Clouds play an important role in determining the Earth’s climate. They can have a number of different effects but perhaps the most basic one of all is that clouds generally reflect solar energy back out into space thus lowering the amount of solar heating that strikes the Earth and is absorbed. This reflection contributes to something called the planetary albedo, which is essentially the overall reflectivity of the planet. The Earth’s albedo is generally taken as having a value of a little more than 0.3 which means that just over 30%, or a third, of all the solar energy that arrives at the Earth is reflected back out to space and clouds contribute a lot of this reflection. If the Earth had a higher albedo, it would absorb less solar energy and would potentially be cooler; if it had a lower albedo it would absorb more solar energy and be warmer.
So far so good, but it is actually really difficult to determine what will happen to clouds when the Earth’s atmosphere warms up (as it is). More heat in the atmosphere could mean less clouds as the air temperature rises and the water droplets in the clouds evaporate (becoming water vapour). On the other hand a warmer world will mean a greater tendency for water to evaporate from the land or ocean surface, increasing the humidity of the overlying air and increasing the potential for clouds to form when air cools (e.g. at high altitude).
Some new research has suggested that, in the north-east Pacific at least, a warming ocean is leading to less cloud which in turn leads to more solar energy reaching the surface (lower local albedo), which in turn leads to a warmer ocean, which in turn leads to less clouds, which in turn… This is an example of “positive feedback” and is a potentially important finding which may help to determine which of the current climate models is doing the best job of predicting future global climate scenarios. But nothing is simple – it’s nigh on impossible to know whether the clouds studied are typical and/or whether the findings can be applied more widely. The work is described briefly in a news item on the BBC Website [24 July 2009].
Tim O'Hare 