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 between Maryland and 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 post mortem on the month. And all 11 states between Maryland and 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.
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.