Small volcanic eruptions account for part of the global warming slowdown since 2000, a new study suggests.

Until now, the climate impacts of small volcanic blasts were overlooked because their planet-cooling particles cluster below the reach of satellites, scientists reported Oct. 31 in the journal Geophysical Research Letters. It turns out, satellites were missing about 30 percent of these particles, called aerosols, the study found.

Volcanoes blast sulfur dioxide aerosols into the stratosphere, where they cool Earth by blocking some of the sun's solar radiation and reflecting it back into space. The stratosphere is the second layer of Earth's atmosphere, above the one in which humans live (the troposphere). Near the tropical latitudes, these layers meet about 9 miles (15 kilometers) above the Earth's surface. Closer to the polar regions, the boundary drops to about 6 miles (10 km), said lead study author David Ridley, an atmospheric scientist at MIT. [Big Blasts: History's 10 Most Destructive Volcanoes]

Water clouds befuddle aerosol-monitoring satellites below about 9 miles above Earth's surface, so any aerosols in low, polar stratosphere were potentially missing, Ridley said. In the new study, Ridley and his co-authors checked aerosol concentrations in the high latitudes more directly, with instruments lofted on balloons and lasers that scan particles from the ground. The research reveals that about 30 percent of the planet's stratospheric aerosol particles reside where the stratosphere dips lower, 6 to 9 miles above the surface.

"About one-third of the aerosols have been missed," Ridley told Live Science. The global aerosol total, plugged into a simple climate model, translates into a cooling impact of between 0.09 and 0.22 degrees Fahrenheit (0.05 to 0.12 degrees Celsius) since 2000. That's less than the slowdown in global warming, but researchers think several factors are responsible for the tardy temperature rise. "This is part of the larger puzzle everyone's been working on," Ridley said.

The so-called "global warming pause" is one of many terms for surface temperatures rising more slowly in recent decades than in the past, despite greenhouse gas emissions continuing to grow.

Ridley and his colleagues also tracked the source of aerosols in the lower stratosphere from volcanic eruptions during the 2000s. The eruptions were significantly smaller than 1991's massive Mount Pinatubo outburst in the Philippines, which had a noticeable cooling effect on the global climate.

The results indicate that many small eruptions do pump aerosols into the stratosphere, especially high-latitude volcanoes. For instance, when Sarychev Peak in the Kuril Islands erupted in 2009, almost all of its sulfur dioxide aerosols reached the lower stratosphere, the study reports. (The low altitude of the stratosphere was a factor.)

"This doesn't necessarily mean that every eruption will be able to get sulfur dioxide into the stratosphere and form aerosols, but they are just neglected entirely in the climate models from the [Intergovernmental Panel on Climate Change]," Ridley said. "The fine nuances make quite a big difference in these eruptions."

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