Recently CNN.co.jp reported the following:
大西洋の海洋循環、今世紀半ばにも停止か 「早ければ２０２５年」
2023.07.26 Wed posted at 11:35 JST
デンマーク・コペンハーゲン大学の物理気候学者、ピーター・ディトレフセン教授らが２５日、英科学誌ネイチャーに発表した。
ＡＭＯＣは地球規模のベルトコンベアーのように、熱帯の暖かい海水と塩分を北大西洋に運ぶ。北大西洋で冷えた海水は深層に沈み込み、再び南下する。
この循環は世界の気象パターンを維持する重要な役割を果たしているため、停止すれば欧米の極端な異常気象や海面上昇、熱帯の季節風の変化など、各地で重大な影響が出る事態が予想される。
科学者らは何年も前から、気候危機が加速するにつれてＡＭＯＣが不安定になり、流れの強さを左右する水温と塩分濃度のバランスが崩れる恐れがあると指摘してきた。
温暖化で氷が解ければ海に淡水が流れ込み、海水の塩分濃度が下がって密度が低くなるため、沈み込む力が弱まる。海水の塩分が薄まりすぎたり温度が上がりすぎたりすると、ベルトコンベアーは止まってしまう。
この現象は、今から１万２０００年以上前にも起きていた。氷河が急速に解けてＡＭＯＣが停止し、北半球の気温は１０年のうちに１０～１５度も変動した。
ＡＭＯＣの停止は地球上の全人口に影響を及ぼすと、専門家らは指摘する。
国連の気候変動に関する政府間パネル（ＩＰＣＣ）は１９年の報告書で、ＡＭＯＣが今世紀中に弱まると予想した。ただし、世紀末までに完全に崩壊する可能性は低いと結論付けていた。
新たな研究は、これよりはるかに厳しい見通しを示している。
ＡＭＯＣの継続的な観測が始まったのは０４年だが、研究チームはより広範囲のデータを調べ、人為的な気候変動が始まる前の海流と比較した。
ディトレフセン教授は「時代をさかのぼる必要があった」と説明する。具体的には、１８７０～２０２０年の１５０年間について、グリーンランドの南側に広がる北大西洋の表面温度を分析した。
その結果、ＡＭＯＣに重大な変動が起きることを示す前兆が見つかったという。チームは「信頼性の高い」予測として、ＡＭＯＣが早ければ２５年、遅くとも９５年には停止し得ると主張した。３９～７０年に停止する可能性が最も高いという。
ディトレフセン教授はＣＮＮとのインタビューで、「実に恐ろしい」「軽々しく論文に書くような話ではない」と語り、この結論に確信があると強調した。
同教授らの研究に関与していない専門家らは、ＡＭＯＣの正確な転換点は不明確で、今のところ観測データにはほとんど変化がみられないと指摘する。ただその一方で、憂慮すべき研究結果であることを認め、転換点が従来の予想より早く到来する可能性を示す新たな証拠になるとの見方では一致した。
研究チームは、温暖化や海氷の融解を抑えるために、温室効果ガスの排出量をゼロにする迅速で効果的な対策が必要だと訴えている。
Translation
(CNN) - A research result reported that if global greenhouse gas emissions continued at the current rate, the North-South Circulation (AMOC), in which seawater in the Atlantic Ocean moved northward at the surface level and moved southward at the deep level, could stop in the middle of this century, or as early as in 2025.
Professor Peter Ditlevsen, a physical climate scientist at the University of Copenhagen, Denmark, announced on the 25th in the British scientific journal Nature.
Like a global conveyor belt, the AMOC could bring tropical warm seawater and salinity to the North Atlantic Ocean. The cold seawater in the North Atlantic sank into the depths and moved south again.
This circulation played an important role in maintaining global weather patterns, so if it stopped, it was expected that there could be serious impacts in various places, such as extreme abnormal weather in Europe and the United States, rising sea levels, and changes in tropical monsoons.
For years, scientists had warned that as the climate crisis accelerated, AMOCs could become unstable, throwing out of balance the water temperature and salinity which influence the strength of currents.
When global warming melted the ice, freshwater flowed into the sea, reducing the salt concentration and density of the seawater, weakening the force of subduction. If the seawater became too diluted or too hot, the conveyor belt will stop.
This phenomenon occurred more than 12,000 years ago. The rapid melting of the glaciers stopped the AMOC, and temperatures in the northern hemisphere fluctuated by 10 to 15 degrees Celsius in a decade.
Experts pointed out that the stopping of AMOC could affect the entire population of the planet.
A 2019 report from the UN's Intergovernmental Panel on Climate Change (IPCC) predicted that AMOC would weaken during the century. However, it concluded that it was unlikely to completely collapse by the end of the century.
A new study offered a much grimmer picture.
Continuous AMOC observations began in 2004, but the researchers looked at a wider range of data and compared them to ocean currents before human-induced climate change.
Professor Ditlevsen explained that "We had to go back in time". Specifically, to analyze the surface temperature of the North Atlantic Ocean south of Greenland for 150 years from 1870 to 2020.
In this area, water temperature rose due to seawater brought from the tropics by AMOC. Therefore, when the water temperature drops, it means that the AMOC is weakening. The research team subtracted the increase in seawater temperature due to global warming from past data to examine changes in AMOC.
As a result, they found early warning signs of a significant shift in AMOC. As a "reliable" prediction, the team argued that AMOC could come to an end as early as 25 and as late as 95. It was most likely to stop between 39 and 70.
In an interview with CNN, Professor Ditlevsen stressed his confidence in the conclusion, saying it was "really terrifying" and "not something to be written down lightly."
Experts who were not involved in the study said the exact tipping point of the AMOC was unclear, they pointed out that there was little change in the observed data so far. On the other hand, they acknowledged the findings as alarming, and agreed they provided new evidence that the tipping point could come sooner than previously thought.
The research team said there was a need for quick and
effective action to reduce greenhouse gas emissions to zero in order to suppress
global warming and sea ice melting.
So, a
research report says that if global greenhouse gas emissions continue at the
current rate, the North-South Circulation (AMOC) could stop in the middle of
this century, or as early as in 2025. This will cause extreme abnormal weather
in Europe and the United States. This is a worrying prediction.
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