Nature Studies by Michael McCarthy: Wet summers conceal a terrible surprise

Aerosol is a word most people associate with the bathroom, the kitchen or the garden shed: we tend to use it to mean a spray can, for deodorants, cleaners, weedkillers or whatever. But it has acquired this meaning by extension, and what it originally signified was the fine cloud of particles which come out of the spray can nozzle.

People who still use the word in this original sense are atmospheric scientists: for them, an aerosol is a cloud or layer of small particles of matter in the atmosphere. Depending on what these particles consist of, and how prevalent they are, they can help heat the atmosphere up, by retaining the sun's warmth, or cool it down, by reflecting the sun's radiation back out into space.

A good example of such a cooling aerosol is the one associated with the eruption of a volcano in the Philippines, Mount Pinatubo, in June 1991. This has not really entered our consciousness in the West, as early evacuation meant casualties were reduced, but in terms of the amount of matter ejected, Pinatubo may have been the biggest volcanic eruption of the 20th century, far bigger than Mount St Helens in 1980 which everyone remembers; and what it spewed out included something like 20 million tonnes of sulphur dioxide, SO2, which turned into a sulphuric acid haze encircling the world, reflecting back sunlight and sending global temperatures downwards, by about half a degree Centigrade.

This was a quite enormous drop, and the Pinatubo sulphur aerosol had a significant effect on the progress of global warming. In the late 1980s, there had been a surge upwards in global temperatures, with, in Britain and Europe especially, 1988, 1989 and 1990 getting steadily hotter, until on August 3, 1990, a new air temperature record was set for Britain of 98.8F, or 37.1C, smashing the previous British record air temperature of 98.1F (or 36.7C) which had stood since 1911.

It was a fair expectation that 1991 might be warmer still; but Pinatubo put paid to that, cooling the next few years right down. You can even see the process at work in something as rarified as French wine. If you are interested in the wines of Bordeaux, you will know that 1988 and 1989 were very good years, leading up to 1990, which was terrific; but 1991 was rubbish, and so was 1992, as Pinatubo wiped out the summers.

But there are much more important things than wine which can be affected by sulphur aerosols, and one of them, we have realised this week, has been the whole progress of the struggle to deal with global warming over the last decade. The Noughties turned out to be a peculiar period for climate change, for during them the climate issue grew and grew around the world, as the science and the risks became better understood and familiar to more and more people; and then suddenly it burst, like an overinflated balloon.

Since the failure of the Copenhagen climate conference in December 2009, the political will to deal with the global warming threat has become enfeebled, and the reason is that most ordinary people have come to consider it as irrelevant to them, as they can see no evidence of it. I mean, can you? After three sodden summers, and two freezing winters?

The great paradox of the Noughties was that, as the climate science became clearer, and the political engagement with it became greater, actual manifestations of warming seemed, to ordinary people anyway, to get fewer and fewer.

Now, thanks to a study by Robert Kaufman of Boston University and others, just published in Proceedings of the National Academy of Sciences , we know why. The colossal, explosive surge in industrial expansion in China during the Noughties, powered by burning coal in power stations, which turned the Chinese into the world biggest CO2 emitters, did not just emit carbon, it emitted vast quantities of sulphur as well; and the consequent sulphur aerosol has cooled the world's climate, just like Pinatubo did except for longer, cancelling out the warming effects of the increased CO2.

That is why, despite all the predictions, despite all the kerfuffle about climate change, global temperatures have not noticeably risen during the last decade, and eventually, public concern has withered on the vine.

But, like Arnie, it'll be back. CO2 emissions are long-lived in the atmosphere, lasting for a hundred years; SO2 emissions are short-lived, and can fall out in weeks or months. As soon as the Chinese start fitting sulphur dioxide scrubbers to the chimneys of their coal-fired power stations, as they will, so that people in Chinese cities can ride their bikes without wearing masks, the aerosol will start to disappear, and the surging temperatures it has been holding back will start to make themselves felt.

Go outside. Look up into the sky. Ask yourself, where's all this global warming, then? The answer is, you're being shielded from it, at the moment, by Chinese sulphur.

It has not gone away. It is building up steadily, behind a Chinese pollution screen, and when that screen disappears, we are going to get a terrible surprise.

Palace holds the prettiest of pictures

Finest nature sight in London at the moment: the blooming of the catalpas, or Indian bean trees, in the courtyard of the Palace of Westminster and Parliament Square. The glossy heart-shaped leaves are fantastically set off by the big white blossoms, even more showy than the "roman candles" of horse chestnuts.

m.mccarthy@independent.co.uk; www.twitter.com/mjpmccarthy