15.3.4 Projected changes on land
Although seasonal snow cover on land is highly variable, it has important effects on the substrate and on local climate, primarily through its insulating properties and high albedo. In Eurasia, and to a lesser extent North America, there has been a persistent 5-6 day/decade increase in the duration of snow-free conditions over the past three decades (Dye, 2002). The reduction of snow residence time occurs primarily in spring. Projections from different climate models generally agree that these changes will continue. Likely impacts include increases in near-surface ground temperature, changes in the timing of spring melt-water pulses, and enhanced transportation and agricultural opportunities (Anisimov et al., 2005a). The projected warming also implies a continuation of recent trends toward later freeze-up and earlier break-up of river and lake ice (Walsh et al., 2005).
Projections of change agree that the retreat of glaciers will continue across Arctic glaciers, with a consequent impact on global sea level (Meehl et al., 2007). Recent changes in the Greenland ice sheet have, however, been complex. The colder interior has thickened, most probably as a result of recently high precipitation rates, while the coastal zone has been thinning. Thus some studies suggest that overall the ice sheet is growing in thickness (Krabill et al., 2000; Johannessen et al., 2005). However, there is a growing body of evidence for accelerating coastal thinning, a response to recent increases in summer melt (Abdalati and Steffen, 2001), and acceleration of many coastal glaciers (Krabill et al., 2004; Howat et al., 2005; Ekstrom et al., 2006; Luckman et al., 2006; Rignot and Kanagaratnam, 2006) suggest that thinning is now dominating the mass balance of the entire ice sheet.
Warming, thawing and decrease in areal extent of terrain underlain by permafrost are expected in response to climatic change in the 21st century (Sazonova et al., 2004; Euskirchen et al., 2006; Lemke et al., 2007). Results from models forced with a range of IPCC climate scenarios indicate that by the mid-21st century the permafrost area in the Northern Hemisphere is likely to decrease by 20 to 35%, largely due to the thawing of permafrost in the southern portions of the sporadic and discontinuous zones, but also due to increasing patchiness in areas that currently have continuous permafrost (Anisimov and Belolutskaia, 2004). Projected changes in the depth of seasonal thawing (base of the active layer) are uniform neither in space nor in time. In the next three decades, active layer depths are likely to be within 10 to 15% of their present values over most of the permafrost area; by the middle of the century, the depth of seasonal thawing may increase on average by 15 to 25%, and by 50% and more in the northernmost locations; and by 2080, it is likely to increase by 30 to 50% and more over all permafrost areas (Anisimov and Belolutskaia, 2004; Instanes et al., 2005).
Current and projected changes in the Antarctic ice sheet are discussed in greater detail elsewhere (Lemke et al., 2007), and are only summarised here. Recent changes in volume of the Antarctic ice sheet are much better mapped and understood than they were in the TAR, but competing theories over the causes still limits confidence in prediction of the future changes. The ice sheet on the Antarctic Peninsula is probably alone in showing a clear response to contemporary climate change (see Section 15.6.3), while the larger West Antarctic and East Antarctic ice sheets are showing changes whose attribution to climate change are not clear, but cannot be ruled out. In West Antarctica, there is a suggestion that the dramatic recent thinning of the ice sheet throughout the Amundsen Sea sector is the result of recent ocean change (Payne et al., 2004; Shepherd et al., 2004), but as yet there are too few oceanographic measurements to confirm this interpretation. Indeed, there is evidence that deglaciation of some parts of West Antarctica, as a response to climate change at the end of the last glacial period, is not yet complete (Stone et al., 2003). There are still competing theories, but the now clear evidence of ice-sheet change, has reinvigorated debate about whether we should expect a deglaciation of part of the West Antarctic ice sheet on century to millennial timescales (Vaughan, 2007). Studies based on satellite observations do not provide unequivocal evidence concerning the mass balance of the East Antarctic ice sheet; some appear to indicate marginal thickening (Davis et al., 2005), while others indicate little change (Zwally et al., 2005; Velicogna and Wahr, 2006; Wingham et al., 2006).
Permafrost in ice-free areas, seasonal snow cover, and lake-ice do exist in Antarctica but in such small areas that they are only discussed in respect to particular impacts.