Plant Particles Slow Global Warming (and Smell Great)

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The warming global climate is causing plants to emit chemicals that lead to more climate-cooling clouds, say researcher in a new study. The cooling effect of more clouds could dampen the warming trend a bit in some remote parts of the world where there are not already ample human-made particles in the air already seeding more cloud formation.

"It's not that much," said Pauli Paasonen of the University of Helsinki in Finland and one of the authors of the study, published in the April 28 issue of Nature Geoscience. But it's another step towards sorting out the roles of small particles -- called aerosols -- in future climate change, he said. It's also one of the very few negative feedbacks found in the current warming world.

Most of the feedbacks to climate change have been positive, meaning they tend to exacerbate the warming as, for instance, melting sea ice does when it allows sunlight to warm sea water rather than reflecting that sunlight back into space; or when permafrost melts, it releases carbon dioxide, which adds to the overall greenhouse gas problem.

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Paasonen and his colleagues came to their conclusions about the plant aerosols after gathering and analyzing temperature and aerosol data from a number of sites in the mid and high-latitudes sites -- in both remote, clean locations and highly polluted locations -- around the world. They found that the abundance of aerosols soared when the temperature increased, probably because plants increase their production and release of the sticky compounds that clump together to make particles large enough for water to condense on.

"Everyone knows the scent of the forest," said Ari Asmi, also a University of Helsinki researcher on the study. "That scent is made up of these gases."

Previous research had predicted the plants would do this, but this is the first study to show that it is happening across continents.

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Among the implications of the work is that as some places clean up the human-made aerosols, plants could take over as things get warmer. In polluted areas there are so many aerosols that the plant contributions make no difference, Paasonen explained. In either case, the cooling effects of clouds is very regional, not global, and only masks warming, rather than eliminates it.

"It's important to note that the cooling effect by aerosols is one of the unknowns right now in assessing the impact of climate warming," said aerosol researcher James Smith of the national Center for Atmospheric Research in Colorado. "Some parts of the world will be more affected than others."

Smith also notes that the amped-up emissions by plants will probably max out as the temperature continues to increase and carbon dioxide reaches levels that not even plants like. That suggests the cooling effect could be very limited and even reverse in the future, further unmasking the full effects of global warming.