The efforts of industrialised nations to cut smog pollution has had a bizarre side-effect - accelerating global warming.
New data show that after years of getting smoggier, our skies have become clearer since about 1990. And one effect has been to allow more solar radiation to reach the surface of the Earth.
The phenomenon known as “global dimming” has gone into reverse, according to research by Martin Wild at the Institute for Atmospheric and Climate Science in Zurich, Switzerland, and been replaced by “global brightening” (Science. vol 308, p 847). “There is no longer a dimming to counteract the greenhouse effect,” he told New Scientist.
Climate scientists say there have been two critical influences on global air temperatures in the past half-century. First, rising atmospheric concentrations of heat-trapping gases like carbon dioxide have warmed the Earth, by preventing more of the heat that reaches the Earth’s surface from escaping back into space.
But a parallel increase in smog particles has shaded the planet, partly offsetting the warming. Past studies have shown an increase in average aerosol particle levels in the atmosphere between 1960 and 1990 that were sufficient to reduce solar radiation reaching the Earth’s surface by about 5%.
The net effect of these two conflicting influences has been a warming of almost 0.5°C since 1960. But the rising levels of aerosols have led to concern that they might be masking greater underlying warming. And now the mask appears to be coming off.
Wild’s new data - assembled from measurements of surface radiation made round the globe - show a widespread reduction in aerosols in the atmosphere since around 1990. His findings are corroborated by satellite data reported by Rachel Pinker, a meteorologist at the University of Maryland at College Park, US, in the same issue of Science (v 308, p 850).
The main cause of the global brightening, says Wild, is the clean-up of air pollution, especially in Europe and the former Soviet Union, where industrial decline has also played a role.
But the same trend emerges in data from North America, Australasia, Japan and, most recently, in China, where smogs have been reduced despite swift industrialisation. The exceptions, where dimming continues to worsen, are mainly in South Asia and Africa.
So should the world hurriedly reinstate smogs to stop global warming from accelerating? Probably not, as Wild points out that carbon dioxide lasts in the atmosphere for a century or more, whereas aerosols typically hang around for only a few days. So as carbon dioxide accumulates in future decades, we would need ever-thicker smogs to counteract it.
And another Science paper reports that current smog levels kill half a million people worldwide each year from heart and lung diseases.