At Bay Elementary in San Lorenzo, California, fans and Otter pops are common purchases for teachers alongside crayons, pencils, and construction paper. The school has no air conditioning, and temperatures in the East Bay can crank up into the 90s at the beginning and end of the school year. The blacktop that surrounds the school, lack of shade, and relentless sun can turn classrooms into ovens.

Alice Johnson is one of the lucky teachers, running her kindergarten class out of an addition tacked on about seven years ago between basketball hoops and four square courts. Unlike teachers in the main building, she has windows on both sides of her classroom that can catch a cross breeze in the morning. She also keeps the lights off and uses a fan.

Still, when the kids come back from recess just after noon, the heat has built up and everyone’s in a different state.

“We’re all hot, all my kids are falling apart,” she told Earther. “I have to remind them to have water, and I also need to remember to drink water myself.” On very hot days, Johnson said, the class will forego afternoon math activities for puzzles. “That means we’re off [schedule] until the weather cools down.”

This scene plays out in classrooms of every grade level all around the U.S. And while the impact of heat on learning has been documented in labs, a recent study presents the first big picture look at how hot weather affects learning. The paper, published in the National Bureau of Economic Research, used weather data and the scores of 10 million PSAT takers to estimate how much each degree of heat adversely affects academic achievement, which in turn affects a lifetime of earnings. The impacts are, perhaps not surprisingly, the worst in impoverished school districts.

Climate change will only make the heat worse, which could further magnify the gap between the rich and poor. But the paper also shows that air conditioning can mitigate the worst of the impacts, raising tough questions about climate adaptation.

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“We see patterns across the U.S. that hot areas of U.S. tend to score lower on standardized exams than cool areas,” Joshua Goodman, a Harvard economist who led the research, told Earther. “We see that replicated across the globe. Our goal was to provide some of first rigorous evidence of the long-term impact of heat on student outcomes. Not just ‘it’s hot today, I score lower on the test’ but cumulative impacts.”

Determining those cumulative impacts required a massive amount of environmental and weather data (the study used temperature records from nearly 3,000 weather stations across the U.S.), plus a standardized metric of academic achievement. Goodman and his colleagues chose the PSAT, a test the study describes as “designed to assess cumulative program on skills learned during secondary school, rather than generalized intelligence, making it arguably better-suited for assessing the effects of formal schooling.”

The test is also administered on the same day of the year, is graded the same way, and most importantly, is frequently taken twice. Even better: the data comes with zip codes, gender, race, and parental education. Put together, all data that can be used to weigh the impact of social factors vs. temperature when it comes to learning. The zip code data also allowed the researchers to have a proxy for income level.

Crunching all this data, the researchers found an strong relationship between heat and learning. When daily maximum temperatures were a single degree Fahrenheit hotter during the school year, students’ test scores were lowered by the equivalent of about one percent of a year’s worth of learning, or being absent for two days

For students living in zip codes with average income in the lowest quintile, heat was three times more damaging to learning than for students from zip codes in the highest income quintile.

Heat is, essentially, an inequality multiplier. The study estimates that heat alone is responsible for up to 13 percent of the education achievement gap between white students and students of color.

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At Johnson’s classroom in Bay Elementary, a third of students are English language learners. The school is a Title I school, which means more than half of its students qualify for free or reduced lunch. Fewer resources means students are already facing greater barriers to suceess. Heat—and lack of air conditioning—is one more barrier standing in their way.

“Part of that is almost certainly that socioeconomically disadvantaged students are more unlikely to have AC in their school, but I don’t think that can be the whole story,” Goodman said. “The wealthier you family or community is, the more ways you have to compensate for lost learning time. If you’re from a higher income family, your parents could have time or hire a tutor. Or maybe you school has a way to help.”

Exactly how heat is causing a drop in learning is something the study did not determine. Goodman and his colleagues suggest heat could be affecting students’ ability to learn or teachers’ ability to teach effectively (or both). Goodman also said schools have been let out early due to heat, meaning less time in the classroom for students to learn.

“Several previous studies have looked at the effect of heat on performance at a point in time,” Janet Currie, a Princeton economist who studies education, told Earther. “What is novel here is looking at heat over the school days in the past year, and showing that it has an effect on children’s test scores. I thought it was particularly convincing that high heat on weekends and summers didn’t seem to affect the test performance, only heat on school days, so that points to an effect on learning.”

There’s one known way to avert most of the heat-related woes. The study found that air conditioning in classrooms offsets nearly all the impacts of heat on learning. That piece of knowledge is crucial for figuring out how to adapt to the global warming we can expect in the coming decades.

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Investing in air condition would offset $25,000 per classroom per year in lost earnings when those students become adults according to the study, which used model projections that show the U.S. could be up to 5 degrees Fahrenheit warmer by mid-century if we continue on a business-as-usual trajectory.

Unfortunately, more air conditioning could also exacerbate climate change because of the potent greenhouse gases air conditioners emit. Running air conditioners also uses lots of electricity, which can lead to further greenhouse gas emissions if the source of that electricity is fossil fuels.

If the world ends up following the Kigali Amendment, which would retire climate-polluting chemicals in air conditioners, then the problem may be partially solved. But right now, it puts education between a rock and hot place. Cool things down and exacerbate climate change, or keep things hot and exacerbate inequality.

“Even in wealthy places like New York City, many schools still lack air conditioning,” Currie said. “So in a world of increasing temperatures, one would expect heat to exacerbate differences between rich and poor kids.”

The study neatly illustrates why climate change is such an intractable problem. It affects everything, and even our best solutions present other problems. Still, there’s hope that win-win scenarios are out there. We just need to start thinking creatively.

“I see school air conditioning as one piece we know for sure how to do,” Goodman said. “It doesn’t mean its the best solution or only solution but it is a potential one. I’ve gotten email from folks about planting trees and shading schools that could help.”