While the planet is still heating up—this year is on track to be the hottest yet on record—the rate at which global temperatuers have risen has slowed in the past 15 years, and it might be thanks in part to small volcanic eruptions. That's the conclusion of a new study led by an atmospheric scientist at the Massachusetts Institute of Technology.
Scientists have long known that volcanoes can cool the atmosphere, mainly by means of sulfur dioxide gas that eruptions expel. Droplets of sulfuric acid that form when the gas combines with oxygen in the upper atmosphere can remain for many months, reflecting sunlight away from Earth and lowering temperatures. However, previous research had suggested that relatively minor eruptions--those in the lower half of a scale used to rate volcano "explosivity"--do not contribute much to this cooling phenomenon.
The team used satellite- and balloon-based particle counters, as well as LiDAR and ground-based sun photometers to look at how volcanic ash in the atmosphere might shield the Earth from warmth. With these tools, the researchers found that small volcanic eruptions could be responsible for slowing global temperature rise by as much as a fifth of a degree Farenheight. That's not as much as larger eruptions—according to one estimate, a 1991 blast from Mt. Pinatubo decreased temperatures by 1.3 degrees—but it's significant compared to the rate at which global warming is raising the temperature of our planet.