Global Warming Cooled Down By Volcanic Eruptions
February 25, 2014

Volcanoes Contribute To Global Warming Slow Down

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Lee Rannals for - Your Universe Online

Researchers at Lawrence Livermore National Laboratory report they have found that volcanic eruptions in the early part of the 21st century have cooled the planet.

The total heat content of the ocean, along with global-mean temperatures at the surface of the planet and in the troposphere have shown relatively little warming since 1998. This “slow-down” has been the topic of political debates and media attention, and the team wrote in the journal Nature Geoscience that volcanoes could be the cause of this hiatus.

When a volcano erupts, it shoots off sulfur dioxide gas into the atmosphere, and large eruptions can add enough of this gas that it forms volcanic aerosols. These droplets reflect some portion of the incoming sunlight back into space, cooling the Earth’s surface and the lower atmosphere.

"In the last decade, the amount of volcanic aerosol in the stratosphere has increased, so more sunlight is being reflected back into space," Lawrence Livermore climate scientist Benjamin Santer, who serves as lead author of the study, said in a statement. "This has created a natural cooling of the planet and has partly offset the increase in surface and atmospheric temperatures due to human influence."

Emissions of greenhouse gases being released into the atmosphere have increased over the past 12 years, causing the troposphere to warm and the stratosphere to cool. However, large volcanic eruptions cool the troposphere and warm the stratosphere. The team said that early 21st century volcanic eruptions have contributed to this warming hiatus, and that most climate models do not accurately account for this effect.

"The recent slow-down in observed surface and tropospheric warming is a fascinating detective story," Santer said. "There is not a single culprit, as some scientists have claimed. Multiple factors are implicated. One is the temporary cooling effect of internal climate noise. Other factors are the external cooling influences of 21st century volcanic activity, an unusually low and long minimum in the last solar cycle, and an uptick in Chinese emissions of sulfur dioxide.”

He said the challenge is to obtain hard quantitive estimates of the contributions of each of these factors to the slow-down. In order to do this, the researchers performed two different statistical tests to determine whether recent volcanic eruptions have cooling effects that can be distinguished from the intrinsic variability of the climate.

The scientists found evidence for the relationship between volcanic aerosol observations and satellite-based estimates of lower tropospheric temperatures as well as the sunlight reflected back to space by the aerosol particles.

"This is the most comprehensive observational evaluation of the role of volcanic activity on climate in the early part of the 21st century," co-author Susan Solomon, the Ellen Swallow Richards professor of atmospheric chemistry and climate science at MIT, said in a statement. "We assess the contributions of volcanoes on temperatures in the troposphere - the lowest layer of the atmosphere -- and find they've certainly played some role in keeping the Earth cooler."