Research from scientists at the University of Washington suggests that pollution from container ships and other shipping traffic is significant enough to change cloud composition and ultimately has a cooling effect on the planet. The study was published 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 pretty 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."
This long-term study is the first to contemplate the question of the cooling effect from shipping emissions at a regional scale. The researchers analyzed data from the Spring cloudy season between 2003-2015 over the common shipping route between Europe and South Africa, which overlaps with that between Europe and Asia.
The trail of fluffy white clouds that a shipping container leaves in its wake look similar to the contrails left by airplanes and show a physical representation of changes in cloud composition. The authors of the study say that they used satellite data to determine that the resulting clouds block an average of two additional Watts of solar energy from reaching each square meter of ocean surface near the shipping lane.
"The difference inside the shipping lane is small enough that we need about six years of data to confirm that it is real," said co-author Hannah Director. "However, if this small change occurred worldwide, it would be enough to affect global temperatures."
The researchers calculate that on a global scale cloud changes caused by particles from all forms of industrial pollution block one Watt of solar energy per square meter of Earth's surface. That has significant implications on current global warming: it blocks nearly one-third of the present-day warming that originates from greenhouse gases.
"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, who is a UW professor of atmospheric sciences.
Of course, these findings beg the question of what do to with this information. Recent research investigating climate intervention tactics often cites marine cloud brightening as a potential strategy to reduce warming, but the scientists of this study warn that we are far from understanding the potential consequences of such action.
"What this study doesn't tell us at all is: Is marine cloud brightening a good idea? Should we do it? There's a lot more research that needs to go into that, including from the social sciences and humanities," Diamond said. "It does tell us that these effects are possible -- and on a more cautionary note, that these effects might be difficult to confidently detect."