An analysis of over 1,000 years of tree ring data confirmed that the 2021 heatwave in western North America was of unparalleled magnitude.


Karen Heeter, the study's lead author, takes a core sample from an ancient mountain hemlock near Crater Lake, Oregon, where at least one tree dates back to the 1300s. Photographer: Grant Harley/University of Idaho

A new study of tree rings from western North America shows that the 2021 heat wave was the worst in at least the past thousand years, according to research published in the journal npj Climate and Atmospheric Science. The study established a year-by-year record of summer average temperatures going back to the year 950, with scores of abnormally hot summers showing up, many grouped into multiyear warm periods. However, the last 40 years, driven by human-influenced warming, have been the hottest, and 2021 was the hottest summer in the entire span. The research confirms that human-driven climate warming has led to hotter and more impactful high-temperature waves.

The period between 1979 and 2021 had the hottest summers in over 1,000 years, as indicated by the tree-ring reconstruction and modern temperature readings. The majority of the hottest years occurred after the year 2000. The second-warmest period was from 1028 to 1096, which was during the Medieval Climate Anomaly, a time when the planet was thought to have undergone a natural warming trend. There was another notable warm period from 1319 to 1307 during this time, but both periods were much cooler than the temperatures seen in recent decades.

Anomalies in summer seasonal temperature found by tree rings and modern weather data, 950-2021. Source: Heeter et al., Climate and Atmospheric Science, 2023.

In the summer of 2021, there was a heatwave that lasted for several weeks from late June to mid-July. Researchers did not specifically analyze short periods in the tree rings but instead used the average seasonal temperatures to determine the intensity of the event. The summer of 2021 set a new record with an average temperature of 18.9 degrees Celsius (66 degrees Fahrenheit), while the hottest summer in prehistoric times occurred in 1080 with an average temperature of 16.9 degrees Celsius (62.4 degrees Fahrenheit).

Although the temperature difference may not seem significant, it should be noted that the researchers mainly used samples collected from mountain elevations above 10,000 feet, as most ancient trees in the lowlands have been destroyed by humans. Temperatures at high elevations are much lower than in the lowlands, where temperatures can be much higher. The 2021 seasonal temperature spike was almost 3 degrees Fahrenheit higher than any annual temperature spike during the Medieval period, but it is reasonable to add a few tens of degrees for areas such as Seattle and Portland. Therefore, it is important to consider the broader context when interpreting the temperature data from the tree rings.

Heeter, along with her husband and a few colleagues, gathered around 50% of the samples for the study over the summers of 2020 and 2021 from sites at high elevations located in national forests and parks. During the 2021 heatwave, Heeter experienced extremely high indoor temperatures of 105 degrees Fahrenheit in her un-air-conditioned apartment located in Moscow, Idaho. Due to the fires in the forests or areas near them, she was hesitant to venture out into the field at the start of the season. In some instances, she was even unable to enter certain locations due to evacuation orders.

Summer heat surge in western North America in 2021. Redder colors reflect higher temperature anomalies, while white Xs show locations where researchers collected tree-ring samples to put the data into context over time. Source: Heeter et al., Climate and Atmospheric Science, 2023.

+The research team collected data by extracting straw-sized samples from around 600 old conifers in northern Idaho, as well as the Cascade ranges of Oregon and Washington. This process did not harm the trees, and the team obtained cross-sections of rings from the samples. To supplement their data, the team also used samples taken in the 1990s by other researchers from British Columbia. The oldest sample was from a Douglas fir on Vancouver Island, dating back to the year 950. 

Unfortunately, the area where the sample was obtained has since been cleared by loggers. The team used a relatively new technique called blue intensity to measure temperature, which involves shining visible light onto a high-resolution scan of each ring and measuring the amount of blue spectrum reflected back. Trees tend to build thicker cell walls in hotter temperatures, which increases the density of the ring. Denser rings reflect less blue light, allowing this method to be used to calculate the temperature.

A recent study conducted by Lamont-Doherty researchers attributed the severity of the 2021 heat wave to progressively increasing temperatures caused by human activities, along with short-term atmospheric patterns that may or may not have been linked to human-driven climate change. The study predicted that by 2050, heat waves like the one experienced in 2021 may occur every ten years. However, the new study used different models to make forecasts and predicts that there is a 50/50 chance of such heat waves occurring each year by 2050.

The writers took a core sample from a Douglas fir in the Tahoma Creek area of Washington's Mt. Rainier National Park. Photographer: Grant Harley/University of Idaho

The region is not accustomed to experiencing extreme weather events and is therefore not well-equipped to handle them. In particular, many people in the region, like Heeter, do not have air conditioning, which may have contributed to the high death rate in 2021. However, Heeter suggests that the long-term record can be used to prepare for future events, for example, by creating refuges where people can go during heat waves.

The authors of the study suggest that the unprecedented temperatures experienced in the study area during the summer of 2021 indicate that no region is immune to the negative effects of rising temperatures. They warn that communities around the world that have not previously been exposed to extreme heat are likely to experience increased rates of illness and death.

Source: Karen J. Heeter et al, Unprecedented 21st-century heat across the Pacific Northwest of North America, npj Climate and Atmospheric Science (2023). DOI: 10.1038/s41612-023-00340-3


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