DEEPENING: Climate Change
What is the Greenhouse Effect?
The sun is the earth's only external source of heat.
Whrn solar radiation, in the form of visible sunlight, reaches the
earth, some is absorbed by the atmosphere and reflected from clouds
and land (especially from deserts and snow).
The remainder is absorbed by the surface which is heated and in
turn warms the atmosphere.
The warm earth also radiates energy back into space, but being much
cooler than the sun, does so by giving off invisible infra-red radiation.
The mean temperature of the earth is determined by the balance between
energy coming in from the sun (mainly as visible sunlight) and invisible
infra-red radiation leaving the earth.
The atmosphere is relatively transparent to solar radiation, but
many atmospheric trace gases absorb some of the infra-red radiation
emitted from the surface.
As a result the atmosphere acts like a blanket, preventing much
of the infra-red radiation from leaving the earth and its atmosphere;
this makes the earth warmer.
Why is it called the greenhouse effect?
In a greenhouse, glass allows sunlight in but keeps some infra-red
radiation from escaping.
The gases in our atmosphere with a similar effect are often called
"greenhouse gases".
These are not nitrogen and oxygen - the bulk of the atmosphere -
but trace gases, including for example water vapour and carbon dioxide
(CO2).
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