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主執筆者の Karen Heeter は、オレゴン州クレーター湖近くの古い山ツガからコア サンプルを採取しました。ここには、少なくとも 1 本の木が 1300 年代にさかのぼります。 クレジット: Grant Harley/アイダホ大学
イベントの歴史的な極限は、他の地域への警告として機能します。
2021 年の夏、前例のない熱波が北アメリカ西部を襲い、ブリティッシュ コロンビア州からオレゴン州ワシントン州に至る地域に影響を与え、通常は温暖な気候を経験する他の内陸部地域に影響を与えました。 気温は多くの場所で記録を打ち破り、広範囲にわたる山火事と少なくとも 1,400 人の命の悲劇的な損失をもたらしました。 科学者は、この出来事を主に人間が引き起こした気候変動に起因すると考えており、これを前例のないものとして分類していますが、前世紀までさかのぼる限られた気象データのため、それが本当に以前に発生しなかったかどうかを確実に判断することは困難です.
最近の調査によると、この地域の年輪の分析に基づいて、2021 年の熱波は過去 1000 年間で最も深刻だったことがほぼ確実です。 ジャーナルに掲載された調査結果 気候および大気科学、西暦 950 年から始まる夏の平均気温の年間記録を確立しました。 この調査では、異常な高温を伴う夏が数多くあったことが明らかになりました。その多くは、数年にわたる温暖な期間に発生しました。 しかし、この研究は、人間が引き起こした地球温暖化により、過去 40 年間が最も暑く、2021 年の夏が最も暑かったことを強調しています。
「太平洋岸北西部がこれまで高温の波を経験したことがないというわけではありません。 しかし、気候変動により、その規模ははるかに大きくなり、地域社会への影響ははるかに大きくなります」と、筆頭著者であり、[{” 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] 夏の気温上昇による経済的および生物学的影響を受けない地域はないことを示唆しています」と著者らは書いています。 これは、「歴史的に極度の暑さにさらされたことのない世界中のコミュニティが、猛暑を経験する可能性が高いことを示唆しています。 [greater] 罹患率と死亡率。」
参照: 「北米の太平洋岸北西部を横切る前例のない 21 世紀の暑さ」カレン J. ヒーター、グラント L. ハーレー、ジョン T. アバツォグロウ、ケビン J. アンチュカイティス、エドワード R. クック、ベサニー L. クルサード、ローラ A. ダイおよび Inga K. Homfeld、2023 年 2 月 17 日、 npj 気候および大気科学.
DOI: 10.1038/s41612-023-00340-3
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