Environmental Engineering Reference
In-Depth Information
New or increased disease and insect infestation
might also follow the warming, but that might
be offset by a decline in existing infestation
problems (Melillo et al. 1990). With warmer and
potentially drier conditions in the forest, an
increase in the frequency of forest fires is a distinct
possibility. One element in particular that requires
study is the rate at which the forests can respond
to the warming. If the temperatures change more
rapidly than the forests can accommodate them,
then the rapid die-off of large numbers of trees
would cause disruption to the northern
ecosystems for perhaps centuries to come. This
in turn would disrupt the patterns of economic
activity in the north, and have a significant effect
on those countries such as Canada, Sweden,
Finland and Russia, where national and regional
economies depend very much on the harvesting
of softwoods from the boreal forest.
In lower latitudes, where the temperature
element is less dominant and changes are
expected to be less, the impact of global warming
will often be experienced through changes in the
amount and distribution of moisture. Soil
moisture levels would decline in areas
experiencing a Mediterranean-type climate—
southern Europe, South Africa, parts of South
America and Western Australia—for example,
as a result of reduced precipitation and the higher
evapotranspiration rates associated with the
warming (Mitchell et al. 1990; Pittock and
Salinger 1991). Although the vegetation in these
areas has adapted to seasonal drought, the
extension of the dryness into the normally wet
winter season would ultimately bring about
changes in the composition and distribution of
the Mediterranean climate biome. Extra
precipitation in monsoon areas, and the increased
poleward penetration of the monsoon rains
expected to accompany global warming (Pittock
and Salinger 1991) would allow the expansion
of the tropical and sub-tropical vegetation of
areas such as northern Australia. Elsewhere—the
Sahel, for example—the impact of the extra
precipitation would be offset, perhaps completely,
by increased evapotranspiration rates at higher
temperatures.
The conditions likely to alter the regional
distribution of natural vegetation are also likely
to change the nature and extent of cultivated
vegetation. A significant expansion of agriculture
is to be expected in mid to high latitudes, where
the greatest warming will be experienced. In the
interior of Alaska, for example, a doubling of
CO 2 levels would raise temperatures sufficiently
to lengthen the growing season by three weeks
(Wittwer 1984), which would allow land
presently under forage crops, or even
uncultivated, to produce food crops such as
cabbage, broccoli, carrots and peas. The growing
season in Ontario, Canada, would be lengthened
by 48 days in the north and 61 in the south. The
reduced frost risk at the beginning and end of
the growing season would be a major benefit in
some areas. By 2050, in New Zealand and the
coastal areas of Australia, for example, the frost-
free season may be 30-50 days longer than at
present (Salinger and Pittock 1991). The greater
intensity of the growing season, along with the
effects of increased CO 2 on photosynthesis,
would lead to increased crop yields. In Europe,
simulations of the effects of a doubling of CO 2
on crops, using grass as a reference, have
indicated an average increase in biomass potential
of 9 per cent, with regional values ranging from
an increase of 36 per cent in Denmark to a
decrease of 31 per cent in Greece (Santer 1985).
The increase in agricultural output in China is
expected to be 2 per cent, brought about by the
greater production of rice, maize and cotton at
higher latitudes, and a northward shift of 50-
100 km in the cultivation of tropical and sub-
tropical fruits (NCGCC 1990).
It may not always be possible for
agriculturalists to take full advantage of the
benefits of global warming, because of the effects
on agricultural production of elements either
unrelated or only indirectly related to climate
change. Warmer climates would allow the
northward expansion of cultivation on the
Canadian prairies, for example, but the benefits
of that would be offset by the inability of the
soils in those areas to support anything other than
marginal forage crops, which are not profitable
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