“AIR-CONDITIONING cannot be a grand success in the [American] South for the reason that the honest natives of the region recognise the natural summer heat as a welcome ally, in that it makes the inside of houses and offices agreeably uninviting.” In the annals of mistaken predictions, this one—made in 1935 by Clarence Cason, author of “90° in the Shade”—merits an honourable mention. In fact, air-conditioning soon became universal south of the Mason-Dixon line, turning the South into an engine of prosperity and even reshaping its politics by luring Republican migrants to a region that had once been a Democratic stronghold.

The stifling summer of 2018 in the northern hemisphere has been a banner season for air-conditioners and a reminder of how they have changed the world. Sales in France in the first three weeks of July were 192% higher than in the same period of 2017. In Japan, the government is helping schools install coolers. In Texas, on the orders of a judge, the state government has been putting them into prisons.

At current growth rates, according to the International Energy Agency (IEA), which advises national governments, 1bn air-conditioners will be installed globally in the next ten years. That would increase the world’s stock—1.6bn in 2016—by two-thirds (see chart). If you include refrigerators and systems that cool food, vaccines and data, the stock could be 6bn units in a decade. The growth in cooling will save lives, improve education and create wealth in the world’s hottest countries. But it brings huge environmental risks, warming the planet even as it cools people.

Lee Kuan Yew, the first prime minister of Singapore, took the view that air-conditioning “changed the nature of civilisation by making development possible in the tropics… The first thing I did upon becoming prime minister was to install air-conditioners in buildings where the civil service worked. This was key to public efficiency.”

When China became cool

In 1990 few Chinese households had air-conditioning. Twenty years later, the country had just under one unit per household. It now accounts for 35% of the world’s stock, compared with 23% for the United States. India and Indonesia are seeing rates of increase similar to China’s in the 1990s. The population of the 800km long southern coast of the Arabian Gulf increased from 500,000 in 1950 to 20m now, thanks to air-conditioned vertical palaces. At current rates, Saudi Arabia will be using more energy to run air-conditioners in 2030 than it now exports as oil.

At the moment, only 8% of the 3bn people in the tropics have air-conditioning, compared with over 90% of households in America and Japan. But eventually, it will be near universal because so many trends are converging behind its spread: ageing, since old people are more vulnerable to heat stroke; urbanisation, since fields cannot be air-conditioned but offices and factories must be; and economic growth, since, after mobile phones, the middle class in emerging markets want fans or air-conditioners next. Even the proliferation of skyscrapers in the developing world’s megacities encourages air-conditioners. Because tall buildings have different air pressures at top and bottom, they usually have to be sealed, and cooled in summer. Shopping malls, open-plan offices and data-processing centres are all inconceivable without air-conditioning.

Environmentalists fret about this. An article in the Washington Post excoriated “the deluded world of air-conditioning”. Another in the New York Times castigated buildings so cold in summer that “I could preserve dead bodies in the office.” Yet air-conditioning makes people, literally, healthier, wealthier and wiser. A study by Tord Kjellstrom of Australian National University found that, in South-East Asia, people without cooling could not work during 15-20% of working hours. It was too darned hot. Solomon Hsiang of the University of California, Berkeley calculated that, in the Caribbean and Central America, GDP falls by 1% for each degree above 26°C. In the tropics, cooling boosts productivity.

The same goes for learning. A recent study in PLOS Medicine, a weekly journal, by Jose Guillermo Cedeño of Harvard University, followed two groups of college students in Boston during the summer of 2016. Those living in air-conditioned rooms did significantly better in a variety of cognitive tests than their peers in uncooled digs. Studies in Denmark showed that air-conditioning schools improved children’s ability to learn mathematics and languages.

Most simply, cooling also saves lives. Western Europe suffered a withering heatwave in 2003; 11,000-17,000 more deaths than normal were attributed to it in France, mainly from cardiovascular and heart disease. There was a public outcry and the government brought in a range of reforms, including making air-conditioning mandatory in old-people’s homes. This year France has been even hotter than in 2003 but excess deaths so far seem to have been much lower: the health minister recently said the number of hospitalisations this summer has been only slightly greater than normal. Europe, it seems, is learning to cope. In Spain, according to a study by Joan Ballester of the Barcelona Institute for Global Health, heat-related deaths fell between 1980 and 2015, though average summer temperatures rose almost 1°C and there were more old people. In south-west Germany, says Stefan Muthers of the German Weather Service, 2003 and 2015 were the two warmest summers in the past 50 years; 1,700 people died in 2003 but the death toll in 2015 was almost 20% lower.

Heatwaves take an even bigger toll in poor countries that are unable to protect themselves. The World Health Organisation (WHO) forecasts that, without adaptation (which is of course happening), over a quarter of a million extra deaths could be attributable to rising temperatures by 2050—about as many as the number of deaths in childbirth now.

