HAIDER Taha has been hard at work. He鈥檚 slapped white paint and light-coloured tiles on millions of rooftops all over Los Angeles and the sprawling communities surrounding it. He鈥檚 replaced thousands of miles of black, asphalt roads with surfaces the colour of weathered concrete. He鈥檚 sneaked into the gardens of millions of private homes and planted trees to shade their roofs. Taha鈥檚 heroic efforts have cooled areas of LA by as much as 4 掳C, slashing air conditioning bills and ridding the region of a sizeable chunk of its air pollution. Not bad for a couple of years鈥� work.
Sadly for southern Californians, Taha鈥檚 city is not the real LA, where the smog is still legendary and summers get hotter year by year. Instead, the cooler version comes from a giant computer simulation started in 1993 by Taha and colleagues at the Lawrence Berkeley Laboratory in Berkeley, California, and the University of California, Los Angeles. Taha鈥檚 trees are fake, and the new roofing is imaginary, digitally daubed onto houses and factories. But one day, says Taha, real cities could follow suit.
To that end, scientists around the US are figuring out cost-effective strategies for combating the so-called 鈥渦rban heat island effect鈥� 鈥� the tendency of cities to get warmer as buildings and roads engulf the countryside. They鈥檙e testing real houses with trees for shade and coatings that reflect sunlight; they鈥檙e analysing new roofing materials and pinpointing the species of trees that would help or hinder their efforts. 鈥淭he urban heat island is a man-made effect,鈥� says Hashem Akbari, leader of LBL鈥檚 Urban Heat Island Project. 鈥淎nd if man is doing it, then man can undo it.鈥�
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Take, for instance, what development has done to LA. In the mid-1800s, before this desert area was built up, summer highs averaged 39 掳C. Then, as river waters were diverted to the basin and vast orchards of fruit trees were planted, the temperatures fell to a nadir of 36 掳C by the 1930s. Thereafter, as orange trees made way for houses and roads, temperatures climbed back to 39 掳C. And they鈥檙e still rising, as building continues. Nor is this effect unique to LA: many cities, including New York and Mexico City, are up to 3 掳C hotter on a summer day than the surrounding countryside.
Akbari and Arthur Rosenfeld, former director of the Center for Building Science at LBL, were the first to convince the research community of the science behind these temperature changes, in the mid-1980s. Black asphalt roads and roofing absorb more sunlight than open fields, sucking up the Sun鈥檚 heat, they said. Trees and vegetation, once cleared, can no longer cool the air via the water that evaporates from their leaves. Reversing this trend is part of President Bill Clinton鈥檚 Climate Change Action Plan announced in 1993, which Rosenfeld is helping to develop as a senior advisor at the US Department of Energy.
Real cool
How do you cool a city like LA? Trees that shade houses keep the sunlight away and the houses cooler. White paint and other reflective coatings deflect much more visible and infrared sunlight than common roofing materials such as black asphalt shingles, so houses gobble up less energy on air-conditioning. Trees, meanwhile, are continuously releasing water from the pores in their leaves, which cools the air as it evaporates. The end result? Hopefully, a cooler city.
And a cleaner city, too. The region鈥檚 factories, refineries, power plants and 8 million vehicles spew out a cocktail of chemicals, including hydrocarbons and nitrogen oxides. These react with sunlight to form a smorgasbord of pollutants that blankets the south coast air basin (LA and the cities surrounding it) 鈥� the most dangerous of these, from a health point of view, is ozone. Chemicals react faster when heated, so the higher the temperature, the faster the smog forms. 鈥淭here鈥檚 never been a smog alert when the average daily temperature was below 21 掳C,鈥� Rosenfeld points out.
Not only that, but the array of organic chemicals evaporating from gas stations and cars, paints and coatings on buildings, fuel and chemical storage tanks, wafts up in even greater quantities when the city is hot, adding to the smog.
Just how much better would things be with decent, reflective roofing and roads and well-placed trees? To figure that out, Taha and colleagues pored over satellite images of the south coast air basin to determine where the trees were and which parts of the region were residential, industrial or undeveloped. And they flew over the basin to estimate the reflectivity of different locations.
They then divided a map of the region into 2600 portions, and estimated how much added vegetation and increased reflectivity 鈥� or 鈥渁lbedo鈥� 鈥� each portion could take. When they simulated the effects of adding moderate amounts of trees and white roofs (up to 15 per cent of all changes thought to be possible), the average summer temperature in the basin dropped by 4 掳C, although the exact change in temperature varied from place to place. Estimated summer energy savings amounted to between $100 000 and $200 000 per hour.
Next, Taha fed his data into the Urban Airshed Model, which is widely used by air quality officials. This calculated the effects of these altered temperatures on the region鈥檚 smog, based on factors such as wind patterns, moisture, temperature, and where the region鈥檚 pollutants are released. This time, he found that a moderate change in albedo reduced the smog 鈥� getting rid, on average, of 5 per cent of the ozone that exceeds the region鈥檚 air quality standards. Planting a moderate number of trees reduced the smog by 3 per cent.
