Stirring Things Up

Big ships can really stir things up—especially in shallow coastal waters. A ship’s wake can extend far below the vessel. It creates turbulence that mixes up the water. It can even extend into the sediments on the sea floor. And that can release a lot of methane—a gas that’s much more efficient than carbon dioxide at warming the atmosphere.

Researchers discovered the connection between ships and methane releases more than a decade ago. They were studying shipping in Neva Bay, a shallow extension of the Baltic Sea on the northwestern coast of Russia. It’s home to Saint Petersburg, which has a busy port.

The researchers found that many ships cruising in the bay produced two bursts of methane. One came from the ship itself—from burning fossil fuels. The other came later, and wasn’t produced by the engines.

Scientists recently took another look at the observations. And they combined them with models of how a ship’s wake churns the water, and how bubbles of methane in the sediments make their way into the air.

They concluded that the wakes were stirring up the sediments. That released bubbles of methane, which floated to the surface, then into the air. There was a 20-fold increase in the amount of methane above the shipping lanes compared to the surrounding area.

Many of the world’s largest ports are built along similar coastal environments. So the churning action of passing ships could add more methane to the air every year.