New York had one of its 10 warmest Augusts ever last year, driven mostly by unprecedented nighttime heat.Shannon Stapleton / Reuters

Every summer until fifth grade, my family took the same vacation to the same town on the Jersey Shore. The days were about as idyllic as possible: lots of sunshine, elaborate sandcastles, afternoon pizza from a few blocks away. At bedtime, when the breeze came in off the ocean, we turned off the cranky AC unit in the living room and cracked the windows. I lay in the top bunk in the salt-clean air and listened. I’m not sure if the sound I heard—a gentle whoosh that rose and fell—came from the ocean or the street. Half-remembered waves, it turns out, arrive and depart at the same pace as light-controlled traffic. Growing up in New Jersey, you learn that a very thin barrier separates the human world from the natural world.

Last summer, my family went back to the same shore town. Without giving it much thought, I looked forward to that old rhythm of hot days and cool evenings, of falling asleep with the windows open, with the Atlantic breeze sneaking in and the lulling patter of waves (or cars) outside. But we were there for two weeks, and we opened the windows at night only once. Even at midnight, temperatures hung in the high 70s Fahrenheit, and the humidity made it feel even hotter.

This wasn’t some fluke. It was happening across the entire Northeast.

Something odd happened in August 2018, the same month my family was down the shore. Five states—Delaware, Connecticut, Rhode Island, Massachusetts, and New Hampshire—notched their hottest August ever measured. Three more states, including New Jersey, recorded their second-warmest August ever. And in fact, every state from Maryland to Maine had at least a top-10 warm August.

Yet there was no severe heat wave last August, nothing like the rash of searing 110-degree highs that helped turn this July into the hottest month ever measured. And if you went purely off the most memorable temperatures—the scorching midday highs that can prompt heat stroke—then last August may have seemed bad, but not entirely without precedent. Ranked just by their daytime high temperatures, August 1980, 1995, and 2001 would all come before August 2018.

But August 2018 was hotter than all those months. What made it exceptional were not the hot days but the warm nights, according to the National Oceanic and Atmospheric Administration. “The record and near-record warmth across the Northeast was driven largely by record warm overnight low temperatures,”says NOAA’s postmortem on the month. And all 11 states from Maryland to Maine broke their all-time August nighttime record.

This pattern is not limited to the Northeast. Across the lower 48 states, summer nights are now warmer than they have been at any other point since 1900, according to data from the climate scientist Zeke Hausfather. Daily minima—in other words, the temperature at the coolest part of the night—are regularly hotter now than they were even during the warmest years of the Dust Bowl. Daytime highs have actually yet to eclipse that mark.

Over the past few decades, warm nights have accumulated most ominously in the United States. In the Southeast, for instance, warm nights are most clearly on the rise, according to the most recent National Climate Assessment.

A graph of the number of warm nights in the Southeast over the last few decades
National Climate Assessment

Warm nights can also be more dangerous, in some cases, than scorching days. If a heat wave strikes only during the day, then elderly or poor people without air conditioning can open their windows and cool off at night, research has found. But if temperatures remain dangerously elevated through the night, then people’s bodies have little time to cool off.

Knowing when any one weather event is related to climate change can be hard. So let’s be clear: Warmer nights—even more than hot days—are a symptom of human-caused climate change. At the most basic level, the planet is warming because carbon dioxide and greenhouse gases trap the sun’s heat and prevent it from radiating back into space. You are noticing that effect now when the sun sets … and temperatures don’t.

Many climate-related trends must be compared with the 1950s or 1880s to stand out. But I’ve noticed this symptom worsen in the few decades I’ve been alive. It’s not like I’m particularly old: I was born in the early 1990s, and many of my childhood summers at the beach date back less than two decades. Yet in the interval between then and now, something about my home changed. An act as simple as sleeping with the windows open—once a cherished everyday ritual—became a rare and special treat. Make no mistake: In the coming years, climate change will hurt things, harm people, and trouble the natural world in ways far worse than this. It already is. But it will also rob us of expectation and tradition, and force us to live on a planet far less lovely than the one we were born to.

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