5.9.2. Responses of Arctic and Alpine Ecosystems and Impacts on their Goods
188.8.131.52. Impacts Resulting from Changes in Climate on Arctic
Changes in climate are likely to be the greatest cause of changes in goods
and services in the arctic (Walker et al., 2001). Projected climatic warming
of 4-10°C by the end of the century probably would cause substantial
increases in decomposition, nutrient release, and primary production. The net
effect on the carbon balance will depend primarily on soil moisture (McKane
et al., 1997; McGuire et al., 2000), which cannot be projected with confidence.
In general, surface soils on slopes are expected to become drier as thaw depth
increases. Lowlands may experience substantial thermokarst, impoundment of water,
and reduced aeration.
Plant production frequently is limited in the arctic by excessive moisture
and slow turnover of nutrients in soils, and warming and drying of soils is
likely to enhance decomposition, nutrient mineralization, and productivity.
Many of these changes in productivity may be mediated by changes in species
composition and therefore are likely to lag changes in climate by years to decades
(Chapin et al., 1995; Shaver et al., 2000). Threshold changes in productivity
associated with poleward movement of the treeline is likely to experience time
lags of decades to centuries because of limitations in dispersal and establishment
of trees (Starfield and Chapin, 1996; Chapin and Starfield, 1997).
The net effect of warming on carbon stores in high-latitude ecosystems depends
on changes in the balance between production and decomposition. Decomposition
initially may respond more rapidly than production, causing trends toward net
carbon efflux (Shaver et al., 1992; Smith and Shugart 1993).
Warming-induced thermokarst is likely to increase CH4 flux to the
atmosphere in lowlands, particularly peatlands of northern Canada and western
Siberia (Gorham, 1991; Roulet and Ash, 1992; see also Section
5.8) and the loess-dominated "yedoma" sediments of central and eastern Siberia
(Zimov et al., 1997). Fires and other disturbances are likely to affect the
thermokarsts; however, the role of these disturbanceswhich can be mediated
with changes in regional climate in inducing thermokarstare poorly understood.
Changes in community composition associated with warming are likely to alter
feedbacks to climate. Tundra has a three- to six-fold higher winter albedo than
boreal forest, but summer albedo and energy partitioning differ more strongly
among ecosystems within tundra or boreal forest than between these two biomes
(Betts and Ball, 1997; Eugster et al., 2000). If regional surface warming continues,
changes in albedo and energy absorption during winter are likely to act as positive
feedbacks to regional warming as a result of earlier melting of snow and, over
the long term, poleward movement of the treeline. Surface drying and a change
in dominance from mosses to vascular plants also would enhance sensible heat
flux and regional warming in tundra (Lynch et al., 1999; Chapin et al., 2000).
Poleward migration of taxa from boreal forest to the Arctic tundra will depend
not only on warming climate but also on dispersal rates, colonization rates,
and species interactions and therefore may exhibit substantial time lags. The
arctic historically has experienced fewer invasions of weeds and other exotic
taxa than other regions (Billings, 1973). Some of the most important changes
in diversity in the arctic may be changes in the abundance of caribou, waterfowl,
and other subsistence resources (see also Section 5.4).
Changes in community composition and productivity may be particularly pronounced
in the high arctic, where much of the surface currently is unvegetated and is
prone to establishment and expansion of additional vegetation (Wookey et al.,
1993; Callaghan and Jonasson, 1995).