Download 18.7 Greenhouse gases and global warming

18.7 THE GREENHOUSE EFFECT AND GLOBAL WARMING
The Earth receives energy from the sun at the rate of about 340
watts per square meter (W/m2) of its surface. This energy
arrives in the form of electromagnetic radiation (Module 14.2),
mainly light and heat. About 30% of the energy is reflected back
into space, mostly by clouds and the polar ice caps. The
remaining 70% is absorbed by, and warms, the atmosphere, the
oceans and the continents. This makes the Earth much hotter
than surrounding space, so it also loses heat by radiation. There
is an equilibrium (a balance) between the heat that the Earth
gains from the sun and the heat that it loses by radiation into
space. This equilibrium keeps the average temperature at the
surface almost constant at about 14ºC.
A greenhouse is an enclosure
for growing crops that need
warmth. It is made of glass or
clear plastic which let in the
heat of the sun and then trap
the warm air inside.
The greenhouse effect is an effect that traps some of the heat which the Earth would otherwise
lose by radiation. This effect was first predicted by the French physicist Joseph Fournier in 1824
to explain why the Earth was warmer than it should be based on its distance from the sun. Details
of the effect were finally worked out by the
Surface
Type of radiation emitted
Swedish chemist Svante Arrhenius in 1896.
temperature
The table on the left shows that the sun has
an average surface temperature of about
Sun
6000 ºC
High energy light and heat
6000ºC. It radiates high energy light and
heat rays. The Earth has an average surface
Earth
14 ºC
Low energy heat
temperature of only 14ºC. It radiates low
energy heat rays. Greenhouse gases in the air let through the incoming high energy rays but
absorb some of the outgoing low energy rays. The energy they absorb makes the greenhouse
gas molecules warmer than the surrounding air and they radiate heat in all directions. Part of this
heat goes back down towards the surface of the Earth as shown in the diagram. This reduces the
amount of heat that the Earth would otherwise lose by radiation. Without the greenhouse effect,
the average temperature at the surface of the Earth would be about 32ºC less than it is; a very
chilly -18ºC! The most important greenhouse
gases are listed in the table below. Their
contributions to the total effect vary depending on
the amount of water vapour in the atmosphere.
Greenhouse gas
Contribution to
greenhouse effect
Water vapour, H2O
Up to 72%
Carbon dioxide, CO2
Up to 26%
Methane, CH4
Up to 9%
SOME ENERGY ABSORBED BY GREENHOUSE
GASES AND RE-EMITTED IN ALL DIRECTIONS
RADIATION FROM EARTH’S SURFACE
Global warming refers to the ongoing rise in
the average temperature of the Earth’s atmosphere and oceans since the start of the last century.
Rising levels of carbon dioxide, due to industrial and commercial activity including deforestation,
are causing an increase in the greenhouse effect. Carbon dioxide (CO2) is produced by the
burning of fossil fuels in motor vehicles and power stations (to make electricity), and in many
industrial processes especially the manufacture of iron, steel and cement. Deforestation by
burning and felling trees not only produces additional CO2, it also reduces the number of trees
removing CO2 from the atmosphere by photosynthesis. The graphs below show that the rise in
CO2 levels, and the rise in temperature,
0.7
380
are accelerating. Global warming has
370
0.6
Rise of
Concentration
started to melt the ice at the poles and
360 of CO
0.5 temperature
to change weather patterns. The effects
350 (parts per
0.4 ( ºC )
of climate change are hard to predict in
0.3
340 million)
detail but will include rising sea levels
0.2
330
and more extreme weather events such
0.1
320
Year
Year
as cyclones, floods and droughts.
2

1. Why is the greenhouse effect so called?

2. Explain in your own words (i) the
greenhouse effect, (ii) global warming, (iii) climate change, (iv) deforestation, (v) fossil fuels, (vi) pollution.
3. Between the years 1960 and 2000, what was the rise in (i) the concentration of CO 2 (ii) the Earth’s temperature?
1900

1920
1940
1960
1980
2000
Trend in temperature of atmosphere
1960
1970 1980 1990 2000
Trend of CO2 in atmosphere
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