Geoscience Reference
In-Depth Information
Plains. In alpine areas, snowlines will rise by 100 to
400 m, depending on precipitation.
species on peaks in the European Alps indicate an
upward migration of alpine plants by 1 to 4 m per decade
during the twentieth century.
Tropical forests are likely to be affected more by
human deforestation than by climate change. However,
decreases of soil moisture are particularly damaging in
hydrologically marginal areas. In the Amazon, climatic
predictions support the idea of increased convection,
and therefore of rainfall, in its western equatorial por-
tion, where present rainfall is most abundant. Because
of the particularly high temperature rises predicted for
the high northern latitudes, boreal forests are expected
to be strongly affected by their advance northward into
tundra regions. This may produce positive feedback
giving further regional warming because of the lower
albedo of forests during the snow season. Climate
change over the twenty-first century is expected to have
least effect on temperate forests.
Wetlands at present cover 4 to 6 per cent of the land
surface, having been reduced by human activities by
more than half over the past 100 years. Climate change
will affect wetlands mainly by altering their hydro-
logical regimes. It is believed that eastern China, the
USA and southern Europe will suffer a natural decline
in the area of wetlands during this century, decreasing
the methane flux to the atmosphere.
Drylands are expected to be profoundly affected by
climatic change. Dry rangelands (including grasslands,
shrublands, savannas, hot and cold deserts) occupy 45
per cent of the terrestrial land surface, contain one-third
of the world's total carbon in their biomass, and support
half of the world's livestock and one-sixth of the world's
population. Low-latitude rangelands are most at risk
both because an increase in CO
2
(increasing the carbon/
nitrogen ratio) will decrease the nutrient value of forage
and because the increasing frequency of extreme events
will cause environmental degradation. Most deserts
are likely to become hotter but not significantly wetter.
Any increases in rainfall will tend to be associated with
increased storm intensity. Greater wind speeds and
evaporation may be expected to increase wind erosion,
capillary rise and salinization of soils. Central Australia
is one of the few places where desert conditions may
improve.
A major consequence of global warming will be
that desiccation and soil erosion will increase in semi-
arid regions, rangelands and savannas adjacent to the
world's deserts. This will accelerate the current rate
of desertification, which is proceeding at six million
3 Hydrology
The difficulty of deducing the possible effects of climate
change on hydrological regimes stems from several
factors. There is the need to adapt large-scale climatic
predictions derived from GCMs to the catchment scales
appropriate to hydrological modelling; from errors in
the climatic and hydrological data; and from converting
climatic inputs into hydrological responses. Predicted
climatic changes are expected to lead to:
1
A more vigorous world hydrological cycle.
2
More severe droughts and/or floods in some places
and less severe ones in others.
3
An increase in precipitation intensities with possibly
more extreme rainfall events.
4
Greater hydrological effects of climate change in
drier areas than in wetter ones.
5
An increase in overall potential evapotranspiration.
6
An increase in the variability of river discharges
along with that of rainfall.
7
A shift of runoff peak times from spring to winter in
continental and mountain areas if snowfall decreases.
8
The greatest falls in lake water levels in dry regions
with high evaporation.
The hydrological impacts of climate change may be
greatest in currently arid or semi-arid regions, implying
that the more severe runoff events there will particularly
exacerbate soil erosion.
4 Vegetation
An increase in CO
2
is expected to enhance global plant
growth up to a value of possibly around 1000 ppmv,
when a saturation limit may be reached. However,
deforestation could decrease the biosphere's capacity
to act as a carbon sink. A sustained increase of only 1°C
can cause considerable change in tree growth, regen-
eration and species extent. Species migrate only slowly
but, eventually, extensive forested areas may change to
new vegetation types, and it has been estimated that 33
per cent of the present forest area could be affected, with
up to 65 per cent of the boreal zone being subject to
change. Alpine tree lines appear to be quite resistant
to climatic fluctuations. However, surveys of plant
