Ronstan

How pollution is making lightning

Published on October 10th, 2019

If your sailing course crosses shipping channels during inclement weather, this report on WIRED.com may give you a shock… literally.


For all the progress humanity has made since Odysseus had a spot of trouble on a long voyage home, life on the high seas remains a largely joyless affair. Twenty-first-century sailors spend weeks away from home. The hours are long, the pay mediocre, the risk of calamity never quite over the horizon.

And, researchers have recently learned, these men and women face a problem not even the King of Ithaca had to deal with: unnaturally large amounts of lightning. Turns out that along some of the world’s busiest shipping lanes, lightning strikes are twice as common as they are in nearby areas with similar climatic conditions.

As usual in such stories, the blame doesn’t fall on a riled up Olympian. It goes to the hubris of humans who, in this case, thought their ships could burn filthy fuel without any judgement raining down.

That’s the takeaway from several years of work by researchers at the University of Washington and NASA, starting with a 2017 paper titled “Lightning enhancement over major oceanic shipping lanes.”

Its authors focused on the northeastern Indian Ocean and the South China Sea, including around Singapore and Indonesia. They picked up the topic when Katrina Virts, a graduate student at the time, created a method to squeeze more resolution out of available data on lightning strikes.

She and Joel Thornton, an atmospheric scientist at the University of Washington, used the method and 11 years worth of data on lightning strikes to make a map of areas with especially high strike rates. And they noticed a pattern. “We instantly recognized that these were shipping lanes,” says Thornton, the paper’s lead author.

A comparison of lightning strikes between 2005 and 2016 in the eastern Indian Ocean and South China Sea (above) and shipping emissions (below) shows a clear correlation between where humans sail and where lightning hits.

This may sound crazy—until you know a little bit about lightning. Under normal conditions, microscopic water droplets in the air grab onto “cloud condensation nuclei,” which are aerosol particles bigger than 50 nanometers, like a bit of dust, or sulphur dioxide. When few particles are present, each one picks up more droplets, and they coalesce into relatively short clouds at low altitudes. Those make rain.

When a lot of aerosol particles are present, each one gets fewer droplets, and can float high enough into the atmosphere to freeze. In the resulting tall clouds, those bits of ice and slush run into each other and transfer electric charges. The differences in charge create an electric field, which results in lightning.

The official term for this is “aerosol convective invigoration.” Thornton also calls it “catalyzing lightning.” You just need to know that more particles means more lightning, and burning fossil fuels is a reliable way to make those particles. Ships are especially culpable because they use bunker fuel to get from port to port.

Made from the dark, viscous stuff that’s left at the bottom of the barrel after the comparatively ethereal gasoline, jet fuel, and kerosene have been distilled off, it contains about 3,500 times as much sulphur as automotive diesel. The world’s fleet burns some 3.3 million barrels of it daily. (At least until December 31—more on that in a flash.)

For the 2017 study, Thornton and his coauthors pulled data on 1.5 × 109 individual strokes (aka discharges) between 2005 and 2016 from the World Wide Lightning Location Network. They compared that to data from the Emissions Database for Global Atmospheric Research, which makes detailed estimates on how much pollution ships create based on real-time info.

Then, in 2018, University of Washington researchers Peter Bloseey and Christopher Bretherton followed up by using a computer simulation to measure the effect of ship emissions in the Indian Ocean on cloud creation, in response to the 2017 study. With support from Thronton and Virts (now at NASA), they found effects on thunderstorms that lined up with the original study.

Other human activity, like agriculture, likely has the same effect on clouds. (Fertilizing soil ramps up bacteria activity in soil, producing more nitrate emissions, which are considered cloud condensation nuclei.) But the results are also easier to see along shipping lanes because most vessels trace the same arcs across the oceans, in places where the effects of other human work remains limited.

And while lightning doesn’t pose a huge threat to large ships or people—it kills just 50 or so a year in the United States—this is just one more reminder that human habits can have hard-to-predict consequences on the world we inhabit. “It just shows how sensitive the earth is,” Thornton says.

For those who prefer their skies calm, the good news is that these particles don’t linger in the atmosphere for more than a few weeks. So if we stop polluting now, the results won’t take ages to kick in. And we just might stop polluting—at least a bit—in the New Year, when an international agreement to make ships burn much cleaner fuel takes effect. “We might see a trend in the reverse,” Thornton says.

It’s essentially a big natural experiment, one whose results might see more sailors safely home—or at least a bit less spooked by the flash and the bang.

Source: WIRED

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