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It wasn’t even a week into the new school year, and already Marsena Toney’s students were being sent home.

But this time, the reason wasn’t Covid. It was extreme heat, which had pushed the temperature in Toney’s Philadelphia classroom to 100 degrees and led the school district to cancel afternoon classes for tens of thousands of students.

“The students are lethargic, it’s extremely difficult to get them motivated,” said Toney, an autistic support specialist at John S. Jenks, one of 118 Philadelphia schools affected by the closures in the district, which serves predominantly Black and Hispanic students. “They’re sweating and trying their best to keep cool, and you have to hold it together yourself because you’re sweating and trying your best to keep cool.”

It’s a pattern that’s becoming more common across the United States, as climate change contributes to record-breaking heat waves. In just the last few weeks, schools in California, Colorado, Pennsylvania and Maryland have closed for “heat days,” and students have suffered through sweltering temperatures in classrooms without air conditioning while trying to get back into the swing of learning.

Many school systems are gravely unprepared for a warming world. While there’s no comprehensive data on how many schools lack air conditioning, some 41 percent of school districts responding to a 2020 Government Accountability Office survey reported that the majority of their school buildings needed heating and cooling upgrades. Wildfires, floods and other extreme weather events that have worsened due to climate change are also straining school infrastructure and disrupting learning.

Related: Climate change is sabotaging education for America’s students

Excessive heat impairs the brain’s functioning and makes it harder for students to learn. Studies by economists Jisung Park of the University of Pennsylvania and Joshua Goodman of Boston University, among others, show that without air conditioning, a 1 degree Fahrenheit increase in temperature reduces a student’s learning by 1 percent.

High temperatures deepen racial divides. Black and Hispanic students tend to live in parts of the country that are more susceptible to extreme heat, and they’re less likely to attend schools or live in homes that are air conditioned. In a 2020 study, Park and Goodman, along with researchers Michael Hurwitz and Jonathan Smith, found that hot school days explain about 5 percent of the racial gap in PSAT scores.

“Air quality and air flow and the working conditions of classrooms that are not 90 degrees to learn – it’s a prerequisite to successful learning and teaching,” Miguel Cardona, the education secretary, said at a briefing last week. 

Schools can use some of the nearly $190 billion in federal pandemic relief funds they received, in three rounds, for infrastructure improvements such as heating and cooling upgrades. An analysis by FutureEd, a think tank at Georgetown University, found that education systems planned to spend about 23 percent of their third tranche of the federal money on infrastructure. 

Some schools, like many in Philadelphia, require electrical rewiring so that they can simply support air conditioning units. And in some cases, school building improvement projects have been slowed down by supply chain issues. 

“The infrastructure of our schools is just very old,” said Jerry T. Jordan, president of the Philadelphia Federation of Teachers, which has called for using federal funds to rewire schools and bring air conditioning to every school building. “They have not been well maintained over the years; they are not equipped to even handle a number of window air conditioners.”

Christina Clark, communications officer with the Philadelphia school district, wrote in an email that the district intends to upgrade wiring and add air conditioning in its schools, in part using federal dollars. Given the extent of the work, though, it will take until 2027 before all schools are air conditioned, she wrote.  

But air conditioning units also present a conundrum: It takes electricity to run them, which is often powered by fossil fuels. In many cases, air conditioning also releases climate-warming chemicals known as hydrofluorocarbons.

“It is critically important that as schools look to add air conditioning that they install options that don’t exacerbate our climate problem,” Laura Schifter, a senior fellow with the energy and environment program at the Aspen Institute, wrote in an email.

Switching to renewable sources of electricity, like geothermal power; installing solar panels with battery storage; and converting asphalt playgrounds to green schoolyards are among the ways schools can cool down while lowering their carbon footprint, Schifter wrote.

Ben Hobbs, a principal with the Kentucky-based engineering company CMTA, said the federal pandemic relief funds have helped nudge more school districts to embrace “net zero” emissions. His firm is working with school systems including those in Bath, Casey and Rowan counties, all in Kentucky, to use geothermal to power upgraded HVAC systems, he said.  

Related: How the effects of climate change threaten student mental health

Without significant building improvements, more learning interruptions are expected, a difficult pill for families after years of Covid disruptions. Cat Wade, a parent in Baltimore, spent recent afternoons bringing her two young sons to work with her after their elementary school closed early due to heat. The school is one of nearly two dozen Baltimore City Schools that lacks adequate air conditioning.

“It’s not possible to parent and work at the same time,” said Wade, a health and human performance coach. “It feels like they are either sitting in front of the TV or getting partial attention or they are being asked to be quiet, and that’s a bad feeling.”

Her kids were just adjusting to school and she worried that they would have trouble building rapport with teachers.

Toney, the teacher in Philadelphia, has a similar worry. She works with students who have autism, and establishing routines is critically important. Moving from half days back to full days caused minor chaos in her classroom.

“Many of the students were upset, they were frustrated, they weren’t sure what was going on,” she said. 

“It was just a lot of confusion to start the new year.”

This story about extreme heat was produced by The Hechinger Report, a nonprofit, independent news organization focused on inequality and innovation in education. Sign up for Hechinger’s newsletter

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Caroline Preston is a deputy managing editor who helps oversee Hechinger's K-12 and higher ed coverage. She previously worked as a features editor with Al Jazeera America’s digital team and a senior...

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