The climatic effects of water vapour

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National Institute for Space Research

2003

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EXTREME variations in local weather and the seasons make it easy for people to mutter

“greenhouse effect”, and blame everything on carbon dioxide. Along with other man-made gases,

such as methane, carbon dioxide has received a bad press for many years and is uniformly cited

as the major cause of the greenhouse effect. This is simply not correct. While increases in carbon

dioxide may be the source of an enhanced greenhouse effect, and therefore global warming, the

role of the most vital molecule in our atmosphere – water – is rarely discussed. Indeed, water

barely rates a mention in the hundreds of pages of the 2001 report by the Intergovernmental Panel

on Climate Change.

Many aspects of the seemingly simple water molecule conspire to make it difficult to model its

effect on our climate. Unlike most other atmospheric gases, the distribution of water in the

atmosphere varies strongly with time, location and altitude (figure 1).Water is also unique among

atmospheric molecules because it changes phase at terrestrial temperatures.This means that it can

transfer energy from its frozen form at the poles to its liquid and vapour forms in the atmosphere.

Once in the atmosphere, water moves with the winds and can even diffuse up to the stratosphere,

where it is responsible for destroying the ultraviolet-shielding ozone layer.

The atmosphere plays a crucial role in the Earth’s radiation budget because it absorbs both the

incoming radiation from the Sun and the outgoing radiation that is reflected from the planet’s

surface. However, the radiation in each of these processes has very different wavelengths. The

Sun radiates approximately as a black body with a temperature of 5800K, which peaks in the optical

region at a wavelength of about 0.6 µm. The reflected radiation profile, on the other hand, is much

closer to a black body at a temperature of 275 K, and has a peak at much longer infrared

wavelengths (about 11 µm). The physical processes that lead to the absorption of radiation in the

two regions are different, but water vapour plays the dominant role in both.