Heat and energy in the atmosphere - Fundamentals of the Physical Environment - page 52

convective heat exchanges. Many of them take place
through evaporation and are discussed later in this
chapter.

appreciably different ( Figure 3.3 ). The general impression
of the maps is of a decrease in energy input towards the
poles, with local anomalies. Most of these anomalies are
caused by the 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. The
seasonal changes as the apparent position of the overhead
sun oscillates between the Tropics are noticeable.
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 ( Figure 2.13 ). 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 radiation. When the sun is

SPATIAL VARIABILITY OF
RADIATION EXCHANGES

Earth is a large spheroidal body which spins on an axis
tilted at 23 1 2

to the vertical and has an elliptical orbit
around the sun. These factors alone have a considerable
influence on how radiation is distributed at Earth's
surface.
Earlier we described the input of solar energy at the
top of the atmosphere and how it was determined by
these astronomic controls. Figure 2.13 showed how the
radiation would be distributed at the surface without an
atmosphere. However, if we look at a map of the average
seasonal short-wave radiation reaching the ground, it is

The greenhouse effect

KEY CONCEPT

The property of the atmosphere that allows the transmission of sunlight, but acts as a partial barrier to the loss
of heat from the surface, has been called the greenhouse effect because of its analogy with a greenhouse,
which was believed to produce warming by a similar process. Subsequent work has shown that greenhouses are
warmed as much by the protection from wind as by any radiational effect but the name of the effect has remained.
Without this natural greenhouse effect Earth's equilibrium temperature would be about -19 s C and the planet would
be almost uninhabitable. We can work this figure out from the amount of long-wave radiation which is lost by the
planet.

As a result of the massive consumption of fossil fuels such as coal, gas 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 3.6 ). Since 1720 the concentrations
of carbon dioxide have increased from about 280 ppmv (parts per million by volume) to the current levels of over
380 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 long-wave 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 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, such as solar output, which are known to occur naturally before it is possible to determine
the precise role of the 'enhanced' greenhouse effect on our climate.

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