Geography Reference
In-Depth Information
back down to Earth again which in turn is absorbed by the Earth
and this further enhances the heating of the atmosphere. A green-
house does a similar thing. The glass allows short-wave radiation to
pass through it to the soil and plants which then absorb the radia-
tion and re-radiate thermal energy back towards the glass.
However, the glass traps the long wavelength heat energy and the
warmer air inside the greenhouse. The natural greenhouse
effect in the atmosphere is a good thing. If this did not happen
then during the day the Sun's energy would be absorbed by the
land, oceans, and vegetation at the surface and then transformed
into heat which would be radiated back into the atmosphere.
However, at night all of this energy would radiate back into space
and so the Earth's surface temperature would fall to extremely cold
levels very quickly. The greenhouse gases prevent this from hap-
pening by retaining some of the energy within the troposphere,
delaying its release back out to space, and keeping the planet at a
good temperature for life. The average temperature of the Earth's
surface is 15°C but without the natural greenhouse effect the
average temperature across the Earth would be around -20°C.
The composition of the atmosphere has changed through time.
The Earth is around 4.6 billion years old. The early atmosphere
mainly consisted of nitrogen gas and carbon dioxide with no
oxygen gas. Oxygen gas did not start to appear in the atmosphere
until about 2 billion years ago. It was at this point that bacteria
evolved and they functioned by absorbing carbon dioxide from the
atmosphere and then releasing oxygen through photosynthesis .
More recently humans have also changed the composition of the
atmosphere slightly through the burning of fossil fuels and release
of other chemicals. This topic is explored further in Chapter 2.
There are many other complex feedbacks between the atmo-
sphere and the Earth which control climate. We will turn to some
of these in later chapters but for now a good example is the growth
of peatlands. Peat is an extremely carbon-rich soil composed of
dead plant matter that has not fully decayed and which builds up
on the land year after year. In some places it has built up deposits
over 10 metres thick. It forms where the conditions are water-
logged because waterlogging slows the rate of decay when plants
die. Plants take carbon out of the atmosphere to form their struc-
ture by incorporating it into carbohydrates. Once the plants die, if
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