Select your localized edition:

Close ×

More Ways to Connect

Discover one of our 28 local entrepreneurial communities »

Be the first to know as we launch in new countries and markets around the globe.

Interested in bringing MIT Technology Review to your local market?

MIT Technology ReviewMIT Technology Review - logo


Unsupported browser: Your browser does not meet modern web standards. See how it scores »

{ action.text }

Getting There

We’ve shown that cool communities measures in Los Angeles could reduce air-conditioning bills by $175 million per year and alleviate $360 million per year of smog-related expenses. How will we get to this happy point?

Part of the solution will be up to the roofing industry. We are working with roofing manufacturers to develop a new generation of cooler shingles and tiles. They will most likely contain a coating of titanium dioxide (TiO2) to provide an attractive light color. Because white surfaces are easily discolored by fungus, these shingles will also need to have a fungicide coating. When fabricated with a smooth surface, these shingles will self-wash and thus stay cool for their entire service lives. The increase in albedo of such shingles can be more than the 35 percentage points assumed in our simulations. (The coolness of a material cannot always be discerned from its apparent lightness. In tests, we have found that “cool” terra-cotta tiles run 6°F cooler than “white” asphalt-fiberglass shingles. The reason: half the heat from the sun arrives as invisible radiation in the near-infrared part of the spectrum, to which architects and roofers have paid little attention. Fortunately, TiO2 reflects well in the infrared. So one can make a cool pastel shingle by adding a little light color to a modern cool white TiO2 shingle.)

Another contributor to the heat island effect is pavement. Asphalt pavement is, by volume, about seven-eighths rock aggregate, cemented together with one-eighth sticky black asphalt. Over a few months, asphalt wears close to the color of the aggregate. By choosing lighter aggregate, we estimate that we can triple the solar reflectivity of worn asphalt pavement. Unfortunately, although there are thousands of pages of specification of the properties of aggregate from quarries and rivers, nobody has thought to list its color. Thus no one knows if there will be a significant cost premium for lighter aggregate.

Even without such knowledge, we should at least urge asphalt resurfacing contractors to discontinue the now common practice of “topping off” their work with black asphalt and carbon black. Better yet would be to switch the binder from asphalt to lighter-colored Portland cement. Although its first cost is higher than asphalt, cement is stronger and lasts longer, so its life-cycle cost is lower. Iowa already requires cement roads as a long-term cost-savings policy.

Local utility companies also can play a big role. Southern California Edison (SCE), which serves two-thirds of the Los Angeles basin, could offer incentives for its customers to plant shade trees and install cool roofs, thus reducing air-conditioning needs. Thanks to California’s efficiency-minded utility regulations, SCE can reap a substantial profit from this lessening of demand. A utility implementing a conservation program that saves its customers money is permitted to raise its rates slightly, so that the savings is shared with the stockholders. Of the roughly $100 million a year that white roofs and shade trees could save in air-conditioning expenses in SCE’s territory, for example, $70 million might go to the utility’s customers and $30 million to its stockholders.

Because cool communities lower smog, some of the impetus should come from the agencies responsible for managing air pollution. In Los Angeles, that means the South Coast Air Quality Management District, or SCAQMD. Fortunately, SCAQMD took a prudent step in 1994 by capping total NOx emissions from the region’s industries, and lowering the cap 8 percent each year. To give businesses flexibility in reducing emissions, SCAQMD started the Regional Clean Air Incentive Market (RECLAIM). Under RECLAIM, companies in compliance with the cap can sell their excess emission-reduction credits to companies that are out of compliance. As in any market, the price of NOx credits is determined by supply and demand.

When RECLAIM started in 1994, it traded only NOx. But the program is now judged a success and is being extended to the other main smog ingredient: VOCs. RECLAIM is also considering giving “cooling credits” for measures that slow the formation of smog from NOx and VOCs. Anything that lowers the temperature of the air would count. The Environmental Protection Administration has urged RECLAIM to adopt these “cooling credits.” If this happens, the Los Angeles roofing contractors association could sell cooling credits on behalf of its members, who would in turn promote cooler roofs and could afford to offer rebates to their customers. An asphalt pavement association could do the same for roads and parking lots. Landscaping contractors could sell credits for trees and other vegetation. Regional air pollution markets like RECLAIM are spreading beyond Los Angeles. Chicago is close to developing one, and a consortium of northeastern states is aiming for 1999. A number of other states have such programs on the drawing boards.

The federal government has a role to play as well. Thus we at the Department of Energy, working with the Environmental Protection Agency, will introduce two sorts of labels. One will be a quantitative “solar reflectance index” that should appear on all roofing material. This will resemble the familiar yellow EnergyGuide labels on all appliances. The other will be called Energy Star. It will adorn only the coolest one-third of the products on the market, and will resemble Energy Star labels already on computers and other efficient products. Over time, better-informed consumers may come to regard hot surfaces as wasteful and ugly thermal polluters.

Los Angeles, or any other large city, cannot be cooled in a day. In fact, the 5° F lowering of the heat-island temperature by the steps we have outlined would take about 15 years. That’s because it is economical to install the cooler surfaces only when normal refurbishing is due, and the lifetime of roofs and pavements are on this time scale. Also, trees take about this time to grow fully.

But it will take a lot longer–forever?–unless businesses and policymakers give cool roofs and tree planting the high priority they deserve. California’s clean-air strategy makes use of two tactics that promise to yield about the same benefits as cool communities. One is reformulated, cleaner-burning gasoline, which was introduced last summer. The new gasoline reduces smog precursors by about 15 percent. California’s other major tactic is to introduce electric cars on a large scale. According to present plans, electric cars are to start at 2 percent of sales in some as-yet-undefined year, and quickly rise to 10 percent of sales. This transition would reduce smog several percent. But as with cool surfaces and trees, there would be a 10- to 15-year delay before the car stock turns over.

The air-pollution benefits of reformulated gasoline and electric cars can be complemented by the planting of trees and installing of lighter-color roofs and roads. These cool-communities strategies not only save energy and clean the air but also yield a more hospitable local climate.

1 comment. Share your thoughts »

Credit: S. Stetson, Global Environmental Management

Tagged: Energy

Reprints and Permissions | Send feedback to the editor

From the Archives


Introducing MIT Technology Review Insider.

Already a Magazine subscriber?

You're automatically an Insider. It's easy to activate or upgrade your account.

Activate Your Account

Become an Insider

It's the new way to subscribe. Get even more of the tech news, research, and discoveries you crave.

Sign Up

Learn More

Find out why MIT Technology Review Insider is for you and explore your options.

Show Me