Schools throughout the United States canceled classes or ended early several days last week as temperatures rose above 90 degrees in parts of the country, exceeding historic averages, according to the New York Times.

The closures are part of what could become a routine phenomenon in the years ahead — heat days.

Just as heavy snowfall keeps kids out of the classroom because of treacherous roads, extremely hot days in schools without adequate air conditioning can endanger students’ health and degrade the learning environment by making people feel lethargic.

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In fact, school year schedules are often crafted to avoid extremely hot days. In states like Connecticut and New Jersey, which aren’t accustomed to such hot days in September, last week’s weather was difficult to navigate and classes ended up being canceled in various schools.

“From Long Island to Central New York, we’re looking at heat indexes of 100 degrees,” Andy Pallotta, president of New York State United Teachers, told the Times. “It’s very hot, and the kids are lethargic.”

Since 2000, 16 of the 17 hottest years in recorded history have occurred, and temperatures are expected to continue rising as climate change intensifies from the ongoing accumulation of greenhouse gas emissions in the atmosphere.

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Earlier in the year, extreme heat waves caused widespread deaths in countries as diverse as Pakistan, the United Kingdom, and Japan.

In Pakistan, for example, a lack of accessible air conditioning combines with a lack of clean water access and electricity to make heat waves especially dangerous.

Heat waves aren’t merely disrupting schools, either. Many industries, including construction, road maintenance, policing, and more, are made considerably more difficult and dangerous by extreme temperatures.

In fact, the global economy could lose trillions of dollars in lost productivity because of climate change.

Read More: Climate Change Is Real. So Why Isn't It Being Taught in US Schools?

Many kids throughout the US no doubt met last week’s heat day closures with glee, but school administrators and public officials are scrambling to figure out this new reality.