The layer of gases surrounding the Earth rises about 500 kilometres
above the surface, although there is no distinct boundary between
the Earth’s atmosphere and space. However, three-quarters
of the
gas in the atmosphere is within 11 kilometres of the Earth’s surface
as the density of gas is very small at high altitudes.
This is why it
becomes more difficult to breathe when you climb mountains and
why people need oxygen masks if they are in the open air at high
altitudes. The lower layer of the atmosphere is called the troposphere
and extends to about 6 kilometres altitude over the poles
and about 15 kilometres over equatorial regions. This layer is the
one in which air mixes most rapidly and where we experience
weather.
The Earth’s atmosphere acts as a filter protecting us from space
debris and harmful radiation. The Earth receives only two-billionths
of the Sun’s total energy release but this energy is the
main driver for water, air and wind motions and most life on Earth.
It is therefore important to understand how the energy from the
Sun drives these processes. When the Sun’s energy reaches the
Earth about 6 per cent of it is scattered and returned to space by
the atmosphere, 21 per cent is scattered and reflected by clouds and
18 per cent is absorbed by the atmosphere and clouds temporarily
before being sent back out to space. Of the 55 per cent that reaches
the Earth’s surface 4 parts are reflected back to space by reflective
surfaces such as ice sheets, snow and dry, light, sandy soils, while
51 parts are absorbed by the surface.
This means that of the total
4 PHYSICAL GEOGRAPHY
solar energy received by the Earth, only around a half makes it all
the way down to warm the Earth’s surface. Some of this energy is
used for processes such as evaporation of water or for plant growth.
However, most absorbed radiation from the Sun (known as short-wave
radiation; an example of short-wave
radiation is visible light)
is transformed by the land, oceans and vegetation and emitted back
into the atmosphere as long-wave
radiation in the form of heat
energy (invisible infrared radiation).
Except for the 19 per cent
of incoming solar energy that is temporarily absorbed, the atmosphere
is mostly transparent to incoming short-wave
radiation. This
means, perhaps surprisingly to many people, that the air is mainly
heated from below by long-wave
heat energy emitted by the
Earth’s surface. Thus, the atmosphere should be warmer close to
the Earth’s surface but cool with altitude in the troposphere. Since
the air is warmed by the surface below, this means that during the
day the air near the surface becomes less dense and more buoyant.
Less dense gases or liquids will naturally seek to rise and more
dense fluids will seek to fall. Hence the less dense air near the
surface seeks to rise above cooler, denser air which in turn sinks
towards the Earth’s surface. As the air rises it in turn cools because
it is able to expand due to the lower air pressure at higher altitudes.
The reason it cools is due to a fundamental law of nature which
means that as the pressure of a gas decreases the temperature will
decrease. The result of these processes is that there is large scale
vertical mixing of the air within the troposphere as rising warm air
is replaced by cooler descending air.
The atmosphere is made up of mainly nitrogen (78 per cent)
and oxygen (21 per cent). The remaining 1 per cent is made up of
mainly argon. There are also small concentrations of other gases
such as hydrogen, water vapour (the gaseous form of water),
methane, nitrous oxide, ozone and carbon dioxide. However,
despite their low concentrations some of these other gases are
important for the climate we experience. While the gases of the
atmosphere are almost unaffected by the short-wave
radiation provided
by the Sun, some of them readily absorb long-wave
radiation
produced by the Earth’s surface. Unlike oxygen and nitrogen, some
gases such as carbon dioxide, methane, water vapour and nitrous
oxide absorb the thermal energy emitted by the Earth’s surface and
provide a sort of blanket over the Earth. They radiate this energy
ATMOSPHERE, OCEANS, WEATHER AND CLIMATE 5
back down to Earth again which in turn is absorbed by the Earth
and this further enhances the heating of the atmosphere. A greenhouse
does a similar thing. The glass allows short-wave
radiation to
pass through it to the soil and plants which then absorb the radiation
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 happening
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 atmosphere
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 waterlogged
because waterlogging slows the rate of decay when plants
die. Plants take carbon out of the atmosphere to form their structure
by incorporating it into carbohydrates. Once the plants die, if
6 PHYSICAL GEOGRAPHY
this carbon is not released again by decay, then it remains stored on
land. In fact peatlands have preserved many interesting archaeological
features including almost perfectly preserved prehistoric human
remains with their leather shoes still intact. The world’s peatlands
are a large store of carbon that was once in the atmosphere, and
they have actually reduced the amount of the greenhouse gas,
carbon dioxide. Peatlands have helped to cool the climate by a few
degrees. However, this all means that these peatlands could also be
a large potential source of carbon dioxide for the atmosphere if
they are rapidly degraded by human action such as through drainage
or extraction for horticulture or fuel.
