Iceland volcano's eruption shows how sulfur particles influence clouds

http://www.eurekalert.org/pub_releases/2015-12/uow-ive120915.php

Public Release: 9-Dec-2015
Iceland volcano's eruption shows how sulfur particles influence clouds
University of Washington

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.

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The results confirm that volcanoes cool the planet not just by emitting particles high in the atmosphere, but also by releasing low-level sulfur to influence cloud formation.

When the air contains aerosol particles, the same amount of water vapor condenses into many small drops, whose larger surface area reflects more sunlight. The difference in reflected solar radiation for September and October 2014 was 2 watts per square meter in the region over Iceland.

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The results may help understand humans' impact on clouds. Human pollution since the Industrial Revolution is believed to have altered skies in the Northern Hemisphere. One uncertainty in climate models is how much human pollution has brightened the clouds, shielding the planet from the effects of the simultaneous rise in carbon dioxide.

"One of the big uncertainties regarding climate change is how much human-produced aerosols have offset the warming until now," Hartmann said. "We hope the data from this eruption will improve the model simulations of cloud effects, and narrow the uncertainties in projections of the future."

The most recent Intergovernmental Panel on Climate Change report was the first to include a chapter on clouds and aerosols, one of the biggest uncertainties in global climate models. This study will provide a benchmark for modelers to check their simulations of clouds and aerosols and improve their algorithms for the next generation of climate models.

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