If you add together the effect of the trees and reflective materials, he says, the figure comes out at 10 per cent. 鈥淚t鈥檚 a huge effect,鈥� says Rosenfeld. 鈥淚t鈥檚 the equivalent of getting about three million cars off the roads.鈥� Taha notes, though, that these numbers are far from uniform across the basin, mainly due to differing wind patterns. His print-out of the results looks like a haphazard chess board of different smog levels, with some unlucky places having worse smog than before.
But simulations have their limitations. 鈥淢odels are models 鈥� they only do what you ask them to do,鈥� says Taha. 鈥淎nd the types of studies I鈥檓 doing cannot be validated 鈥� no one has painted LA white for me to double-check my results.鈥� And even if his findings are right on target, implementing the 鈥渃ool cities鈥� scheme has its complications. By planting the wrong kinds of trees, cities may do the equivalent of adding polluting cars to the area. Many trees emit highly reactive hydrocarbons, terpenes and isoprene, that react with nitrogen oxides and contribute to the smog. Today trees may give off as much as 10 per cent of the polluting hydrocarbons in the LA region.
And so one strand of the project is to figure out which trees are naughty, and which nice, from the point of view of smog. This task has fallen to Arthur Winer, director of the environmental science and engineering program at UCLA. Winer has studied tree emissions for 15 years and is collaborating with the LBL group. Spreading the word about low-emitting and high-emitting trees is tricky, he says, without being perceived as a tree-hater.
鈥淲hen I first started to talk about this work to urban forestry people, at times 鈥� looking out at the audience 鈥� I thought they were going to start throwing tomatoes, or take a big hook and haul me off stage,鈥� he says. 鈥淧eople love trees. They have their favourite trees and they get upset to be told that an oak or a weeping willow or a eucalyptus is a 鈥榟igh-emitting tree鈥�.鈥�
But the fact remains, he says, that the highest emitters, including weeping willow, release ten thousand times more terpenes and isoprene than the lowest emitters, such as ash. And while nobody鈥檚 suggesting that the thousands of trees already standing should be chopped up for firewood, planting high emitters en masse will not help improve air pollution. Taha鈥檚 鈥済ood鈥� simulations were performed with low-emitting trees. But when he used moderate emitters in his model, ozone concentrations in certain areas increased by as much as 25 per cent.
Planting spree
Already, large-scale planting schemes are taking root in US cities with a view to saving energy as well as creating a more leafy environment. California鈥檚 Sacramento Municipal Utility District, for instance, is well on the way to its goal of planting 500 000 trees by the year 2000 鈥� all of them, SMUD maintains, low-emitting species. LA has similar schemes on the drawing board.
And so each of the 375 species of trees and shrubs that are commonly planted in the LA basin must be rated, which is no easy task. For each measurement, the scientist must encase a branch in a transparent, gas-tight bag, take samples, and run the sample in a gas chromatograph to quantify the terpenes or isoprene. The leaves must be stripped and dried, to measure leaf mass. Variation from day to day and branch by branch is great, due to a host of factors, such as how much sunlight falls on any particular branch. No wonder, then, that only 150 species 鈥� measured by Winer and others 鈥� have been tallied. For the other species, Winer has classified them in high, low, or medium groups based on the emission properties of their nearest measured relatives.
Meanwhile, other researchers are doing what they can to test all these theories in the field. For instance, Akbari鈥檚 team has measured air-conditioning costs before and after surrounding houses in Sacramento, California鈥檚 state capital, with 6-metre shade trees loaded onto flat-bed trucks, and coating the roofs with a reflective, white, plastic polymer. The researchers found that the air-conditioning in the houses used 40 per cent less energy, with the reflective roofing causing 50 to 60 per cent of the saving, on average.
Experiments in Florida gave similar figures. Jack Parker, professor of chemistry and environmental science at Florida International University in Miami, planted vegetation round a childcare centre, then monitored the costs of air conditioning as trees and shrubs matured over the years. The centre reduced its air conditioning costs by 58 per cent at the height of summer. Danny Parker, a senior research scientist at the Florida Solar Energy Center in Cape Canaveral, monitored houses all over Florida before and after coating their roofs with bright white polymers. He saw average air-conditioning savings of 19 per cent, ranging from 3 to 43 per cent depending on insulation and roof type.
Measuring the direct effects on houses is one thing. But it鈥檚 much trickier to test whether the vegetation and reflective surfaces will alter temperatures of entire neighbourhoods. That鈥檚 because the effects may be overshadowed by day-to-day and place-by-place changes in the climate.
鈥淵ou鈥檙e moving towards 鈥榳eather鈥� when you鈥檙e trying to measure these mesoclimates,鈥� says Danny Parker. 鈥淭hese are very inscrutable systems.鈥� Nonetheless, researchers are gamely trying to gather data to see whether more trees and white roofing produce measurably cooler air temperatures. The LBL group, meanwhile, has shown that local temperature differences can definitely be detected in the more uniform desert climate. They have monitored the air above New Mexico鈥檚 White Sands National Monument, a vast stretch of highly reflective sand, and compared it with that of the surrounding desert. They found that the morning air, was 3 掳C cooler over the reflective sand.
