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.