Environmental Engineering Reference
In-Depth Information
Enhanced warming at the poles appears to be a complex positive feedback situation with
several factors involved. Warming reduces the extent of land and sea ice, so reducing the
surface albedo, leading to more warming. Warming of soils and peats leads to increased
rates of mineralization of the soil organic matter and peat, hence increasing CO
2
and
methane release. Low absolute humidity in the atmosphere enhances the CO
2
effect, and
atmospheric haze, especially in the Arctic, is an important factor. It consists of nitrogen
oxides, sulphur oxides and particulate material, derived from pollution sources in mid and
low latitudes. The haze becomes trapped beneath the inversion and acts as a greenhouse,
especially in spring.
Predicting the future effects on ecosystems of not yet known environmental changes is
an almost impossible task. Taking the example of arctic-breeding shorebirds or waders,
which are relatively high up the ecological food web, there are numerous direct and
indirect pathways, which will either reinforce each other by positive feedback or
counteract each other by negative feedback. Figure 1 shows the predictions from the
latest IPCC model for annual mean temperature rises for the years 2021-50 relative to
1961-90 with a scenario of increased greenhouse gases and sulphate aerosols. Figure 2
shows the complex effects of this rise in temperature. The summer will be warmer, wetter
(though surprisingly on fewer rain days), and soil moisture and snow will decrease. In
addition, windiness and UV-B will increase. Whereas changing climate will affect birds
directly, indirect effects through rising sea levels, habitat changes and food abundance
will be larger. Figure 2 illustrates that the abundance of insects, a major food source for
waders, will itself be affected directly and indirectly via plants by climate change. Whilst
one might expect the abundance of the insects at the breeding grounds to increase under a
warmer climate, higher CO
2
and UV-B levels may counteract this effect.
In the Antarctic the ice sheet covers 13·5 million km
2
and has an average thickness of
2·3 km. Its volume occupies 30 million km
3
, and world sea level would rise by 80 m if
the entire ice sheet melted. There are also associated ice shelves whose stability is very
important, and 2000 to 3000 gigatonnes (Gt) of ice are lost by iceberg calving each year,
only 50 Gt by surface ablation and 500 Gt per year by basal melting. Estimates of the
overall mass balance vary greatly; some say it is negative, some say it is in balance and
some believe there is a deficit as high as 400 Gt per year. The floating ice shelves are
retreating, as are certain parts of the grounded ice sheet. The nature of the mass balance
depends very much on the area being considered.
One of the major uncertainties facing climate change scientists is the future behaviour
of the Antarctic ice sheet, and especially the West Antarctic Ice Sheet (WAIS) and its
associated ice shelves. Figures 15.1 and 15.5 show the distribution of ice sheets and ice
shelves in the Antarctic. Since the last glacial maximum of 20,000 years ago, the WAIS
has lost
two-thirds of its mass, a volume of ice sufficient to raise sea level by 11 m. It is not
clear when retreat started significantly, with estimates varying from 10,000 to 7500 years
ago. The interior of Antarctica does not seem to have lost much ice, with the main losses
being on the edge, especially on the Ross and Ronne-Filchner ice shelves. The ice
shelves are fed by ice streams from the interior, and it is the behaviour of these streams
which is very difficult to monitor and predict. Temperature data from scientific stations
for the period 1945 95 show no warming trend
except for stations on the Antarctic
