While world leaders fret over the 20th century's one-degree Fahrenheit temperature climb, localized high temperatures in large North American cities have risen up to two degrees Fahrenheit each decade since 1950. Global warming could double that rate, warns E. Gregory McPherson, a research forester with the U.S. Department of Agriculture Forest Service, in the book The Ecological City.
The density of our cityscapewhere we live and work stacked floor-upon-floor and commute in packed subways and busesfosters energy efficiencies unheard of elsewhere in America. On a per capita basis, our gasoline consumption matches the national average of 1920 and our electricity usage is half of contemporary San Francisco's. Our per capita greenhouse gas emissions are a third of those seen nationally today.
Yet we roast our relatively virtuous skins on vast acres of concrete between asphalt and glass towers that convert nearly 45 percent of incoming sunlight into sensible heat. Dead-end or poorly oriented canyons interfere with air convection, trapping warm and dirty air. Meanwhile, our physiological vulnerability to heat has grown. Hypertension, diabetes, depression, obesity, and other modern ailmentsand the treatments for themcompromise our ability to flush and sweat, the primary coping mechanisms.
By many measures, heat deaths have actually dropped, thanks to one of the least sustainable contraptions ever invented: the air conditioner. In 1970, 40 percent of NYC households had at least one. Last year, 80 percent did.
But switching on the AC starts a negative feedback loop.
"Anyone who has ever stood in the West 4th Street station in the middle of summer knows it feels like 120 degrees down there, and it's because of the air conditioners blowing out hot air from the subway cars," says Joyce Rosenthal, an urban planning doctoral student and research investigator with the Cool Cities Project at Columbia University.
Now imagine that phenomenon on a citywide scale. An Okayama University study found air conditioners raised ambient temperatures in Tokyo by almost three degrees Fahrenheit. The Heat Island Group at Lawrence Berkeley Laboratory, in California, calculated that the efforts to offset the phenomenon in Los Angeles cost more than $100 million annually. By comparison, New York City is denser and lacks the steady ocean breezes. "I would not be surprised if in New York the number was on the order of $150 million," says lead scientist Hashem Akbari.
Heat is also an issue of local environmental justice. Just look at Hunts Point in the South Bronx, where poverty is the handmaiden of heat death. Hunts Point is chockablock with low-slung warehouses, virtual ovens. Without adequate street trees, asphalt turns to goo. The exhaust of 60,000 weekly truck trips, power plants (to keep New York's air conditioners chugging), and sewage treatment plants is transmuted by heat into thick smog. The children there, mostly black and Latino, are afflicted with asthma so severe that they require hospitalization for it at a rate four times the national average.
photo: Ira McCrudden
Heat preys on the most vulnerable among us. Nearly half of all heat victims live alone. People older than 65 account for more than half of heat deaths. The city health department reports that 28 percent of the 2006 victims had known cognitive or psychiatric disorders. Pregnant women are also vulnerable, and lab mice have shown extreme deformities in fetuses where the mother experienced a six-degree body temperature rise for just 30 minutes. Children, like the elderly, overheat faster.
BETTER SWEAT IT OUT
Lessons gleaned from nature's billions of years of evolutionand innovations from our preindustrial pastmight help us survive on our urban heat islands, and even cool them a bit.
You can almost take comfort in the knowledge that humans remain one of the most astonishingly heat-tolerant mammals. Our simple ability to sweatfive times that of horses and double that of camelsis alone absolutely remarkable. And it makes no particular difference where your ancestors came from or what they looked like. "All humans appear to function as tropical animals," writes Dr. Sarah A. Nunneley, an editor with the journal Aviation, Space, and Environmental Medicine.
So why are we wilting and dying?
"We're born with a lot of sweat glands, and that will be true if you are Nordic, African, or Indian. You'll retain that ability, regardless of your ethnicity, if you're raised in a tropical environment," says Mark S. Blumberg, author of Body Heat: Temperature and Life on Earth. "But we lose some of that capacity through disuse, especially in the first few years of life." If you grow up under an AC, you risk losing the ability to function without one.
Being able to adjust to your climate means being able to survive. An Indiana State University study of leaf-cutter ants found that those coming from cities functioned in 108- degree heat for 20 percent longer than their country cousins. The effect in city-dwelling humans can be just as profound.
