Geoscience Reference
In-Depth Information
The Earth's natural greenhouse
The temperature of the Earth is determined by the balance between energy from Sun and its
loss back into space. Of Earth's incoming solar short-wave radiation (mainly ultraviolet
(UV) radiation and visible 'light') nearly all of it passes through the atmosphere without in-
terference (see
Figure 1
). The only exception is ozone that luckily for us absorbs energy in
the high-energy UV band restricting how much reaches the surface of the Earth as it is very
damaging to cells and DNA. About one-third of the solar energy is reflected straight back
into space. The remaining energy is absorbed by both the land and ocean. This warms them
up, and they then radiate this acquired warmth as long-wave infrared or 'heat' radiation.
Atmospheric gases such as water vapour, carbon dioxide (CO
2
), methane (CH
4
), and ni-
trous oxide are known as greenhouse gases (GHGs) as they can absorb some of this long-
wave radiation, thus warming the atmosphere. This effect has been measured in the atmo-
sphere and can be reproduced time and time again in the laboratory. We need this green-
house effect because without it, the Earth would be at least 35°Celsius (C) colder, making
the average temperature in the tropics about -10°C. Since the industrial revolution we have
been burning fossil fuels (oil, coal, natural gas) deposited hundreds of millions years ago,
releasing the carbon back into the atmosphere as CO
2
and CH
4
, increasing the 'greenhouse
effect' and elevating the temperature of the Earth. In effect we are burning fossilized sun-
light.
