Weather wise

THE GREENHOUSE effect is a good thing. Without it, life almost certainly could not have evolved on Earth, as, for the most part, water could have not have existed on its surface.

A planet orbiting our sun at the same distance as our planet, spinning once a day and equipped with a thermally transparent atmosphere, say of pure nitrogen, could expect to have an average surface temperature of about -18C.

Happily, large amounts of gases which absorb outgoing radiation at various wavelengths have always been present in Earth's atmosphere, raising average surface temperatures by 33C to a much more hospitable 15C.

The most important of these gases, by far, is water vapour, which alone accounts for 21C of global warming.

Water absorbs infra-red heat radiation across a wide range of wavelengths, and is the primary agent in rendering Earth habitable. Other natural gases to which we should be grateful include carbon dioxide (a 7C warming), ozone (2C) and trace gases such as nitrous oxide (3C).

Deep in the pre-Cambrian era, three billion years ago or more, Earth's mean surface temperature was much higher than today - so warm, in fact, that polar ice caps probably did not exist.

This was because concentrations of water vapour and methane - an extremely powerful greenhouse gas, one of the main sources of which today is the rectums of farm animals - were high.

Ammonia, another greenhouse gas, now almost completely absent from our atmosphere, was also present in large concentrations.

The early Earth, as well as being rather muggy, must have been a very smelly place.