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Climate: Study finds plant ‘emissions’ counteract small percentage of global warming

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Plant-based aerosols seen as factor in global warming puzzle. Bob Berwyn photo.

Regional effects of plant-based aerosol formation could be significant

By Summit Voice

FRISCO — A warmer climate will spur plants to release more gases that help form clouds, counteracting about 1 percent of effect of heat-trapping greenhouse gases, according to research by IIASA and the University of Helsinki.

On a regional scale, the effect is more significant, the scientists concluded, finding that in places like Finland, Siberia, and Canada, the negative feedback loop could counteract  up to 30 percent of warming in more rural, forested areas where anthropogenic emissions of aerosols were much lower in comparison to the natural aerosols.

Scientists had known that some aerosols – particles that float in the atmosphere – cool the climate as they reflect sunlight and form cloud droplets, which reflect sunlight efficiently. Aerosol particles come from many sources, including human emissions. But the effect of so-called biogenic aerosol — particulate matter that originates from plants — had been less well understood.

Plants release gases that, after atmospheric oxidation, tend to stick to aerosol particles, growing them into the larger-sized particles that reflect sunlight and also serve as the basis for cloud droplets. The new study showed that as temperatures warm and plants consequently release more of these gases, the concentrations of particles active in cloud formation increase.

“Everyone knows the scent of the forest,” saud Ari Asmi, University of Helsinki researcher who  worked on the study. “That scent is made up of these gases.”

While previous research had predicted the feedback effect, until now nobody had been able to prove its existence except for case studies limited to single sites and short time periods. The new study showed that the effect occurs over the long-term in continental size scales.

“This does not save us from climate warming,” said IIASA and University of Helsinki researcher Pauli Paasonen, who led the study. “Aerosol effects on climate are one of the main uncertainties in climate models. Understanding this mechanism could help us reduce those uncertainties and make the models better.”

The researchers collected data at 11 different sites around the world, measuring the concentrations of aerosol particles in the atmosphere, along with the concentrations of plant gases, the temperature, and reanalysis estimates for the height of the boundary layer, which turned out to be a key variable.

The boundary layer refers to the layer of air closest to the Earth, in which gases and particles mix effectively. The height of that layer changes with weather. Paasonen said.

“One of the reasons that this phenomenon was not discovered earlier was because these estimates for boundary layer height are very difficult to do,” he explained. “Only recently have the reanalysis estimates been improved to where they can be taken as representative of reality.”

Paasonen, P., et. al. 2013. Evidence for negative climate feedback: warming increases aerosol number concentrations. Nature Geoscience doi: 10.1038/NGEO1800

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