Environmental Engineering Reference
In-Depth Information
Figure 1
Carbon dioxide measurements at Mauna Loa, Hawaii.
As a result of the massive consumption of fossil fuels such as coal and oil, the waste
products of combustion have been released into the atmosphere, where they slowly
accumulate. Changing land use through deforestation, particularly of the temperate and
tropical forests, has a similar effect, as less carbon is stored in the replacement crops.
From this, the composition of our atmosphere has been changing (Figure 1). Since 1720
the concentrations of carbon dioxide have increased from about 280 ppmv (parts per
million by volume) to the current levels of over 367 ppmv and those of methane from 0·7
ppmv to 1·7 ppmv. Other minor constituents of the atmosphere with similar effects such
as chlorofluorocarbons (CFCs) and nitrous oxide have increased, too. As there is now a
greater concentration of gases in the atmosphere which have the capacity to absorb
longwave radiation from Earth's surface, it may be expected that Earth will warm.
Actual changes of climate and their causes will be discussed in Chapter 9, but it is
clear that this 'enhanced' greenhouse effect has the potential to warm our planet. It is
much more difficult to
prove
that the increase in the concentration of greenhouse gases
has been significant in the variations of global temperature in the last 300 years.
Mathematical models of the climate system demonstrate that a doubling of the proportion
of carbon dioxide in our atmosphere should lead to an increase of global temperatures of
between 2° and 4° C, but there is considerable uncertainty about the accuracy of the
predictions. We need to know much more about the causes of short-period temperature
changes which are known to occur naturally before it is possible to determine the precise
role of the 'enhanced' greenhouse effect on our climate.
surface albedo and the distribution of clouds. High values are found over the tropical
oceans. The lowest values occur in regions of high albedo such as Greenland, or of high
cloudiness such as Amazonia.
What is it that produces this spatial pattern of radiation? Obviously the astronomic
factors have a great effect, giving rise to the poleward decline. But the decrease is far
greater than one would expect from the distribution at the top of the atmosphere. We have
to look for other reasons. One of the most important is the angle between the sun's rays
and Earth's surface. The input is greatest whenever the surface is at right-angles to the
sun's rays. If the sun is overhead a horizontal surface will receive the highest intensity of