If the definition of air-conditioning is widened to include cold chains for food, industrial processes or vaccines, the overall market increases—and so does its capacity for good. According to Toby Peters of Birmingham University in Britain, cooling for things such as industry, food and data storage uses only a little less energy than air-conditioning. Refrigerated transport is not far behind. These have as many benefits as cooling buildings.

Between farm and fork

In many developing countries, half the food crop is lost to rats and insects after harvest. Reducing that through refrigeration in storage or transport could do more to boost overall food availability than a new green revolution. In the process, it would also limit the greenhouse-gas emissions from wasted agricultural production. According to the WHO, 600m people fall ill at some point each year—and over 400,000 die—from eating contaminated food. A quarter of liquid vaccines are spoiled because they are not kept properly chilled. The death toll from diseases that could be vaccinated against but aren’t, says the WHO, is 1.5m a year, more than die in road accidents. Better cooling would reduce all these harms.

However, as Homer Simpson, an American philosopher, said of alcohol, air-conditioning is the cause of, and solution to, many of life’s problems. In “Losing Our Cool”, a book from 2010, Stan Cox, an agricultural scientist, listed some of its damages: “emission of greenhouse gases…ozone-depleting chemicals [and] a lever to open ecologically vulnerable parts of the country to reckless growth.” He even blamed it for obesity caused (he said) by sitting around in artificially cooled refuges. A recent article in PLOS Medicine, by David Abel and colleagues at the University of Wisconsin-Madison, calculated that, by 2050, there will be 17,000 excess deaths in the eastern half of the United States from an increase of ozone and PM2.5 in the atmosphere (pollutants with a diameter of 2.5 microns or less). Air-conditioning, the authors think, will be responsible for almost 1,000 of those deaths.

Air-conditioners produce greenhouse gases in two ways. First, they are responsible for a share of the CO 2 generated in the power stations that produce the electricity they run on. At the moment, according to the IEA, it takes about 2,000 TWhs (terawatt hours) of electricity to run all the world’s cooling machines for a year. This produces 4bn tons of CO 2 , 12% of the total. Without drastic improvements in air-conditioners’ efficiency, the IEA reckons, they will be burning up 6,000 TWhs by 2050.

On hot days in Riyadh, air-conditioners account for 70% of electricity demand during peak hours, usually the early evening. Peak hours matter because countries must build enough power stations to meet the maximum demand. But most of the time full capacity is not used, meaning firms earn nothing from it. So energy companies build peak capacity as cheaply as possible, which often means using coal or diesel. So demand for air-conditioning is pushing countries to build not just more power plants, but more polluting ones.

Second, air-conditioners use so-called “F gases” (such as hydrofluorocarbons, or HFCs) as refrigerants. When—as is common—the machines leak in use or on disposal, these gases escape, doing vast damage. HFCs trap between 1,000 and 9,000 times as much heat as the same amount of CO 2 , meaning they are much more potent causes of global warming. On this basis, Paul Hawken of Project Drawdown, a think-tank, calculates that improving air-conditioners could do more than anything else to reduce greenhouse gases.

Hitting the fan

Fatih Birol, the head of the IEA, calls the insatiable energy demands of air-conditioning “one of the most critical blind spots in today’s energy debate”. Slowly, that blind spot is being opened up. In 2017, the Lawrence Berkeley National Laboratory in California, a research centre, calculated the extra carbon emissions that could be saved if air-conditioners were better. If HFCs were phased out and all units were as efficient as the best ones, the world could be spared around 1,000 average-sized (500MW capacity) power stations by 2030. There would be many more air-conditioning units, but each would use less energy. In India, this would save three times as much in carbon emissions as the prime minister’s much-vaunted plan to install 100 gigawatts of solar capacity by 2022. In China, it would save as much as eight Three Gorges dams (the largest dam in the world).

Such gains will not be easy to achieve. A common way to improve energy efficiency is to impose minimum energy standards or energy codes for buildings. But these vary from country to country (they are stricter in Japan and Europe than in America, for example). And most poor, hot countries do not even have them.

Getting rid of toxic refrigerant gases also depends on regulation, in this case an international agreement called the Kigali amendment (after the Rwandan capital where it was approved). The deal sets a timetable for phasing down the toxic gases. The trouble is that, to win the backing of tropical countries, the agreement allows them more than a decade to phase the gases out. Since air-conditioners often have a useful life of more than ten years, it could take until 2038 before the full effects of the Kigali amendment come into force—a long time to wait when demand for cooling is growing so fast. With the deal, argues Dan Hamza-Goodacre, boss of the Kigali Cooling-Efficiency Programme, a non-governmental organisation, air-conditioners can produce better health, higher productivity and more food while limiting the rise in global temperatures. In its absence, people cannot have those benefits without the environmental costs.

Correction (August 28th, 2018): A previous version of this article misstated the name of Project Drawdown, a think-tank, as Project Download. This has been corrected.