Environmental Engineering Reference
In-Depth Information
States or the Tibetan plateau. The westerly circulation would certainly be different,
probably with less meridional exchange and a different pattern of precipitation as areas
favouring convergence and divergence changed. The ideas could be tested by using
climate models with different surface topography, though the validity of the conclusions
would depend upon the reliability of the model. Similarly, ocean basin shape may vary,
as it did to some extent when sea level dropped markedly during glacial phases. Shape
changes may affect ocean current patterns and - something which has been appreciated
only recently - they may affect salinity levels and interactions between surface and deep-
sea waters (Figure 11.9).
Changes in surface features may have drastic effects on climate over rather shorter
time periods than orography and ocean shape. Deforestation is a clear case of a change in
surface properties which, by changing surface albedo and heat budget, could affect
climate. Clearance of temperate forests in Europe, and currently of tropical forest in
South America and East Asia, are believed to have the potential to modify climate. Some
model estimates predict that a change of Brazilian rain forest to savannah would lead to a
decrease of evapotranspiration of up to 40 per cent, an increase of run-off from 14 per
cent of rainfall to 43 per cent, and an average increase in soil temperature from 27° C to
32° C, but the precise figures depend upon the assumptions embodied in the models.
Degradation of vegetation has also been given as a factor in influencing the climatic
change in the Sahel.
Another surface change which would have clearer effects is when snow or ice melts. If
a surface is deglacierized, its albedo will decrease from relatively high values to much
lower ones; as a result more solar energy will be available to warm the surface. With
snow or ice on the ground much energy will be reflected or consumed in melting or
ablating. This energy will become available for heating when the surface changes. All
these factors produce a marked increase of temperature at the surface. Conversely, a
change to snow and ice would trigger a positive feedback to enhance cooling. It has been
suggested that the regional cooling over Europe between 11,000 and 10,000 years BP
may have resulted from a breakdown of the North Atlantic Gulf Stream due to a sudden
massive surge of fresh pro-glacial meltwater from the St Lawrence and the break-up of
sea ice from northern Canada.
It is well known that atmospheric composition can and does change through time,
though precise levels of measurement may not be available. One major influence is
volcanic activity. When a volcano erupts it may expel vast quantities of dust and gases
such as carbon dioxide and sulphur dioxide into the atmosphere. How significant the
eruption is for climatic conditions depends particularly on how much material is ejected
into the stratosphere and how long it is able to survive there as well as the latitude of the
eruption. If dust and sulphate particles can survive in the stratosphere they are able to
reduce by reflection the amount of solar radiation reaching the ground. The longer they
survive the greater will be the impact of the eruption. Major eruptions can result in
surface cooling of about 0·2° C for a few years after the event. The eruption of Mount
Pinatubo in the Philippines in 1991 led to a reduction in the recent trend of increasing
global mean temperatures (Figure 9.6). Although large amounts of smoke were released
from the Kuwaiti oilfield fires in the Gulf War, none of it was able to penetrate into the
stratosphere and its climatic effects were largely local. Volcanic eruptions at high
latitudes send dust into the circumpolar vortex which tends to get trapped rather than
