The Earth's temperature record for the 20th century shows a period of slight cooling from about 1940-1970:

In the HADCRUT3 data set shown above, this appears as a sharp fall in the 1940s, but in other data sets there is a more gradual decline. 

In AR4 FAQ3.1, Chapter 3 p. 252-3, the IPCC says: "An increase (0.35°C) occurred in the global average temperature from the 1910s to the 1940s, followed by a slight cooling (0.1°C)" and then adds in a figure caption, "From about 1940 to 1970 the increasing industrialisation following World War II increased pollution in the Northern Hemisphere, contributing to cooling...". Remarkably, the IPCC provides no data or references to support this claim. The suggested aerosol explanation for the mid-century temperature drop is based on theory alone. There is no observational physical evidence for strong anthropogenic aerosol cooling on a global basis during this period. The IPCC position appears to be that industrialisation can produce cooling (by aerosol pollution) or warming (by carbon dioxide production) as required, and can therefore explain any upward or downward temperature changes!

Of course, this position makes no sense. Carbon dioxide was being emitted in increasing quantities in the post-war period, during an economic boom period when fossil fuel costs were low. Conversely, air pollution increased strongly in the late 20th century, particularly in places such as China and India. In fact, emissions of the main atmospheric aerosol pollutant, sulphur dioxide, are greater now than in the post-war period.

Furthermore, the claim about pollution from 1940 to 1970 is in contradiction with a statement on p. 238 near the start of Chapter 3, which talks of "increases in air pollution (aerosols), especially from 1970 to 1990."

Climate scientist Judith Curry discussed this point in a seminar she gave at MIT in 2011. She pointed out that aerosols do not explain the mid-century cooling because the observed cooling was larger in the Southern Hemisphere.