Additional info from Skeptical Science
Although temperatures increased overall during the 20th century, three distinct periods can be observed. Global warming occurred both at the beginning and at the end of the 20th century, but a cooling trend is seen from about 1940 to 1975. As a result, changes in 20th-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 20th-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 sulfate aerosols, which reflect incoming solar energy back into space and lead to cooling. This increase was the result of two sets of events:
1. 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).
2. 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 daytime temperature, aerosols should have a noticeable cooling impact on it. Minimum nighttime 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 daytime temperatures fell during this period but minimum nighttime temperatures carried on rising.
The introduction of pollution control measures reduced the emission of sulfate 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.
Adapted from © John Cook and Skeptical Science