A blanket of smog hanging over India means the country is getting less and less sunlight, warn researchers. But this phenomenon, known as "solar dimming" may also protect against global warning.
India is getting about 5% less sunlight than it did 20 years ago, according to a study by Padma Kumari and colleagues at the Indian Institute of Tropical Meteorology in Pune. They studied data from the India Meteorological Department, measuring differences in solar radiation at 12 stations across India between 1981 and 2004.
They found that the amount of solar radiation reaching India's land mass dropped on average by 0.86 watts per square metre each year. The decrease was greater during the 1990s than the 1980s, and on average corresponded to a 5% drop in sunshine over the two decades.
India is losing out on sunshine because a cloud of tiny air-borne particles released by the nation's industries hovers above the subcontinent, blocking light from reaching the Earth.
Similar dimming has been caused by Western nations too. The smog produced by the US and Europe during the 20th century spread worldwide and was responsible for a phase of "global dimming". But when the West cleared up its smog pollution in the 1980s and 1990s, as smog pollution in India was increasing, clearer skies returned. Researchers have described this as "global brightening".
According to research published in February 2007 by Martin Wild of the Institute for Atmospheric and Climate Science in Switzerland, this global brightening was accompanied by an accelerated rise in global temperatures (Geophysical Research Letters DOI: 10.1029/2006GL028031).
Similarly, Kumari and colleagues believe that the real degree to which India is being warmed by greenhouse gas emissions is being masked by the cooling effect of the smog. Looking at temperature records since the 1980s, they found that maximum and minimum temperatures have both increased, but to different extents.
Maximum temperatures, which occur during the day and are driven by sunshine, have risen by just 0.04°C because of the protective effect of the smog. Meanwhile minimum, night-time temperatures, which are independent of sunshine, have risen by a much greater 0.31°C.
The researchers also found that solar dimming over India was lessened during the monsoon season because the torrential rains brought the fine particles back down to Earth, allowing more sunshine to get through.
Other Asian countries are also likely to be experiencing solar dimming. Preliminary data that Wild and colleagues have from China suggests it is getting less and less sun as well because of rising particle pollution linked to industrialisation.
So will the rising smog over India and China bring about a new phase of global dimming? "There are good chances there indeed," says Wild. He and his team are currently in the process of updating data in the Global Energy Balance Archive, which tracks sunshine levels at 2500 sites worldwide.