Global Warming Hiatus May Have Been Caused by Volcanic Eruptions

Over the past 15 years, a warming "hiatus" has impacted the world. Now, scientists have found out what might have caused this stall in global warming: volcanic eruptions.

Researchers have long known that volcanoes cool the atmosphere due to the sulfur dioxide that's expelled during eruptions. Droplets of sulfuric acid that form when the gas combines with oxygen in the upper atmosphere can persist for months, reflecting sunlight away from Earth and lowering temperatures at the surface. Now, scientists believe that early 21^st-century eruptions may have caused this same effect, explaining the hiatus.

The warmest year on record was 1998. After that, though, the steep climb in temperatures over the 20^th century seemed to level off. In the past, scientists have proposed that factors such as weak solar activity and increased heat uptake by the oceans could explain the lull in temperature increase. Now, though, scientists have found that it could be due to volcanic eruptions.

The researchers essentially found the missing piece of the puzzle; they discovered that at the intersection of two atmospheric layers, the stratosphere and the troposphere, there were a myriad of aerosols. Cirrus clouds can largely interfere with satellite aerosol measurements, which means that the scientists had to combine observations from ground- air- and space-based instruments. This allowed them to improve the estimates of total volcanic aersols.

"The fact that these volcanic signatures are apparent in multiple independently measured climate variables really supports the idea that they are influencing climate in spite of their moderate size," said Mark Zelinka, one of the researchers, in a news release. "If we wish to accurately simulate recent climate change in models, we cannot neglect the ability of these smaller eruptions to reflect sunlight away from Earth."

The findings are published in the journal Geophysical Research Letters.

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