The climate is full of feedback loops. When a warming climate melts sea ice, the water that's left behind reflects far less sunlight, leading to a further warming. Now, some researchers at the University of California Berkeley have looked at a human feedback loop: the relation between climate change and air conditioning. Using Mexico as an example, they find that the rising use of air conditioning may boost the country's electricity use and carbon emissions by 80 percent before the century is over—but only if economic growth continues at a pace that allows people to buy air conditioners.

Understanding the margins

The study involves combining two types of economic figures, the intensive and extensive margins. Intensive margins, in this case, focused simply on what happens to electricity use as the temperature goes up. Here, the authors took advantage of the complete household-level billing records of 25 million residential electricity customers in Mexico. That data was cross-correlated with local temperature data.

Mexico contains everything from high-elevation deserts to tropical forests at sea level, so customers are exposed to a range of climate conditions. The authors found that electricity use remained relatively flat until the temperatures reached about 24 degrees Celsius (75 degrees Fahrenheit), after which they rose to the point where each day of the month with temperatures at or above 35 degrees Celsius (95 degrees Fahrenheit) boosted electricity use by 3.2 percent.

On the low end, cooler days (10 degrees Celsius/50 degrees Fahrenheit or less) had no effect on electricity use. This is in contrast with the US, where cold days see an increase in electrical heating use. It's not clear how much of this is due to a lack of electrical heating in Mexico, how much is a cultural tendency to just deal with the cold, and how much is just that it doesn't get that cold in many areas of Mexico.

You could extrapolate this pattern to the temperature changes later in the century, but that would be unlikely to tell us much, as air conditioner use in middle-income economies is booming. China, for example, has seen it double in the last five years, and last year, its consumers bought eight times the number of air conditioners sold in the US.

So here is where an analysis of the extensive margins comes in. The authors tracked the number of air conditioners installed in communities relative to their average income. In areas of the country where temperatures were generally cool, there was only a slight increase in installations as annual income neared $25,000 a year. By contrast, hot areas saw installations start growing before annual income reached even $5,000 a year, and the growth increased in a linear manner as income rose past $25,000 a year.

Projections

Over the course of this century, two things are highly likely: incomes of Mexicans will continue to rise, and so will the temperatures. The authors handled the former very simply: they assumed a very conservative two percent growth in income a year over the rest of the century. For the latter, they used climate model projections for two IPCC scenarios: RCP 4.5, in which emissions are limited to stabilize the atmosphere at 500 parts-per-million CO 2 , and RCP 8.5, where emissions continue at current growth rates, leaving us at 900 ppm and rising at the end of the century.

The regional data for Mexico was obtained from the Climate Wizard website, a joint project between universities and the Nature Conservancy.

The effect of temperature changes alone would be pretty minor: a boost in electricity use of 7.5 percent. That change would add $357 million to the $4.8 billion that Mexicans spend on electricity and add 2.7 million tons to the country's carbon emissions, which stand at 35.9 million tons.

But the rising temperatures would also shift more locations into the warm category, meaning they'd see air conditioner installations go up with income. And annual income growth would mean that just about everyone in those locations would be installing air conditioners well before the end of the century. The result? Air conditioners will be installed in at least 70 percent of Mexican households by the end of the century.

This makes for a pretty radical difference. Even under RCP 4.5, the lower-emissions scenario, electricity consumption from air conditioning will go up by 64 percent, adding another $3 billion to the money Mexicans spend on it. Carbon emissions from feeding this demand would rise by 23 million tons (a boost of 63 percent). Under RCP 8.5, the increases would be larger: $4 billion in new electricity and a boost of carbon emissions by nearly 80 percent.

And while Mexico is meant to be representative, the real action will be elsewhere. India has over 10 times the population of Mexico. Compared to the US, the average Indian town sees three times the number of days that would typically trigger air conditioning use. Assuming these and other countries in the tropics see the same pattern of use as Mexico, where cool days don't boost electricity consumption, then the global increase could be staggering.

The authors are keen to point out that while this scenario is what's likely to happen without any changes, many changes are possible. Air conditioners have increased in efficiency, and there's always the prospect of switching to a different cooling (and/or insulating) technology entirely. The growth of renewable energy may at least cut down on the carbon emissions feedback, if not the added costs. And the government could also set policies that will discourage air conditioning purchase or use (either directly or through the price of energy). Finally, people may simply move to the cooler areas of the country or even other countries entirely.

Still, the analysis is a bit shocking in terms of the effect that just a single lifestyle habit can have on national carbon emissions. And since the majority of the impact comes in the lower emissions scenario, it reinforces how hard we'll have to work to avoid business-as-usual emissions growth.

PNAS, 2015. DOI: 10.1073/pnas.1423558112 (About DOIs).