A possible solution to global warming
It was Jean Fourier who, in 1827, first realised that it was the atmosphere that protects us from the coldness of space. In the 1850s, John Tyndall recognised that only gases with three or more atoms in their molecules are greenhouse gases – these reflect the Earth’s infra-red radiation and help to keep us warm. It followed that O2 and N2 are not greenhouse gases (which is just as well!), but CO2, H2O and O3 are.
Greenhouse gas emissions and climate change
In the latter years of the 19th century, Svante Arrhenius warned of the danger of accumulating too much CO2 in the atmosphere, and, in the 1930s, George Callendar estimated how much of it had been added to the atmosphere by man’s activity between 1880 and 1935. He compared this with measured changes in global temperatures, suggesting a correlation between the two sets of figures. He was adamant that such studies should be continued.
Unfortunately, for those who wished to establish a relationship between the build up of CO2 from human activity and global warming, from 1940-75, the average global temperature first fell, and then remained approximately constant, before starting to rise again in the late 1970s. This was in spite of the huge consumption of fossil fuel during and after World War II, and the increase in population and industrial activity in the post-war years.
One possible explanation for the absence of warming, despite increasing CO2 levels, is that any greenhouse effect was swamped by massive cooling due to an unknown and short-lived natural cause. It is noteworthy that starting in the second half of the 1970s, global warming resumed and has continued to the present time at more or less the expected rate.
Global warming and sulphate aerosols
An alternative explanation is more satisfactory and more interesting. Most fossil fuels contain impurities, particularly sulphur, which end up in the atmosphere, first as sulphur dioxide and then as a sulphate aerosol. Individual particles interrupt the sun’s rays and reflect them back into space. In other words, the aerosols in the atmosphere keep the earth cool and counteract the warming effect of the additional CO21.
The lifetime of a CO2 molecule in the atmosphere is up to 200 years, while sulphate aerosols are washed out of the air in a matter of weeks. This explains the hitherto puzzling fact that the temperature of areas of high population and industrial activity that produce huge quantities of CO2 are cooler than the surrounding countryside (where the short-lived aerosols fail to penetrate).
Potential ways of dealing with global warming
Would it be worthwhile putting up with smog and acid rain for the sake of keeping the planet cool? I do not think so – one recalls the five-day smog in London in December 1952 which resulted in the death of more than 4,000 people (a casualty rate that exceeded that of 9/11).
The chemistry Nobel Laureate, Paul Crutzen, has come up with a novel suggestion. He advocates firing into the stratosphere, ten kilometres above the Earth’s surface, quantities of sulphur that would form relatively long-lived aerosols and defend the world against global warming. He calculates that about five tons of sulphur would need to be injected into the stratosphere each year which, incidentally, is about 10% of what man releases annually into the atmosphere. The columnist and savant, George Monbiot, is worried about possible side-effects, but the idea certainly justifies further investigation.
1. Since writing this column I have been informed that Professor Tom Wigley was the first person to suggest that sulphate aerosols might be the cause of the 1940-75 cooling. See for example, T. Wigley, Nature, vol 339, 1 June 1989, p365.