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Scientists who looked at the whole history of climate in the 20th century have come up with a new possible explanation for the apparent slowdown in global warming. It is because, they say, of the Montreal Protocol that banned chlorofluorocarbons, also known as CFCs, and saved the ozone layer.

The ozone layer was the second great atmospheric crisis of the late 20th century (the first involved the urban smog and acid rain that triggered clean air legislation in Europe and the US). CFCs were the safe, enduring gases used as refrigerants: their only problem was that – once they reached the stratosphere – they unexpectedly began to destroy the ozone layer that screens out harmful ultraviolet light.

Within four years of the dramatic discovery of a huge “hole” in the ozone layer over the Antarctic, these gases were phased out, in a rare act of international agreement, by the 1989 Montreal Protocol.

But CFCs had a second unexpected property. They were greenhouse gases of unusual potency: molecule for molecule, one of them was rated at more than 17,000 times more effective at trapping infra-red radiation than carbon dioxide. CFCs were released only in comparatively tiny quantities, but they were calculated to account for up to 24% of global warming.

Francisco Estrada of the National Autonomous University of Mexico and colleagues report in Nature Geoscience [link available after publication] that they spotted the signal of the missing CFCs when they used statistical analysis to examine average temperature records in the two hemispheres from 1880 to 2010.