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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

What the science says...

Mid-century cooling involved aerosols and is irrelevant for recent global warming.

Although temperatures increased overall during the 20^th century, three distinct periods can be observed. Global warming occurred both at the beginning and at the end of the 20^th century, but a cooling trend is seen from about 1940 to 1975. As a result, changes in 20^th century trends offer a good framework through which to understand climate change and the role of numerous factors in determining the climate at any one time.

Early and late 20^th century warming has been explained primarily by increasing solar activity and increasing CO2 concentrations, respectively, with other factors contributing in both periods. So what caused the cooling period that interrupted the overall trend in the middle of the century? The answer seems to lie in solar dimming, a cooling phenomenon caused by airborne pollutants.

The main culprit is likely to have been an increase in sulphate aerosols, which reflect incoming solar energy back into space and lead to cooling. This increase was the result of two sets of events.

Industrial activities picked up following the Second World War. This, in the absence of pollution control measures, led to a rise in aerosols in the lower atmosphere (the troposphere).

A number of volcanic eruptions released large amounts of aerosols in the upper atmosphere (the stratosphere).

Combined, these events led to aerosols overwhelming the warming trend at a time when solar activity showed little variation, leading to the observed cooling. Furthermore, it is possible to draw similar conclusions by looking at the daily temperature cycle. Because sunlight affects the maximum day-time temperature, aerosols should have a noticeable cooling impact on it. Minimum night-time temperatures, on the other hand, are more affected by greenhouse gases and therefore should not be affected by aerosols. Were these differences observed? The answer is yes: maximum day-time temperatures fell during this period but minimum night-time temperatures carried on rising.

The introduction of pollution control measures reduced the emission of sulphate aerosols. Gradually the cumulative effect of increasing greenhouse gases started to dominate in the 1970s and warming resumed.

As a final point, it should be noted that in 1945, the way in which sea temperatures were measured changed, leading to a substantial drop in apparent temperatures. Once the data are corrected, it is expected that the cooling trend in the middle of the century will be less pronounced.

Updated on 2010-08-21 by Anne-Marie Blackburn.

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