During Europe’s mid-July heat wave—when temperatures topped 40°C—countries such as Spain and Germany recorded thousands of excess deaths as people succumbed to heat-related injuries and illnesses. Earlier this year, India and Pakistan experienced their hottest March on record, with an unusually early heat wave that killed at least 90 people.

By the 2050s, large swaths of the world will need some form of air-conditioning (AC) to ride out these extreme heat waves or face deadly consequences, according to new research published in Energy and Buildings. But few countries have anywhere near enough cooling capacity to protect residents.


Heating Beyond Human Limits

With our unique ability to sweat off excess heat, humans have adapted to life in a wide range of climates. But there is a limit to what we can tolerate before our internal cooling system can no longer operate. Evaporative cooling of sweaty skin is particularly inefficient when the surrounding air is already heavy with humidity.

When the mercury spikes, people around the world turn to shade, swimming, and fans to keep cool. Relatively few rely on air-conditioning. In the hottest regions of the world, including Indonesia and India, only about 8% of homes currently have an AC unit. In Europe, that number tops out at 20%.

With more frequent heat waves on the horizon due to climate change, mechanical cooling may become a necessity, however. “Severe heat has acute impacts, and air-conditioning is an extremely effective way to protect against those impacts,” said Noah Diffenbaugh, a climate scientist at Stanford University who was not involved in the study.


An Air-Conditioned Future

To assess how actual heat stress and risks will change, scientists turned to the perceived temperature, called the wet-bulb temperature, which conveys how hot it feels considering humidity, temperature, Sun angle, wind, and cloud cover. On days when the wet-bulb temperature exceeds the threshold above which the human body cannot easily adapt, air-conditioning would be needed to limit excess deaths.

In the new study, the researchers used climate models to predict the number of days when that threshold would be passed in urban areas around the world under both a “business-as-usual” greenhouse gas emissions scenario and one in which emissions increased drastically by the 2050s. They then calculated the cumulative electricity demand from air conditioners in each country during these extreme heat events, assuming each unit was on for 6 hours a day.

As part of the Harvard-China Project, the scientists had been researching how to decarbonize India when they realized they needed to understand how energy demand would increase because of climate change, said Peter Sherman, a climate scientist at Harvard University and lead author of the study.

Previous studies had projected demand from gross domestic product and population growth, he said, “but what’s often not considered is increased demand for electricity associated with air-conditioning.”

Because climate models are global, the group decided to provide these estimates for countries around the world, Sherman said.

By the 2050s, in the high-emissions scenario, more than 80% of urban residents in most countries (including wealthier countries such as the United States and Japan, and developing countries such as India and Indonesia) will need air-conditioning to ride out extreme heat waves. In the United States and Japan, 90% of homes already have at least one air conditioner, so installing additional units does not pose a major challenge. However, in Indonesia and India, only 8% of homes have units. In Indonesia, air-conditioning during days when the wet-bulb temperature exceeds the threshold could account for up to 75% of the country’s current total electricity demand, according to the study.

Considering population growth, urbanization, and climate change over the next 30 years, some of the world’s hottest places will need major investments in air-conditioning to meet demand during extreme heat events, according to the study. The tropics are particularly vulnerable because their high heat and humidity make for deadly wet-bulb temperatures, and some of these areas have the lowest current rates of AC use. These areas are also highly sensitive to the different emissions scenarios the study evaluated.


Residents of urban centers in the tropics, therefore, face a particularly tough cooling future, especially if the world does not curb greenhouse gas emissions.

“The largest source of uncertainty in the future climate is the human dimension,” said Diffenbaugh.

Cooling Begets Warming

When calculating energy demand, the researchers assumed that increased air-conditioning usage will come in the form of ductless mini split units—wall-mounted air conditioners designed to cool a single room. However, these units have a high up-front cost, and in developing countries, cheaper, less efficient window units will likely fill the demand, said Shelie Miller, an environmental engineer at the University of Michigan who was not involved in the study. “If anything, [the study’s researchers] may be underestimating the electricity demand because they are assuming a much more efficient unit than window ACs.”

But blanketing the world in even high-efficiency air conditioners would strain electrical grids and generate greenhouse gases.

Air conditioner manufacturing and usage account for 4% of global greenhouse gas emissions—more than the aviation industry produces, said Jason Woods, a research engineer at the National Renewable Energy Laboratory who was not involved in the study.

“More air-conditioning means more carbon dioxide emissions, which then means more climate warming, which means more air-conditioning energy use,” Woods said. To break that cycle, we need to develop more efficient cooling technologies and power those systems with renewable energy, he added.

Unequal Cooling

The findings highlight widely disparate access to cooling between wealthy and developing nations. Some studies have shown that the gap exists even within individual countries or cities.

“It’s the more affluent people who have access to this cooling,” said Neil Jennings, a geographer at Imperial College London who was not involved with the study. “The presence of that cooling then means that more heat [from the units] is being dumped out into the streets, which then means that those who don’t have access to cooling are exposed to even higher temperatures.”

Renewable energy sources, more efficient cooling solutions, and a return to traditional building practices—such as using dense building materials, windows, and shade to maximize insulation and airflow—may be needed to address the growing number of extreme heat days projected to hit some countries, Woods noted.

“I think [the study] really highlights the need to have action now,” Miller said, “rather than as things continue to get worse over time.”

—Jennifer Schmidt (@DrJenGEO), Science Writer