The Pacific Northwest is sweltering under a record-breaking heat wave. Portland reached 116 degrees Fahrenheit this week. Seattle reached 108 degrees. Vancouver reached 89 degrees. The searing heat has buckled roads, melted power cables, and led to a spike in deaths. It’s especially concerning in a region like the Pacific Northwest, where few buildings have air conditioners.

This follows weeks of extremely high temperatures across the Northern Hemisphere and an early-season heat wave in North America that triggered heat warnings for 50 million people. Scientists say these record highs align with their expectations for climate change, and warn that more scorchers are coming.

There’s more to heat waves like this than high temperatures. The forces behind them are complex and changing. They’re a deadly public health threat that can exacerbate inequality, cause infrastructure to collapse, and amplify other problems of global warming. Even more worrying is that in the context of the hot century ahead, 2021 may go down in history as a relatively cool year.

Heat waves, explained

Extreme heat might not seem as dramatic as hurricanes or floods, but the National Weather Service has deemed it the deadliest weather phenomenon in the US over the past 30 years, on average.

What counts as a heat wave is typically defined relative to local weather conditions, with sustained temperatures in the 90th to 95th percentile of the average in a given area. So the threshold for a heat wave in Tucson is higher than the threshold in Seattle.