More than 1,000 years of tree rings confirm historic extremes of 2021 western North American heat wave
Unprecedented nature of events contains warnings for other regions
In the summer of 2021, an astonishing heat wave swept across western North America, from British Columbia to Washington, Oregon, and beyond the generally mild interior. Temperatures in many places set records of tens of degrees, wildfires broke out, and at least 1,400 people died. Scientists have largely blamed the event on human-driven warming, declaring it unprecedented. But is it really without precedent without reliable weather data dating back more than a century?
A New study of tree rings Climate change in the region suggests the event will almost certainly be the worst for at least the past millennium.The study was published in the journal Climate and Atmospheric Sciences, established year-by-year records of summer mean air temperatures dating back to 950 years. Dozens of unusually hot summers have occurred, many of them divided into multi-year warm periods. But the new study shows that the past 40 years have been the hottest – driven by human-influenced warming – and 2021 is the hottest summer of the entire span.
“That’s not to say the Pacific Northwest hasn’t experienced heat waves before. But with climate change, they’re much stronger and their impact on communities is much greater,” said the lead author Karen HitePostdoctoral Research Fellow, Columbia University Lamont-Doherty Earth Observatory“There’s tremendous power in being able to go back in time and compare it to climate models and come to similar conclusions.”
Lead author Karen Heeter took core samples from an ancient mountain hemlock near Crater Lake, Oregon, where at least one tree dates back to the 1300s. (Grant Harley/University of Idaho)
Tree-ring reconstructions and modern temperature readings suggest that the period 1979-2021 experienced a sustained hot summer unmatched in the past 1,000-plus years. Most of the warmest years have occurred since 2000. The second warmest period shown by the tree rings was 1028-1096 – the peak of the so-called Medieval Climate Anomaly, when natural warming trends are thought to have developed across much of the planet. Another notable thermal span during the Medieval Climate Anomaly is from 1319 to 1307. But even those periods were much cooler than in recent decades.
The 2021 heat wave lasted for several weeks from late June to mid-July. While the researchers didn’t try to pinpoint such short periods of time in the rings, they say average seasonal temperatures are a good proxy for such events. Summer 2021 holds the annual record at 18.9 degrees Celsius, or about 66 degrees Fahrenheit. By comparison, the hottest summer in prehistoric times was in 1080, at 16.9 degrees Celsius or 62.4 degrees Fahrenheit.
That might not sound impressive—unless you consider that due in part to the near-total destruction of ancient lowland trees by humans, the researchers primarily used samples collected in mountainous regions above 10,000 feet above sea level. The temperature here is much lower than in the densely populated lowlands. There is often snow in June. “You have to think about it in a broader context,” Heeter said. She noted that places like Seattle and Portland could legitimately see tens of degrees increase. According to tree rings, the seasonal temperature peak in 2021 was nearly 3 degrees Fahrenheit higher than any annual peak shown by tree rings in the medieval period.
Top, 2021 summer heat wave in western North America. Redder colors indicate higher temperature anomalies; white Xs indicate where researchers took tree ring samples to place in long-term environments. Above, summer seasonal temperature anomalies revealed by tree rings and modern weather data, 950-2021. (both modified from Heeter et al., Climate and Atmospheric Sciences, 2023)
Hieter, her husband, and several colleagues collected about half of the study samples during the summers of 2020 and 2021 from high-altitude locations in national forests and parks. She experienced the 2021 heat wave firsthand when she sweltered in a 105-degree indoor temperature in her non-air-conditioned apartment in Moscow, Idaho. She was afraid to go into the wild until later in the season, as many of the target forests or nearby forests were on fire, and in some cases evacuation orders prevented her from entering.
To obtain the data, the team drilled straw-sized samples that provided tree-ring cross-sections from about 600 ancient conifer trees in northern Idaho and the Cascade Mountains of Oregon and Washington. (The coring process doesn’t harm the tree.) Their oldest sample came from a mountain hemlock near Crater Lake in Oregon, which took root in the 1300s. They supplemented these samples with samples collected in the 1990s by other Lamont-Doherty researchers, mainly in British Columbia. The oldest of these is Douglas fir from Vancouver Island, dating back to 950 years. The area has since been cleared by loggers.
Most traditional tree-ring studies have focused on ring width, with wider rings generally indicating wetter years. To measure temperature, Heeter and her colleagues used a relatively new technique called blue intensity. This involves shining visible light onto high-resolution scans of each ring and measuring how much of the blue spectrum is reflected back. Trees typically build thicker cell walls at higher temperatures, increasing ring density. Dense rings reflect less blue light, which translates to temperature.
Another recent Lamont-Doherty study Attribute the extremes of the 2021 heat wave to gradual increases in human-induced temperatures, as well as short-term atmospheric patterns that may or may not be driven by human-driven climate change. Such heat waves could hit once every 10 years by 2050, the study suggests. The new model uses a different model to make predictions, estimating a 50/50 chance of relapse each year through 2050.
The center is a Douglas fir near Tahoma Creek in Mount Rainier National Park, Washington, from which the authors collected core samples. (Grant Harley/University of Idaho)
The region is not well prepared for such an event, as the climate is usually quite mild. For one thing, like Hitter, few people have air conditioning — which could be one reason for the high death rate in 2021. “We can use our long-term record to prepare,” Hieter said. “For example, it might not be practical to install air conditioning everywhere, but communities can create shelters where people can go when these things happen again.”
“The unprecedented nature of summer temperatures in 2021 [the study area] This suggests that no region is immune to the economic and biological impacts of higher summer temperatures,” the authors wrote. It suggests, they say, that “communities around the world that have not historically been exposed to extreme heat may experience [greater] Morbidity and Mortality. “
Additional authors on the study are Grant Harley of the University of Idaho; John Abatzaglou of the University of California, Merced; Kevin Anchukaitis of the University of Arizona; Bethany Coulthard and Laura Dye of the University of Nevada; ; and Edward Cook of Lamont-Doherty.
media inquiries 
Kevin Krajic
(917) 361-7766
kkrajick@ei.columbia.edu
Caroline Adelman
(917) 370-1407
ca2699@columbia.edu
