1,000 years of tree rings confirm 2021 heat wave was historic

“Being able to look at the past and compare that with climate models and come to similar conclusions, there’s a lot of power in that.”

Unmatched with the previous years

The tree-ring reconstruction and current temperature measurements reveal that 1979–2021 experienced a run of exceptionally hot summers that were unmatched in the previous 1,000–plus years. Most of the hottest years have occurred since 2000.

The second-warmest period, indicated by the tree rings, was 1028-1096—at the height of the so-called Medieval Climate Anomaly, when a natural warming trend is thought to have taken hold across large parts of the planet. Another notable hot span during the Medieval Climate Anomaly ran from 1319 to 1307. But even these periods were considerably cooler than temperatures in recent decades.

The 2021 heat wave spanned several weeks, from late June to mid-July. While the researchers did not try to pick out such small periods in the rings, they say average seasonal temperatures are a fair proxy for such events. Summer 2021 held the annual record at 18.9 degrees Celsius, or nearly 66 degrees Fahrenheit. By contrast, the hottest summer in prehistoric times was in 1080, at 62.4 Fahrenheit (16.9 degrees Celsius).

“The unprecedented nature of summer 2021 temperatures across [the study area] suggests that no region is impervious to the economic and biological impacts of increasing summer temperatures,” the authors write. This suggests, they say, that “communities across the world that have not been historically exposed to extreme heat are likely to experience [greater] morbidity and mortality.”

The full study was published in Nature and can be found here.

Study abstract:

Extreme summer temperatures are increasingly common across the Northern Hemisphere and inflict severe socioeconomic and biological consequences. In summer 2021, the Pacific Northwest region of North America (PNW) experienced a 2-week-long extreme heatwave, which contributed to record-breaking summer temperatures. Here, we use tree-ring records to show that summer temperatures in 2021, as well as the rate of summertime warming during the last several decades, are unprecedented within the context of the last millennium for the PNW. In the absence of committed efforts to curtail anthropogenic emissions below intermediate levels (SSP2–4.5), climate model projections indicate a rapidly increasing risk of the PNW regularly experiencing 2021-like extreme summer temperatures, with a 50% chance of yearly occurrence by 2050. The 2021 summer temperatures experienced across the PNW provide a benchmark and impetus for communities in historically temperate climates to account for extreme heat-related impacts in climate change adaptation strategies.