In summer 2021, a stunning heat wave swept western North America, from British Columbia to Washington, Oregon and beyond into other inland areas where the climate is generally mild. Temperature records were set by tens of degrees in many places, wildfires broke out, and at least 1,400 people died. Scientists blamed the event largely on human-driven climate warming, and declared it unprecedented. But without reliable weather data going back more than a century or so, did it really have no precedent?