Climate depends less on the Sun

IF THE Sun got hotter, would we care? Probably not, according to Hsien-Wang Ou of the Lamont-Doherty Earth Observatory in Palisades, New York. Water, he suggests in the Journal of Climate, minimises the climatic effects of a cooler or warmer sun.

The Sun has got about 30 per cent hotter since the world began. Four billion years ago it was a younger star, and burned less brightly.

Geological imprints of global temperatures at that time indicate that our planet was then warm enough to support liquid water. Global average temperatures seem not to have varied much in either direction since then.

Somehow the planet has stayed indifferent to the Sun's changes. Some explain this so-called `faint young sun paradox' by assuming that the early atmosphere contained more greenhouse gases such as carbon dioxide, which trapped a greater proportion of solar heat. Ou thinks that such considerations may not be necessary.

Water alone is enough to buffer the global temperature against reduced or increased solar heating, he says. Water establishes lower and upper boundaries on how far the temperature can drift from today's. Water vapour is, in fact, the most important greenhouse gas, although unlike carbon dioxide it is not produced directly in large amounts by human activity.

Most water vapour in the atmosphere is the result of evaporation from the oceans. As long as the oceans do not freeze, says Ou, there will always be plenty of water vapour in the air, mitigating the effect of a dimmer Sun.

As the Sun warms, we might expect the effects of water vapour to increase, as more water will evaporate and enhance the greenhouse effect.

In Ou's model, however, clouds counteract such warming. More water vapour in the air enhances cloud formation; clouds reflect sunlight, and so can offset an increase in solar output.

Some previous studies have argued that clouds can act as a kind of climate thermostat. Ou's model treats the climatic effects of clouds in much more detail. He takes into account the fact that there are, crudely speaking, two different kinds of cloud high and low which affect climate differently. Strong updrafts of warm, moist air form high clouds close to the top of the troposphere (at a height of about 15 kilometres). Flat, low clouds form closer to the ground.

In a warmer climate caused by a hotter Sun, high clouds become less extensive and the amount of low cloud increases, Ou calculates. Overall, this counteracts warming because of sunlight reflection.

Even if the Sun became 50 per cent more intense, the average temperature would rise by only 10 degrees a significant change, but not nearly as great as that without water's moderating influence.

However, this is unlikely to be the whole story. Some researchers believe that all or most of the oceans froze over at some time in the past.

Ou's model does not apply to an ice-bound planet, and he has not looked at how increases in other greenhouse gases would effect global warming.

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