Climate Change 2001: Impacts, Adaptation and Vulnerability

16.3. Synthesis

16.3.1. Feedbacks and Interactions—Polar Drivers

Climate change will affect some key polar drivers, creating impacts in the wider global arena. Many of these impacts will be self-amplifying and, once triggered, will affect other regions of the world for centuries to come. These impacts relate to probable changes in the cryosphere, sea level, thermohaline circulation and ocean ventilation, exchange of GHGs, and cloudiness:

Snow/ice-albedo feedback: The amount of absorbed solar radiation, and thus surface heating, depends on surface albedo, which is very high for snow or ice surfaces and much lower in the absence of snow and ice. Thus, absorbed shortwave radiation over the vast areas of snow and ice in polar regions is about three times lower than over non-snow-covered surfaces. Warming will shrink the cryosphere, particularly in the Arctic, causing additional heating of the surface, which in turn will further reduce ice and snow cover. Thus, any significant alteration in albedo over large areas will have the potential to produce a nonlinear, accelerated change.
Sea-level rise: Projected climate change in polar regions will have a critical impact on global sea levels. Expected sea-level rise is in the range 0.09-0.88 m by 2100, using the SRES scenarios (TAR WGI Chapter 11). Increased melting of the Greenland ice sheet and Arctic glaciers, as well as possible thinning of the West Antarctic ice sheet, is expected to make important contributions. However, increased snow accumulation on the East and West Antarctic ice sheets is a major process that can offset sea-level rise. Sea level will continue to rise long after atmospheric GHGs are stabilized, primarily because of the large heat capacity of the ocean and the slow response of glaciers and ice sheets. The rate of downwelling of cold, dense waters in polar regions is a major control on thermal expansion of the ocean and hence the rate of sea-level rise over the next centuries. Feedbacks that link sea-level rise to the size and health the West Antarctic ice sheet remain the subject of research and debate and are discussed in more depth in TAR WGI Chapter 11.
Ocean circulation: Ocean-climate models predict increased stability of the surface mixed layer, reduction in salt flux, less ocean convection, and less deepwater formation. This could lead to a prolonged, major reduction in thermohaline circulation and ocean ventilation (O'Farrell et al., 1997; Budd and Wu, 1998; Hirst, 1999). Such changes will affect surface ocean currents and climates of Europe and mid-latitude landmasses in the southern hemisphere, where it could slow warming in some regions (Murphy and Mitchell, 1995; Whetton et al., 1996) but amplify it in others. Changes in runoff from large Arctic rivers (especially in Siberia) and increased melt from the Greenland ice sheet and glaciers will to lead to more freshwater in the Arctic Ocean. This may further weaken the thermohaline circulation in the North Atlantic. With less overturning in the ocean, there will be a reduction of upwelling in temperate and subtropical latitudes. Wood et al. (1999) present simulations of present-day thermohaline circulation, using a coupled ocean-atmosphere climate model without flux adjustments. The model responds to forcing with increasing atmospheric concentrations of GHGs with a collapse of circulation and convection in the Labrador Sea. These changes are similar in two separate simulations with different rates of increase of CO₂ concentrations. Although various models give differing results, any changes in the thermohaline circulation will have profound consequences for marine biology and fisheries because of inevitable changes in habitat and nutrient supply. Perturbations caused by projected climate change, such as a marked increase in freshwater inputs in polar regions, may cause reorganization of the global ocean thermohaline circulation, leading to abrupt climate change (e.g., Manabe and Stouffer, 1993; Wright and Stocker, 1993; Stocker and Schmittner, 1997). Palaeoclimatic effects of past large freshwater inputs are widely discussed for the Atlantic (e.g., Broecker et al., 1990; Rasmussen et al., 1996; Bianchi and McCave, 1999) and for extra melt from the Antarctic ice sheet (Mikolajewicz, 1998). These studies show that with past climate changes, shifts from one circulation mode to another have caused large, and sometimes abrupt, regional climate changes. Although there is low confidence that such events will occur, the associated impacts would be substantial.
