Ships’ emissions create measurable regional change in clouds

Pollution from ships create lines of clouds. — This satellite image was taken January 16, 2018, off the coast of Europe. Pollution from ships creates lines of clouds that can stretch hundreds of miles. The narrower ends of the clouds are youngest, while the broader, wavier ends are older.

A container ship leaves a trail of white clouds in its wake that can linger in the air for hours. This puffy line is not just exhaust from the engine, but a change in the clouds that’s caused by small airborne particles of pollution.

New research led by the University of Washington is the first to measure this phenomenon’s effect over years and at a regional scale. Satellite data over a shipping lane in the south Atlantic show that the ships modify clouds to block an additional 2 Watts of solar energy, on average, from reaching each square meter of ocean surface near the shipping lane.

The result implies that globally, cloud changes caused by particles from all forms of industrial pollution block 1 Watt of solar energy per square meter of Earth’s surface, masking almost a third of the present-day warming from greenhouse gases. The open-access study was published March 24 in AGU Advances, a journal of the American Geophysical Union.

“In climate models, if you simulate the world with sulfur emissions from shipping, and you simulate the world without these emissions, there is a sizable cooling effect from changes in the model clouds due to shipping,” said first author Michael Diamond, a UW doctoral student in atmospheric sciences. “But because there’s so much natural variability it’s been hard to see this effect in observations of the real world.”

Averaged globally, they found changes in low clouds due to pollution from all sources block 1 Watt per square meter of solar energy — compared to the roughly 3 Watts per square meter trapped today by the greenhouse gases also emitted by industrial activities. In other words, without the cooling effect of pollution-seeded clouds, Earth might have already warmed by 1.5 degrees Celsius (2.7 F), a change that the Intergovernmental Panel on Climate Change projects would have significant societal impacts. (For comparison, today the Earth is estimated to have warmed by approximately 1 C, or 1.8 F, since the late 1800s.)

“I think the biggest contribution of this study is our ability to generalize, to calculate a global assessment of the overall impact of sulfate pollution on low clouds,” said co-author Rob Wood, a UW professor of atmospheric sciences.

Other co-authors are Ryan Eastman, a UW research scientist in atmospheric sciences. The research was funded by NASA and the National Science Foundation.