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Writing in 1862, John Tyndall described the key to our modern understanding of why the Earth's surface is so much warmer than the effective radiating temperature.
"As a dam built across a river causes a local deepening of the stream, so our atmosphere, thrown as a barrier across the terrestrial rays, produces a local heightening of the temperature at the Earth's surface."
(Tyndall, 1862, quoted in Weart, 2004)
Tyndall's careful experimental work had established what others only suspected: expressed in modern scientific terms, certain atmospheric gases absorb infrared radiation with wavelengths in the range spanned by outgoing terrestrial radiation (about 4 to 100 μm). These are the greenhouse gases.
Tyndall identified water vapour and carbon dioxide (CO2), but the list of natural greenhouse gases (naturally present in the atmosphere long before human activities began to make their mark) also includes methane (CH4), nitrous oxide (N2O) and ozone (O3). The main mechanism by which these gases absorb infrared radiation is through the vibrations of their molecules.
The natural greenhouse gases absorb infrared wavelengths throughout most of the terrestrial range; there is only one region, between 8 and 13 μm, where absorption is weak.
Known as the ‘atmospheric window’, this allows some of the longwave radiation from the surface to escape directly to space, but most of it is intercepted by the atmosphere.
Now most of the longwave radiation from the surface is effectively ‘trapped’ and recycled by the atmosphere, being repeatedly absorbed and re-emitted in all directions by the greenhouse gases. This warms the atmosphere.
Some of the re-emitted radiation ultimately goes out to space, maintaining an overall radiation balance at the top of the atmosphere. This prevents the whole Earth-atmosphere system from heating up without limit.
The crucial difference is that much of the re-emitted radiation goes back down and is absorbed by the surface. It is this additional energy input - over and above the absorbed solar radiation - that keeps the Earth's GMST over 30 °C warmer than it otherwise would be.
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