Hot Paper: Xiaohong Liu on Modeling Atmospheric Aerosols

September 2013
Dr. Xiaohong Liu

According to a recent analysis of InCites/Essential Science Indicators (a subset of Thomson Reuters Web of Science), a 2012 report entitled “Toward a minimal representation of aerosols in climate models: Description and evaluation in the Community Atmosphere Model CAM5” (X. Liu et al., Geoscientific Model Development, 5[3]: 709-39, 2012) has distinguished itself as a New Hot Paper in the main field of Geosciences. To qualify as a Hot Paper, a report must be indexed by Thomson Reuters within the last two years and must be cited at a level notably above reports of comparable type and age published in the same journal. To date, this paper has been cited 51 times in Web of Science.

Corresponding author Xiaohong Liu, who was affiliated with the Pacific Northwest National Laboratory at the time the report was published, recently moved to the University of Wyoming, where he is now professor in the Department of Atmospheric Science, holding the Wyoming Excellence Chair in Climate Science. His collaborators on this paper included 21 co-authors representing eight institutions in the United States and Sweden.

Below, Liu answers a few questions about this New Hot Paper in Geosciences.

SW: Why do you think your paper is highly cited?

Atmospheric particles from human activities and from other sources (e.g., forest fires, deserts) play an important role in the Earth’s climate system. However, their effect is still one of the largest sources of uncertainty in model predictions of climate change. Our paper describes and evaluates an advanced aerosol module for the Community Earth System Model (CESM), one of the major climate models participating in the Intergovernmental Panel on Climate Change (IPCC) 5th Assessment Report (IPCC AR5). The CESM is widely used in the climate modeling community and is an important tool for understanding and simulating climate variability and climate change. The role of aerosols in the climate system, such as effects on atmospheric temperature, drought/flooding, atmospheric circulation, air quality and visibility, snow/ice cover and biogeochemical cycles, is among the hottest topics in atmospheric and climate research.

SW: Does it describe a new discovery or new synthesis of knowledge?

Our aerosol module is able to treat the complicated physical and chemical processes governing aerosol lifecycles and aerosol effects on climate.

Our aerosol module is able to treat the complicated physical and chemical processes governing the aerosol lifecycles and aerosol effects on climate. It requires a minimal increase in computer time, which is essential for global climate models simulating many complex and interacting components (e.g., atmosphere, ocean, land, land ice and sea ice, and carbon/nitrogen cycles) for multiple centuries of climate change. The module treats all important aerosol processes in the atmosphere such as emission, new particle formation, gas and aqueous chemical reactions, transport, scavenging through interactions with clouds and precipitation, and deposition on surface, all based on our best understanding of the processes, and is validated with data obtained from long-term satellite, aircraft, and surface observations. It is an important step towards the realistic representation of aerosols and aerosol effects in climate models.

SW: Would you summarize the significance of your paper in layman's terms?

Our paper describes an advanced aerosol module in a widely used climate model (CESM). Our paper provides a foundation for multi-century simulations of climate change driven by anthropogenic aerosols as well as greenhouse gases such as carbon dioxide.

SW: How did you become involved in this research, and how would you describe the particular challenges, setbacks, and successes you've encountered along the way?

In order to reduce the uncertainties in model prediction of climate change, aerosol effects must be realistically represented in climate models. In 2005 we were asked by the developers of the CESM to implement an aerosol module to the CESM, and the US Department of Energy funded us from 2007-2011 to do this. One major challenge during the development was debugging the CESM, which is a complicated system with millions of lines of code running on parallel platforms. Calibrating the CESM was also a major challenge because aerosols interact with clouds and other components, which make the results of parameter changes hard to anticipate. After several years of hard work, we were able to show that including the aerosol effects on clouds and climate was essential for realistically simulating the climate change observed for the last 150 years. 

SW: Where do you see your research leading in the future?

We are working to improve the secondary organic aerosol in the aerosol module, which is a major submicron aerosol species and has an important impact on climate, but is under-represented in global climate models. We are also adding other aerosol species such as nitrate, which will become more important in the next few decades as sulfur emissions are reduced. We are also improving the treatment of aerosols from natural sources such as mineral dust and sea salt to enable more accurate calculations of anthropogenic aerosol forcing of climate change and to study the biogeochemical feedbacks involving aerosols.

SW: Given that climate research tends to be such a charged topic, how would you describe the social or political implications of this work?

Some anthropogenic aerosol (black carbon) warms the climate and some (sulfate) cools. The cooling by sulfate aerosol has been masking some of the warming due to increasing carbon dioxide, which might explain why the observations have indicated little warming in the last decade as sulfur emissions from China have increased. Since China is expected to reduce sulfur emissions in the next few decades to improve local air quality, the cooling by sulfate is expected to decline, revealing more of the warming by carbon dioxide. The warming by black carbon suggests the potential for rapid reduction in warming by reducing black carbon emissions, which would also improve human health and air quality. Geoengineering of the Earth’s climate by introducing cooling aerosol such as sea salt has been suggested as a last resort should efforts to reduce carbon dioxide emissions fail and climate warming become unbearable. Our aerosol module in the CESM can be used to explore these mechanisms and guide efforts to prevent or mitigate excessive climate change.

Dr. Xiaohong Liu
Professor & Wyoming Excellence Chair in Climate Science
Department of Atmospheric Science
University of Wyoming

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