Air conditioning units in Salvador, Brazil, on July 2, 2014.  (Reuters/Sergio Moraes)

To get through the 21st century in some semblance of a stable state, humanity needs to slash its greenhouse gas emissions. In every part of the world, countries will have to shift their energy sources and cut back on carbon. And more and more, they openly acknowledge as much.

But what, in coming decades, if some major energy trend emerges that cuts in the opposite direction — pushing emissions upward? In a new study in the Proceedings of the National Academy of Sciences, two researchers from the University of California, Berkeley and the National Bureau of Economic Research may have found one.

“People are getting richer all around the world, and they’re buying air conditioners,” explains lead study author Lucas Davis of the Haas School of Business at the University of California, Berkeley. “It’s pretty startling, actually.”

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Air conditioners in the United States are commonplace — they’re in some 90 percent of our homes, where they consume 5 percent of total U.S. electricity. But we’re not where the growth will happen. Rather, it’s in warm or tropical countries with large populations, like India and Brazil. And you can’t really blame them: As incomes rise, people everywhere seek more comfort — and more and more, they’ll be able to afford it.

China, for instance, has seen a doubling of air conditioner sales in just a half decade — 64 million of them were sold in the year 2013, according to the study. Demand will also be huge in India, the paper finds, where there are “more than three times as many cooling degree days per person” than in the United States. (A “cooling degree day” is a measurement used to determine the demand for air conditioning by calculating how many degrees the average temperature is over 65 —and thus how much cooling you’d need to maintain your home at 65.)

The study looked in particular at Mexico, where air conditioning penetration is currently only at about 13 percent. It found that as people get richer, those living in warm areas will flock to air conditioning. “In warm areas … we find a close relationship between household income and air conditioner adoption, with ownership increasing 2.7 percentage points per $1,000 of annual household income,” write Davis and his colleague Paul Gertler, also based at U.C. Berkeley.

For Mexico, if incomes increase 2 percent each year, this leads to a projection of between 71 and 81 percent of households having air conditioning by the end of the century, versus just 13 percent today. This means that residential energy use overall would increase between 64 and 83 percent — and carbon dioxide emissions would increase, annually, by 23 to nearly 30 million tons. In 2012, according to the U.S. Energy Information Administration, Mexico’s total emissions were 453 million metric tons — so this is fully 6 percent of that overall number.

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It is important to note that this future trend is not forecast to occur because the world will be warmer — though it will be — but rather, because it will be richer. “The income increases alone explain the lion’s share of the predicted increases in air conditioning saturation,” Davis says. “It’s mainly that people are growing richer. And we think this pattern will hold not only in Mexico but in low and middle income countries around the world. The potential is just enormous.”

In fact, the study notes, there are many countries with much greater potential to increase overall air conditioning levels than Mexico — either because of larger populations, or larger numbers of cooling degree days each year. The paper provides a list of 12 based on “air conditioning potential,” including India, China, Indonesia and Brazil. “Most of the countries on the list are within the income range where in Mexico we begin to observe a rapid increase in air conditioning adoption,” notes the paper.

Thus, the growth of air conditioning nestles into a much broader global trend — the rise of highly populous developing countries, their huge economic growth and the many knock-on effects from that growth, environmental and otherwise.

On the environmental front, unless the electricity to power all of these new air conditioners somehow manages to come strictly from renewable sources — and it seems pretty unlikely that this will be the case — there could clearly be a substantial added contribution to the world’s greenhouse gas emissions (although the study does not quantify that contribution).

So the good news is that the world will be a lot more comfortable in the future. And with global warming, we’ll need it. But the bad news is that with millions upon millions of air conditioners, there will be a huge growth in electricity use to keep them humming.