It has long been suspected that sulfur emissions can brighten clouds. Water droplets tend to clump around particles of sulfuric acid, causing smaller droplets that form brighter, more reflective clouds.
But while humans have pumped sulfur into Earth's atmosphere since the Industrial Revolution, it's been hard to measure how this affects the clouds above. New University of Washington research uses a huge volcanic eruption in Iceland to measure the change.
The new study, to be published in Geophysical Research Letters, a journal of the American Geophysical Union, shows that sulfur emissions do indeed result in smaller cloud droplet size, leading to brighter clouds that reflect significantly more sunlight.
"This eruption is a chance to nail down one of the big uncertainties in climate models," said first author Daniel McCoy, a UW doctoral student in atmospheric sciences.
The study takes advantage of a unique geologic event. During six months from summer 2014 until early 2015, a crack in the Bardarbunga volcano seeped lava and sulfur gas. This was not one of Iceland's huge explosive eruptions that fill the skies with ash and shut down airplane routes. Instead it was a long, slow, low-elevation seep of sulfur emissions that produced an amount of lava second only to Laki in the recent history of Iceland eruptions.
The UW researchers looked at data for that region recorded by NASA's MODIS, or Moderate Resolution Imaging Spectroradiometer, instrument to measure the size of droplets in the marine cloud layer. While the volcano was spewing sulfur, the droplets were the smallest in the 14-year record of observations.
"You can see the effect over an entire ocean for a two-month period," McCoy said. "It was a pretty unique geophysical event within the satellite record."