What’s black and dirty and messing with the climate?

Soot, derived from many sources, needs to be looked at more closely by atmospheric researchers.

PRNewsFoto/Green Works/Laura Morton
On walls, soot can make for beautiful nature-inspired murals, like this one in San Francisco on Earth Day 2008. But in the atmosphere, soot can mess with weather and global warming.

Soot nags at climate scientists like a child demanding a parent’s attention. While soot has played a minor role compared with the rock stars of climate change – carbon dioxide and other greenhouse gases – that perspective is changing. Recent research suggests that black carbon soot must be reckoned with.

Soot particles affect cloud formation and precipitation. Because it absorbs sunlight, soot can cool the surface while warming air aloft. The balance between such warming and cooling affects regional climates.

Last week, the Proceedings of the National Academy of Sciences carried a report by Renyi Zhang at Texas A&M University and colleagues on the first study of what happens when the dirty black particles soak up sulfuric acid. This combination significantly changes the role these particles play in the atmosphere.

These changes could have “profound implications on visibility, human health, and direct and indirect climate forcing,” the scientists note, adding that the details of these processes “remain highly uncertain, considerably hindering efforts to assess their impact on visibility, human health, and climate.”

Clearing up that uncertainty should be a research priority.

Last March, V. Ramanathan at Scripps Institution of Oceanography and Greg Carmichael at the University of Iowa reviewed what’s known about soot’s effects in Nature Geoscience. They noted that black carbon pollution could be the most powerful agent driving global warming after carbon dioxide. They said it’s time “to examine if black carbon is also having a large role in the retreat of arctic sea ice and Himalayan glaciers as suggested by recent studies.”

Part of the problem in assessing soot’s environmental role is knowing where it’s coming from. Wildfires, fossil fuel combustion, and burning vegetation for farming all produce the black particles. Many such sources are known on all continents. Yet scientists have literally been missing the boat on one source. Last month, Daniel Lack with NOAA’s Earth System Research Laboratory and colleagues published the first extensive study of ship emissions in

Geophysical Research Letters. They find that large ships emit twice as much soot as anyone thought. They singled out tugboats as special offenders because they emit more soot per gallon of fuel burned than any other vessels.

Ship soot has a big effect off coastlines and in harbors where there is major shipping traffic. The scientists warn that this could be a new and important environmental factor in the Arctic when global warming melts enough ice to open the Arctic Ocean to significant summertime shipping. Yet they note that “commercial shipping emissions have been one of the least studied areas of all combustion emissions.”

Ship soot would add to the wildfire smoke that already is increasing over the Arctic. The fire season seems to be intensifying as the Arctic warms. Whether more sootiness would retard or enhance that warming is not known. Finding out what might happen is one more reason that soot should join the other “rock stars” on center stage of the climate change show.

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