"That morning power walk or run on a warm April day in the low 80s feels terribly hot, but in July or August won't because you'll have acclimated," says Dr. Thomas Matte, of the NYC Department of Health and Mental Hygiene. When you're used to the heat, electrolytes make up less of your sweat. Instead of sweating so much around your torso, you'll sweat more efficiently from your arms and legs. And your trigger point for sweating will rise.
"But if you stay in your chair in an air-conditioned room and never have to turn on your hormones and heat-loss mechanism, you're not going to be a fit organism," Blumberg says.
YOUR BRAIN ON ICE
We also hamper ourselves with lousy diets. Instead of consuming junk food and fatty ice cream (and then rushing to gulp down sports drinks and electrolyte supplements), we should drink plenty of water or juice, and eat dark leafy greens, nuts, seeds, whole grains, vegetables, blackstrap molasses, and bananas to get the calcium, magnesium, and potassium adults need.
And don't fetishize ancestral lore. Drinking hot tea on a hot day won't cool you down, but rather will burden your body with extra heat to disperseit's a matter of basic physics. Chinese skullcap is believed in Asia to stimulate flushing, but Western scientists aren't yet convinced. Capsicum, the operative component in cayenne pepper and other hot spices, stimulates neural channels designed to receive signals of thermal stress. Some argue that the chemical can be beneficial by tricking the body into producing a protective sweat without exertion. But, asks Hunter College nutrition professor Arlene Spark, "Why would you need to trick the body into sweating during a heat wave?"
Michael Caterina, a professor of biology at Johns Hopkins University School of Medicine and a leading capsicum researcher, says, "One thing that's pretty clear in lab animals is that when newborn rats are injected with capsicum, they lose some of their temperature-sensing neurons, and for the rest of their lives they're missing this subpopulation of neurons. If you put them into an excessively warm environment, they can't protect themselves against overheating." In adult humans, capsicum temporarily dulls or deadens those receptors in a localized fashion, such as on the tongue.
Apart from toughing it out and allowing our bodies to care for themselves, we can take cues from critters. The most basic advice regarding summer clothingthat it should be light-colored, thin, and loosenicely mimics a camel's fur, which provides sun protection and prevents sweat from simply rolling off before it can absorb body heat and transfer it to the air.
Nature is full of tricks for beating the heat. Blumberg is particularly admiring of the gazelle's carotid rete, an intricate web of thin blood vessels (read: increased surface area) below the brain stem. This web allows a gazelle fleeing a lion to protectively chill its brain to more than seven degrees Fahrenheit below its body temperature, even as its muscles are pumping frantically.
"That advantage is important because the brain gets damaged at a lower temperature than the rest of your body. It's a bit like the neck cooler you see in the Sharper Image catalogue. It's cooling the blood that goes to your brain. There's a conceptual validity to the product, but I don't know of any scientific studies on it," he says. "But we do know that it feels damn good."
Not that the brain always gets pride of place. As Blumberg writes, "A ram continues panting as long as its scrotum is overheated, even if its body temperature decreases more than two degrees Celsius (four degrees Fahrenheit). It appears that the ram is more committed to maintaining the temperature of its testes than to maintaining its deep body temperature."
SO MUCH FOR SHADE Buildings and cities, like bodies, are in large part plumbing. In ancient Egypt, wealthy pallace dwellers installed cooling systems that circulated water from aqueducts through the walls. Today's cutting-edge "green" buildings in Tribeca, Harlem, and Chelsea, among other neighborhoods, save electricity by employing geothermal cooling, looping water through coils deep underground to chill the air in the chambers above. Seven World Trade Center, a model sustainable building, uses rainwater for cooling and for watering plants.
New York City is, by world standards, ridiculously rich in fresh water. Our civil engineers spend much of their time racing to find ways to get rid of it. Household "gray water" from showers and sinks and storm water could slake the thirst of the 1 million new trees Mayor Bloomberg is calling for. Many sidewalks could be lined with "nearly continuous greenswales," says Rohit Aggarwala, director of the Office of Long-term Planning and Sustainability. Bloomberg also wants a tax credit for so-called "green roofs," which would be covered with grass and other plants.
Just don't expect even a newly verdant metropolis to spell the end of our heat island.
"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.