On Valentine's Day, Indonesia's Mount Kelud blew its top and coated villages up to 500 kilometers away with ash. At the same time, the eruption injected a small but consequential amount of sulfur dioxide 28 kilometers up into the stratosphere. Tiny droplets of sulfuric acid then reflected away incoming sunlight, helping to cool the planet. Such “small” eruptions—along with others at places like Manam, Soufrière Hills, Jebel at Tair and Eyjafjallajökull, to name a few of the 17 between 2000 and 2012—have helped slow the pace of global warming, according to work published in Nature Geoscience. (Scientific American is part of Nature Publishing Group.)

“The uptick in early 21st-century volcanism clearly was a contributing factor to the hiatus,” says atmospheric scientist Benjamin Santer of Lawrence Livermore National Laboratory, lead author of the report. The volcanoes did not act alone. There was also an unusually quiescent sun, air pollution from China's coal-fired power plants and the mysterious workings of the ocean. Santer adds, “The net impact was to offset part of the human-caused greenhouse gas warming.”

In the meantime, global warming continues to gather strength, hidden behind volcanoes that may shutter their tops at any moment. Based on supersized eruptions such as Mount Pinatubo in the Philippines in 1991, reflective aerosols would then fall to Earth within a few years at most, leaving the planet exposed to the full heat-trapping effects of greenhouse gases from human activities.

If the volcanoes do not do their part, a last resort may be required—bring our own aerosols. Advocates of one form of geoengineering want to step in, injecting sulfate aerosols in the stratosphere to augment or replace eruptions. Such deliberate tinkering with planetary-scale systems has been proposed as a fallback plan if climate change were to turn catastrophic, though at the cost of the stratospheric layer that helps to shield life from ultraviolet light. Sulfuric acid high in the sky has the unfortunate side effect of eliminating ozone. But given the inertia in reducing greenhouse gas pollution, the debate around geoengineering will undoubtedly linger longer than the aftermath of these small volcanic eruptions.