After rising rapidly during the first part of the 20th century, global average temperatures did cool by about 0.2?C after 1940 and remained low until 1970, after which they began to climb rapidly again.
The mid-century cooling appears to have been largely due to a high concentration of sulphate aerosols in the atmosphere, emitted by industrial activities and volcanic eruptions. Sulphate aerosols have a cooling effect on the climate because they scatter light from the Sun, reflecting its energy back out into space.
The rise in sulphate aerosols was largely due to the increase in industrial activities at the end of the second world war. In addition, the large eruption of Mount Agung in 1963 produced aerosols which cooled the lower atmosphere by about 0.5?C, while solar activity levelled off after increasing at the beginning of the century
The clean air acts introduced in Europe and North America reduced emissions of sulphate aerosols. As levels fell in the atmosphere, their cooling effect was soon outweighed by the warming effect of the steadily rising levels of greenhouse gases. The mid-century cooling can be seen in this NASA/GISS animation, which shows temperature variation from the annual mean for the period from 1880 through 2006. The warmest temperatures are in red.
Climate models that take into account only natural factors, such as solar activity and volcanic eruptions, do not reproduce 20th century temperatures very well. If, however, the models include human emissions, including greenhouse gases and aerosols, they accurately reproduce the 1940 to 1970 dip in temperatures.
How aerosols will influence the climate over the coming century is unclear. While aerosol emissions have fallen in Europe and the US (and in the former Soviet Union after 1991), they are now rising rapidly in China and India.
The picture is complicated because different kinds of aerosols can have different effects: black carbon or soot has warming rather than a cooling effect, for instance. Then there is the question of how all the different aerosols affect clouds. Climate scientists acknowledge that the aerosol issue is one of the key uncertainties in their understanding.
UPDATE: The sudden drop in temperatures in 1945 now appears to be an artefact of a switch from using mainly US ships to collect sea surface temperature data to using mainly UK ships. The two fleets used a different method. The temperature record is currently being updated to reflect this bias, but in essence it means that the cooling after 1940 was more gradual and less pronounced than previously thought.
