Important ocean circulation in the Atlantic Ocean is showing signs of collapse, researchers warn (2/2)

Recently CNN.co.jp reported the following:

大西洋の重要な海洋循環、崩壊の予兆を示しつつある　研究者ら警鐘 (2/2)

2024.02.10 Sat posted at 20:07 JST

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ＡＭＯＣ崩壊の影響は破滅的なものになる可能性がある。欧州の一部地域では１００年間で気温が３０度低下し、わずか１０～２０年の間にまったく別の気候に変化するかもしれない。

論文の著者らは「どんな適応策もそこまで急激な気温の変化には現実的に対応できない」と指摘する。

一方、南半球の国では温暖化が加速する可能性がある。アマゾンでは雨期と乾期が逆転し、生態系に深刻な影響を及ぼす可能性がある。

ファンウェステン氏によると、海水面は約１メートル上昇する可能性があるという。

論文に関わっていないドイツ・ポツダム大学の海洋物理学者、ステファン・ラームストルフ氏は、今回の研究成果を「ＡＭＯＣの安定性に関する科学で大きな進展」と評価する。「北大西洋が淡水で薄まれば、ＡＭＯＣが崩壊するという転換点があることを確認した」

以前にも転換点の存在を示す研究はあったが、はるかに簡単なモデルを採用していた。科学者の間では、複雑なモデルでは転換点が存在しないのではないかと期待する声もあったが、今回の研究でそれが打ち砕かれたと同氏は語る。

英国の国立海洋学センターで海洋システムのモデル形成に携わるジョエル・ハーシ氏は、大洋に比較的少量の淡水が入ることでＡＭＯＣが「オン」から「オフ」に切り替わることを、複雑な気候モデルで示した初の研究だと評価した。

ただ注意すべき点もあり、複雑なモデルといってもまだ解像度が低く、一部の海流ではモデルとしての限界がありうるという。

近年、ＡＭＯＣが転換点に近づいている可能性を示す研究が増えている。

２０２１年の研究では、ＡＭＯＣが過去１０００年で最も弱くなっていることが示された。昨年７月に公開された論文は、早ければ２０２５年にもＡＭＯＣが崩壊する可能性があると警鐘を鳴らし、議論を呼んだ。

ただ、まだ不確実性も大きい。米アリゾナ州にある惑星科学研究所のジェフリー・ガーゲル氏は、ＡＭＯＣの停止が迫っているとする理論は、それが起きているとわかるまでは論議を呼ぶものであり続けるのではと考える。

ＡＭＯＣの崩壊の可能性はいわば「株式市場の荒っぽい変動のうち、大暴落の前触れになるもの」にあたり、どの変化が元に戻せ、どの変化が大災害の前触れになるのかを見極めるのは不可能に近いとの認識を示す。

ハーシ氏によると、最近のデータからはＡＭＯＣの強さに変動が見られるものの、まだ減退が生じているとの観測上の証拠はない。「気候の温暖化が進む中、過去にあったようなＡＭＯＣの突然の変化が起きるかどうかは、まだ解の出ていない重要な問題」と言及する。

ラームストルフ氏は、今回の研究はそのパズルを埋めるピースの一つになると語る。研究は「ＡＭＯＣの崩壊がそう遠くない将来に起きるとの懸念に拍車をかけるもの」であり、「我々がこのリスクを無視すれば危険を冒すことになる」と続けた。

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The effects of AMOC's collapse could be catastrophic. Temperatures in some parts of Europe could drop 30 degrees in 100 years, and the climate could change to a completely different climate in just 10 to 20 years.

The study's authors pointed out that ``no adaptation measures can realistically respond to such rapid changes in temperature.''

Meanwhile, countries in the southern hemisphere might see an acceleration of global warming. The rainy and dry seasons in the Amazon could be reversed, with serious consequences for the ecosystem.

Van Westen said sea levels could rise by about 1 meter.

Stefan Rahmstorf, a marine physicist at the Potsdam University in Germany who was not involved in the paper, called the research results a ``major advance in the science of AMOC stability.'' He ``confirms that there is a tipping point where the AMOC may collapse if the North Atlantic becomes diluted with fresh water.''

Previous studies had shown the existence of tipping points, but they used much simpler models. Among the scientists, some had hoped that there would be no tipping point in a complex model. The study, he said, shattered that hope.

Joel Hirschi, an ocean system modeler at the UK's National Oceanography Centre who was involved in the formation of marine system model, commented that relatively small amount of freshwater entering the ocean could switch the AMOC from "on" to "off", and it was the first study presented with a complex climate model.

However, there were some points to be aware of; even though it was a complex model, the resolution was still low, and there might be limitations as a model in some ocean currents.

In recent years, a growing number of studies had suggested that AMOC might be approaching a tipping point.

A 2021 study showed that AMOC was at its weakest in 1,000 years. A paper published in July last year caused controversy by warning that AMOC could collapse as early as 2025.

However, there was still a great deal of uncertainty. Jeffrey Kargel of the Planetary Science Institute in Arizona, on the theory that AMOC was about to shut down, thought that until we know what was happening, it would continue to be controversial.

When talking about the possibility of AMOC's collapse, so to speak it might correspond to "among the wild fluctuations in the stock market, one occasion could be a harbinger of a major crash," and to acknowledge that it would be nearly impossible to determine which changes could be reversible and which might be a harbinger of disaster.

Hirsi said that although recent data showed fluctuations in AMOC's strength, there was no observational evidence that a decline had occurred yet. He noted that `` as the climate continues to warm, whether or not the AMOC will suddenly change as it has in the past is an important question that remains unanswered.''

Rahmstorf said this research was one filling piece in that puzzle. The study was" fueling fears that the collapse of the AMOC could happen in the not-too-distant," and added that "If we ignore this risk, we do so at our peril."

              So, in the Atlantic Ocean, sea water that moves northward at the surface and move southward at deep down may already be on its way to collapse. This collapse could have a serious impact on sea level rise and temperature changes in many parts of the world. It seems that so far, the melting of ice sheets in Antarctica and the collapse of AMOC are the two more noticeable geographical changes at sea as global warming is continuing.

Note:

The Planetary Science Institute is a 501 non-profit research institute based in Tucson, Arizona, focusing on planetary science. As of 2018, its director is Dr. Mark V. Sykes. (Wikipedia)