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Capturing heat islands in climate models

Two satellite views of Atlanta that show urban heat island effect.

This pair of satellite images provides two views of Atlanta, with the city’s urban core at the center of both images. The top image is a photo-like view of the area, while the bottom image is a land surface temperature map with cooler temperatures in yellow and hotter temperatures in red. Because vegetation cools Earth’s surface through evaporation, the most densely vegetated areas (darkest green in top image) are the coolest areas (palest yellow in bottom image). (Images courtesy NASA Earth Observatory.)

A team of scientists led by NCAR’s Keith Oleson has incorporated urban areas into a global climate model. The development is important because most models used for predicting future climate change do not account for the urban “heat island” effect. The study will be published in the International Journal of Climatology.

Oleson and colleagues used the Community Climate System Model, an NCAR-based model that uses trillions of calculations to simulate the chemical and physical processes that drive Earth’s climate. After inserting a parameterization for urban surfaces into the CCSM’s land surface component, the researchers ran the model from present day to 2100 under the Intergovernmental Panel on Climate Change A2 emissions scenario, which assumes that global fossil fuel emissions will continue to rise at high levels over the coming century.

Results from the modeling experiment show that present-day annual mean urban air temperatures are up to 4°C (7.2°F) warmer than temperatures for surrounding rural areas, a finding that is important for verifying the model’s accuracy since scientists already have observational evidence that urban areas are warmer than surrounding rural areas.

The study found that both urban and rural areas warm substantially by the end of this century as emissions rise, with rural areas warming slightly more than urban—resulting in a decrease in the urban-to-rural contrast. In addition, nighttime urban warming is much greater than daytime urban warming, resulting in a reduced diurnal range in temperature compared to rural areas.

“This study demonstrates that climate models need to begin to account for urban surfaces to more realistically evaluate the impact of climate change on people in the environments where they live,” Oleson says.

He cautions that the study does not account for urban growth or changes in urban form or function; nor does it account for changes in the atmosphere other than increased carbon dioxide concentrations, such as aerosols or other kinds of pollution.

Oleson, Keith, 2012: Contrasts between Urban and Rural Climate in CCSM4 CMIP5 Climate Change Scenarios. J. Climate, 25, 1390–1412. doi: http://dx.doi.org/10.1175/JCLI-D-11-00098.1

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