15.2 Current sensitivity/vulnerability
15.2.1 Climate, environment and socio-economic state
Arctic
For several decades, surface air temperatures in the Arctic have warmed at approximately twice the global rate (McBean et al., 2005). The areally averaged warming north of 60°N has been 1-2°C since a temperature minimum in the 1960s and 1970s. In the marine Arctic, the 20th-century temperature record is marked by strong low-frequency (multi-decadal) variations (Polyakov et al., 2002). Serreze and Francis (2006) have discussed the attribution of recent changes in terms of natural variability and anthropogenic forcing, concluding that a substantial proportion of the recent variability is circulation-driven, and that the Arctic is in the early stages of a manifestation of a human-induced greenhouse signature. This conclusion is based largely on the relatively slow rate of emergence of the greenhouse signal in model simulations of the late 20th and early 21st centuries.
The most recent (1980 to present) warming of much of the Arctic is strongest (about 1°C/decade) in winter and spring, and smallest in autumn; it is strongest over the interior portions of northern Asia and north-western North America (McBean et al., 2005). The latter regions, together with the Antarctic Peninsula, have been the most rapidly warming areas of the globe over the past several decades (Turner et al., 2007). The North Atlantic sub-polar seas show little warming during the same time period, probably because of their intimate connection with the cold, deep waters. Temperatures in the upper troposphere and stratosphere of the Arctic have cooled in recent decades, consistent with increases in greenhouse gases and with decreases in stratospheric ozone since 1979 (Weatherhead et al., 2005).
Precipitation in the Arctic shows signs of an increase over the past century, although the trends are small (about 1% per decade), highly variable in space, and highly uncertain because of deficiencies in the precipitation measurement network (McBean et al., 2005) and the difficulty in obtaining accurate measurements of rain and snow in windy polar regions. There is no evidence of systematic increases in intense storms in the Arctic (Atkinson, 2005) although coastal vulnerability to storms is increasing with the retreat of sea ice (see Section 15.4.6). Little is known about areally averaged precipitation over Greenland. The discharge of Eurasian rivers draining into the Arctic Ocean shows an increase since the 1930s (Peterson et al., 2002), generally consistent with changes in temperature and the large-scale atmospheric circulation.
Reductions of Arctic sea ice and glaciers (see Lemke et al., 2007), reductions in the duration of river and lake ice in much of the sub-Arctic (Prowse et al., 2004; Walsh et al., 2005), and a recent (1980s to present) warming of permafrost in nearly all areas for which measurements are available (Romanovsky et al., 2002; Walsh et al., 2005) are consistent with the recent changes in Arctic surface air temperatures. Although there is visual evidence of permafrost degradation (Lemke et al., 2007), long-term measurements showing widespread thickening of the active layer are lacking. Changes in vegetation, particularly a transition from grasses to shrubs, has been reported in the North American Arctic (Sturm et al., 2001) and elsewhere (Tape et al., 2006), and satellite imagery has indicated an increase in the Normalised Difference Vegetation Index (NDVI, a measure of photosynthetically active biomass) over much of the Arctic (Slayback et al., 2003). This is consistent with a longer growing season and with documented changes in the seasonal variation in atmospheric CO2 concentrations as reported in the TAR. Broader ecosystem impacts of climate change in both polar regions are summarised by Walther et al. (2002) and were documented more extensively for the Arctic by the Arctic Climate Impact Assessment (ACIA, 2005).
Recent analysis of air-borne data (Krabill et al., 2004), satellite data (Howat et al., 2005; Luckman et al., 2006; Rignot and Kanagaratnam, 2006) and seismic data (Ekstrom et al., 2006) indicate thinning around the periphery of the Greenland ice sheet, where summer melt has increased during the past 20 years (Abdalati and Steffen, 2001; Walsh et al., 2005), while there is evidence of slower rates of thickening further inland (Johannessen et al., 2005).
The Arctic is now home to approximately 4 million residents (Bogoyavlenskiy and Siggner, 2004). Migration into the Arctic during the 20th century has resulted in a change of demographics such that indigenous peoples now represent 10% of the entire population. This influx has brought various forms of social, cultural and economic change (Huntington, 1992; Nuttall, 2000b). For most Arctic countries, only a small proportion of their total population lives in the Arctic, and settlement remains generally sparse (Bogoyavlenskiy and Siggner, 2004) and nomadic peoples are still significant in some countries. On average, however, two-thirds of the Arctic population live in settlements with more than 5,000 inhabitants. Indigenous residents have, in most regions, been encouraged to become permanent residents in fixed locations, which has had a predominantly negative effect on subsistence activities and some aspects of community health. At the same time, Arctic residents have experienced an increase in access to treated water supplies, sewage disposal, health care facilities and services, and improved transportation infrastructure which has increased access to such things as outside market food items (Hild and Stordhal, 2004). In general, the Arctic has a young, rapidly growing population with higher birth rates than their national averages, and rising but lower than national average life-expectancy. This is particularly true for indigenous populations, although some exceptions exist, such as in the Russian north, where population and life-expectancy has decreased since 1990 (Einarsson et al., 2004).
