Climate mover. Less-than-huge eruptions in recent years have sent climate-cooling debris into the stratosphere.

Taro Taylor

A Bit of Shade for a Warming Planet

Recent volcanic activity has beefed up a sun-blocking layer of haze 20 kilometers above the ground, according to a new study. This cloud of debris has been keeping the planet cooler than it would have otherwise been, but experts caution that greenhouse warming will still prevail.

Volcanoes have long been seen as climate players. The massive 1991 eruption of Mount Pinatubo in the Philippines blew lots of ash and gas directly into the stratosphere. There, sulfurous gases turned into a haze of tiny droplets of sulfuric acid that stayed up long enough to spread around the world. By reflecting some sunlight back to space, that added haze cooled the globe by 1°C for a year or two before dissipating.

Eruptions much smaller than Pinatubo didn't seem powerful enough to put a lot of debris into the stratosphere, but atmospheric scientist Susan Solomon of the University of Colorado, Boulder, and her colleagues report online today in Science that less-than-colossal eruptions did just that in recent years. The team examined laser-based measurements from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite launched in 2006. The data show that debris from two 2006 tropical eruptions managed to spread globally through the stratosphere, while two eruptions in 2008 and 2009 at higher, northern latitudes spread through much of the Northern Hemisphere's stratosphere.

The added stratospheric haze was enough to affect global climate, the group found. They put the stratospheric hazes from CALIPSO and from longer ground-based records into a climate model with increasing greenhouse gases. Without the recent extra stratospheric haze, they found, 20% more greenhouse warming would have occurred since 1998 or about 0.07°C.

The smaller eruptions "did get some stuff in the lower stratosphere," concludes climate scientist Alan Robock of Rutgers University in New Brunswick, New Jersey, "and it cooled the planet. Here's one more thing we have to add to the mix" of forces that have been counteracting the strengthening greenhouse lately. The list had included: Pinatubo-scale eruptions, a small decrease in the sun's brightness at the minimum of the solar cycle, an increase in short-lived pollutant hazes from Asia that stay in the lower atmosphere, and decade-long variations of climate spurred by El Niño and other natural climate movers. And now Solomon and her colleagues allow that some of the lower-atmosphere pollution may be leaking up into the stratosphere as well, as the late David Hofmann—an atmospheric scientist at the National Oceanic and Atmospheric Administration in Boulder—recently suggested.

But even as climate-moderating influences proliferate, researchers remain adamant about the future course of global climate. Such transitory influences may have been holding down global warming the past decade. But they will be overwhelmed by inexorably building greenhouse gases, Robock says. It will get warmer still in the long run.