THE blackouts that left hundreds of millions of Indians sweltering in the dark last month underscored the status of air-conditioning as one of the world's most vexing environmental quandaries.Fact 1: Nearly all of the world's booming cities are in the tropics and will be home to an estimated one billion new consumers by 2025. As temperatures rise, they -- and we -- will use more air-conditioning. Fact 2: Air-conditioners draw copious electricity, and deliver a double whammy in terms of climate change, since both the electricity they use and the coolants they contain result in planet-warming emissions. Fact 3: Scientific studies increasingly show that health and productivity rise significantly if indoor temperature is cooled in hot weather. So cooling is not just about comfort. Sum up these facts and it's hard to escape: Today's humans probably need air-conditioning if they want to thrive and prosper. Yet if all those new city dwellers use air-conditioning the way Americans do, life could be one stuttering series of massive blackouts, accompanied by disastrous planet-warming emissions. We can't live with air-conditioning, but we can't live without it. ''It is true that air-conditioning made the economy happen for Singapore and is doing so for other emerging economies,'' said Pawel Wargocki, an expert on indoor air quality at the International Center for Indoor Environment and Energy at the Technical University of Denmark. ''On the other hand, it poses a huge threat to global climate and energy use. The current pace is very dangerous.'' Projections of air-conditioning use are daunting. In 2007, only 11 percent of households in Brazil and 2 percent in India had air-conditioning, compared with 87 percent in the United States, which has a more temperate climate, said Michael Sivak, a research professor in energy at the University of Michigan. ''There is huge latent demand,'' Mr. Sivak said. ''Current energy demand does not yet reflect what will happen when these countries have more money and more people can afford air-conditioning.'' He has estimated that, based on its climate and the size of the population, the cooling needs of Mumbai alone could be about a quarter of those of the entire United States, which he calls ''one scary statistic.'' It is easy to decry the problem but far harder to know what to do, especially in a warming world where people in the United States are using our existing air-conditioners more often. The number of cooling degree days -- a measure of how often cooling is needed -- was 17 percent above normal in the United States in 2010, according to the Environmental Protection Agency, leading to ''an increase in electricity demand.'' This July was the hottest ever in the United States. Likewise, the blackouts in India were almost certainly related to the rising use of air-conditioning and cooling, experts say, even if the immediate culprit was a grid that did not properly balance supply and demand. The late arrival of this year's monsoons, which normally put an end to India's hottest season, may have devastated the incomes of farmers who needed the rain. But it ''put smiles on the faces of those who sell white goods -- like air-conditioners and refrigerators -- because it meant lots more sales,'' said Rajendra Shende, chairman of the Terre Policy Center in Pune, India. ''Cooling is the craze in India -- everyone loves cool temperatures and getting to cool temperatures as quickly as possible,'' Mr. Shende said. He said that cooling has become such a cultural priority that rather than advertise a car's acceleration, salesmen in India now emphasize how fast its air-conditioner can cool. Scientists are scrambling to invent more efficient air-conditioners and better coolant gases to minimize electricity use and emissions. But so far the improvements have been dwarfed by humanity's rising demands. And recent efforts to curb the use of air-conditioning, by fiat or persuasion, have produced sobering lessons. Since 2005, Japan had been promoting energy conservation through its annual summer ''cool biz'' campaign: air-conditioning thermostats in government offices were set to between 75 and 77 degrees and workers were told they could forsake business suits for looser, cooler clothes. So far so good. But in the past year, the country became an unwitting laboratory to study even more extreme air-conditioning abstinence, and the results have not been encouraging. After the Fukushima earthquake and tsunami knocked out a big chunk of the country's nuclear power, the Japanese government mandated vastly reduced energy consumption. To that end, lights have been dimmed and air-conditioners turned down or off, so that offices comply with the government-prescribed indoor summer temperature of 82.4 degrees (28 Celsius); some offices have tried as high as 86. Unfortunately, studies by Shin-ichi Tanabe, a professor of architecture at Waseda University in Tokyo who has long been interested in ''thermal comfort,'' found that while workers tolerated dimmer light just fine, every degree rise in temperature above 25 Celsius (77 degrees Fahrenheit) resulted in a 2 percent drop in productivity. Over the course of the day that meant they accomplished 30 minutes less work, he said. Other studies have found that with office temperatures between 82 and 86 degrees, symptoms like headache, drowsiness and difficulty concentrating increase, which may explain the drop in performance.
