Antarctica sets the stage for the world’s greatest waterfall. The action takes place beneath the surface of the ocean. Here, trillions of tonnes of cold, dense, oxygen-rich water cascade off the continental shelf and sink to great depths. This Antarctic “bottom water” then spreads north along the sea floor in deep ocean currents, before slowly rising, thousands of kilometres away.
南極洲為世界上最大落水奠定基礎。此作用發生於海洋表面之下。在這裡，數萬億噸寒冷、稠密、富含氧的水，從大陸棚上傾瀉而下，沉入極深海中。之後，這種南極“底部水”，在緩慢上升前，隨深處洋流，沿著海底向北，不斷擴散數千公里。
In this way, Antarctica drives a global network of ocean currents called the “overturning circulation” that redistributes heat, carbon and nutrients around the globe. The overturning is crucial to keeping Earth’s climate stable. It’s also the main way oxygen reaches the deep ocean.
以此方式，南極洲驅動一種，被稱為“翻轉環流”之全球洋流網絡，於全球各地，重新分配熱量、碳及養分。就保持地球氣候穩定而言，此種翻轉是至關重要的。這也是，氧及於海洋深處的主要途徑。
But there are signs this circulation is slowing down and it’s happening decades earlier than predicted. This slowdown has the potential to disrupt the connection between the Antarctic coasts and the deep ocean, with profound consequences for Earth’s climate, sea level and marine life.
不過，有諸多跡象，這種循環正在放緩，而且比預期早幾十年發生中。這種放緩具有，破壞南極海岸與海洋深處間之聯繫的可能性。對地球氣候、海平面及海洋生物具深遠影響。
Our new research, published today in the journal Nature Climate Change, uses real-world observations to decipher how and why the deep ocean around Antarctica has changed over the past three decades. Our measurements show the overturning circulation has slowed by almost a third (30%) and deep ocean oxygen levels are declining. This is happening even earlier than climate models predicted.
我們今天(2023年5月25日)發表於《自然•氣候變化》期刊的新研究，利用現實世界的觀察，來解讀南極洲周遭海洋深處，在過去三十年間，為何及如何已經發生變化。我們的諸多測量顯示，翻轉環流已經減慢近三分之一(30%)，海洋深處的氧含量正下降中。這甚至比氣候模型預測的，更早發生中。
We found melting of Antarctic ice is disrupting the formation of Antarctic bottom water. The meltwater makes Antarctic surface waters fresher, less dense, and therefore less likely to sink. This puts the brakes on the overturning circulation.
我們發現，南極冰的融化正在擾亂南極底部水的形成。融水使得南極地表水鹽分更清淡、稠度更低。因此，下沉的可能性更少，這對翻轉循環造成抑制作用。
Now that’s a waterfall: dense water flowing from the continental shelf into the deep ocean in the Ross Sea. Consortium for Ocean-Sea Ice Modelling in Australia (COSIMA) and National Computational Infrastructure.
(短片請參閱原文)目前，那是一種落水：稠密水從大陸棚流入羅斯海(南極洲之南大洋的一個深海灣)的海洋深處。澳大利亞海洋冰層模型製作(COSIMA)暨國家計算基礎設施聯盟提供。
As the flow of bottom water slows, the supply of oxygen to the deep ocean declines. The shrinking oxygen-rich bottom water layer is then replaced by warmer waters that are lower in oxygen, further reducing oxygen levels.
當底部水的流動變緩慢時，供應到海洋深處的氧減少。而後，減少之富氧的底部水層，被含氧量較低的暖和水取代，而進一步降低氧含量。
Ocean animals, large and small, respond to even small changes in oxygen. Deep-ocean animals are adapted to low oxygen conditions but still have to breathe. Losses of oxygen may cause them to seek refuge in other regions or adapt their behaviour. Models suggest we are locked in to a contraction of the “viable” environment available to these animals with an expected decline of up to 25%.
海洋動物，無論大小，對氧方面甚至微小的變化，皆做出反應。海洋深處的動物適應低氧環境，不過仍必需呼吸。氧減少可能導致它們，於其他地區，尋求庇護處或調整行為。諸多模型暗示，我們正處於一種，對此些動物可資利用之具有“能存活”的環境，預計下降幅度高達25%的減縮。
Slowdown of the overturning may also intensify global warming. The overturning circulation carries carbon dioxide and heat to the deep ocean, where it is stored and hidden from the atmosphere. As the ocean storage capacity is reduced, more carbon dioxide and heat are left in the atmosphere. This feedback accelerates global warming.
此翻轉的放緩也可能加劇全球暖化。這種翻轉的環流，將二氧化碳及熱量帶到海洋深處，被儲存於那裡，而被隱藏於大氣之外。當海洋儲存能耐減少時，更多的二氧化碳及熱量被留置於大氣中。這種反饋加速了全球暖化。
Reductions in the amount of Antarctic bottom water reaching the ocean floor also increases sea levels because the warmer water that replaces it takes up more space (thermal expansion).
及於海洋底部之南極底層水量的減少，也會提升海平面。因為，較暖和的水取代它，佔據更多空間(熱膨脹)。
圖1. 大陸棚水的變淡，降低了稠密水的流動，而減緩了翻轉環流的最深部分，同時也減少了深層的氧合作用。 (圖援用自原文)
Freshening of shelf waters reduces the flow of dense water and slows the deepest parts of the overturning circulation while also reducing deep oxygenation.
Making observations of bottom water is challenging. The Southern Ocean is remote and home to the strongest winds and biggest waves on the planet. Access is also restricted by sea ice during winter, when bottom water forms.
