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Does heating or cooling our buildings contribute more to climate change?

Globally, heating creates much more climate pollution than air conditioning, and that's often true at the level of a single building too. But emissions from air conditioning are rising faster.

December 18, 2024

Humans are a Goldilocks species. There is a small band of temperatures where we feel comfortable, which is why societies spend tremendous amounts of energy heating or cooling our homes and other buildings. We also generate lots of climate pollution in the process: over 5 billion tons of climate-warming carbon dioxide (CO₂) a year, or close to 15% of all the CO₂ emissions from humanity’s energy use.¹

Most of those emissions come from heating. According to the most recent figures from the International Energy Agency, for every one ton of CO₂ our cooling equipment emits, heating technologies contribute another four. But, says Christoph Reinhart, MIT professor and director of the building technology program, that is poised to change as the Earth heats up.

“We heat more right now, but there are various trends that explain why this is going to change,” he says. “It's very likely that we are going to cool more in the not distant future.”

From a pure physics standpoint, heating a space is simpler than cooling it, Reinhart says. Even on a cold day, the sun is out and creates some heat. Our own bodies generate heat, as do our computers and appliances (including the ones whose purpose is to cool, like air conditioners and refrigerators). Fire is as old as civilization, and humans have spent centuries burning climate-warming fossil fuels like coal and oil for heat.

But air conditioning, which made active cooling of our homes and offices possible, was only invented in the early 20^th century. “This process is still really magical,” Reinhart says. “There's nothing in nature that you can turn on and suddenly it becomes ice cold on a hot day, right? Because it works against entropy. So it's just really fascinating that we can do this at all.”

Historically, then, heating has contributed more to climate change than cooling has, simply because heating was more available. And some research suggests that heating an individual building or home takes more energy than cooling one.² One reason is that heating tends to be inefficient: Reinhart notes that the world has many buildings that are not properly insulated to keep the heat in. “If we live in a cold climate, we just lose a lot of heat to the outside,” he says. “And right now the overwhelming majority of the heat in residential buildings is still provided by fossil fuels,” like oil and natural gas, which directly produce CO₂ whenever they’re burned.

Furthermore, heating a home in winter means overcoming a huge temperature difference. Say your favorite temperature to set your thermostat to is 70° F. On a frigid 20° day in Minnesota, the outside temperature is 50 degrees away from that ideal. By contrast, a hot 95° summer day is only 25 degrees away.

Nevertheless, cooling could soon eclipse heating as a climate change contributor. According to the International Energy Agency, the energy demand from cooling could jump as much as 40 percent by the year 2030.³ That’s largely because incomes and living standards are rising in hot regions of the world from South Asia to Africa and Latin America, which means a growing number of people are able to afford much-needed air conditioning.

Plus, as the planet’s average temperature increases, more people will need air conditioning—including those who live in places from Spain to San Francisco where they previously could get by without it. For example, Reinhart says, MIT scientists have studied this issue with colleagues in Portugal. “This is definitely one of these countries where people traditionally don't cool—it's in the Mediterranean, and it's at a sweet spot where it's cool enough at night,” he says. “That is changing rapidly.” And once people have air conditioning, he adds, they typically use it all the time to stay comfortable, and not just on the most extreme days when they can’t live without it.

If there is a silver lining to this trend, it is the potential for cooling (and heating) to emit much less climate-warming CO₂ than they do today. Adding more clean energy to the grid would help to counterbalance the extra electricity demand of all the additional air conditioning on the way. Replacing gas-burning furnaces with electric heat pumps makes our heating systems more efficient, while at the same time turning our buildings over from fossil fuels to electricity. “Since we are decarbonizing the electric grid,” Reinhardt says, “the electricity overall goes down” in terms of how much CO₂ we emit with every kilowatt-hour of power we consume. “And that benefits heating and cooling to the same degree."

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