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Smoke on the water

发布时间:2019-03-07 04:15:01来源:未知点击:

By Jon Copley THE FULL impact of global warming has been curbed by the emissions from ship’s smokestacks, according to researchers in Pennsylvania. Sulphur from burning marine fuel has been seeding clouds over the open ocean, which then reflect sunlight back into space rather than allowing it to heat the planet. Oceanographers have recently spotted linear clouds on satellite pictures marking out the tracks of ships across the oceans. The clouds are made of water vapour like aircraft vapour trails, but they are much lower in the atmosphere and last for days instead of hours. Marine fuel is rich in sulphur and produces sulphur dioxide and sulphate particles when it burns. Cloud droplets condense around these particles. “Everybody assumed this was not an important process,” says Spyros Pandis of Carnegie Mellon University in Pittsburgh. Previous models of climate change incorporated the sulphur from burning marine fuel, but assumed that it was released at the ports where the fuel was sold, rather than spread over the oceans. “There was the correct amount of sulphur, but in the wrong places,” says Pandis. Pandis and his colleagues decided to correct this oversight. They reconstructed the traffic over the oceans for a year and added this information to a model of atmospheric sulphur chemistry and cloud formation. Estimating the amount of solar radiation that ship trails reflect back into space, Pandis argues that we might have seen a global temperature rise of 2 to 3 °C over the past century without them, instead of half a degree ( Nature, vol 400, p 743). “In a sense, we have been counteracting global warming,” says Pandis. Although burning marine fuel also releases carbon dioxide, these emissions have already been included in models of climate change, as their effects do not depend on where they are released. The team’s results may also explain why there is so much sulphur dioxide in the atmosphere over remote areas of the ocean. Some plankton produce dimethyl sulphide, which then breaks down into sulphur dioxide and sulphate, but estimates of what they produce have always fallen far short of actual measurements. “It solves this puzzle,” says Pandis. “Human fingerprints are all over the planet.” Other atmospheric scientists are more sceptical, however. “There is some uncertainty in the model,” says Peter Liss of the University of East Anglia in Norwich. Crucially, the relationship between particles and clouds is not linear—where there are already plenty of particles in the atmosphere, the cloud-generating effect will be much less noticeable. “In the northern hemisphere, there are very few places where the effect will be substantial,” Liss argues. “In the southern hemisphere, the effect will be much greater, but that is where you have less shipping.” But he admits that ships’ trails are intriguing and provide an opportunity to study the processes behind cloud formation. Pandis and his group are now studying the effect of shipping on ozone. In the open ocean, nitrous oxides from ship emissions may promote oxide formation, but in polluted ports they may eat it up. “I can argue that boats in Athens actually decrease ozone,