What Caused the 2021 Pacific Northwest Heat Wave?

Scientists have identified the physical drivers behind a record-shattering heat wave in Western North America. The analysis may help elucidate how climate change is connected to the magnitude and frequency of such extreme events.

In late June and early July 2021, parts of the Pacific Northwest suffered nine days of extreme heat, setting new records on consecutive days. Typically, temperature records are broken incrementally. But this event, which topped out at temperatures of 49.6°C (121.3°F), exceeded past peaks by tens of degrees Celsius. At the time, many considered this impossible under current climate conditions.

Now, a new study led by Samuel Bartusek, a PhD candidate in the Dept. of Earth and Environmental Sciences at Columbia Univ., explains how various climate and weather mechanisms came together to enable this outsized heat wave.

Bartusek and his colleagues found that a combination of unprecedented conditions was responsible, including atmospheric factors, land conditions, and rising global temperatures. “When all these things interact with each other, this heat wave was allowed to add up to more than the sum of its parts,” he says.

One key driver to the event was an unusual jet stream pattern. In late June 2021, a meteorological anomaly stretched across the entire Northern Hemisphere, forming the polar jet stream into a large and wavy configuration. This structure, which is associated with wildfires, is an essential precondition to the weather phenomena known as a “heat dome.”

Heat domes are created when the jet stream bends into a...