Worse still, perhaps, Mr. Tanabe calculated that the suffering was all for naught: When offices were kept above about 82 degrees, so many people were using inefficient fans at their desks that the total electricity consumption could be higher than if the building had been better cooled. ''That's just stupid,'' he said. Some studies from hot and humid Singapore also show that cooler is better when it comes to office work, said Mr. Wargocki, who is currently a visiting researcher at the National University there. Though people in Singapore tend to identify a range from about 68 to 75 degrees as ''neutral'' temperature -- neither hot nor cold -- studies found that work improved if the thermostat was lowered to about 72. ''It's a huge problem if we have to cool buildings in tropical environments to that level, in terms of energy use and climate,'' Mr. Wargocki said. And that is where the bulk of new demand will occur. A new report by the McKinsey Global Initiative predicts that one billion city dwellers ''will enter the global consuming class by 2025.'' And, for most of them, an air-conditioner will most likely be a first purchase since almost all of the cities with the highest potential cooling needs, according to Mr. Sivak's research, are in developing countries that are in hot climates. These include Chennai, India; Bangkok; Manila; Jakarta, Indonesia; Karachi, Pakistan; Lagos, Nigeria and Rio de Janeiro. Sales of air-conditioning units are already growing by double digits annually in many emerging economies. So researchers say the best hope is that we all adjust our air-conditioning expectations and behavior. Building managers could increase airflow in hot buildings, for example, which improves comfort. Workers could wear lighter, looser clothing to work in summer -- instead of carrying sweaters to protect themselves from over-chilled air. Architects could design office blocks using materials that did not conduct so much heat and where humans could open the windows to take advantage of natural ventilation and breezes. Stan Cox, author of ''Losing Our Cool,'' suggests that one solution might be a return to room air-conditioning, so we only use energy to cool spaces that people are actually using. He believes that people are accustomed to working in frigid offices but could acclimatize to warmer conditions. Mr. Wargocki says that an office temperature in the mid to high 70s should be fine. The comfortable temperature for sleeping (naked) is around 84, Mr. Tanabe says, if a fan is on. Those suggestions are a good deal warmer than the norms in the United States, which underlines a cultural differences in cooling preferences. ''The temperature many Americans find most comfortable indoors in summer -- 70 degrees -- feels uncomfortable to most Europeans, who find it too cold,'' said Mr. Sivak, who suggested that Europe's greater environmental awareness might make people more inclined to put on an extra sweater in winter or tolerate a bit more heat in summer. Unfortunately many tropical places -- including Singapore, Malaysia and Hong Kong -- seem to have followed the United States' lead in cooling preferences, Mr. Tanabe said, holding cooler to be better. But certainly if I deserve to have an air-conditioner here in New York, my counterpart in Mumbai deserves to have one, too. So individuals need to be coaxed to make new choices. ''We need to educate people there are other ways to be comfortable than just turning up the A.C., you have to use it wisely,'' said Mr. Wargocki, speaking from his condo in Singapore with the windows open late one evening to create natural drafts for cooling. He began telling me about how the European Union was effectively forcing companies to use less cooling, by mandating that new buildings meet stricter energy-use standards. Since air-conditioning gobbles up far more electricity than heating in many office buildings, one way for architects to achieve compliance is to avoid an over-reliance on air-conditioning -- for example, building with materials that do not absorb heat or pumping in cool air from deep underground. I was listening from my living room in New York on a steamy Sunday morning. Given the topic of our conversation, I had the air-conditioner off, and the temperature was 85 or so. I couldn't concentrate. DRAWING (DRAWING BY VICTO NGAI) (SR1); CHART: CRANK IT UP: Cooling degree days -- an oddly named index that measures (but not in actual days) the need for air-conditioning -- in large urban areas. Developing regions are in bold. (Source: Michael Sivak, University of Michigan)(SR7)