river hydrographsdocument strong variabilityof river inflows into the Arctic without showing a distinctive long-term trend.
- The precipitation-CHL mechanism (balancing, regional, contested) Increasing atmospheric temperatures and the following reduction of ice volume in Northern Hemisphere determine that the space of ice free water expands. Thus the evaporation from extended open water bodies increase leading to precipitation increase. If precipitation exceeds evaporation it leads to increased river runoff thus providing additional freshwater forcing. The CHL is strengthened by the cold freshwater input from the river runoff. Nevertheless as a result of wind energy input over large areas of open water internal wave induced mixing is enhanced that results in removing the CHL. This is proposed to increase Atlantic layer heat loss and ice-ocean heat exchange affecting the surface energy and mass balance of sea ice in Arctic. Thus increased precipitation and river runoff that strengthen the CHL could slow the depletion of Arctic ice by avoiding the warm Atlantic waters to intervene with the ice sheet. This is a balancing mechanism which means that after maintaining ice volume the open water surface will decline thus affecting precipitation and weakening CHL that further declines ice volume. Therefore other mechanisms could be dominating while this mechanism is in the balancing phase of ice decline instead of ice increase. If the wind-CHL mechanism is dominant over this mechanism then the loss of ice volume due to absence of CHL would still increase.
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). 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. Greenhouse gas concentrations in the atmosphere and increasing temperatures are considered to be slow variables underlying this regime shift. The decrease in albedo and the resulting increase in the absorption of solar energy are the fast variables that are directly resulting in the decrease in Arctic sea ice.
The main external direct drivers that contribute to the shift include:
- Greenhouse gas emissions (global; well established): Resulting increase in average global temperatures extend the area of open water in summer which in turn decreases the albedo that affects the absorption of solar energy therefore further degrading sea ice.
Slow internal system changes that contribute to the regime shift include:
- Atmospheric temperatures (global; well established) This variable mainly affects ice volume variations throughout the year and in case of continuous depletion of Arctic sea ice it triggers various mechanisms that maintain the new regime of Arctic without summer sea ice.
- Atmospheric temperatures: threshold at which thermal balance is established for promoting ice depleting or maintenance of arctic conditions
- Atmospheric temperature (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
- River runoff (regional, ?): essential to alter in order to minimize the freshwater input in cold halocline layer and thus further decline of ice volume.
- Albedo (local/regional, well established): altering low albedo would decrease the absorbtion of solar radiation thus avoiding atmospheric temperature increase and continuous loss of arctic ice.
Ecosystem service impacts
Local knowledge and spiritual and aesthetic values might be lost as the local communities have to adapt to the new circumstances and thus their lifestyle. Local wild animal and plant food diversity and fisheries would be affected as warmer conditions and less saline water would alter biodiversity and certain species would be extinct.
On the other hand, summer ice free Arctic would provide the access to new fisheries. The loss of ice cover could affect the Arctic's freshwater system and surface energy budget, andmanifest in middle latitudes as altered patternsin atmospheric circulation and precipitation (Francis and Vavrus 2012). This presents the way how water and atmospheric circulations could be altered as ecosystem services.