Greenhouse gases—reduced uptake by the Southern Ocean: Projected climate change will alter vast areas of oceans, wetlands, and permafrost in the polar regions that act as major sources and sinks for atmospheric CO₂ and CH₄. Projected climate change will alter these features, thereby altering the exchange of these gases. Model results (Sarmiento and Le Quere, 1996) show that of all oceans, the Southern Ocean is likely to experience the greatest slowing in CO₂ uptake with climate change. Reduced downwelling also will limit the ability of the ocean to sequester anthropogenic CO₂ (Sarmiento et al., 1998). Changing marine biology also must be considered. Using coupled climate model output under the IPCC IS92a GHG scenario, Matear and Hirst (1999) calculate that by the year 2100, there could be a reduction in cumulative oceanic uptake of carbon of 56 Gt. This reduced uptake is equivalent to a 4% yr^-1 increase in CO₂ emissions for 1995-2100.
Greenhouse gases—emission by Arctic landscapes: Whether the Arctic will be a net sink or a net source of CO₂ will depend largely on the magnitude and direction of hydrological changes and the rate of decomposition of exposed peat in response to temperature rise (Oechel et al., 1993; McKane et al., 1997a,b). Tundra ecosystems have large stores of nutrients and carbon bound in permafrost, soil, and microbial biomass and have low rates of CO₂ uptake as a result of low net primary production (Callaghan and Jonasson, 1995). The 25-year pattern of net CO₂ flux indicates that tundra in Alaska was a net sink during the cool, wet years of the 1970s; a net source of CO₂ during the warm, dry 1980s; and a net sink during the warm but less dry 1990s (Vourlitis and Oechel, 1999). These responses also may reflect a decrease in the rate of decomposition of soil organic matter because decay potential decreases with depth, as older, more recalcitrant carbon is exposed in soil profiles (Christensen et al., 1998, 1999a,b). Sink activity will be altered by changes in soil water content, temperature, or longer term adjustment of biotic processes (Oechel and Billings, 1992; Shaver et al., 1992; Oechel and Vourlitis, 1994; Chapin et al., 1995; Jonasson, 1996; Nadelhoffer et al., 1996; Rastetter et al., 1996; Waelbroeck et al., 1997). Increased frequency of disturbances such as fire in boreal forest also could contribute to increased seasonal amplitude of atmospheric CO₂ (Zimov et al., 1999). CH₄ production also is related to the position of the water table in the active layer (Torn and Chapin, 1993; Vourlitis et al., 1993; Johnson et al., 1996). The gas is oxidized in unsaturated soils and in the uppermost layer of the soil water column (Gilmanov and Oechel, 1995; Rouse et al., 1995; Tenhunen, 1996). Hence, the future magnitude of soil emissions of CH₄ and CO₂ will reflect the net outcome of anaerobic and aerobic processes. Thawing of permafrost has the potential to release considerable quantities of CH₄ and CO₂ (Fukuda, 1994; Michaelson et al., 1996; Anisimov et al., 1997; Goulden et al., 1998; Bockheim et al., 1999). Considering all of these effects, future warming is likely to further increase natural GHG emissions. Fluxes of these gases eventually may revert, however, to current levels after an initial pulse (Waelbroeck et al., 1997).
Hydrates of greenhouse gases: Substantial amounts of natural gas may be released to the atmosphere as a result of climate-induced destabilization of gas hydrates beneath the surface of the Earth. On the continents, stable gas hydrates can be found only at depths of several hundreds of meters, making it unlikely that they will be released by climate change in the coming centuries. In the northern seas, gas hydrates may be deposited in the near-bottom zone, and their decomposition is likely to occur if deepwater temperature rises by even a few degrees. Because there are large uncertanties in the estimated amounts of the near-bottom gas hydrates, their role in providing positive feedback to the climate system cannot be evaluated with reasonable accuracy. There is evidence of methane hydrate destabilization and release with warming of coastal ocean bottomwater from other parts of the world (Kennett et al., 2000).

Other reports in this collection