Hit the Brakes

The roughly 103,000 ships that transport goods around the world are heavy polluters. But vessels could cut their emissions significantly if they stick to a lower speed limit, researchers say.

Ships release carbon dioxide and other pollutants such as nitrogen oxides, sulfur oxides, and particulate matter. Shipping lanes often take these vessels near shore, prompting concerns that the pollution could endanger the health of people living along the coast. To reduce emissions, some ports have already begun asking vessels to slow down near the coast.

In a study published in Environmental Science & Technology, researchers monitored emissions from two vessels travelling from San Pedro Bay to the Port of Oakland, California. The ships slowed down to 12 or fewer knots when they entered a zone extending 24 nautical miles from the coast.

At the lower speed, carbon dioxide emissions fell by 61 percent, nitrogen oxides by 56 percent, and particulate matter by 69 percent. When a ship used fuel that contained less sulfur and travelled at 11 knots, particular matter emissions dropped by 97 percent.

Emissions depended partly on the current, the team found. When a vessel had to fight the current, it released 21 percent more carbon dioxide and 10 percent more nitrogen oxides. — Roberta Kwok | 29 October 2012

Source: Khan, M.Y. et al. 2012. Greenhouse gas and criteria emission benefits through reduction of vessel speed at sea. Environmental Science & Technology doi: 10.1021/es302371f.

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