10.2.1.2. Deserts and Desertification
Cold deserts and semi-deserts are widespread in west-central Temperate Asia.
With projected shifts to warmer and drier conditions in Mongolia, the Gobi would
change to warm temperate desert scrub, and the area of cool temperate desert
scrub would expand to the Khangai mountains, supplanting forest areas (Ulziisaikhan,
1996). At the same time, low- and midland parts of central Asia are likely to
change gradually into a more arid interior desert as the disappearance of mountain
glaciers reduces the volume of summer runoff. Projection of potential changes
in the total extent of cool semi-deserts varies among models: Whereas BIOME
1.1 suggests an almost 50% reduction in its extent, IMAGE 2.0 projects no change.
However, desertification arises from adverse climatic conditions and from human
abuse of the land; the latter can accelerate the desertification process (IPCC
1996, WG II, Section 2.6).
Evapotranspiration rates over the grasslands in inner Mongolia of China are
two to three times higher in midsummer than in spring or autumn, and the heat
budget of the grassland under such a desertification process is close to that
of a sand dune (Harazono et al., 1993). In northWestern China, the number of
days each year when windstorms (greater than 17.2 m/sec) occur averages 20-40
days in arid regions and 10-25 days in semi-arid regions-but reaches more than
100 days in areas most affected (Xia, 1993; Zhao, 1996). These numbers are expected
to increase under global warming because dryer and stronger-wind conditions
are expected, resulting in accelerated soil degradation.
10.2.1.3. Lakes, Streams, and Wetlands
Significant shortening of the duration of ice cover in lakes is expected. Enhanced
evaporation, as well as ground thaw, would cause some arctic lakes to disappear.
Lakes in northern latitudes may change from a vertically homogeneous state to
a stratified state (vice versa for those in southerly areas). These changes
in mixing regimes are likely to have very important effects on biota (IPCC 1996,
WG II, Section 7.4.4).
Lake water levels are very sensitive to climate. In many regions, lake levels
are likely to decline or fluctuate more widely because of changes in precipitation
or increases in evapotranspiration. In lakes experiencing declines or rapid
changes in water level, vegetation and habitat characteristics of highly productive
inshore littoral areas may change significantly-with profound but highly lake-specific
effects on food webs and productivity. Water-level declines in lakes with extensive
bordering wetlands are likely to reduce lake productivity and populations of
fish and invertebrates dependent on these wetlands for spawning and nursery
grounds (IPCC 1996, WG II, Sections 10.5.2 and 10.6.2).
Climate change would result in a wide range of changes in the hydrological
regimes of streams, with associated impacts on their ecosystems. For snowmelt
streams, for example, a shift toward more rainfall and less snowfall in winter
would result in a shift of the seasonality of stream discharge regimes toward
higher winter flows and lower spring and summer flows. These changes may alter
the timing of biogeochemical fluxes that could produce very important, but highly
uncertain, effects on the biota in streams, lakes, and estuaries (IPCC 1996,
WG II, Section 10.3.5).
Model results based on six different climate change scenarios (precipitation
increasing or decreasing by 10% and temperature increasing 1°C, 2°C, and 3°C)
suggest that the areal extent of herbaceous wetlands in eastern China would
decline as a result of reductions in total rainfall and increases in evapotranspiration
(IPCC 1996, WG II, Section 6.3.3).
Local extinctions and extirpations of cold-water and cool-water species of
fish and invertebrates may be expected as a consequence of warming in the temperate
zone. This effect will be most pronounced in shallow lakes, streams, and rivers
in which appropriate thermal refugia (well-oxygenated deep waters, groundwater
vents, access to higher elevations) are not available. Although biodiversity
theoretically should increase with warming-particularly at mid- and high latitudes-initially
there may be a loss in total diversity in cool temperate and boreal regions,
if northward migration of warm-water species cannot keep pace with the rate
of loss of cool-water and cold-water species due to limitations on dispersal
(physiological limitations, lack of north-south corridors) and genetic adaptation
(IPCC 1996, WG II, Sections 10.6.1 and 10.6.3).
Productivity and biodiversity in streams and rivers will likely be reduced
in humid parts of Temperate Asia, which are expected to experience reductions
in total rainfall and/or increases in evapotranspiration that produce longer,
more severe droughts during the warm season. Declines in biodiversity would
result from severe water quality deterioration (e.g., low dissolved oxygen levels,
high concentrations of toxic substances, high temperatures) during extended
low-flow periods in the summer, as well as from drying of previously perennial
streams. These effects will be exacerbated in systems that are strongly affected
by human activities (IPCC 1996, WG II, Sections 10.3.7, 10.4.2, 10.6.2, and
10.6.4).
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