‘Effects of aerosols on clouds, climate change unclear’

Team of Israeli, US and Australian researchers releases new study.

Planet Earth 311 (photo credit: Courtesy)
Planet Earth 311
(photo credit: Courtesy)
While aerosols do combat some of the warming effects of greenhouse gases, the overall effect of such substances on climate change remains uncertain, a team of Israeli, American and Australian researchers has determined in a new study.
“Climate Effects of Aerosol- Cloud Interactions,” published in the journal Science on January 24, explores the ways in which aerosols – particulate pollutants – impact the climate through their complex interactions with clouds. In part, aerosols are able to counteract the warming effects of greenhouse gases by increasing the amount of sunlight reflected back into space, yet a clearer understanding of aerosol-cloud interactions is still hampered by limited observational capabilities and coarse climate models, the authors concluded.
Scientists quantify aerosol- cloud interactions by using global satellite measurements to calculate changes in cloud properties that can be explained by variability in aerosol amounts, Prof. Daniel Rosenfeld of the Hebrew University’s Fredy and Nadine Hermann Institute of Earth Sciences told The Jerusalem Post on Sunday.
One such change in properties, he explained, could be the amount of solar radiation that the clouds reflect back to space.
Rosenfeld collaborated on the report with Dr. Steven Sherwood of the University of New South Wales, Dr. Robert Wood of the University of Washington and Dr. Leo Donner of the US National Oceanic and Atmospheric Administration.
Depending on meteorological conditions, aerosols can have dramatically different effects on the clouds – either increasing or decreasing the sun-deflecting effect, Rosenfeld and his colleagues wrote. The inability of researchers to differentiate between the effects of aerosols and the effects of meteorology on the cloud properties is the main cause of the uncertainty of aerosol effects, Rosenfeld told the Post.
Another problematic issue the researchers faced is the fact that little information is available about aerosol levels prior to the industrial era, and obtaining this level as a reference is crucial to estimating the radiative forcing of aerosol.
Further clarification on the response of the cloud cover to the loss of water by rainfall and to the formation of ice is also necessary, the researchers explained. As of now, simulations of these processes at the scale of a one-cloud or a multicloud system would require hundreds of hours on the most powerful computers, so conducting simulations for the entire planet would not be feasible, they said.
Scientists have, however, been working on groundbreaking simulation techniques that simplify the schemes of cloud-aerosol interactions and might provide the potential for models that resolve clouds on a global scale for up to several years’ time, the researchers said. Climate simulations on the scale of a century are still not feasible, however.
Recognizing the uncertainty in aerosol-cloud interactions is crucial because of their potentially direct relationship to global warming from greenhouse gases. If, for example, the cooling effect of aerosols hides an amount of heat, it means a smaller amount of heat than the Earth is actually emitting is responsible for global warming thus far, Rosenfeld told the Post.
“This means a greater climate sensitivity, which is the amount of temperature increase for a given amount of heating,” he said.
Although global air pollution occurring through aerosol pollution particles has already reached its peak, carbon dioxide greenhouse gas will continue to rise, Rosenfeld explained.
With a growing amount of greenhouse gases in the air but stagnant or decreasing aerosol supplies performing the mediated cooling effect, a stronger- than-anticipated global warming situation could occur, he warned.
Moving forward, Rosenfeld said that he felt the way to decrease uncertainty about the aerosol-cloud interactions was to launch better satellites, with stronger resolutions in more wavelengths and multiple angles of view for the same clouds.
“This is within our technological reach, and in fact, I have already designed and proposed such a satellite,” he said.
“The main obstacle is the cost, but the alternative of not having this essential knowledge is much more costly.”