Dev Millstein and Surabi Menon investigated, on a continental scale, the effects on climate of increasing surface reflectivity due to adoption of cool roofs and pavements in urban areas. They also looked at the effect of decreasing surface reflectivity by installing large solar arrays in desert areas.

The model the pair used ran over a relatively long period (12 years), involves a large but finely resolved spatial scale (the US), and included detailed representations of the physics of surface–atmosphere interactions. "This framework allows us to investigate the potential for surface reflectivity changes to cause feedback effects by changing weather patterns," Millstein told environmentalresearchweb. "We did find the feedback effect to be important."

The researchers found different regions of the country responded differently to the installation of cool roofs. For example, cities in Texas were less sensitive to reflective roofs – experiencing less summer cooling – than similar cities in California or the north east of the US. "Even when including feedback effects of cool roofs, we still found that installation of cool roofs could lead to significant increases in radiation reflected back to space," said Millstein.

With adoption of cool roofs and pavements, domain-wide annual average outgoing radiation increased by 0.16±0.03 W m–2 and afternoon summertime temperature in urban locations was reduced by 0.11 to 0.53 °C, although some urban areas showed no statistically significant temperature changes. In response to increased urban albedo (surface reflectivity), some rural locations showed summer afternoon temperature increases of up to +0.27 °C and these regions were correlated with less cloud cover and lower precipitation.

Millstein admits that the scenario that he and Menon modelled was extreme – it was based on the presumption that new regulation was introduced that stipulated all new roofs in every city in the US had to use cool roof technology. "We calculated that, if cool roofs were installed across the US, the emissions offset by the increase in outgoing radiation is 3.3±0.5 Gigatonnes of carbon dioxide," he said.

The researchers also modelled the effect of installing large photovoltaic arrays (1 Terawatt peak capacity) in the southern Californian desert. This reduces the reflectivity of the surface and will have the opposite effect to cool roofs. "In this case we could track consistent feedback effects over the scale of a few hundred kilometres," said Millstein. "But changes to climate at the continental scale were obscured by year-to-year variability."

The researchers published their research in Environmental Research Letters (ERL).