Arctic and alpine ecosystems are characterized by low human population densities. However, they provide important goods and services locally and globally. At the local scale, they are the resource base of many indigenous cultures and provide recreation, food, and fiber to people in adjacent regions. At the global level, arctic and boreal regions play an important role in the world's climate system. They contain 40% of the world's reactive soil carbon (McGuire et al., 1995); influence global heat transport through their impact on regional water and energy exchange with the atmosphere; and determine freshwater input to the Arctic Ocean, which influences bottom-water formation and thermohaline circulation of the oceans. Alpine regions are important sources of freshwater and hydropower for surrounding lowlands. Changes in these goods and services would have socioeconomic impacts throughout the world.
Chapter 16 presents information on the effects of climate on the physical environment of polar regions. The impacts of climate change on high-mountain systems and arctic tundra are extensively reviewed in the SAR by Beniston and Fox (1996) and Allen-Diaz (1996), respectively. This section emphasizes the effects of climatic change particularly on ecosystem production, carbon stores, biodiversity, and water flowwhich, in turn, can affect the productivity especially of alpine ecosystems.
Arctic and alpine tundra each occupy 4 million km2 (Körner, 1999). Approximately 25% of the tundra is ice-covered, 25% shrub-dominated, and the rest dominated by herbaceous plants. Soil carbon stocks in boreal and tundra peatlands are large (see Sections 5.6 and 5.8).
High-latitude warming that has occurred in the Arctic region since the 1960s (Chapman and Walsh, 1993; Serreze et al., 2000) is consistent with simulations of climate models that predict increased greenhouse forcing (Kattenberg et al., 1996). Climatic change has been regionally variable, with cooling in northeastern North America and warming in northwestern North America and northern Siberia. The warming results from a change in the frequency of circulation modes rather than gradual warming (Palmer, 1999). Precipitation (P) and surface evaporation (E) have increased at high latitudes, with no significant temporal trend in the balance between the two (P-E) (Serreze et al., 2000).
Permafrost underlies 20-25% of the northern hemisphere land area (Brown et al., 1997a). Ice that forms during periods of cold climate frequently constitutes a high proportion (20-30%) of the volume of these frozen soils (Brown et al., 1997a). Consequently, melting of permafrost can lead to surface collapse of soils forming thermokarst, an irregular topography of mounds, pits, troughs, and depressions that may or may not be filled with water, depending on topography. Permafrost temperatures have warmed in western North America by 2-4°C from 1940 to 1980 (Lachenbruch and Marshall, 1986) and in Siberia by 0.6-0.7°C from 1970 to 1990 (Pavlov, 1994), whereas permafrost cooled in northeastern Canada (Wang and Allard, 1995)patterns that roughly parallel recent trends in air temperature. However, the magnitude of warming and patterns of interannual variation (roughly 10-year oscillations) in permafrost temperatures are not readily explained as a simple response to regional warming (Osterkamp et al., 1994). Changes in permafrost regime probably reflect undocumented changes in the thickness or thermal conductance of snow or vegetation, in addition to changes in air temperature (Osterkamp and Romanovsky, 1999). Thermokarst features are developing actively in the zone of discontinuous permafrost (Osterkamp and Romanovsky, 1999), particularly in association with fire and human disturbance, but there are no long-term records from which to detect trends in the regional frequency of thermokarst.
In contrast to the long-term trend in tundra carbon accumulation during the Holocene, flux measurements in Alaska suggest that recent warming may have converted tundra from a net carbon sink to a source of as much as 0.7 Gt C yr-1 (Oechel et al., 1993; Oechel and Vourlitis, 1994). The direction and magnitude of the response of carbon exchange to warming may be regionally variable, depending on climate, topography, and disturbance regime (Zimov et al., 1999, McGuire et al., 2000). During recent decades, peak-to-trough amplitude in the seasonal cycle of atmospheric CO2 concentrations has increased, and the phase has advanced at arctic and subarctic CO2 observation stations north of 55°N (Keeling et al., 1996). This change in carbon dynamics in the atmosphere probably reflects some combination of increased uptake during the first half of the growing season (Randerson et al., 1999), increased winter efflux (Chapin et al., 1996), and increased seasonality of carbon exchange associated with disturbance (Zimov et al., 1999). This "inverse" approach generally has concluded that mid-northern latitudes were a net carbon sink during the 1980s and early 1990s (Tans et al., 1990; Ciais et al., 1995; Fan et al., 1998; Bousquet et al., 1999; Rayner et al., 1999). At high northern latitudes, these models give a wider range of estimates, with some analyses pointing to a net source (Ciais et al., 1995; Fan et al., 1998) and others to a sink (Bousquet et al., 1999; Rayner et al., 1999).
High-latitude wetlands and lakes account for 5-10% of global CH4 fluxes to the atmosphere (Reeburgh and Whalen, 1992). These fluxes increase dramatically with thermokarst (Zimov et al., 1997), acting as a potentially important positive feedback to global warming.
Satellite imagery suggests an increase in NDVI (a measure of "greenness") from 1981 to 1991 (Myneni et al., 1997), although interpretation is complicated by changes in sensor calibration (Fung, 1997). If these changes in satellite imagery are an accurate reflection of vegetation activity, changes in NDVI could help explain the increase in seasonal amplitude in atmospheric CO2 observed at high northern latitudes (Keeling et al., 1996; Randerson et al., 1999). This also is consistent with the increased biomass of shrubs in the arctic tundra (Chapin et al., 1995).
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