10 October 2016

Revisiting the Climate Impacts of Cool Roofs around the Globe using an Earth System Model

Atmospheric aerosols offset the climate cooling effects of bright (cool) roofs.

Science

Solar reflective bright roofs absorb less sunlight than traditional dark roofs, reducing solar heat gain, and decreasing the amount of heat transferred to the atmosphere. This study investigated the climate impact of bright roofs at urban, continental, and global scales, using an Earth System Model (CESM) that includes a sophisticated treatment of the land surface type in urban areas and aerosol processes.

Impact

Widespread adoption of bright roofs could reduce temperatures in urban areas, partially mitigating the urban heat island effect, and contribute to reversing the local impacts of global climate change. The impacts of bright roofs on the global climate remain debated by past research and are uncertain. This study indicates that adoption of bright roofs around the globe would lead to statistically insignificant reductions in the global mean air temperature.

Summary

The researchers, including a Department of Energy scientist at Pacific Northwest National Laboratory, modified the land surface albedo in the Community Earth System Model (CESM) to represent the change induced by implementing bright roofs. The prognostic aerosol treatment in CESM allowed the researchers to consider the dampening effect of aerosols more realistically. To estimate the uncertainty of the climate impact, they performed long-term ensemble slab ocean model simulations. They found that the global adoption of bright roofs in urban areas would reduce urban heat islands everywhere, with an annual- and global-mean decrease from 1.6 to 1.2 K. Those decreases are statistically significant, except for some areas in Africa and Mexico where urban fraction is low, and some high-latitude areas during wintertime. Analysis of the surface and top of atmosphere energy budget in urban regions at continental-scale shows bright roofs cause increases in solar radiation leaving the Earth−atmosphere system in most regions around the globe, though the presence of aerosols and clouds are found to partially offset increases in upward radiation. Aerosols dampen bright roof-induced increases in upward solar radiation, ranging from 4% in the United States to 18% in more polluted China. Adoption of cool roofs also causes statistically significant reductions in surface air temperatures in urbanized regions of China and the United States. India and Europe show statistically insignificant changes in that regard. The research suggests that while bright roofs are an effective tool for reducing building energy use in hot climates, reducing urban heat islands, and regional air temperatures, their influence on the global climate is likely negligible.

Contact
Phil Rasch
Pacific Northwest National Laboratory (PNNL)
Funding
Publications
Zhang, J, K Zhang, J Liu, and G Ban-Weiss.  2016.  "Revisiting the climate impacts of cool roofs around the globe using an Earth system model."  Environmental Research Letters 11(8), doi:10.1088/1748-9326/11/8/084014.