In the summer of 2021, a historic heat wave baked the Pacific Northwest. A heat dome stalled over the United States and Canada, causing temperatures to soar to 40°C–50°C (104°F–121°F). More than 650 people died, infrastructure buckled, wildfires sparked, agricultural fields withered, and forests were damaged.
“It was incredible,” said Karen Heeter, a dendrochronologist at Lamont-Doherty Earth Observatory at Columbia Climate School. That summer, Heeter was finishing up her doctorate at the University of Idaho while living in a brick building with no air conditioning. “That experience was the jumping point for me,” she said, and she decided to investigate the history of heat in the Pacific Northwest.
In a new study in npj Climate and Atmospheric Science, Heeter and her colleagues used tree ring records from the Pacific Northwest to reconstruct summer temperatures for the past 1,000 years. They found that 2021 was a scorcher—hotter than any other year in the past millennium. The data hint at a future full of heat waves.
A Millennium of Temperatures
Tree rings capture years of climate signatures: temperature, soil moisture, and wind stressors. A tree records summer temperature in its ring density. Typically, denser rings mean warmer temperatures.
Heeter and her colleagues collected cores from 29 conifer trees in the United States and Canada. “In the Pacific Northwest, there are long-lived species of Douglas fir and mountain hemlock,” said Heeter, adding that land preservation efforts helped protect these old trees. To extend the record back, the team also sampled dead wood.
To make sure they had a continuous record, the researchers overlapped the cores from dead and live trees. Tree rings are like barcodes, and comparing the codes is like pattern matching, Heeter explained. “Wherever they match up, you can validate the dates. Once you’re confident in that, you can keep pushing it back in time.”
Using a method called blue intensity, the team measured ring densities to infer past temperatures. Heeter describes blue intensity as a “very, very up-close remote sensing,” in which the blue wavelet of the visible light spectrum is reflected off the tree ring and measured. The reflectance of a ring’s surface is related to the ring’s density and is used as a proxy for the average ambient summer temperature—when a tree does most of its growing—during that year.
The rings showed that Pacific Northwest conifers grew throughout temperature fluctuations over the past 1,000 years, though there has been significantly more warming in the past few decades.
Since at least 1000 CE—as far back as their samples recorded—the Pacific Northwest has experienced prolonged periods of warmer-than-average temperatures, Heeter said. “But if you are comparing the magnitude, we’re much warmer now than we were back then.”
A Hot Future
The team took the research a step further, using their 1,000 years of data to simulate future climate with the Coupled Model Intercomparison Project Phase 6 (CMIP6) under two different emissions scenarios.
The tree ring data indicate that the 2021 Pacific Northwest heat wave, which reached 3.6°C above the 1951–1980 average, was a 1-in-1,000-year event. However, “we found the 3.6° anomaly becomes so much more likely to happen within the next couple of decades,” Heeter said. The future climate simulation shows that with intermediate levels of emissions (Shared Socioeconomic Pathways 2–4.5), the Pacific Northwest will have a 50% chance of experiencing a heat wave each year over the next few decades.
“This is really a step forward,” said Irina Panyushkina, a dendrochronologist at the University of Arizona who was not involved in the study.
As a check, the team compared the modeled temperatures, measured near-surface air temperatures (from 1950 to 2021), and the 1,000-year tree ring temperature record and found good overlap. Because there are lots of uncertainties in modeling, this check is a way to confirm that the modeling approach was good, Panyushkina said. “It shows that the prediction is quite accurate.” The researchers’ approach could be used in other areas around the world, she added, even expanding to the hemisphere scale.
The likelihood of future heat waves has important implications for ecosystems and human health. Frequent and extreme heat will continue to put stress on the environment, people, and infrastructure. Heeter said she’s already seen trees in heat distress in the Pacific Northwest. “The rate of change at which the Pacific Northwest is warming is really striking,” she said, adding that the trees cannot adapt to abrupt temperature rise.
—Sarah Derouin (@Sarah_Derouin), Science Writer