Cool Roofs: ‘Geoengineering’ I Can Support

by Philip Duffy

Geoengineering — large-scale engineering measures intended to offset the climate altering effects of increased greenhouse gases — is a potential “solution” to climate change that we experts love to hate. But new initiatives in New York and other cities to moderate the local climate in urban environments are smart, safe, and could reduce local climate-change impacts while saving money. Unlike the planetary-scale geoengineering proposals that may create more problems than they solve, I can get behind such city-scale efforts.

Global climate change occurs when the exchange of energy between the planet and space is altered. Greenhouse gases, for example, tend to reduce outgoing but not incoming energy, thus causing warming. In principle, some of the warming from greenhouse gases such as carbon dioxide (CO2) could be avoided by measures that increase the amount of sunlight reflected into space. Naturally, about 30 percent of incoming sunlight is reflected back into space by either the land surface or atmosphere (especially clouds). Increasing this to about 32 percent would be enough to offset most of the warming caused by doubling the atmospheric concentration of CO2.

A diagram of various planetary-scale geoengineering proposals (by Kathleen Smith/Lawrence Livermore National Laboratory)

However, this could have significant negative consequences, such as reductions in worldwide precipitation. And there would still be harmful consequences of CO2 itself, such as ocean acidification, even if warming were reduced. Beyond these important generic problems that apply no matter how additional sunlight were reflected into space, the most commonly-discussed method for increasing reflection — adding small reflective particles to the atmosphere — would create other problems, such as slowing the recovery of the ozone hole and increasing acid rain.

And those are only the problems we know about. On a scientific level, the biggest worry with these schemes is probably “unintended consequences” — who knows what else might happen? Instinctively, most scientists are deeply reluctant to try to fix one environmental experiment by trying another. The political/legal/ethical problems with global-scale climate modification seem intractable. Who decides that this is a good idea? (There would certainly be widespread dissent). Who pays for it? Who is responsible for maintaining a system into perpetuity? Do people who are negatively affected get compensation?

If we can’t agree internationally on reducing greenhouse gas emissions, it’s hard to imagine reaching agreement on something like this.

But urban-scale climate modification is quite different. Roughly 50 percent of the global population lives in cities, and this percentage is growing. Many cities are warmer than the surrounding environment, due to the well-known “urban heat island effect.” (However, the heat island effect is not why global temperature measurements show warming). Heat islands or not, cities are warming along with the rest of the planet, and some important societal impacts of climate change — in particular heat stress mortality and worsening of air quality—are concentrated in cities. Simple measures that reduce urban temperatures can minimize these impacts, make cities more livable, and also save money.

Cities like Atlanta, shown here, are heat islands whose temperatures are significantly higher than in surrounding regions. (NASA graphic)

The most widely discussed example is “cool roofs” or “white roofs”—reflective rooftop coatings that bounce more sunlight back into space. These measures could reduce global temperatures for the same reason that adding reflective particles to the atmosphere would: more sunlight is reflected. In practice, however, any impact on global temperatures would be negligible; there simply isn’t enough rooftop area to make a worldwide difference.

Locally however, the effect can be significant. Reflective coatings can dramatically reduce the very high temperatures often found right above typical dark urban rooftops. In warm climates, this can have important benefits: less need for air-conditioning, which saves money and further reduces the input of heat into an already stifling urban environment, as well as fewer greenhouse gas emissions. (In cold climates, the increased need for winter heating can outweigh summer savings on air conditioning, so read the prospectus before investing!)

Cool roofs are catching fire, so to speak: New York City recently announced a “cool roofs initiative”, and Philadelphia is also taking steps to reduce its urban heat island effect. California is encouraging cool roofs as well. And if you think this is idea is only for “tree-huggers” in New York and Berkeley – think again. According to the New York Times, Wal-Mart, the nation’s largest retailer, has cool roofs on 75% of its stores. Wal-Mart is a lot hipper environmentally than you might think, by the way.

Whitening roofs is not the only way to moderate the urban climate. Asphalt pavement, which is normally a very effective absorber of sunlight, can be made lighter in color and much cooler. Permeable pavements, which allow water to pass through them, can increase evaporation and result in significant cooling (and less tendency for vehicles to skid). They also improve drainage, reducing the likelihood of urban flash floods. Similarly, planting trees and other vegetation increases evaporation and results in lower daytime temperatures, and can provide shade and other benefits.

Unlike global-scale climate-modification schemes, these modest urban measures seem far more immune to serious negative unintended consequences. (And if I am wrong about this, they’re easy to reverse.) Furthermore, they generate no international political controversy. Individual cities — even individual homeowners —can choose to adopt these measures, and the effects on their neighbors should only be positive and non-disruptive.

If all this seems too good to be true, it isn’t! Although urban-scale climate modification measures won’t solve the climate-change problem on their own, even in cities where they’re applied heavily, they should help, while hurting nothing and saving money along the way. We need more ideas like that.