Tree rings confirm the historic significance of 2021 heat wave

A study examining tree rings in western North America has revealed that the record-breaking heat wave experienced in the summer of 2021 was the most extreme in over a millennium.

The research, published in the journal Climate and Atmospheric Science, establishes a year-by-year record of summer average temperatures dating back to the year 950. The study indicates that the last 40 years, driven by human-influenced climate change, has been the hottest on record, and that the summer of 2021 was the hottest in over 1,000 years.

Lead author Karen Heeter, a postdoctoral researcher at Columbia University’s Lamont-Doherty Earth Observatory, explained the significance of the findings: “It’s not that the Pacific Northwest has never before experienced waves of high temperature. But with climate change, their magnitude is much hotter, and they have a much greater impact on the community.”

The tree-ring reconstruction and modern temperature readings show that the period between 1979 and 2021 saw a sustained increase in hot summers that were unrivaled in the last 1,000-plus years. Most of the hottest years have occurred since 2000. 

The second-warmest period, as indicated by the tree rings, was from 1028 to 1096, during the so-called Medieval Climate Anomaly, when a natural warming trend is thought to have affected large parts of the planet. However, even these historical periods were considerably cooler than temperatures experienced in recent decades.

The researchers in this study collected tree-ring data from about 600 old conifers in northern Idaho and the Cascade ranges of Oregon and Washington. Using a relatively new technique called blue intensity, they measured the density of the tree rings, which can be translated into temperature data. 

The summer of 2021 held the annual record, with an average temperature of 18.9 degrees Celsius (66 degrees Fahrenheit), while the hottest summer in prehistoric times was in 1080, with an average temperature of 16.9 degrees Celsius (62.4 degrees Fahrenheit).

Heeter experienced the 2021 heat wave firsthand, suffering in her un-air-conditioned apartment in Moscow, Idaho, as indoor temperatures reached 105 degrees Fahrenheit. She also noted the challenges faced in collecting data during that summer, as many target forests were on fire, and some areas were under evacuation orders.

The research highlights the vulnerability of communities that have not been historically exposed to extreme heat, as they are often poorly prepared to cope with such events. Heeter suggests that creating refuges where people can go when extreme heat events occur again could be a more feasible solution than installing air conditioning everywhere. 

The experts warn that “communities across the world that have not been historically exposed to extreme heat are likely to experience greater morbidity and mortality.”

The study’s findings also underscore the global nature of the climate crisis, as even regions with historically moderate climates are experiencing unprecedented temperature extremes. As climate change continues to progress, the likelihood of such heat waves occurring more frequently increases, necessitating adaptation and preparedness measures in vulnerable communities around the world.

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