The contrails (vapour streams) from aircraft are gradually covering the skies of the more industrially developed countries with a haze. This haze both reflects sunlight back into space and traps heat near the Earth's surface. It is not yet clear whether this would result in net global warming or not. Measurements taken during the relatively plane-free days following the terrorist attack on the New York Trade Center (11.9.01) suggest that they might.

A BBC2 Horizon programme (13.1.05) entitled Global Dimming extended this issue to all atmospheric pollution. Measurements from around the world testify to significant reductions in the intensity of sunlight reaching the Earth's surface, even on apparently cloud-free days. Air pollution increases the reflective properties of natural cloud and, to a lesser extent, reflects sunlight itself. Climate scientists still debate whether air pollution will eventually encourage or hold back global warming, but those interviewed on the programme all agreed that its possible role either way had been seriously underestimated.

Could it be that the cooling effect of one type of pollutant, air pollution in the classic sense, has been protecting us against the warming effect of another greenhouse gas, carbon dioxide (CO2)? The rise in CO2 levels recorded so far has resulted in a temperature rise of just 0.6° Centigrade (C). During the last ice age, a similar rise led to a temperature rise of 6°C. This suggests that the climate may be more sensitive to greenhouse gases than previously thought.

If so, this could be extremely bad news. According to leading climate modeller Dr Peter Cox. CO2 levels are still projected to rise strongly over coming decades, whereas there are encouraging signs that chemical air pollution is finally being brought under control. "That means that, unless we act urgently to curb our emissions of greenhouse gases, we'll get reduced cooling and increased heating at the same time," he says. "A temperature rise of 10°C by 2100 would be possible, giving the UK a climate like that of North Africa, and rendering many parts of the world uninhabitable."

And it's not just global warming. Scientists are now worried that 'global dimming' may be disrupting world rainfall patterns by shielding the oceans. Some suggest that dimming was behind the droughts in sub-Saharan Africa which claimed hundreds of thousands of lives in the '70s and '80s. Others detect disturbing hints that the same thing may be happening today in Asia, home to half the world's population.