When solar radiation falls on the Earth the energy is
distributed in a number of ways. Some is absorbed by the atmosphere, some is reflected by the
land and the remainder is absorbed by the Earth's surface. This warms the land which then re-
radiates this energy.
The radiation from the Sun's surface at a temperature 6000 K has a
peak on the energy-wavelength curve at 420 nm. The Earth may rise to a temperature of
27o C (300 K) and so the radiation is emitted with a wavelength of 6000/300 = 20 times
that of the incident radiation - in other words 8400 nm - well into the region where the glass will be
opaque. Of this radiated energy some escapes into space but the rest is absorbed by the
atmosphere. This known as the greenhouse effect.
Not all gases absorb infra red. The most
important absorbers are the so called "Greenhouse gases" such as carbon dioxide, water vapour,
methane, sulphur dioxide, nitrous oxide and the chlorofluorocarbons (CFCs). At present some 100
000 million tons of carbon dioxide every year from biological processes but people add a further
5000 million tons. The destruction of the rain forests removes a most important absorber of carbon
dioxide - trees.
The fact that the low energy infrared cannot escape means that the
temperature of the Earth's surface will rise – known as global warming. The results of this may be
a radical change in the climate of out planet and steps must be taken to prevent this. The raising of
the mean sea level due to the melting of the south polar ice cap is just one effect, and this would
result not only in the flooding of many major cities that are sited at "sea level" but also the
disappearance of low lying islands. Storms and unseasonal temperature have also been
predicted.
It remains to be seen just how accurate these prophecies are!
