50 years ago, scientists puzzled over a slight global cooling

Sulfate pollution turned out to be the culprit

smoke stacks

Sulfate particles from fossil fuel burning briefly lowered global temperatures during the mid-twentieth century.

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Earth’s cooling climate, Science News, November 15, 1969 —

The average temperature for the entire Earth rose gradually from the 1880s until the early 1940s. At that time, a cooling trend suddenly set in which is continuing today.… The amount of dust and other particulate matter in the atmosphere has increased dramatically in recent decades, a change that could counteract the thermal effect of carbon dioxide buildup.


From 1940 to about 1975, the average global surface temperature decreased by about 0.1 degrees Celsius, interrupting a decades-long warming trend even as carbon emissions continued to rise. Many scientists thought the cooling trend was possibly caused by sulfate particles from the burning of fossil fuels that can scatter sunlight and reduce atmospheric warming (SN: 11/21/09, p. 5). That hunch proved correct: When the United States and other countries began to lower sulfur emissions in the 1970s to reduce acid rain and respiratory illnesses, the cooling ended abruptly. Since 1975, the average global temperature has risen by about 0.6 degrees C.

Today, the average surface temperature is 1.1 degrees C warmer than it was in pre-industrial times (SN: 9/25/19). However, ongoing sulfate emissions, particularly from China and India, may still be slowing greenhouse gas-driven warming. Removing all aerosol emissions from the world at once could add about 0.7 degrees C to global temperatures. 

Carolyn Gramling is the earth & climate writer. She has bachelor’s degrees in geology and European history and a Ph.D. in marine geochemistry from MIT and the Woods Hole Oceanographic Institution.

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