IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group II: Impacts, Adaptation and Vulnerability

15.4.2 Terrestrial ecosystems and their services Historical and current changes in Arctic terrestrial ecosystems

Climatic changes during the past 20,000 years and more have shaped current biodiversity, ecosystem extent, structure and function. Arctic species diversity is currently low, partly because of past extinction events (FAUNMAP Working Group, 1996). As a group, large mammals are in general more vulnerable to current change than in the past when the group contained many more species. Also, tundra ecosystem extent, particularly in Eurasia, is now less than during the glacial period, when extensive tundra-steppe ecosystems existed (Callaghan et al., 2005). Modern habitat fragmentation (e.g., Nellemann et al., 2001), stratospheric ozone depletion, and spread of contaminants, compound the ongoing impacts of anthropogenic climate change and natural variability on ecosystems and their services.

Traditional ecological knowledge (TEK, see Section 15.6.1) from Canada has recorded current ecosystem changes such as poor vegetation growth in eastern regions associated with warmer and drier summers; increased plant biomass and growth in western regions associated with warmer, wetter and longer summers; the spreading of some existing species, and new sightings of a few southern species; and changing grazing behaviours of musk oxen and caribou as the availability of forage increases in some areas (Riedlinger and Berkes, 2001; Thorpe et al., 2001; Krupnik and Jolly, 2002).

In northern Fennoscandia the cycles of voles, and possibly also of lemmings, have become considerably dampened since the 1980s, due to low spring peak densities, and it is likely that these changes are linked to poorer winter survival due to changing snow conditions (Yoccoz and Ims, 1999; Henttonen and Wallgren, 2001; Ims and Fuglei, 2005). Arctic fox, lesser white fronted goose and shore lark have declined dramatically (Elmhagen et al., 2000) and moose, red fox and some southern bird species have spread northwards (Hörnberg, 1995; Tannerfeldt et al., 2002), although the specific role of climate change is unknown. Some migrant bird populations, particularly Arctic waders, have declined substantially (Stroud et al., 2004) due to various causes including climate change and loss of habitat on migration routes and wintering grounds (Morrison et al., 2001, 2004; Zöckler, 2005). In contrast, populations of Arctic breeding geese, which in winter increasingly feed on agricultural crops or unharvested grain, have shown a geometric increase in numbers in Europe and North America, which has led to intense foraging in Arctic coastal breeding habitats, loss of vegetation and the occurrence of hypersaline soils (Jefferies et al., 2006). Although there are examples from temperate latitudes, evidence for early arrival of migratory birds in the Arctic is weak (Gauthier et al., 2005), emphasising the need for adequate monitoring programmes (Both et al., 2005). Some populations of caribou/reindeer, which are essential to the culture and subsistence of several Arctic peoples, are currently in decline (Russell et al., 2002; Chapin et al., 2005a), mainly due to social and cultural factors. However, climate impacts have also affected some populations. Icing events during warmer winters that restrict access to frozen vegetation have impacted some reindeer/caribou populations and high-Arctic musk oxen populations (Forchhammer and Boertmann, 1993; Aanes et al., 2000; Callaghan et al., 2005 and references therein).

Evidence of recent vegetation change is compelling. Aerial photographs show increased shrub abundance in Alaska in 70% of 200 locations (Sturm et al., 2001; Tape et al., 2006). Along the Arctic to sub-Arctic boundary, the tree line has moved about 10 km northwards, and 2% of Alaskan tundra on the Seward Peninsula has been displaced by forest in the past 50 years (Lloyd et al., 2003). In some areas, the altitude of the tree line has risen, for example by about 60 m in the 20th century in sub-Arctic Sweden (Callaghan et al., 2004; Truong et al., 2007), although the tree line has been stable or become lower in other localities (Dalen and Hofgaard, 2005). Bog growth has caused tree death in parts of the Russian European Arctic (Crawford et al., 2003). The pattern of northward and upward tree-line advances is comparable with earlier Holocene changes (MacDonald et al., 2000; Esper and Schweingruber, 2004) (see below for rates of advance). In addition to changes in woody vegetation, dry-habitat vegetation in sub-Arctic Sweden has been partly displaced by wet-habitat vegetation because of permafrost degradation in the discontinuous permafrost zone (Christensen et al., 2004; Malmer et al., 2005). Similarly, in northern Canada, up to 50% of peat plateau permafrost has thawed at four sites in the discontinuous permafrost zone (Beilman and Robinson, 2003).

There is also recent evidence of changes in growing season duration and timing, together with plant productivity, but patterns are spatially variable. Analyses of satellite images indicate that the length of growing season is increasing by 3 days per decade in Alaska and 1 day per decade in northern Eurasia (McDonald et al., 2004; Smith et al., 2004; McGuire et al., 2007), but there has been a delayed onset of the growing season in the Kola Peninsula during climatic cooling over the past two decades (Høgda et al., 2007). Remote sensing estimates of primary productivity also show spatial variability: there were increases in the southern Arctic and decreases in the central and eastern Russian Arctic between 1982 and 1999 (Nemani et al., 2003).