Nitrogen dioxide is a nasty gas produced largely by internal-combustion engines. A small whiff of NO2 is enough to anesthetize the nose, and its chemical byproducts can harm the cardiovascular system and trash a good pair of lungs.
This pernicious pollutant is rampant throughout the planet, but nowhere moreso than above the heavily trafficked lanes of international shipping vessels. While we don't often see (or even think about) the smoke-belching monsters that bring us our Japanese tuna belly and cheap Indian shirts, NASA has found a nifty way to visualize the collective pollution that boats pump into the atmosphere.
The above image represents NO2 readings that the space agency's Aura satellite made from 2005 to 2012 (large version). It's easy to see the pollution signature above major shipping lanes – the long orange track in the Indian Ocean between Signapore and Sri Lanka, the smudges from Singapore to various ports in China, the L-shaped path through the Red Sea into the Gulf of Aden. The fact that there aren't similar trails in deep Atlantic and Pacific waters doesn't mean there aren't any ships there; it's just that the routes are varied and more spread out, with sea captains steering around large ocean storms.
If you're wondering about the faint but humongous NO2 plume coming off of Africa, that's the emissions signature of large-scale agricultural burning. The sickeningly dense clouds of nitrogen around the coasts of various countries arise from cities, obviously, with off-shore oil rigs producing intense concentrations off of Europe, China and the United States.
It's thought that shipping accounts for 15 to 30 percent of all the oxides of nitrogen in the atmosphere. What this map isn't showing are the other climate-bending gases churned out by huge, bunker-fuel burning vessels: The United Nations estimates they produce about 1 billion tons of greenhouse gas each year. (And, yep, international shipping is still mostly unregulated.)
The eyes behind the satellites have been observing the vaporous footprints of ships for a while now. You can see boat-made clouds in this 2001 NASA image of the Pacific Ocean a thousand miles west of San Francisco. The upper panel is a stereo anaglyph, so it'll really pop if you have a pair of 3-D glasses handy:
(NASA/GSFC/LaRC/JPL, MISR Team)
And these are ship tracks that formed in January 2013 off of the Pacific Northwest:
(eff Schmaltz, LANCE MODIS Rapid Response)
A video of the same, from the GOES Project Science Team:
Top NASA Earth Observatory image by Jesse Allen, using NO2 data provided by Lok Lamsal of the Aura Project Science Office.