Major eruption cooled the climate but went unnoticed

A large, previously unknown volcanic eruption somewhere in the tropics helped make the 1810s the coldest decade of the past 500 years, a new analysis suggests.

Scientists have long known about the 1815 eruption of Indonesia’s Mount Tambora, an eruption whose climate-cooling effect was so large that 1816 is often called “the year without a summer” (SN: 8/30/2008, p. 16). Now, evidence from ice cores taken from polar regions suggests that another major eruption occurred in a remote, unpopulated region of the tropics just a few years before Tambora blew its top, says Jihong Cole-Dai, an environmental chemist at South Dakota State University in Brookings.

Precipitation that fell on Greenland and Antarctica in 1810 and 1811 contained higher-than-normal amounts of sulfates, Cole-Dai and his colleagues report online and in the Nov. 28 Geophysical Research Letters. Some scientists had suggested that those sulfates came from small, local eruptions that happened to occur half a world apart but at the same time, he notes.

But new analyses of those sulfates reveal a shift in ratios of sulfur isotopes that indicate the sulfur had undergone chemical reactions high in the atmosphere. Those results show that the sulfates came from a single, massive eruption large enough to send aerosols into the stratosphere over both the Northern and Southern hemispheres, the researchers contend. That eruption, which probably occurred sometime around February 1809, was about half the size of Tambora and cooled the climate substantially, says Cole-Dai. “Then, before temperatures had a chance to fully recover, Tambora happened,” he adds.