Mehr als 1.000 Jahre Baumringe bestätigen die beispiellose Natur der Hitzewelle 2021 im Westen Nordamerikas

Die Hauptautorin Karen Heeter entnimmt eine Kernprobe von einer alten Gebirgsschierlingstanne in der Nähe von Crater Lake, Oregon, wo mindestens ein Baum aus dem 14. Jahrhundert stammt. Bildnachweis: Grant Harley/Universität Idaho

Das historische Ende des Ereignisses dient als Warnung für andere Regionen.

Im Sommer 2021 fegte eine beispiellose Hitzewelle über den Westen Nordamerikas und betraf Regionen von British Columbia bis Washington, Oregon, und darüber hinaus auch andere Innengebiete, in denen normalerweise ein mildes Klima herrscht. An vielen Orten brachen die Temperaturen Rekorde, was zu weit verbreiteten Waldbränden und dem tragischen Verlust von mindestens 1.400 Menschenleben führte. Während Wissenschaftler dieses Ereignis in erster Linie dem vom Menschen verursachten Klimawandel zugeschrieben und es als beispiellos bezeichnet haben, ist es aufgrund begrenzter Wetterdaten, die nur bis ins letzte Jahrhundert zurückreichen, schwierig, mit Sicherheit festzustellen, ob es wirklich kein vorheriges Ereignis gegeben hat.

Laut einer aktuellen Studie, die auf der Analyse von Baumringen aus der Region basiert, war die Hitzewelle 2021 mit ziemlicher Sicherheit die schwerste der letzten tausend Jahre. Die Ergebnisse, die in der Zeitschrift veröffentlicht wurden Klima- und Atmosphärenwissenschaft, stellte ab 950 n. Chr. eine jährliche Aufzeichnung der Sommerdurchschnittstemperaturen auf. Die Forschung ergab zahlreiche Sommer mit ungewöhnlich hohen Temperaturen, von denen viele in mehrjährigen Wärmeperioden auftraten. Die Studie hebt jedoch hervor, dass die letzten 40 Jahre aufgrund der vom Menschen verursachten globalen Erwärmung die heißesten waren, wobei der Sommer 2021 der wärmste von allen war.

„Es ist nicht so, dass der pazifische Nordwesten noch nie Wellen mit hohen Temperaturen erlebt hat. Aber mit dem Klimawandel ist ihr Ausmaß viel heißer und sie haben viel größere Auswirkungen auf die Gemeinschaft“, sagte Hauptautorin Karen Heeter, Postdoktorandin bei[{” attribute=””>Columbia University’s Lamont-Doherty Earth Observatory. “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.”

Summer seasonal temperature anomalies revealed by tree rings and modern weather data, 1950-2021. Credit: Modified from Heeter et al., Climate and Atmospheric Science, 2023

The tree-ring reconstruction and modern temperature readings show that 1979-2021 saw a sustained period of hot summers unrivaled for the last 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 a several weeks from late June to mid-July. While the researchers did not try to pick out such short periods in the rings, they say average seasonal temperatures are a good proxy for such events. Summer 2021 held the annual record, at 18.9 degrees Centigrade, or about 66 degrees Fahrenheit. By contrast, the hottest summer in prehistoric times was in 1080, at 16.9 degrees C, or 62.4 F.

This perhaps does not sound very impressive—until you consider that due in part to the near-complete human destruction of ancient trees in the lowlands, the researchers used mainly samples collected at mountain elevations above 10,000 feet. Here, temperatures are drastically lower than in the populous lowlands; there is often still snow cover in June. “You have to think about it in the broader context,” said Heeter; one can reasonably add a few tens of degrees for places like Seattle and Portland, she noted. According to the tree rings, the 2021 seasonal temperature spike was nearly 3 degrees F greater than any annual spike shown by tree rings during the Medieval period.

The summer 2021 western North America heat wave. Redder colors represent higher temperature anomalies; white X’s indicate sites where researchers took tree-ring samples to put it into a long-term context. Credit: Modified from Heeter et al., Climate and Atmospheric Science, 2023

Heeter and her husband and a few colleagues collected about half of the samples for the study during the summers of 2020 and 2021, from high-elevation sites in national forests and parks. She got a personal taste of the 2021 heat wave as she sweltered in 105-degree indoor temperatures in her un-air-conditioned apartment in Moscow, Idaho. She feared going into the field until later in the season, since many target forests or ones near them were on fire, and in some cases, she was blocked from entering by evacuation orders.

To obtain data, the team bored out straw-size samples that provided cross sections of rings from about 600 old conifers in northern Idaho and the Cascade ranges of Oregon and Washington. (The coring process does not hurt the trees.) Their oldest sample came from a mountain hemlock near Oregon’s Crater Lake, which took root in the 1300s. They supplemented these with samples taken in the 1990s by other Lamont-Doherty researchers, mostly in British Columbia. The oldest of these was from a Douglas fir on Vancouver Island, dating to the year 950. The area has since been clear-cut by loggers.

Most conventional tree-ring studies focus on ring widths, with wider annual rings generally indicating wetter years. To measure temperature, Heeter and her colleagues instead used a relatively new technique called blue intensity. This involves shining visible light onto a high-resolution scan of each ring, and measuring how much of the blue spectrum is reflected back. Trees generally build thicker cell walls in hotter temperatures, increasing the density of the ring. Denser rings reflect less blue light, and this can be translated into temperature.

Center, a Douglas fir in the Tahoma Creek vicinity of Washington’s Mt. Rainier National Park, from which the authors took a core sample. Credit: Grant Harley/University of Idaho

Another recent Lamont-Doherty study attributed the extremity of the 2021 heat wave to progressively heightening temperatures caused by humans, combined with shorter-term atmospheric patterns that may or may not have been driven by human-driven climate change. That study suggested that by 2050, such heat waves may hit every 10 years. The new one, which used different models to make forecasts, estimates a 50/50 chance of recurrence each year by 2050.

With a climate that is usually quite moderate, the region is poorly prepared to cope with such events. For one thing, like Heeter, few people have air conditioning—possibly one reason for the high mortality rate in 2021. “We can use the long-term record to prepare ourselves,” said Heeter. “For instance, maybe it’s not realistic to put air conditioning everywhere, but communities could create refuges where people could go when these things happen again.”

“The unprecedented nature of summer 2021 temperatures across [the study area] legt nahe, dass keine Region immun gegen die wirtschaftlichen und biologischen Auswirkungen steigender Sommertemperaturen ist“, schreiben die Autoren. Dies deutet darauf hin, dass „Gemeinschaften auf der ganzen Welt, die historisch keiner extremen Hitze ausgesetzt waren, wahrscheinlich davon betroffen sein werden [greater] Morbidität und Mortalität.”

Referenz: „Beispiellose Hitze des 21. Jahrhunderts im pazifischen Nordwesten Nordamerikas“ von Karen J. Heeter, Grant L. Harley, John T. Abatzoglou, Kevin J. Anchukaitis, Edward R. Cook, Bethany L. Coulthard, Laura A. Dye und Inga K. Homfeld, 17. Februar 2023, npj Klima- und Atmosphärenwissenschaft.
DOI: 10.1038/s41612-023-00340-3