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"In terms of urban heat, I'm not a big green roof fan," says Cornell University urban-forestry researcher Thomas Whitlow. "If you had green roofs on 80 percent of the feasible buildings in Manhattan, you could conceivably lower air temperatures by half a degree. A half a degree doesn't make a diddlysquat worth of a difference."
And we're asking a lot of trees too, Whitlow argues. "That tree you plant in front of a 50-story building, that huge thermal mass, what the hell is it going to do? The tree is going to do zilch," he says. For what it's worth, trees in skyscraper canyons should be tall and narrow to encourage convection of hot air away from the surface and avoid trapping exhaust from vehicles.
Not that architects are giving up on cooling buildings with plants. They're designing green terraces, ivy-shaded windows, and "living walls." These new methods might shave some degrees from towers, but nothing would be as effective as simply building shorter ones, Whitlow notes. "But at some point you've no longer got a city," and we're back to environmentally unsustainable sprawl.
Green roofs (and roofs covered with white paint) are excellent means of cooling top-floor apartments, and shade trees planted close to buildings can cool ground floor rooms. That will produce measurably lower demand on the electricity grid, as demonstrated by Philadelphia's "Cool Homes" project.
"You also have to be sober about how you extrapolate the benefits of one tree. Two trees don't necessarily give you double the effect of one tree. A thousand trees might give you two hundred times the effect of one tree in terms of energy balance with the hardscape," Whitlow estimates.
But he doesn't deny the powerfully beneficial presence of parks, with their evaporative colling from leaves and water bodies, and their distance from those overheated skyscrapers.
"I've been out where it's in excess of 100 degrees on Columbus Avenue, but in Central Park, at the castle, temperatures are 20 degrees cooler," he says. "Certifiably, inside the park, things are a whole lot better."
These cool and beautiful oases draw people out from their steamy apartments, lowering air-conditioning demand and combating the social isolation we know is a killer. On hot nights in generations past, people slept in parks and on docks even more often than on their fire escapes. Yet before we rhapsodize about parks too much, we must acknowledge that the lowly and suspect air conditioner has probably saved more lives than a radically green urban makeover could. Much of the solution lies in what's at the other end of the power line.
Or in what's coming down the pike, whether it's dress suits made with the same nanofibers that keep athletes cool or gizmos straight out of Star Wars.
Scientists at Stanford University, with the backing of the Pentagon's Defense Advanced Projects Agency, have created a cooling device that boosts athletic performance as dramatically as steroids. Just slip your hand into the RTX glovethat's short for Rapid Thermal Exchangeand let the technology do the rest. Designed to help people with cystic fibrosis, the RTX creates a mild vacuum around your hand and a special cone in which ice water circulates. Grab ahold, and your overall body temperature will start falling. Like other hairless skin regions, the human palm is densely populated with heat-shedding structures called venous plexuses and arteriovenous anastomoses.
Call it a new way to get a grip on local warming.