COMPOSITION OF THE ATMOSPHERE

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.