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June 2008

short takes

An overview of projects throughout the organization

Mexico City

A team of researchers in ESSL/CGD is developing a model to study the effects of global warming on cities. The model appears to do a good job of simulating solar radiation, heat fluxes, and temperatures in Mexico City, shown here.

Tropospheric ozone and climate. Helen Worden (ESSL/ACD) and colleagues at the Jet Propulsion Laboratory (JPL) recently published a satellite-based analysis of the role played by tropospheric ozone in boosting the greenhouse effect. The results, which appeared online in Nature Geoscience on April 20, help clarify a significant uncertainty in the climate-change picture.

Along with its ill effects on human health, ozone in the troposphere—the lowest atmospheric layer—also acts as an important greenhouse gas, ranking third behind carbon dioxide and methane. However, it varies dramatically by region and over time, and it’s unclear how much tropospheric ozone existed before the industrial era.

In 2007, the Intergovernmental Panel on Climate Change estimated that, on average, changes in tropospheric ozone from human activities have increased the radiation trapped by the atmosphere anywhere from 0.25 to 0.65 watts per square meter, roughly the same as the radiative forcing from methane and several times less than that from carbon dioxide.

Helen and her collaborators drew on recent data from the Tropospheric Emissions Spectrometer, launched in 2004 on NASA’s Aura satellite. They found that the radiative forcing from tropospheric ozone (from both natural and human sources) between the latitudes 45°S and N averaged between 0.34 and 0.62 watts per square meter. The analysis focused on clear-sky radiances observed over the ocean in these latitudes. Although not directly comparable to IPCC estimates for anthropogenic radiative forcing, this measurement provides confirmation that the IPCC understanding of the impact of tropospheric ozone on climate change is essentially correct.

“These are the first global observations to partition the role of tropospheric ozone in the greenhouse effect,” says Helen, who spent 14 years at JPL before joining ACD last year. “These measurements provide new observational constraints for climate models, and we are working to extend the analysis beyond clear-sky ocean radiances to cloud and land scenes, where models have the largest uncertainties.”

Cities and climate change. Because of the large number of people who live in urban areas, scientists want to examine the likely impacts of global warming on cities. But cities are not represented in climate models, both because they are too small for the coarse resolution of these models and because urban surfaces respond to the atmosphere in different ways than undeveloped areas.

A team of scientists, led by Keith Oleson in ESSL/CGD, is making important progress toward adding an urban component to the Community Climate System Model. The team has created a model that represents an urban canyon, consisting of roofs, sunlit and shaded walls, and a canyon floor that is divided into both pervious surfaces such as lawns and impervious surfaces such as roads. One of the most difficult aspects of creating such a model is to correctly account for the trapping and reflection of solar radiation by various urban surfaces.

Testing has revealed that the model appears to do a good job of simulating solar radiation, heat fluxes, and temperatures in two cities—one based on the architecture of Mexico City, and the other on Vancouver. The team is continuing to refine the model, focusing on the height-to-width ratios of city buildings and the thermal properties of various surfaces. In the next year or so, they hope to integrate the model into CCSM, enabling researchers to study the effect of a warming world on different urban areas.

The research is summarized in two recent papers that appeared in the April issue of the Journal of Applied ­Meteorology and Climatology.

Permafrost and sea ice. A study led by David Lawrence (ESSL/CGD) has found that the rate of climate warming over northern Alaska, Canada, and Russia could more than triple during periods of rapid sea ice loss. The research raises concerns about the thawing of permafrost (permanently frozen soil), which has potential consequences for sensitive ecosystems, human infrastructure, and greenhouse gas emissions.

Side-by-side polar projections

Accelerated Arctic warming. [ENLARGE] (Image by Steve Deyo, ©UCAR.)

“Our study suggests that if sea ice continues to contract rapidly over the next several years, Arctic land warming and permafrost thaw are likely to accelerate,” David says.

The research was spurred in part by events last summer, when the extent of Arctic sea ice shrank to more than 30% below average, setting a modern-day record. From August to October, air temperatures over land in the western Arctic were also unusually warm, reaching more than 3.6°F (2°C) above the 1978–2006 average and raising the question of whether or not the unusually low sea-ice coverage and warm land temperatures were related. To investigate the question, David and colleagues generated climate change simulations with the Community Climate System Model.

The image below shows simulated autumn temperature trends during periods of rapid sea-ice loss, which can last for 5 to 10 years. The accelerated warming signal reaches nearly 1,000 miles inland. In contrast, the image at right shows the comparatively milder but still substantial warming rates associated with rising amounts of greenhouse gases in the atmosphere and the moderate sea-ice retreat that is expected during the 21st century. Most other parts of the globe still experience warming, but at a lower rate of less than 0.9°F (0.5°C) per decade. (Image by Steve Deyo, COMET).

For more about the study, visit www.ucar.edu/news/releases/2008/permafrost.jsp.


In this issue...

Measuring the Arctic's haze and smoke

NCAR names three new senior scientists

UCAR readies new financial management tools

Bluefire burns hot - with less energy

Researchers study monsoon in Taiwan

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