Low-lying clouds have expanded since 1979, reflecting sunlight and reducing energy absorbed at Earth's surface by 0.32 watts per square metre. Researchers now warn this cooling effect could soon reverse and speed up global warming.
Gregory Cesana and Assia Arouf at Columbia University analysed satellite data and other records to estimate low cloud cover back to 1979. They found that low clouds have increased over tropical regions such as the eastern Pacific at a rate of 0.18 per cent per decade.
The increase has created a negative feedback that opposes warming, Cesana said. Human activities are currently heating Earth by about 2.72 watts per square metre, so the clouds have offset a fraction of that rise.
Scientists expect the eastern Pacific to warm more in coming years, partly because of reduced sulphur pollution. This change could reduce low cloud cover and remove the cooling shield, researchers said.
Paulo Ceppi at Imperial College London noted that low cloud cover has already declined globally since 2003. He added that the effect might mean future climate impacts are slightly underestimated.