In fact ships pollute nearly half as much as all the world’s cars. We're talking smog-type pollution. The kind that causes premature deaths from heart disease and asthma. A new study [pdf] estimates the total contribution of commercial maritime shipping and it adds up to about 2.2 million pounds of particle pollution a year.

Since more than 70 percent of shipping traffic takes place within 250 miles of the coastline, ship spew is a serious health issue for nearly half the people of the world—the number who live near the coast [pdf]

The problem starts with sulfates, the same gunk emitted by diesel engines on land. Sulfates already have some measure of regulations attached to them. But more than half of shipping pollution comes from organic pollutants and sooty black carbon. These aren't targeted by today’s regulations.

When you consider that our world is a giant fluid dynamics experiment, then it makes sense that what happens at sea flows ashore. And vice versa. There are all kinds of ways to address this problem. Kevin Drum talks cap and trade and that could work for shipping too. But keep in mind that one upside to the global downturn in the economy is decreased shipping and therefore cleaner air. So why not recast the recession as a long-overdue refreshment for our weary planet?