原文網址：https://www.ucl.ac.uk/news/2024/jul/gulf-stream-wind-powered-and-could-weaken-climate-change
由倫敦大學學院的科學家主持的新研究，發現了新的證據可以用來了解末次冰期期間灣流如何變化，也顯示出未來氣候變遷可能會對灣流造成更多影響。
這篇發表在《自然》(Nature)的研究發現大約在距今20,000年前的末次冰期期間，由於北大西洋副熱帶地區上空的風比現在更強，使得灣流的強度增加。這意味著如果氣候變遷造成未來副熱帶地區的風力減弱(某些研究的初步成果已經如此顯示)，那麼灣流就會跟著減弱。結果便是熱帶地區帶到歐洲的熱能變少，造成歐洲大陸的氣溫下降並讓北美的海水面上升。不過這些潛在影響的規模可能會有多大還不清楚。
灣流是在海水表面流動的洋流，它先沿著美國東岸往北，然後帶著熱帶的海水橫跨大西洋前往歐洲。這些溫暖的海水會把熱能釋放到大氣，使得歐洲的氣溫溫暖宜人。
研究人員發現末次冰期期間，北半球大部分地區都被冰層覆蓋的時候，此區域的風變得更強，造成灣流的強度增加並到達更深的地方。然而就算灣流變得更強，總體來說地球的溫度還是比現在低非常多。
主要作者，倫敦大學學院地理系的Jack
Wharton博士說：「我們發現末次冰期期間，由於北大西洋副熱帶上空的風變強，使得灣流強度也增加許多。結果是雖然地球其他地方的溫度下降許多，但是灣流仍然可以把許多熱量輸往北方。我們的成果也顯現出未來氣候變遷對風場模式的影響，可能會讓灣流出現變化。比方說，如果未來的風力就像最近氣候模型的研究結果所示會變弱，就意味著灣流的強度可能會下降，使歐洲的氣溫變低。」
灣流也是巨大的北大西洋經向翻轉環流(Atlantic
Meridional Overturning Circulation ，AMOC)的一部分，其動力來源一為深層水的形成，也就是在北大西洋的副極地地區，因為溫度降低造成表層海水密度變高而下沉；另一個動力來源即為風力。科學家早就擔憂氣候變遷可能會讓AMOC變弱，因為從格陵蘭傾瀉而出的融冰會擾亂深層水的形成，使得溫暖的熱帶海水無法到達歐洲，進而讓歐陸的溫度下降。
風力變弱與深層水變少加總起來的效應可能會讓灣流強度大幅下降。如果AMOC停擺——雖然一般認為此情境的機率不大，但可能性並非為0——那麼歐洲的溫度可能會下降10到15℃，而對陸上的農業和天氣模式造成重大災難。灣流由風驅動的部分強度下降也會讓此情況變得更加嚴重。
倫敦大學學院地理系的教授Mark
Maslin是共同作者，他說：「我們通常不會意識到洋流對於把熱能傳遞到地球各處，以及塑造我們所處的氣候來說相當重要。但是氣候暖化可能會造成歐洲許多地方的溫度降低，這項聽起來矛盾的結果便是來自於AMOC受到阻擾。我們的新研究讓人們對洋流的重要性有更深的理解，並且顯示出驅動灣流的風力減弱可能會減緩熱能的流動，進而影響到陸地。」
雖然AMOC和組成它的各洋流(包括灣流)有時候會被比喻成一條巨大的輸送帶，這項研究強烈顯示此系統其實相當複雜，每一部分的洋流受到氣候變遷影響的時候可能都會產生獨特的反應。
倫敦大學學院地理系的教授David
Thornalley是共同作者，他說：「與其以慣用的『輸送帶』來比喻AMOC，把它想成是串連在一起的環帶也許更加恰當。比方說灣流所屬的副熱帶部分是一條環帶，接著把熱量往北帶到北極的副極區部分又是另一條。在末次冰期期間，我們的發現顯示副熱帶環帶比現在還強，而之前的研究則認為副極地環帶變得比較弱。因此在探討人為氣候變遷與AMOC的關係時，我們需要考慮這些不同的部分未來可能有什麼變化，各自又會造成什麼樣的氣候衝擊。」
為了偵測有歷史紀錄之前的灣流強度，研究人員分析了一種住在海底的微生物——有孔蟲的化石遺骸。他們跟麻塞諸塞州伍茲霍爾海洋研究所的科學家合作，從北卡羅來納以及佛羅里達外海取回沉積物岩芯，再從裡面採集有孔蟲的化石。
研究人員發現地點不同但上方都有灣流經過的沉積物岩芯之中，年代為末次冰期的段落所取出的有孔蟲，其同位素訊號都指出當時的灣流所能到達的深度以及流動的速度皆為現在的兩倍。(同位素訊號為氧-18跟氧-16的比例，由溫度和鹽度共同決定。)
研究經費來自NERC、利華休姆基金會、美國國家科學基金會，以及歐盟的「展望歐洲」與「展望2020研究與創新計劃」。
The Gulf Stream
is wind-powered and could weaken from climate change
New evidence of changes to the Gulf
Stream during the last ice age could indicate additional sensitivity to future
climatic changes, finds a new study led by UCL researchers.