進行底層水的觀測具有挑戰性。南大洋(也稱為南極海洋)地處偏遠，是地球上風最強及海浪最大的發源地。於冬季期間，在底部水形成時，通路也遭到海冰的限制。
This means observations of the deep Southern Ocean are sparse. Nevertheless, repeated full-depth measurements taken from ship voyages have provided glimpses into the changes underway in the deep ocean. The bottom water layer is getting warmer, less dense and thinner.
這意味著，南大洋深處的觀測是欠缺的。儘管如此，從船舶航行中，進行的重複全深度測量已經提供，有關海洋深處正發生中之變化的諸多窺探。此底部水層正變得較暖和、稠度較小及較薄。
Satellite data shows the Antarctic ice sheet is shrinking. Ocean measurements taken downstream of regions of rapid melt show the meltwater is reducing the salinity (and density) of coastal waters.
衛星數據顯示，南極冰層正縮減中。在快速融化區域之下游進行的海洋測量顯示，融化水正在降低沿海水域的鹽度(也就是稠度)。
Antarctic ice mass loss over the last few decades based on satellite data, showing that between 2002 and 2020, Antarctica shed an average of ~150 billion metric tonnes of ice per year, adding meltwater to the ocean and raising sea-levels (Source: NASA).
(短片請參閱原文)根據衛星數據，過去幾十年間，南極冰塊流失。顯示在2002與2020間，南極洲每年平均排出1500億公噸的冰，向海洋增添融水並提高海平面(來源：NASA)。
These signs point to a worrying change, but there are still no direct observations of the deep overturning circulation. We combined different types of observations in a new way, taking advantage of each of their strengths.
此些跡象指向一種令人擔憂的變化，不過仍然沒有，這種深層翻轉環流的直接觀察。我們以一種新方式結合了，諸多利用其每一長處之不同類型的觀察。
The full-depth measurements collected by ships provide snapshots of ocean density, but are usually repeated about once a decade. Moored instruments, on the other hand, provide continuous measurements of density and speed, but only for a limited time at a particular location.
由船舶收集之全深度測量提供了，海洋稠度的快照。不過，通常大約十年重複一次。另一方面，繫泊的儀器提供了，稠度及速度的連續測量。不過，僅在特定位置，供有限的時間測量。
We developed a new approach that combines ship data, mooring records, and a high resolution numerical simulation to calculate the strength of Antarctic bottom water flow and how much oxygen it transports to the deep ocean.
我們開發了一種，結合了船舶數據、繫泊記錄及一種高解析度數值模擬，來計算南極底部水流的強度，及其向海洋深處輸送多少氧的新方法。
Our study focused on a deep basin south of Australia that receives bottom water from several sources. These sources lie downstream of large meltwater inputs, so this region is likely to provide an early warning of climate-induced deep ocean changes.
我們的研究著重於，澳大利亞南部一處，從多個來源接收底部水的深海盆地。此些來源位於，諸多大融水輸入的下游。因此，該區域可能提供氣候誘發之海洋深處諸變化的早期預兆。
The findings are striking. Over three decades, between 1992 and 2017, the overturning circulation of this region slowed by almost a third (30%) causing less oxygen to reach the deep. This slowing was caused by freshening close to Antarctica.
此些發現是令人震驚的。三十多年來，在1992年與2017年間，該地區的翻轉環流減慢了近三分之一(30%)，導致更少的氧及於海洋深處。這種放緩是，由於靠近南極洲之海水鹽分變淡導致的。
We found this freshening reduces the density and volume of Antarctic bottom water formed, as well as the speed at which it flows.
我們發現，這種海水變淡，除了其流動的速度之外，也降低了形成之南極底部水的密度與體積。
The observed slowdown would have been even greater if not for a short-lived climate event that drove a partial and temporary recovery of bottom water formation. The recovery, driven by increased salinity, further illustrates the sensitivity of bottom water formation to salinity changes on the Antarctic continental shelf.
倘若不是因為短暫之氣候事件，驅動底部水形成的一部分及暫時恢復，那麼被觀察到的放緩可能一直是更大的。由鹽度增加驅動的恢復，進一步說明了，底部水的形成，對南極大陸棚鹽度變化的敏感性。
Worryingly, these observations show that changes predicted to occur by 2050 are already underway.
令人擔憂的是，此些觀察顯示，預計到2050年發生的變化，已經在進行中。
Abyssal ocean warming driven by Antarctic overturning slowdown.
(短片請參閱原文)由南極翻轉減慢驅動之深海底層的海洋變暖。
Ice loss from Antarctica is expected to continue, even accelerate, as the world warms. We are almost certain to cross the 1.5℃ global warming threshold by 2027.隨著世界暖化，冰層從南極洲的流失預計會繼續，甚至加速。我們幾乎肯定，到2027年，會跨過1.5℃全球暖化門檻。
More ice loss will mean more freshening, so we can anticipate the slowdown in circulation and deep oxygen losses will continue.
更多的冰流失將意味著，更多的海水變淡。因此，我們能預期在循環上的放緩，及深海的氧流失會繼續。
The consequences of a slowdown will not be limited to Antarctica. The overturning circulation extends throughout the global ocean and influences the pace of climate change and sea level rise. It will also be disruptive and damaging for marine life.
放緩的後果將不僅限於南極洲。此翻轉環流及於全球海洋，且影響氣候變化及海平面上升的步調。對海洋生物而言，這也會具破壞性及殺傷性。
Our research provides yet another reason to work harder – and faster – to reduce greenhouse gas emissions.
我們的研究更提供了另一種，更加努力及更快運作，來減少溫室氣體排放的理由。
網址：https://theconversation.com/antarctic-alarm-bells-observations-reveal-deep-ocean-currents-are-slowing-earlier-than-predicted-206289
翻譯：許東榮
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