In the recent geologic past, the tundra was a carbon sink. Recent climatic warming in the Arctic, coupled with the concomitant drying of the active layer and the lowering of the water table, has shifted areas of the Arctic from sinks to sources of CO2 (Oechel et al., 1993). Extrapolating results from the Alaskan tundra to the circumpolar Arctic, regional warming could have caused a net flux from the land to the atmosphere of about 0.2 GtC/yr during the 1980s (IPCC 1996, WG I, Section 126.96.36.199). However, unpublished evidence from the Land-Atmosphere-Ice-Interactions component of ARCSS indicates that there may be substantial interannual variability in the source-sink relationship of Arctic tundra. This concern is reinforced by recent study by Myneni et al. (1997). Loss of a sizable portion of more than 50 Gt of carbon in Arctic soils and 450 Gt of carbon in soils of all tundra ecosystems could cause an appreciable positive feedback on the atmospheric rise of CO2 (IPCC 1996, WG II, Section 7.4.3). It should be noted, however, that the preceding projections are based on an assessment for all high-latitude ecosystems, not just the tundra, and could be misleading as an indication of the potential for tundra ecosystems to act as a positive feedback. In addition, Schimel et al. (1994) and McGuire et al. (1995) have modeled the temperature sensitivity of soil carbon responses and find levels much less than 450 GtC.
Enhanced decomposition of soil organic matter leads to the release of trace gases and feedbacks on the global climate. A change in total CH4 flux from Arctic wetlands can be expected if the areal extent of wetlands changes, the duration of the active period changes, or the per-unit-area production or oxidation of CH4 changes (IPCC 1996, WG II, Section 188.8.131.52.2). The limited available data for the CH4 content of permafrost has substantial variability. High CH4 concentrations in ice-bonded sediments and gas releases suggest that pore-space hydrate may be found at depths as shallow as 119 m. These data raise the possibility that gas hydrates could occur at much shallower depths and may be more rapidly influenced by climate change than previously thought (IPCC 1996, WG II, Section 7.4.3).
In Antarctica, the terrestrial ecosystem is comparatively simple, constrained by an exposed land area that is very cold. Only 2% of the Antarctic surface is not covered by ice. This limited regime is mostly rocky areas where the temperature is below freezing except for periods of a few days or weeks during the Austral summer. These "Antarctic oases" provide a natural laboratory for assessing the vulnerability and response of this ecosystem to climate change. The mainland plays host to a number of microscopic plants that are found mainly in crevices and cavities of exposed rocks. Even the poorly developed soil of Antarctica harbors bacteria, algae, yeast and other fungi, lichens, and even moss spore (though usually in a dormant stage). The coastal region is particularly hospitable to the vegetation of lichens and mosses. Meltwater in the area helps to support herbaceous species such as grass. Some species of invertebrates survive in the harsh environment by super-cooling or anhydrobiosis mechanisms (Walton and Bonner, 1985). The Dry Valley's environmental conditions resemble those on Mars; this area is one of the world's most extreme desert regions. It was formed by the advances and retreats of glaciers through the coastal mountain ranges. The ecosystem there consists of microorganisms, microinvertebrates, mosses, and lichens. Climate change will impact on the physiology, distribution, and species composition of this terrestrial ecosystem.
The ranges of many species in lakes and streams are likely to shift poleward by about 150 km for every 1�C increase in air temperature (IPCC 1996, WG II, Technical Summary, Section 3.3.1). This axiom is not useful for extrapolation in oceanic waters. If global warming effects continue, the sea surface temperature increases would be about 3�C in the North Pacific in 50 years. This change represents the same effect that would be seen in a present isotherm shift northward of about 500 km in mid-latitudes (Kim, 1995). Climate change or regime shifts might change distribution, species composition, and productivity in the North Pacific as well as adjacent subarctic areas.
Water temperatures have a direct impact on the productivity of fish species and the relative abundance of different fish species. Climatic changes are likely to affect not only water temperatures but also salinity and seasonal water cycles. Temperature influences biological production and can have a profound effect on growth and metabolic processes. Observations show that biological rates double or halve with a 10�C increase or decrease in temperature, respectively. Over the range of temperatures encountered in the ocean (-2 to +30�C), maximum growth rates of plankton vary by about a factor of 10. At high latitudes, where the temperature range is smaller (-2 to +3�C), temperature changes may have a relatively larger effect. Even small changes in temperature could have pronounced effects on biological rates of growth and development. Further, glacial meltwater is known to affect the nearshore zonation of Antarctic marine invertebrates (Berkman, 1994) and increased meltwater production may further impact species in the Antarctic coastal zone. Climate impacts on long-lived benthic invertebrate species such as sponges and some calcareous species, some of which have decadal and perhaps century-long lifespans, are unknown.
Greenhouse warming of the troposphere would be accompanied by cooling of the stratosphere. Changes in the stratospheric ozone layer have occurred over both the Arctic and the Antarctic. In the Antarctic, the occurrence of a persistent stratospheric ozone hole occurs during the spring and is due to ozone-depleting substances, including chlorofluorocarbons. The low human and species populations in the Antarctic have limited the biological damage, although there is evidence that significant changes in ecosystem function could occur. In the Arctic, where considerably more biological activity may be affected, ozone depletion and increased uv radiation have been observed over the past decade. Ozone depletion has occurred both as a steady decline and also with short, isolated areas of very low ozone (Weatherhead and Morseth, 1997). Climate change may further ozone depletion. The cooling of the stratosphere is likely to increase this depletion with current chlorine loading. However, chlorine loading can be expected to decline considerably in the future. Some of the episodes of low ozone observed in the Arctic are not associated with chemical depletion but are due to the influx of low-ozone air from lower latitudes (Taalas, 1993; Taalas et al., 1995). Whether these episodes will increase or decrease will depend on stratospheric circulation patterns near the Arctic; thus, it also may be influenced by climate changes. The chemically induced and the dynamically induced episodes of low ozone occurring in the Arctic both appear to be increasing in frequency and severity (Taalas et al., 1997; Weatherhead and Morseth, 1997). These depletion events are most prevalent in the spring, when biological activity is highly sensitive to UV-B radiation. Increased levels are likely to affect the human populations as well as the aquatic and terrestrial species and ecosystems (Taalas, 1993; SCOPE, 1996).
Sea ice is important in the development and sustenance of temperate-to-polar ecosystems. Ice conditions conducive to ice-edge primary production provide a primary food source in polar ecosystems. Ice-dependent activities of organisms ensure energy transfer from primary producers (algae and phytoplankton) to higher trophic levels (fish, marine birds, and mammals). As a consequence, the ice-dependent habitat maintains and supports abundant biological communities (IPCC 1996, WG II, Section 8.3.2).
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