Growing interest
But neighbourhood effects or no, the house-by-house energy savings seem clear. Even the utility companies 鈥� who say they want their customers to save energy so they don鈥檛 have to build new power plants 鈥� are getting interested in roofing and trees. Air quality officials are showing interest too. 鈥淚t鈥檚 an exciting idea that has to be pursued,鈥� says Bernard Bloom, air resources engineer with the Montgomery County Department of Environmental Protection, in Rockville, Maryland. He is helping to develop plans to reduce pollution in the Washington DC area. 鈥淭aha has shown that maybe we can knock off 5 to 15 parts per billion ozone for each 1 掳C drop in temperature, which would be about 25 per cent of our reduction need in Washington. It鈥檚 very significant.鈥�
But there are still many questions to be answered, Bloom adds. It鈥檚 not clear, for instance, if other cities would behave like LA. Cities that have colder winters will have to make sure that houses don鈥檛 become more chilly in the winter; reflective roofing will absorb less sunlight when it鈥檚 cold outside as well as hot. This might cause energy demand to increase in winter, offsetting energy savings in the summer. If that鈥檚 so, says Bloom, the answer might be to develop 鈥渟mart鈥� roofing materials that reflect sunlight in the summer but switch to absorbing it when temperatures fall past a critical level. And such substances do exist.
It鈥檚 also important, he says, to ensure that reduced summer temperatures don鈥檛 alter air patterns so that pollutants hug closer to the ground, increasing ozone in cities. Taha鈥檚 simulations in LA took such changes into account and showed that pollution plummeted nonetheless. But every city should be modelled in turn, argues Bloom.
Another problem is that muck tends to build up on roofs and roads, and in humid places, like Florida, roofing becomes progressively streaked with algae and fungi. How well will sunlight be reflected when this happens? To get round the fungus problem, Danny Parker is developing paints that use zinc oxide, which acts as a fungicide and algicide, as a partial substitute for titanium dioxide, the conventional white pigment in paint.
And what if people don鈥檛 want bright cities, with gleaming white buildings as far as the eye can see? In fact, cities don鈥檛 have to be white to be cool. 鈥淲e don鈥檛 have to make LA look like Casablanca,鈥� says Rosenfeld (although most LA residents would probably consider this an improvement). While roughly half of sunlight鈥檚 energy is in the visible spectrum, most of the rest is in the infrared. By tinkering with their raw materials roofs can be made to reflect infrared and thus be cool even though they鈥檙e not white. Even a black asphalt shingle, which reflects 5 per cent of the Sun鈥檚 rays, can be made 11 掳C cooler at noon if covered by a coating that reflects in the infrared.
Which just goes to show that you can鈥檛 just look at a substance and know what its reflectivity will be, says Paul Berdahl, a materials scientist at LBL. 鈥淚 think we perhaps know more about the solar reflectance on the surface of Mars than we do about the solar reflectance of the roofs of our buildings,鈥� he says.
His group has found, for instance, that a so-called 鈥渨hite鈥� asphalt shingle which is really asphalt covered with tiny white stones 鈥� is actually fairly hot. With a dark undersurface, it reflects only 30 per cent of sunlight compared with 80 per cent for a good white paint. Two red terracotta tiles that look identical may in fact reflect different amounts of light, due to greater or lesser amounts of magnetite, which absorbs a lot of infrared.
Eliminating these impurities would be one way to make roofing cooler, says Berdahl. Another way would be to alter the size of pigment particles, since substances tend to reflect more light when the particles are of a certain size. Smoother roofing surfaces also reflect sunlight better, since a photon is more likely to be deflected cleanly away if the surface is flat, rather than hilly.
Heat labels
Ultimately, the LBL group would like to see the day when the solar reflectance of all roofing materials is well understood, and when minimum standards for reflectance are built into the California building and material codes. They鈥檙e working on a ratings scale right now, in collaboration with the American Society for Testing Materials. They鈥檝e also been talking to roofing manufacturers, who are interested in devising temperature labels for paints and other materials.
Urban heat island projects are taking off. Public utilities around the country, such as Sacramento鈥檚 SMUD, are beginning to put their money into tree and roofing efforts. Southern California鈥檚 South Coast Air Quality Management District is starting to incorporate the LBL data into air quality policies. By encouraging home owners to use the right materials when they reroof their houses, and by promoting low-cost, volunteer programmes to plant trees, Rosenfeld, Akbari and colleagues think they can make cool city measures highly cost-effective. Roads and roofs must sooner or later be replaced, at which time new reflective coatings can be added at little cost. If implemented nationwide, these could help clean the air and save $10 billion in annual energy costs within 20 years.
鈥淭his has the chance of being the most astoundingly productive single thing that you can do to improve air quality,鈥� says Rosenfeld. 鈥淭he country鈥檚 going to a lot of trouble to try to figure out how to make hybrid cars or clean cars 鈥� and this is a cheaper way to do it. Frankly, in terms of potential, it鈥檚 probably the nicest, most important single piece of environmental science on the market.鈥� (see Diagram)
