Rising air pollution clouds climate debate ?

Darker skies have uncertain effect on global warming.

Quirin Schiermeier

Air pollution that is harmful to human health has increased over all populated continents except Europe since 1973, according to an extensive survey.

I can see dimly now: air pollution is on the rise in most areas of the world.punchstock

The results play into a long-standing debate over whether the Earth's skies are dimming or brightening, how this affects the amount of sunlight reaching the planet's surface and what that means for climate change.

Two studies published in Science in 2005 concluded that a global dimming trend that began in the 1950s has been replaced since 1990 by global brightening^1,2. The likely effect of all that extra solar radiation reaching the Earth's surface would be faster global warming.

Now a study published in Science concludes that in fact skies became dimmer over most land areas between 1973 and 2007³. Only in Europe have skies become cleaner than they were some 30 years ago; there, industrial production declined sharply after the collapse of communist governments around 1990, and air quality regulations have had a big effect since. Air quality in North America has changed little during the study period.

Kaicun Wang, an atmospheric scientist at the University of Maryland in College Park, and his colleagues based their conclusions on visibility measurements, a good proxy for aerosol pollution, from 3,250 meteorological stations around the world. They found that visibility has tended to decrease over that period, the most pronounced dimming having occurred in South Asia and South America. Various types of aerosol have contributed to the trend, but sulphate and soot particles from fossil fuel burning are the main culprits, the team found.

"Most countries have realized by now that air pollution is a serious health risk," says Wang. "But attempts, such as China's, to regulate air quality have not yet borne fruit."

Cloudy understanding

Significant questions remain about what these results means for climate change, because soot and different kinds of aerosols can affect cloud formation in very different ways.

In come circumstances, aerosol particles can act as seeds for clouds, which help to reflect the Sun's rays back into space and so cool the planet. Aerosols can also reduce cloudiness, however, as probably happens in northern China, meaning that the net effect of aerosol pollution on global temperatures is worryingly uncertain.

Another complication is that soot from burning biofuels, widely used for cooking and heating in India and Africa, tends to absorb sunlight rather than scattering it back into space. This means that it warms the troposphere in much the same way as carbon dioxide and other greenhouse gases⁴.

Wang suspects that the poor understanding of these effects might explain why previous studies, which measured incoming solar radiation rather than visibility, concluded that the skies have brightened over most land areas, including China.

"The issue of global dimming versus global brightening is not just a question of aerosols," agrees Martin Wild, an atmospheric scientist at the Swiss Federal Institute of Technology in Zurich, who led one of the Science studies in 2005. "One must indeed consider clouds as well."

Wang is optimistic, however, that the air in China could become cleaner in the near future — as it did in Europe in the 1980s and previously in North America — if coal is replaced by oil and natural gas as an energy source, and if particulate filters in cars and factories become more common.