Political and administrative regimes in Arctic regions vary between countries. In particular, indigenous groups have different levels of self-determination and autonomy. Some regions (e.g., northern Canada and Greenland) now have formalised land-claim settlements, while in Eurasia indigenous claims have only recently begun to be addressed (Freeman, 2000). Wildlife management regimes and indigenous/non-indigenous roles in resource management also vary between regions. Nowadays, large-scale resource extraction initiatives and/or forms of social support play significant roles in the economies of many communities. Despite these changes, aspects of subsistence and pastoral livelihoods remain important.
Regardless of its small and dispersed population, the Arctic has become increasingly important in global politics and economies. For example, the deleterious effect on the health of Arctic residents of contaminants produced in other parts of the world has led to international agreements such as the Stockholm Convention on Persistent Organic Pollutants (Downey and Fenge, 2003). Furthermore, significant oil, gas and mineral resources (e.g., diamonds) are still to be developed in circum-Arctic regions that will further increase the importance of this region in the world (e.g., U.S. Geological Survey World Energy Assessment Team, 2000; Laherre, 2001).
Antarctic
Direct measurements reveal considerable spatial variability in temperature trends in Antarctica. All meteorological stations on the Antarctic Peninsula show strong and significant warming over the last 50 years (see Section 15.6.3). However, of the other long-term (>30 years) mean annual temperature records available, twelve show warming, while seven show cooling; although only two of these (one of each) are significant at the 10% level (Turner et al., 2005). If the individual station records are considered as independent measurements, then the mean trend is warming at a rate comparable to mean global warming (Vaughan et al., 2003), but there is no evidence of a continent-wide ‘polar amplification’ in Antarctica. In some areas where cooling has occurred, such as the area around Amundsen-Scott Station at the South Pole, there is no evidence of directly attributable impacts, but elsewhere cooling has caused clear local impacts. For example, in the Dry Valleys, a 6 to 9% reduction in primary production in lakes and a >10%/yr decline in soil invertebrates has been observed (Doran et al., 2002). Although the impacts are less certain, precipitation has also declined on sub-Antarctic islands (Bergstrom and Chown, 1999).
Recent changes in Antarctic sea-ice extent are discussed in detail elsewhere (Lemke et al., 2007), but evidence highlighted in the TAR (Anisimov et al., 2001) gleaned from records of whaling activities (de la Mare, 1997) is no longer considered reliable (Ackley et al., 2003). So, for the period before satellite observation, only direct local observations (e.g., Murphy et al., 1995) and proxies (e.g., Curran et al., 2003) are available. For the satellite period (1978 to present) there has been no ubiquitous trend in Antarctic sea-ice duration, but there have been strong regional trends (see Figure 15.2). Sea-ice duration in the Ross Sea has increased, while in the Bellingshausen and Amundsen Seas it has decreased, with high statistical significance in each case (Parkinson, 2002; Zwally et al., 2002). This pattern strongly reflects trends in atmospheric temperature at nearby climate stations (Vaughan et al., 2003).
Increasing atmospheric CO2 concentrations are leading to an increased draw-down of CO2 by the oceans and, as a consequence, sea water is becoming more acidic (Royal Society, 2005). As is the case in other parts of the world’s oceans, coccolithophorids and foraminifera are significant components of the pelagic microbial community of the Southern Ocean and contribute to the draw-down of atmospheric CO2 to the deep ocean. Experimental studies (Riebesell et al., 2000) indicate that elevated CO2 concentration reduces draw-down of CO2 compared with the production of organic matter. Recent investigations suggest that at the present rate of acidification of the Southern Ocean, pteropods (marine pelagic molluscs) will not be able to survive after 2100 and their loss will have significant consequences for the marine food web (Orr et al., 2005). Similarly, cold-water corals are threatened by increasing acidification. Furthermore, increasing acidification leads to changes in the chemistry of the oceans, altering the availability of nutrients and reducing the ability of the oceans to absorb CO2 from the atmosphere (Royal Society, 2005; see also Chapter 4 this volume).