GoKawiilAir conditioners generate heat and are expensive to run.Credit: Ezra Acayan/Getty
As large swathes of Europe and Asia swelter through record heatwaves this summer, discussions have centred around an obvious way to keep people cool: more air conditioners. Popular in high-income countries such as the United States, Japan and Australia, these units are less common in homes in Europe or in low-income countries. But air conditioners are expensive to run, use vast amounts of energy and generate heat, so installing them in every home that needs one is not the answer, say scientists. Instead, they say that, over the past few years, there have been advances in materials that passively cool buildings, offering a solution without the drawbacks.
Cooling buildings accounts for nearly 10% of electricity use globally, according to a review published earlier this month in Nature Reviews Clean Technology1. That is expected to grow as heatwaves become more frequent, intense and long-lasting. By 2050, the electricity needed to cool buildings globally is expected to have increased by 210% above levels recorded in 2024, with greenhouse-gas emissions from air conditioning likely to triple over the same period.
Matthaios Santamouris, an architecture researcher at the University of New South Wales in Sydney, Australia, who co-authored the review, says air conditioning can be life-saving for people who are at high risk of dying from extreme heat, including those aged more than 65, but that the technology has several drawbacks. During temperature spikes, when there is a surge in air-conditioner use, the sudden demand for electricity puts pressure on the grid, leading to blackouts that endanger people vulnerable to heat stress.
Air conditioning also raises temperatures in cities, says Negin Nazarian, an urban climatologist at the University of New South Wales. Split systems, in which an air-conditioning unit cools down an individual room, work by extracting heat from the building and “dumping it right next door”, into the surrounding environment, Nazarian adds. This creates a “vicious loop”, she says, in which temperatures outside increase, prompting greater use of air conditioning to cool indoor spaces.
The cost of using air conditioners also makes them harder to access for low-income populations, who are particularly vulnerable to harm from exposure to extreme heat. For example, up to 100 million families in India, Mexico, Indonesia and Brazil will remain unable to afford air conditioning by 2040, despite access to electricity2.
An alternative solution
Santamouris says that passive cooling of buildings — in which materials are designed to control temperatures — uses minimal to no energy. Automated blinds and window slats that track the position of the Sun are already used in commercial buildings, offices and universities. These control how much heat enters a building, reducing the reliance on air conditioning.
One of the most exciting advances is the development of materials that can achieve passive radiative cooling, says Nicole Miranda, a sustainable cooling specialist at the University of Oxford, UK. Most of the heat released on Earth is absorbed, except for waves in the ‘atmospheric window’ (those with wavelengths between 8 and 13 micrometres), which escape directly into space. Passive radiative cooling technologies exploit this, with special paints, glasses, gels and other building materials made specifically to reflect these wavelengths.
Materials consisting of several layers, or with bubbles or pockets of air, have been developed by scientists to reflect heat. For example, paints that incorporate micro- or nano-sized spheres of silicon oxide and aluminium oxide can enhance light scattering and increase how much heat is released into the atmosphere3. One coating decreased the temperature of the surface it was applied to by 5.26 °C, reflecting 97% of the solar energy that it was exposed to4.