March 15, 2016 | By spitting gases into the atmosphere that form sunlight-blocking particles, volcanoes have a cooling influence on the world’s climate. But scientists warn that satellite observations may be failing to detect the particles (called aerosols) at lower levels of the atmosphere—which means that the computer models used by climate scientists may also be underestimating their cooling effects.

To get a better handle on how smaller volcanic eruptions are affecting the atmosphere, a team of scientists led by the National Center for Atmospheric Research (NCAR) has developed a novel method that incorporates alternative ways to measure the aerosols.

Through an innovative combination of computer modeling and observations—both from the ground and from satellites—they found that satellites are often missing more than half of the sulfates and other aerosols that result from volcanic eruptions, according to a new paper in the Journal of Geophysical Research: Atmospheres.

Scientists have used an innovative combination of observations and computer models to quantify atmospheric aerosols (tiny particles) from volcanic eruptions. This video shows amounts of those sulfate aerosols circulating in the atmosphere from late 2004 to late 2015. Volcanic eruptions are indicated by red triangles; the first (at 0:06) occurs in February 2005.  Because aerosols play a role in cooling Earth's surface, volcanic activity since 2005 may have significantly reduced rates of global warming. (@UCAR. Scientific visualization by Michael Mills, NCAR.  This video is freely available for media & nonprofit use.)

This indicates that standard climate models, which depend on satellite observations to simulate the effect of eruptions, are underestimating the role of volcanoes. And it could help explain why global warming appeared to temporarily slow down early this century, as volcanic activity has increased.

“In light of these results, the impact of volcanoes on reducing the rate of global average temperature increases since the year 2000 should be revisited,” said NCAR scientist Michael Mills, the lead author of the new study.

What satellites can’t see

Most climate models rely on analyses of satellite measurements to represent volcanic aerosol. While satellites can globally monitor stratospheric aerosol, they have trouble measuring aerosol in the lowermost stratosphere, where clouds interfere with measurements. As a result, models are failing to include volcanic aerosols below about 15 kilometers (9 miles), thereby missing the majority of volcanic aerosol outside of the tropics.

Scientists had previously suspected that the amount of aerosols missed by satellites could be large enough to affect model simulations. But they could not quantify the extent of the problem.

To determine how much is being overlooked by satellites, Mills and his co-authors compiled a database of emissions from 171 volcanic eruptions from 1990 to 2014. They drew on measurements from an array of satellites and databases, such as the Smithsonian Global Volcanism Program and NASA’s Global Sulfur Dioxide Monitoring website, which monitor eruptions but do not measure the resulting aerosols.

They then produced a series of simulations of the gases that would likely have been lofted into the atmosphere by the eruptions, as well as the resulting chemical reactions that would have generated aerosols. For the simulations, they turned to an advanced computer model, the NCAR-based Whole Atmosphere Community Climate Model, which enables scientists to study the atmosphere from Earth’s surface all the way up to 140 kilometers (87 miles).

“This is the first time that this many volcanic eruptions occurring over the past quarter century have been simulated interactively from sulfur emissions in a global climate model,” Mills said.

The research showed that satellites are missing 50-95 percent of stratospheric aerosols at middle and higher latitudes, where cloud cover makes it more difficult for satellites to observe that low in the atmosphere. Volcanic aerosols also impact depletion of the ozone layer, and the authors show that models have been failing to account for 60 percent of that effect.

To verify the accuracy of the simulations, the study team used measurements taken with ground-based lidars and balloon-borne instruments. They found that the model simulations corresponded remarkably well with these observations, which show a significant increase in volcanic aerosol since 2005.

“This represents an important new capability for climate modeling,” Mills said. “It can greatly advance our understanding of how volcanic eruptions influence the climate system.”

About the article

Michael J. Mills, Anja Schmidt, Richard Easter, Susan Solomon, Douglas E. Kinnison, Steven J. Ghan, Ryan R. Neely III, Daniel R. Marsh, Andrew Conley, Charles G. Bardeen, Andrew Gettelman, 2016: Global volcanic aerosol properties derived from emissions, 1990-2014, using CESM1 (WACCM), Journal of Geophysical Research: Atmospheres, 121, doi: 10.1002/2015JD024290

Collaborating organizations
University of Leeds
Pacific Northwest National Laboratory
Massachusetts Institute of Technology

Funder
National Science Foundation

Writer/contact
David Hosansky, Manager of Media Relations

See all News