Excerpt from “Global
Environment Outlook 3” Chapter 2, p214. UNEP, 2002.
Scientists have known about the natural ‘greenhouse
effect’ for more than a century (Arrhenius, 1896): the
Earth maintains its equilibrium temperature through a
delicate balance between the incoming solar energy (short
wavelength radiation) it adsorbs and the outgoing
infra-red energy (long wavelength radiation) that it emits
and some which escapes into space. Greenhouse gases (water
vapour, carbon dioxide, methane and others) allow solar
radiation to pass through the Earth’s atmosphere almost
unimpeded but they absorb the infra-red radiation from the
Earth’s surface and then re-radiate some of it back to the
Earth. This natural greenhouse effect keeps the surface
temperature about 33°C warmer than it would otherwise be –
warm enough to sustain life.
Since the industrial revolution the concentration of CO2,
one of the major greenhouse gases, in the atmosphere has
increased significantly. This has contributed to the
enhanced greenhouse effect known as ‘global warming’
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