Climate change is making Mexico City unbreathable

 This article originally appeared on GlobalPost.

Global Post MEXICO CITY, Mexico — On a sweaty May morning in this sprawling mountain capital, residents heard a painfully familiar warning on the radio and TV. Air pollution was at dangerous levels, environmental authorities said. People were advised to stay indoors as much as possible and avoid exercise. Asthma sufferers should take particular care.

On the city streets, this pollution could be seen in dirty clouds hanging amid grid-locked traffic.

The “environmental pre-contingency” on May 9 was the fourth so far this year, compared to three in all of 2014. The warnings are a reminder of the long uphill battle against dirty air in North America’s largest city — which has been a laboratory for pollution in megacities around the planet.

This rise comes after years in which Mexico City air has been getting cleaner, thanks to concerted campaigns. But while some problems have been resolved, others appear.

One issue is that Mexico is getting steadily hotter, apparently due to global warming. Last year was the hottest in Mexico since records began, with average temperatures of 71.78 degrees Fahrenheit.

Higher temperatures mean that pollutants release faster into the air.

“The rate of emission of certain types of pollutants goes up. … Secondly, chemical reactions tend to be faster,” says Christopher Cappa at the Air Quality Research Center in Davis, California. “This makes existing problems more challenging.”

Last year, Stanford scientists published a paper predicting that global warming will increase air pollution levels in cities around the world. It highlighted Mexico, along with India, as places of particular concern.