As heat wave ends in the West, scientists try to make sense of its intensity

The scorching spring heat dome that ravaged the West for more than a week has finally progressed, having set more than 1,500 temperature records in 11 states, according to the research group Climate Central.

In its wake, climatologists, irrigation managers and local officials are taking stock of a looming water crisis and trying to understand just how exceptional the heatwave has proven to be. Even before high temperatures arrived, Western states were reporting some of the lowest snowfall numbers in modern history. Today, in many places there is little snow left.

Researchers have long recognized that climate change intensifies heat waves. But some scientists wonder whether new or little-understood climate dynamics could help explain the duration, intensity and scale of this month’s event.

Jennifer Brady, senior data analyst at Climate Central, said the widespread footprint and duration of the heat wave made it an outlier “even with what we’re experiencing now with climate change and what many people are calling our new normal, or our new baseline.”

The organization has developed a metric it calls the Climate Shift Index, which rates the influence of climate change on average daily temperatures from 1 to 5.

About 29% of the country experienced maximum temperatures classified by Climate Central as a “5,” meaning they were at least five times more likely due to climate change. Based on data dating back to 1970, this is the largest part of the U.S. landscape that has experienced such temperature anomalies, the group found.

“This is unprecedented and it can be very dangerous,” Brady said.

The World Weather Attribution group – a consortium of scientists that presents statistical analyzes quantifying the influence of climate change on a particular event – said the first part of the March heatwave was so statistically improbable that it would be considered “virtually impossible” without climate change. Temperatures were 20 to 30 degrees Fahrenheit above average in some locations during this period.

In a report released March 20, the group found that heat waves in the West increased in intensity by more than 7 degrees Fahrenheit due to climate change, and that this event was 800 times more likely than it would have been in a world without global warming.

Climate change is responsible for a change in the distribution of temperatures in different parts of the world. Land is warming faster than oceans, and the western United States is warming faster than other regions, said Karen McKinnon, an associate professor in the department of atmospheric and oceanic sciences at the University of California, Los Angeles.

So even though the global average temperature last year was about 1.47 degrees Celsius (2.65 degrees Fahrenheit) warmer than between 1850 and 1900, “depending on where you are … we can already achieve, you know, 4 or 5 degrees Fahrenheit warming,” McKinnon said.

But she added that scientists are increasingly interested in whether other factors, beyond that baseline warming, contribute to severe heat waves like this one. Some are investigating whether climate change also causes changes in atmospheric dynamics.

This month’s heat wave is the result of what’s known as a heat dome — a system in which a high-pressure system on clear, sunny days parks itself over an area, trapping air like a stove lid and amplifying the warming.

Some scientists believe that climate change is causing large-scale changes in atmospheric circulation patterns that cause heat domes, like the movement of the jet stream. The polar jet stream is a ribbon of air that surrounds the northern hemisphere at high altitudes and determines atmospheric pressure patterns. It often serves as a barrier between the cold air of the Arctic and the warmer air of the South.

Some researchers have hypothesized that climate change may have made the jet stream wavier. The idea suggests that the jet stream is sinking lower and rising higher in latitude than in the past, causing dramatic fluctuations across the continental United States.

McKinnon said climate scientists are making progress investigating these questions but do not yet have satisfactory answers. Many scientists have competing theories, she added, and it could take several years to reach a consensus.

“That’s basically the million-dollar question,” McKinnon said. “Are these circulation patterns made more likely due to climate change?