Important ocean circulation in the Atlantic Ocean is showing signs of collapse, researchers warn (1/2)

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 (1/2)

2024.02.10 Sat posted at 20:07 JST

（ＣＮＮ） 大西洋の海水が表層で北上し、深層で南下する大西洋子午面循環（ＡＭＯＣ）は既に崩壊に向かっている可能性がある――。そんな新たな報告が発表された。ＡＭＯＣの崩壊は海面上昇や世界の気象に深刻な影響を及ぼし、各地で気温の急激な低下や上昇をもたらす可能性があるという。

米科学誌サイエンス・アドバンシズに９日発表された研究結果によると、研究チームは極めて複雑で高価な計算システムを使用し、ＡＭＯＣ崩壊の予兆を捉える指標（ＥＷＳ）を検出する新たな方法を発見した。

ＡＭＯＣは巨大な地球規模のベルトコンベアのような働きをしている。ＡＭＯＣが熱帯地域の暖かい海水を遠く北大西洋まで運ぶと、海水はそこで冷却されて塩分濃度が高まり、海中深くに沈み込んで再び南へ向かう。

こうした海流は地球上のさまざまな地域に熱や栄養分を運んでおり、北半球の広い地域の気候を比較的温暖に保つうえで欠かせない役割を果たす。

気候変動に伴う海洋温暖化や氷の融解により、海流の強さを決定する熱と塩のバランスが乱れる中、科学者はＡＭＯＣの安定性について長年警鐘を鳴らしてきた。

科学者の間では気候変動の影響でＡＭＯＣのスピードが遅くなり、場合によっては停止する可能性もあるとの見方が多いが、それがいつ、どれだけ早く起きるのかについては依然不明な点が多い。ＡＭＯＣの継続的な観測が始まったのは２００４年に過ぎない。

ただ、氷床コアや海洋堆積（たいせき）物のような手掛かりを使って過去を再現した結果、ＡＭＯＣが今から１万２０００年以上前、氷河の急速な融解の後に停止したことは分かっている。

これと同じことが再び起きる可能性がないか、いま研究者は解明を急いでいるところだ。

今回の新研究は「重要な突破口」になる――。そう語るのはオランダ・ユトレヒト大学の海洋大気研究者で、論文の共著者を務めたレネ・ファンウェステン氏だ。

研究チームはスーパーコンピューターを駆使して３カ月にわたり複雑な気候モデルを走らせ、シミュレーションの中でＡＭＯＣに徐々に淡水を加えていった。淡水の追加は氷の融解や降雨、流入する河川水を表している。これらは海水の塩分濃度の希薄化し、海流を弱体化させる原因となる可能性がある。

モデル内で淡水の量を少しずつ増やしていくと、ＡＭＯＣは徐々に弱まり、やがて突然崩壊した。こうした複雑なモデルでＡＭＯＣの崩壊が確認されたのは初めてで、報告書は「気候システムと人類にとって悪い知らせ」だと指摘する。

ただ、この研究では崩壊までの時間枠は示していない。ファンウェステン氏はＣＮＮの取材にさらなる研究の必要性を指摘し、地球温暖化の進行を含む気候変動の影響を考慮したモデルも必要になるとの見方を示した。

それでも「気候変動の下、我々は（ＡＭＯＣの崩壊という）転換点の方向に進んでいるということは少なくとも言える」という。

(to be continued)

Translation

(CNN) The Atlantic Meridional Overturning Circulation (AMOC), in which ocean water moved northward at the surface and moved southward at deep down, might already be on its way to collapse. A new report had been released. The collapse of the AMOC could have a serious impact on sea level rise and global climate, leading to sharp drops or rises in temperature in various regions.

Using an extremely complex and expensive computational system, a research team discovered a new indicator (EWS) to detect AMOC's early signs of collapse, according to a study published in the US journal Science Advances on the 9th.

AMOC acted like a giant global conveyor belt. When the AMOC transported warm seawater from the tropics to the North Atlantic, the water cooled and became more salty, sinking deeper into the ocean and heading south again.

These ocean currents transported heat and nutrients to different parts of the globe, and played an essential role in keeping the climate relatively warm across a large area of the Northern Hemisphere.

Scientists had long sounded the alarm about the stability of the AMOC, as ocean warming and ice melting caused by climate change disrupted the balance of heat and salt that determined the strength of ocean currents.

Scientists believed that the effects of climate change would slow down AMOC, there were many views that there could be a possibility that it would be stopped in some situation, there were still many unknowns about when and how quickly this could happen. Continuous observation of AMOC began only in 2004.

However, by reconstructing the past using clues such as ice cores and marine sediments, we knew that the AMOC stopped more than 12,000 years ago after rapid melting of glaciers.

Researchers were currently rushing the work to find out whether something similar to this could happen again.

This new research represented an ``important breakthrough.'' So said Rene van Westen, an ocean and atmosphere researcher at Utrecht University in the Netherlands and co-author of the paper.

The research team used a supercomputer to run a complex climate model for three months, gradually adding freshwater to the AMOC during the simulation. The addition of fresh water represented melting ice, rainfall, and inflowing river water. These diluted the salinity of seawater and was a possibility in weaken ocean currents.

By gradually increasing the amount of fresh water in the model, the AMOC gradually weakened until it suddenly collapsed. This was the first time such a complex model had confirmed AMOC collapse, and the report pointed out that it was "bad news for the climate system and for humanity."

However, the study did not provide a time frame for collapse. Van Westen pointed out the need for further research in an interview with CNN, saying that models that took into account the effects of climate change, including the progression of global warming, would also be needed.

Also, he said, "we can at least say that with climate change, we are moving towards a tipping point [of the collapse of AMOC]."

(to be continued)

Note:

 The Atlantic meridional overturning circulation (AMOC) is a system of surface-level and deep currents in the Atlantic Ocean which are driven both by changes in the atmospheric weather and thermohaline changes in temperature and salinity. The AMOC is characterized by a northward flow of warm, salty water in the upper layers of the Atlantic, and a southward flow of colder, deep waters. (Wikipedia)