Cool roofs in warming cities? They may come at a cost, study finds.

As cities confront global warming, how they cool their roofs – reflecting sunlight or planting gardens – could add to winter heating costs or even decrease rainfall in some regions, the study finds.

"Cool" roofs and roofs covered with vegetation can be effective tools for keeping people who live in America's broad, heat-generating urban corridors cooler as rising greenhouse-gas emissions warm Earth's climate.

But this cooling effect carries trade-offs, including the potential for less rain in some regions and lower annual energy savings, according to a new study. How these trade-offs play out depends on a region's population growth, geography, and current climate, as well as the mix of roofing approaches used.

The study highlights the unintended consequences of attempts to adapt to global warming. It also underscores the point that regional and local climate also is affected by human decisions that alter land use and the amount of sunlight absorbed and returned to the atmosphere as heat.

As burgeoning US "megapolitan" areas – such as the Boston-Washington corridor, South Florida's Gold Coast, or the urban-suburban expanse between San Diego and Los Angeles – grapple with global warming, plans to adapt to a warmer world have become as important as plans to reduce a community's carbon emissions.

Communities are exploring the adoption of both cool and green roofs to deal with a warming climate. Cool roofs are made of materials or sport colors that reflect much of the sunlight they receive back into space. Green roofs incorporate vegetation to keep a building cool and provide moisture to the atmosphere that cools their surroundings.

But even a hypothetically perfect "green" city or megapolitan area won't exist in isolation from its wider environment, notes Matei Georgescu, an assistant professor at Arizona State University in Tempe and a senior scientist at the university's Global Institute of Sustainability.

Defining the ideal green city at this point is difficult "because we're still peeling layers off the onion" of trade-offs, says Dr. Georgescu, who is the lead author of the new study, which appears in the current issue of the Proceedings of the National Academy of Sciences.

At issue is the urban heat-island effect, long recognized as raising temperatures in urban areas above those of the surrounding countryside. The largest impact comes at night, when sunlight absorbed during the day rises from dark pavement and roof materials, and combines with heat generated by the production and use of energy.

The impact of urban heat on rising global average temperatures is tiny, many researchers find. In a study published two years ago, for example, researchers at Stanford University estimated that globally, the urban heat-island effect represents between 2 and 4 percent of the warming that rising greenhouse gases have driven.

But locally, it can be significant.

In the new study, Georgescu and colleagues from the US Environmental Protection Agency – which has an active information program on the benefits of cool roofs, benefits that include a federal tax credit for installing them – explore the effect that 100 percent adoption of cool and plant-covered roofs, or some combination of the two, would have on the future of urban warming.

The team examined the outcomes based on two projections of urban expansion through 2100, which are keyed to projections of population growth by then. One scenario is based on modest population growth to some 380 million people from today's 315 million. The other assumes the population will reach a business-as-usual 690 million.

With the larger population, the urban heat-island effect would add an average of 1 to 2 degrees Celsius (1.8 to 3.6 degrees Fahrenheit) to summertime temperatures across all major urban regions in the US by the last two decades of the century, compared with the average for 1990 to 2010. This would come on top of any general warming attributable to climate change. For some metropolitan areas, such as Chicago and Detroit, the urban heat-island effect can add up to 3 degrees C.

If population trends followed the lower trajectory, urban heat-island warming would fall by one-third to half of the warming the higher population and expansion of urban areas would trigger.

When the team applied cool, plant covered, or a hypothetical mix of the two to all the roofs in their six megapolitan areas, the additional heating from the urban heat-island effect vanished – although the extent of cooling depended on roof type and whether the general climate in the region was arid or humid.

For instance, cool or hybrid roofs were consistently more effective than green roofs in cooling summertime temperatures – in some reasons by wide margins.

Game, set, match?

Not quite.

If all the roofs were designed as cool roofs, they would continue to reflect sunlight back into space during winter, cooling the air more than would be the case with plant-covered or typical roof covers. That could increase the demand for winter heating, offsetting energy savings achieved in the summer. In the mid-Atlantic states, this cooling could amount to an additional 1.5 degrees C. over winter temperatures typical of the 1990-2010 period.

Roofs dominated by plants, rather than cool or mixed-type roofs, showed no additional winter cooling at all.

Moreover, the shift in roof types could have a marked effect on rainfall.

In a study published in 2012, Georgescu and colleagues from Arizona State looked at the potential impact on rain from the summer monsoons of the extensive use of cool roofs throughout Arizona's expanding Sun Corridor, which runs from Tucson through the Phoenix metropolitan area. By 2050, the wholesale adoption of cool roofs could reduce monsoon rainfall by up to 4 percent, on top of a 12 percent reduction from population growth and an increased expanse of asphalt and concrete surfaces. Urban expansion replaces vegetation, which even in a desert can return moisture to the atmosphere through evapotranspiration.

Essentially, urban sprawl removed locally available atmospheric moisture, which is added to the moisture the Southwest monsoon winds bring in from the Pacific Ocean and Gulf of California. Meanwhile, cooler temperatures over the Sun Corridor reduced the intensity of updrafts over the region, retarding the formation of the thunderheads that bring the badly needed rain.

For an arid region like the Southwest, which already is projected to dry out further with global warming, the additional combined loss from growth and reflective roofs is significant.

Under the scenario with the largest population growth, the new study projects a 1 millimeter-per-day decline in summertime rain averages across all of the megapolitan areas the team included. The biggest losses occurred along the East Coast, from Florida through the Northeast. The study indicated that summertime rainfall would ease by 2 to 4 millimeters a day.

Georgescu and colleagues set their findings in a broader context of global population growth and the urban expansion likely to result from it. Other studies have explored the cool-roof issue globally.

The Stanford study, conducted by researchers Mark Jacobson and John Ten Hoeve, looked at 100 percent adoption of cool roofs globally. They found that while this adaptation approach cooled population centers by an average of about 0.02 degrees C., global average temperatures rose by 0.07 degrees C.

Among the reasons: cooling inhibited cloud formation, particularly in the Southern Hemisphere, and the clouds that did form were thinner. And black soot, an atmospheric warming agent, would absorb reflected light from beneath, as well as from sunlight shining from above – and reradiate both as heat.