For many years now, scientists have used satellites to chronicle "ship tracks" — bright and easily visible atmosphere-bound emissions similar to the vapor trails produced by airplanes. But ships also emit a less obvious signature, one that's not so easy to see. As data from the Dutch and Finnish-built Ozone Monitoring Instrument (OMI) on NASA's Aura satellite now shows, ships are leaving long tracks of elevated nitrogen dioxide (NO2) along certain shipping routes. And alarmingly, a bi-product of NO2 — what's called NOx — often leads to cardiovascular and respiratory problems in humans.

OMI has been tracking these highly-reactive oxides of nitrogen since 2005, allowing researchers to put together this high-resolution bird's eye view of the Earth's hot zones. The researchers believe that shipping accounts for 15 to 30 percent of global NOx emissions — but they're not entirely sure. The team is hoping that their data — and their new map — will reduce the uncertainty associated with this potentially escalating problem.

NASA explains the map:


The map above is based on OMI measurements acquired between 2005 and 2012. The NO2 signal is most prominent in an Indian Ocean shipping lane between Sri Lanka and Singapore, appearing as a distinct orange line against (lighter) background levels of NO2. Other shipping lanes that run through the Gulf of Aden, the Red Sea, and the Mediterranean Sea also show elevated NO2 levels, as do routes from Singapore to points in China. These aren't the only busy shipping lanes in the world, but they are the most apparent because ship traffic is concentrated along narrow, well-established lanes.

The Atlantic and Pacific Oceans also have heavy ship traffic, but OMI doesn't pick up NO2 pollution tracks because the shipping routes are less consistent. The shapes of landmasses force ships into narrow paths in the Indian Ocean, while ships in the Atlantic and Pacific tend to spread out over a broad areas as they navigate around storms.

In addition, the air over the northeastern Indian Ocean is relatively pristine. Heavy NO2 pollution (dark red in the map) from cities and off-shore drilling activity along the coasts of China, Europe, and the United States obscures the ship tracks that might otherwise be visible to OMI. In the map, the Arctic is gray because the lack of light during the winter and frequent cloudiness during the summer prevented OMI from collecting usable data in the area.

Source and image: NASA.