The humble and charming tugboat emits more soot for the amount of fuel it uses than any other commercial vessel, according to a new study that also finds cargo ships emit more than twice as much as previously thought.

The findings bode ill for the Arctic, in particular, where a rapidly melting ice cap allow for more commercial ship travel. In particular, a long-sought Northwest Passage opened last summer and could become navigable again this year. In fact the very North Pole could be ice-free later this year, scientists said recently.

The researchers worry that the extra soot could exacerbate Arctic melting.

Small dark soot particles absorb sunlight, create haze, and affect how clouds form and make rain, further altering a region's heat balance, the researchers say. If commercial shipping extends new routes through Arctic waters as they become navigable, soot emissions there could increase. Previous studies have concluded that soot could hasten Arctic melting.

Dirty shipping

Commercial shipping releases roughly 130,000 metric tons of soot per year, or 1.7 percent of the global total--much of it near highly populated coastlines, the authors estimate. In the coming years global shipping is expected to grow two to six percent annually.

Oceangoing tankers and container ships emit half a gram of soot per kilogram of fuel burned when at dock and slightly less when traveling, according to scientists from the National Oceanic and Atmospheric Administration (NOAA) and the University of Colorado (CU) who conducted the new study. Tugs emit nearly a gram of soot per kilogram of fuel burned — twice as much as any other vessel type, the authors find.

"Tugboats are a huge source of black carbon that may be under- reported or not reported at all in emissions inventories compiled by ports," says the study lead author Daniel Lack of NOAA's Earth System Research Laboratory (ESRL) and the NOAA-CU Cooperative Institute for Research in Environmental Sciences.

The finding will be detailed in the July 11 issue of the journal Geophysical Research Letters, a journal of the American Geophysical Union. The research was funded by NOAA.

Exceptionally high soot levels from tugboats point to their low- quality fuel — a thick, black tar left over from crude oil after the gasoline and kerosene have been removed. Engine age and maintenance also play a role. Tugboats have a disproportionate impact on air quality because they travel within ports, emitting potentially harmful particles near populous urban areas, the researchers point out.

About the study

To investigate ship emissions, the researchers observed plumes from commercial vessels in open ocean waters, channels, and ports along the southeastern United States and Texas during the summer of 2006. From the NOAA research vessel, Ronald H. Brown, the team measured black carbon emitted by tankers, cargo and container ships, large fishing boats, tug boats, and ferries, many of them in the Houston Ship Channel.

"Commercial shipping emissions have been one of the least studied areas of all combustion emissions," Lack said. "The two previous studies of soot emissions examined a total of three ships. We reviewed plumes from 96 different vessels."

A 2007 study by American and German scientists linked particle pollution from shipping to tens of thousands of premature deaths each year, most of them along coastlines in Europe, East Asia, and South Asia. Soot makes up a quarter of that pollution, Lack said.

The primary sources of soot, or small particles of black carbon, are fossil fuel combustion, wildfires, and burning vegetation for agricultural purposes. On a global scale, soot currently traps about 30 percent as much heat as does carbon dioxide, the most important greenhouse gas, according to the latest assessment of the Intergovernmental Panel on Climate Change.