Lightning traces busy shipping lanes

Atmospheric scientists have known for decades that aerosolized particulate emissions from ships can seed clouds. The low-lying clouds that form as a result, called ship tracks, appear as puffy white lines over the ocean. But researchers have long suspected that the influence of human-dispersed aerosols extends much higher in the atmosphere, up to tens of kilometers. In general, the addition of condensation-inducing particles causes cloud droplets to become smaller and more numerous. Those small droplets should be more easily lofted to great heights, where they can freeze, electrify, and form thunderstorm-producing cumulonimbus clouds.

The clearest evidence to date in support of that hypothesis has now come from flashes of lightning. The University of Washington’s Joel Thornton and his colleagues analyzed 12 years of lightning data from the World Wide Lightning Location Network and found two lines of enhanced lightning activity (upper map) that lie directly over two of the world’s busiest shipping lanes (lower map)—one running east–west in the Indian Ocean and one running southwest–northeast in the South China Sea. The two lanes see lightning two times as much as adjacent regions of the ocean. After eliminating winds, atmospheric temperature structures, and other meteorological factors as sources of the enhancement, Thornton and his colleagues concluded that the emissions from ships had to be the cause.

On land, researchers studying how aerosol pollution interacts with clouds have to contend with terrain effects, heating of the lower atmosphere by the ground, and other confounding factors. Thus the shipping lanes present a rare opportunity to observe in isolation the consequences of human-made aerosols to weather and climate. (J. A. Thornton et al., Geophys. Res. Lett. 44, 9102, 2017 .)