The World Will Soon Need More Cooling Than Heating

The world has always needed more heating than cooling, which was fortunate because it was always easier to heat than it was to cool. You could just throw another log or lump of coal on the fire, whereas cooling had to wait until the 20th century and Willis Carrier, the inventor of modern-day air conditioning.

It usually took more energy to heat than it did to cool because the change in temperature was bigger. In a place like the northern U.S., there might be a 20-degree difference between comfort and really hot and a 50- or 60-degree difference between comfort and really cold. And most people in the global south couldn't afford air conditioning or get power for it, so way more energy was used for heating than for cooling.

But things are changing fast. A new study, "Population-weighted degree-days: The global shift between heating and cooling," explains how. But first, an explanation of degree-days is in order, because they are fascinating.

I learned about degree-days in architecture school when you still looked them up on maps like this. You take a base temperature—65 degrees Fahrenheit in the U.S.—and then you total up all the average temperatures that are below that base. So, a 50-degree average day would add 15 degrees to the total for the year, the Heating Degree Days (HDD). You could use this to figure out how much energy a given building was going to use in a year; the same building in New York City, at 5,500 degree-days, is going to use 11 times as much energy to heat as if it were in Miami at 500 degree-days.

Cooling degree-days (CDD) work the same way, except they total the number above 65 degrees. Compare the two charts, and it is clear that there are way more heating degree-days than cooling. Even the hottest spots in the U.S. are at 4,000 cooling days, whereas half the U.S. is over that in heating degree-days.

If you multiply the number of people in a given area by the degree-days, you get the population-weighted degree-days, telling you the total amount of energy needed by a given population to heat or cool. It is obviously an approximation; not everyone is going to set their thermostat at the 65-degree base temperature, and not everyone has an air conditioner. But it is a useful tool to see where things are going.

The study finds that the population is growing fastest in the hotter parts of the world, as those parts get hotter still due to climate change.

"The analysis shows that mean global area-weighted heating degree-days have fallen 8.46°C days/year, whereas population-weighted heating degree-days have fallen by 12.5°C days/year. At the same time, mean global area-weighted cooling degree-days have risen by 3.0°C days/year, while population-weighted cooling degree-days have risen at 6.0°C days/year."

Population movements had much more impact than climate change; for example, in the U.S., the massive population shift to the Sunbelt made possible by air conditioning resulted in a decrease in heating demand and an increase in cooling demand, but the increase in cooling is less than the decrease in heating because the degree-day totals are smaller. But it is the global trends of population growth that are more significant.

The paper concludes that over the last 40 years, "HDDs have decreased, and CDDs have increased. The changes in area-weighted degree-days are in line with the observed global warming in our study period. Population-weighted CDDs are increasing at a faster rate than area-weighted CDDs, which is explained by increases in populations in warmer regions."

Where it gets scary is that, while the CDDs and the need for cooling increased, people didn't have the money or the electricity supply to meet the need until recently, as they got richer and AC units got cheaper. As the paper notes, "Historically, it has been far easier to actively heat a building than cool it. At a global scale, rapid urbanization and the increased use of air conditioning worldwide could lead to substantial increase in energy demand for cooling, while existing heating demand in colder regions would persist."

All over the world, from India to Italy, they are shoveling coal as fast as they can to make the electricity needed to meet the demand for air conditioning. According to this study, at some point in the next decade, the need for cooling will overtake heating: "Providing zero carbon cooling is essential to avoiding more carbon emissions and making global warming even worse."

We get zero carbon cooling by cleaning up our electricity supply and by reducing demand with better buildings. And given the way these numbers are trending, we are going to need a lot of both.