Have volcanoes slowed global warming? That’s what a new study says.
The study, led by Lawrence Livermore National Laboratory, found that volcanic eruptions in the early part of the 21st century cooled the planet.
The cooling partly offset the warming produced by greenhouse gases.
Despite greenhouse gases and the total heat content of the ocean increasing, global mean temperatures at the surface of the planet and the toposphere (the lowest portion of Earth’s atmosphere) have not warmed much since 1998. This has been referred to as a “slow-down” or “hiatus.”
Published in Nature Geoscience, the new study says that the eruptions injected dioxide gas into the atmosphere. If the eruptions were large enough to add the dioxide to the stratosphere (the atmospheric layer above the toposphere), then the gas forms tiny droplets of sulfuric acid. These reflect a portion of the incoming sunlight back into space, cooling the Earth’s surface and 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,” said Lawrence Livermore climate scientist Benjamin Santer, who serves as lead author of the study, 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.”
From 200 to 2012, greenhouse gases in the atmosphere, as from the start of the Industrial Revolution, have increased. This has the opposite effect of eruptions, warming the troposphere and cooling the stratosphere. The researchers say that most climate models haven’t accurately accounted for the effect of the eruptions.
“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.”
“The real scientific challenge is to obtain hard quantitative estimates of the contributions of each of these factors to the slow-down,” he added.
According to the laboratory, 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 team found evidence for significant correlations 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,” said co-author Susan Solomon, the Ellen Swallow Richards professor of atmospheric chemistry and climate science at MIT. “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 Earth cooler.”