There were 3.6 billion air conditioners in the world in 2020, and their number is expected to increase four-fold, to 14 billion, by 2050.

Most modern air conditioners and air heat pumps use the refrigerant R32 also known as HFC 32, as their coolant. The global warming potential of R32 is 675 in one hundred years and approximately 2,400 within a period of 20 years. This means that each kilogram of R32 leaking into the atmosphere heats the planet as much as 2 400 kilograms of carbon dioxide during the next 20 years.

However, the atmosphere’s carbon dioxide content is currently 420 parts per million or ppt. The R32 content of the air is less than 15 parts per trillion and it has been growing at roughly 1 part per trillion per year. If the amount of R32 in the atmosphere is 30 million times less than the concentration of carbon dioxide, it cannot make a major contribution to global warming, especially because its average lifespan in the air is five years only.

There are already 2 billion air heat pumps and air conditioners using R32.  Besides this, most of the older models use the refrigerant R 410A, 50 per cent of which is R32.  These figures mean that even if the number of air conditioners and air heat pumps increased 10-fold, their coolant leaks won’t be a problem for the climate as long as they use R32. A typical air heat pump only contains about 1 kilogram of R32 and leaks are very rare. Most of the 21 000 tons of R32 that is annually released into the air probably comes from the air conditioning systems in cars. Cars are not designed to stand still at the same site but move around. Cars receive hundreds of millions of small bumps during their lifetime, and their air conditioning systems begin to leak, almost inevitably, when they become older. So we shouldn’t worry about the HFC 32 in air heat pumps and air conditioners.

However, the power consumed by these devices is a different matter, because most of it is still produced by burning fossil fuels.

The average global temperatures are expected to rise by at least 2 and possibly by 6 degrees Celsius before the year 2100, due to the build-up of carbon dioxide and other greenhouse gases into the atmosphere. This means that air conditioners must soon work much harder to achieve the desired results.

The problem is aggravated by the fact that most people will be living in large cities. Large cities tend to be warmer than the surrounding countryside.  The phenomenon is known as the urban heat island effect.  It happens partly because cities have so much asphalt and other dark surfaces that they have a smaller reflectivity (albedo) than rural areas. Cities absorb a larger percentage of solar radiation and reflect a smaller share of it straight back to space. Densely populated cities also consume so much energy, that the waste heat from millions of buildings has a collective local heating impact.  The combined impact of these two factors can amount to 12 degrees Celsius, during heat waves. Inside the larger urban heat islands there can be even warmer, small hot spots.

Cooling the great megalopolises during extreme heatwaves will become a growing challenge during the next decades. Air conditioners will have to work harder to keep the apartments cool and the people alive, but the harder they work the more waste heat they produce and the more they contribute to the urban heat island effect.

Humans can tolerate dry heat relatively well, but humid heat is a problem for us. This is because our body’s heat regulation system is based on evaporating sweat. Whenever a gram of sweat evaporates from our skin, 2260 joules of heat go with it.  But if the relative humidity in the air increases too much, sweat can no longer evaporate but only drops from the skin, and at this point our heat regulation system more or less breaks down. Even if the sweat would be produced from very cold water, we have just drunk, each gram of sweat dropping from the skin could still only remove a maximum of 160 joules of heat from our body.

This means that we are extremely vulnerable to the combination of high humidity and heat.  Normal heat waves kill people who are already sick or very old and fragile, but a combination of a relative humidity of 100 per cent and a temperature of 35 degrees Celsius kills even young and healthy people in six hours, unless they can get inside air-conditioned buildings. 40 degrees Celsius combined with the relative air humidity of 75 per cent would be an equally lethal combination.

Such TW35 temperatures were seen for the first time since the time humans have existed on the planet, in the beginning of 2021 in two towns:  in Jacobabad (Pakistan) and in Ras al Khaimah (United Arab Emirates). In both cases the lethal combination of humidity and heat persisted for less than two hours, but they were a frightening demonstration on how close of making large parts of the tropics uninhabitable for humans we are getting.

The concept TW35 refers to the wet bulb temperature of 35 degrees Celsius. The wet bulb temperature is the temperature shown by a thermometer wrapped in a wet blanket. TW35 is considered as the limit of a lethal combination of heat and humidity, because our skin has to remain well below 35 degrees Celsius in order to prevent our body’s core temperature from reaching lethal levels.

How can our power grids handle the longest and the most extreme heat waves in a world that is several degrees warmer than now, and in which most people are living in large cities with very strong urban heat island effects?

It might be a good idea to complement air conditioners with other ways to reduce the temperature and humidity.