Global average temperatures have been rising in recent years, but not as much as they might have, thanks to a series of small to moderate volcanic eruptions that have spewed sunlight-blocking particles high into the atmosphere.
That's the conclusion of a new study, which also finds that microscopic particles derived from industrial smokestacks have done little to cool the globe.
Between 2000 and 2010, the average atmospheric concentration of carbon dioxide - a planet-warming greenhouse gas - rose more than 5 percent, from about 370 parts per million to nearly 390 parts per million.
If that uptick were the only factor driving climate change during the period, global average temperature would have risen about 0.2°C, says Ryan Neely III, an atmospheric scientist at the University of Colorado-Boulder. But a surge in the concentration of light-scattering particles in the stratosphere countered as much as 25 percent of that potential temperature increase, he notes.
According to satellite data, a measure of the light-scattering ability of the stratospheric particles, called aerosols, rose on average between 4 percent and 7 percent each year between 2000 and 2010. The more incoming sunlight is scattered back into space, the stronger the cooling effect.
But researchers have strongly debated the source of those aerosols, Neely says. While many teams have suggested that the aerosols came from small-to midsized volcanic eruptions, a few have proposed that they originated in Asian smokestacks.
Their rationale: Emissions of sulfur dioxide in India and China grew about 60 percent during the decade, and atmospheric convection associated with the region's summer monsoon provides a way for watery droplets containing that gas to reach the stratosphere then diffuse around the world.
Now, by using a computer model, Neely and his colleagues show that the human contribution of aerosols to the stratosphere was minimal between 2000 and 2010.