The United States has long consumed more energy each year for air conditioning than the rest of the world combined. In fact, we use more electricity for cooling than the entire continent of Africa, home to a billion people, consumes for all purposes. Between 1993 and 2005, with summers growing hotter and homes larger, energy consumed by residential air conditioning in the U.S. doubled, and it leaped another 20 percent by 2010. The climate impact of air conditioning our buildings and vehicles is now that of almost half a billion metric tons of carbon dioxide per year.

Yet with other nations following our lead, America’s century-long reign as the world cooling champion is coming to an end. And if global consumption for cooling grows as projected to 10 trillion kilowatt-hours per year—equal to half of the world’s entire electricity supply today—the climate forecast will be grim indeed.

Because it is so deeply dependent on high-energy cooling, the United States is not very well positioned to call on other countries to exercise restraint for the sake of our common atmosphere. But we can warn the world of what it stands to lose if it follows our path, and that would mean making clear what we ourselves have lost during the age of air conditioning. For example, with less exposure to heat, our bodies can fail to acclimatize physiologically to summer conditions, while we develop a mental dependence on cooling. Community cohesion also has been ruptured, as neighborhoods that on warm summer evenings were once filled with people mingling are now silent—save for the whirring of air-conditioning units. A half-century of construction on the model of refrigerated cooling has left us with homes and offices in which natural ventilation often is either impossible or ineffective. The result is that the same cooling technology that can save lives during brief, intense heat waves is helping undermine our health at most other times.

The time window for debating the benefits and costs of air conditioning on a global scale is narrowing—once a country goes down the air-conditioned path, it is very hard to change course.

China is already sprinting forward and is expected to surpass the United States as the world’s biggest user of electricity for air conditioning by 2020. Consider this: The number of U.S. homes equipped with air conditioning rose from 64 to 100 million between 1993 and 2009, whereas 50 million air-conditioning units were sold in China in 2010 alone. And it is projected that the number of air-conditioned vehicles in China will reach 100 million in 2015, having more than doubled in just five years.

As urban China, Japan, and South Korea approach the air-conditioning saturation point,the greatest demand growth in the post-2020 world is expected to occur elsewhere, most prominently in South and Southeast Asia. India will predominate—already, about 40 percent of all electricity consumption in the city of Mumbai goes for air conditioning. The Middle East is already heavily climate-controlled, but growth is expected to continue there as well. Within 15 years, Saudi Arabia could actually be consuming more oil than it exports, due largely to air conditioning. And with summers warming, the United States and Mexico will continue increasing their heavy consumption of cool.

Countries are already struggling to keep up with peak power demand in hot weather. This summer, India is seeing a shortfall of 17 gigawatts, with residential electricity shut off for 16 hours per day in some areas. China is falling short by 30 to 40 gigawatts, resulting in energy rationing and factory closings.

In most countries, the bulk of electricity that runs air conditioners in homes and businesses is generated from fossil fuels, most prominently coal. In contrast, a large share of space heating in cooler climates is done by directly burning fuels—usually natural gas, other gases, or oil, all of which have somewhat smaller carbon emissions than coal. That, together with the energy losses involved in generation and transmission of electric power, means that on average, an air conditioner causes more greenhouse emissions when pushing heat out of a house than does a furnace when putting the same quantity of heat into a house.