Led by the University of Washington, the research looked at satellite data over a shipping lane in the south Atlantic from 2003 to 2015. It found that particles from shipping emissions caused clouds to ‘seed’ around these shipping routes more readily, blocking an additional 2W of solar energy, on average, from reaching each square metre of ocean surface near the lanes.
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The UW team extrapolated that globally, industrial pollution blocks 1W of solar energy per square metre of the Earth’s surface, masking almost a third of the present-day warming from greenhouse gases. The work is published in AGU Advances.
“In climate models, if you simulate the world with sulphur 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.”
The study analysed cloud properties detected over 12 years by the MODIS instrument on NASA satellites and the amount of reflected sunlight at the top of the atmosphere from the CERES group of satellite instruments. The researchers compared cloud properties inside the shipping route with an estimate of what those cloud properties would have been in the absence of shipping based on statistics from nearby, unpolluted areas.
“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, a UW doctoral student in statistics. “However, if this small change occurred worldwide, it would be enough to affect global temperatures.”
The study implies that, without the cooling effect of pollution-seeded clouds, Earth might have already warmed by 1.5 degrees Celsius, the target to keep below that was set out in the Paris climate agreement. It also has implications for strategies to temporarily slow global warming by spraying salt particles to make low-level marine clouds more reflective, known as marine cloud brightening.
“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,” said Diamond. “It does tell us that these effects are possible — and on a more cautionary note, that these effects might be difficult to confidently detect."
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