The research, published in Nature, found that during the last ice age about 20,000 years ago,
the Gulf Stream was stronger than today because of more powerful winds across
the subtropical North Atlantic. This could mean that if climate change causes a
reduction in subtropical winds in the future, as initial research is beginning
to indicate¹, the Gulf Stream could also weaken. This would limit the amount of
tropical heat that reaches Europe, cooling the continent and causing higher sea
levels in North America, but the likely size of this potential effect is still
unclear.
The Gulf Stream is a surface current that flows up
the east coast of the US then crosses the Atlantic to Europe, carrying warm
tropical water with it. This warm water releases heat into the atmosphere,
warming Europe.
The researchers found that during the last ice age,
when ice sheets covered much of the northern hemisphere, stronger winds in the
region resulted in a stronger and deeper Gulf Stream. However, despite the
stronger Gulf Stream, overall, the planet was still much colder than today.
Lead author Dr Jack Wharton (UCL Geography) said: “We
found that during the last ice age, the Gulf Stream was much stronger because
of stronger winds across the subtropical North Atlantic. As a result, the Gulf
Stream was still moving lots of heat northwards, despite the rest of the planet
being far colder. Our work also highlights the Gulf Stream’s potential
sensitivity to future changes in wind patterns. For example, if in the future
winds are weaker, as shown in a recent study using climate models, it could
mean a weaker Gulf Stream and a cooler Europe.”
The Gulf Stream is also part of the vast Atlantic
Meridional Overturning Circulation (AMOC), which is driven by both deep water
formation in the subpolar North Atlantic, where cooling causes surface waters
to become dense and sink, as well as winds. Scientists have previously raised
concerns about how climate change could weaken the AMOC as melting glacial
water pouring off Greenland could disrupt deepwater formation, preventing warm tropical
water from reaching Europe and thus cooling the continent.
Together, the combined effect of weakening winds and
reduced deep water formation could significantly weaken the Gulf Stream. If the
AMOC were to collapse – considered an unlikely but possible future scenario -
European temperatures would cool by 10 to 15 degrees Celsius, wreaking havoc on
continental agriculture and weather patterns, and the decrease in the
wind-driven part of the Gulf Stream would further exacerbate this.
Co-author Professor Mark Maslin (UCL Geography) said:
“It’s not always recognised how much ocean currents are responsible for
transferring heat around the planet and shaping our climate. Paradoxically, the
warming of the climate could cool down much of Europe by disrupting the AMOC.
Our new research adds to this understanding, and shows that the weakening of
the winds which drive the Gulf Stream could reduce the circulation of heat,
further affecting the continent.”
Though the AMOC and its constituent currents,
including the Gulf Stream, is sometimes referred to as a giant conveyer belt,
this study highlights the system’s complexity, with each part of the current
capable of having its own unique response to climate change.
Co-author Professor David Thornalley (UCL Geography)
said: “Rather than the established conveyor belt metaphor, perhaps it is better
to think of the AMOC as a series of interconnected loops. There is the
subtropical loop—that the Gulf Stream is part of—and a subpolar loop, which
carries heat further northwards into the Arctic. During the last ice age, our
findings show that the subtropical loop was stronger than it is today, whereas
the subpolar loop is thought to have been weaker. Therefore, when investigating
anthropogenic climate change and the AMOC, we need to consider how these
different parts may change and what climate impacts each is associated with.”
To gauge the strength of the prehistoric Gulf Stream,
the researchers analysed the fossil remains of foraminifera—microorganisms that
live at the bottom of the ocean—taken from sediment cores recovered off the
coast of North Carolina and Florida, in collaboration with researchers based at
Woods Hole Oceanographic Institute in Massachusetts.
The researchers found that foraminifera taken from
layers dating to the last ice age in sediment cores from different locations
beneath the Gulf Stream had isotopic signatures (the ratio of oxygen-18 to
oxygen-16, which is controlled by a combination of temperature and salinity)
that indicated a Gulf Stream that was twice as deep and flowing twice as fast
compared to today.
This research was supported by NERC, the Leverhulme
Trust, the National Science Foundation, and the European Union’s Horizon Europe
and Horizon 2020 Research and Innovation Programmes.
原始論文：Jack H. Wharton,
Martin Renoult, Geoffrey Gebbie, Lloyd D. Keigwin, Thomas M. Marchitto, Mark A.
Maslin, Delia W. Oppo, David J. R. Thornalley. Deeper and stronger
North Atlantic Gyre during the Last Glacial Maximum. Nature,
2024; DOI: 10.1038/s41586-024-07655-y
引用自：University College London. "The Gulf
Stream is wind-powered and could weaken from climate change."
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