A century of average surface temperatures of the Northern Hemisphere, looking back from 1980. (Shown as differences in °C from the 1946-60 mean.) Decades later, scientists concluded that the drop after 1940 was largely due to a rise in industrial pollution, which blocked some sunlight, augmented by a long-term cycle in the Pacific Ocean. Later data also showed that the decline was not seen in the Southern Hemisphere. For a graph of temperatures into the 21st century click here.

From P. D. Jones et al., Monthly Weather Review 110 (1982), p. 67,
reproduced by permission American Meteorological Society.