IN A bizarre twist to the climate change saga, cutting smog in industrialised nations is actually accelerating global warming.
In the past half-century, say climate scientists, rising levels of greenhouse gases like carbon dioxide have warmed the Earth. But a corresponding increase in smog particles has shaded the planet, partly offsetting the warming. The net effect has been a global warming of almost 0.5 掳C since 1960.
This was despite the fact that the amount of sunlight reaching the Earth鈥檚 surface declined by about 5 per cent between 1960 and 1990 鈥� easily explained by rising levels of aerosol particles in the atmosphere over the same period. Now Martin Wild at the Institute for Atmospheric and Climate Science in Zurich, Switzerland, and colleagues say that this trend has reversed. 鈥淭here is no longer a dimming to counteract the greenhouse effect,鈥� says Wild.
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The team analysed data from a global network of instruments measuring the radiation reaching our planet鈥檚 surface and found that on average the surface had brightened by about 4 per cent over the past decade (Science, vol 308, p 847). Rachel Pinker, a meteorologist at the University of Maryland at College Park, and her colleagues have satellite measurements to back these figures up. The satellite records, which run from 1983 to 2001, show that while dimming did occur during the early years, the trend reversed in 1992 (Science, vol 308, p 850).
Ironically, cleaner air is letting in more solar radiation, especially in Europe and the former Soviet Union, where industrial decline has also played a role, says Wild. But the same trend emerges in data from North America, Australasia, Japan and, most recently, in China, where smog has been kept in check despite rapid industrialisation. The exceptions, where dimming continues to worsen, are mainly in south Asia and Africa.
鈥淩ising smog levels meant the amount of sunlight reaching the Earth鈥檚 surface declined by 5 per cent between 1960 and 1990鈥�
That doesn鈥檛 mean that a return to smoggy air will halt global warming. Wild points out that CO2 persists in the atmosphere for a century or more, whereas aerosols typically hang around for only a few days. That would mean smog would have to get ever thicker to counteract rising levels of CO2, not to mention the seriously bad effects on health 鈥� even at existing levels smog kills half-a-million people worldwide each year.
