The Aerosols Conundrum

Research shows that aerosols not only cool, but also heat the planet — a finding that may cloud the validity of climate-change models.

MIT News Office 7/8/10
 
Just how much warmer Earth will become as a result of greenhouse-gas emissions — and how much it has warmed since preindustrial times — is much debated. In a 2007 report, the Intergovernmental Panel on Climate Change, an agency formed by the United Nations to assess climate change, said that the planet’s average surface temperature will rise by between 2 and 11.5 degrees Fahrenheit by 2100, with a best estimate at between 3.2 to 7.2 degrees F. However, the IPCC’s computer models have a record of overestimating warming: If the IPCC models were right, the planet should now be hotter than it is.


The IPCC attributes the discrepancy to aerosols — microscopic particles in the atmosphere that are created by both nature (dust blown by desert winds) and human activity (liquid droplets created from fuel combustion). Because aerosols help cloud droplets form into icy particles and reflect sunlight back into space, they help to cool Earth and possibly mitigate warming caused by emissions. But Richard Lindzen, the Alfred P. Sloan Professor of Meteorology in MIT’s Department of Earth, Atmospheric and Planetary Sciences, is among those who question the accuracy of the IPCC models, and he has been critical of the aerosols argument.

In a paper published last month in the Proceedings of the National Academy of Sciences Lindzen and his former postdoctoral researcher, Yong-Sang Choi, suggest that aerosols not only cool the Earth-atmosphere system — the system by which the atmosphere and oceans interact and affect the global climate — but also heat it. By describing the potential dual effects of aerosols, the research questions the IPCC’s models.

“Current climate models generally overpredict current warming and assume that the excessive warming is cancelled by aerosols,” the researchers say in their paper. “[Our research] offers a potentially important example of where the secondary effect is to warm, thus reducing the ability of aerosols to compensate for excessive warming in current models.” That is, the degree to which aerosols can compensate for model over-prediction of warming remains open, the research suggests.

While Thomas Stocker, co-chair of the IPCC’s Working Group I that is examining the physical scientific aspects of the climate system and climate change, declined to comment on the study, he says Lindzen and Choi’s research is part of relevant peer-reviewed work that the group will assess in its Fifth Assessment Report about climate change to be published in 2013.

Pinning down aerosols

In their research, Lindzen and Choi analyzed data about cloud formation and dust aerosols, or tiny particles of sand and silicate in the atmosphere, that were collected by NASA’s Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite from June 2006 through May 2007. Their analysis revealed that there were about 20 percent fewer “super-cooled” cloud particles — droplets that are a mixture of water and ice, but reflect more sunlight than ice — in regions that had dust aerosols. Such a difference, Lindzen and Choi suggest, could warm the atmosphere in those regions.

According to the researchers, the decrease in super-cooled particles occurs when aerosols travel to a layer of the atmosphere where the temperature is around minus 20 degrees Celsius, and they “effectively kill” super-cooled cloud droplets by causing them to form into ice. Fewer super-cooled cloud droplets would mean that clouds reflect less sunlight, which could have a warming effect on the climate. That effect, the researchers believe, needs to be incorporated into climate-change models. “The IPCC assumed that all the secondary effects of aerosols would be to increase reflectivity, so it has left out a very important factor that could lead to the opposite effect,” Lindzen says.

The work is important to the global-warming debate because it sheds light on the uncertainties of climate sensitivity, which is the term the IPCC uses to describe the change that a doubling of carbon dioxide would have on global average temperatures (the IPCC’s 2007 report estimates that change to be between 3.6 and 8.1 degrees F by the end of the century, with a best estimate of about 5.4 degrees F). According to Yale climate scientist Trude Storelvmo, “aerosol effects on climate, particularly via their influence on clouds, currently represent the most uncertain forcing of climate change.” Although the IPCC models assume that aerosols cool the Earth-atmosphere system, she cautions that “unless we can quantify this supposed aerosol cooling counteracting the warming due to increasing greenhouse gases, we cannot say what the climate sensitivity of the Earth-atmosphere system is.”

Because satellite data can be limited, she suggests that future research should include measurements of aerosol and cloud properties taken by instruments onboard aircraft that travel to the upper atmosphere. She thinks this combination could help address one question that remains unanswered in the paper: why few super-cooled clouds were detected over South America even though the satellite didn’t detect dust or carbon aerosols over that region.

Lindzen agrees that climate scientists can’t rely solely on remote sensing techniques to get “solid, incontrovertible data” about aerosols and clouds. Even so, he is eager for the launch of better satellites and instruments so that he and his colleagues can gather as much data as possible about how clouds evolve “so that we can better pin down what aerosols do.” Until scientists figure out that missing piece of the climate change puzzle, it will be difficult to predict the effects of future warming.