研究人員對全球氣候最近一次的長期變化原因有了新的瞭解

原文網址：http://www.exeter.ac.uk/news/featurednews/title_697437_en.html

研究人員對全球氣候最近一次的長期變化原因有了新的瞭解

全球氣候最近一次的長期變化大約發生在一百萬年前。追查背後原因的研究得出了全新且不同於以往的發現。

由艾希特大學Sev Kender博士領導的研究團隊，對於「中更新世轉型期」(Mid-Pleistocene Transition ,MPT)的原因有了引人注目的新觀點。地球因為MPT這個現象使得極為寒冷的氣候條件的循環時間變得更加漫長，強度也更加劇烈。

雖然科學家尚未完全瞭解MPT的成因，不過最主流的理論之一認為可能是冰河造成二氧化碳的排放量降低所導致。

Kender博士和他的團隊最近發現這段期間白令海峽因為冰河擴張而關閉，可能導致北大西洋出現分層現象，使得大氣中的二氧化碳減少。他們認為這造成了全球冷化。

團隊相信這項最新發現不只對MPT的成因提供了關鍵知識，也讓我們對於全球氣候變遷背後的驅動因素有了嶄新的觀點。

研究於2018年12月19日刊登在《自然通訊》(Nature Communications )。

Kender博士是這篇研究的共同作者，目前任職於康瓦爾郡艾希特大學彭林校區的坎伯恩礦業學院。他說：「副極地的北大西洋擁有地球上某些年代最久的海水。這些海水跟大氣隔離了相當久的時間，使得深部可以累積溶解態的二氧化碳到很高的濃度。當它們往上湧至表層，一部份的二氧化碳就會釋放出來。這種機制在地質時間中是相當重要的作用，在過去的冰河期發生之後它可以造成某種程度的全球暖化。」

「我們從白令海峽的深部海底取出沉積物岩芯，並從中得到此區的歷史文獻。研究岩芯沉積物和有孔蟲(一種海洋原生生物)殼體化石的化學性質，我們可以重建浮游生物的生產力，以及海洋表層和底層水體的性質。此外，我們也可以對沉積物進行更佳的定年，如此一來就可以比較白令海和全球其他區域當時出現的變化。」

「我們發現在MPT時白令海區域的分層現象變得更加劇烈。中層水體的擴張大幅抑制了導致全球暖化的重要因子之一：北大西洋的副極地湧升流。」

地球氣候一直以來都有劇烈的變化發生。在過去60萬年以來和更早之前的許多時期，氣候經常在類似現今的暖期以及氣溫較低的冰河期之間來回動盪。冰河期時大片的陸地會覆蓋在數公里厚的冰層之下。

地球氣候在自然條件下的規律變化受控於地球繞日軌道的變化，以及地球跟其他星體彼此間的重力作用對自轉軸造成的變化。

這些稱為軌道循環的變化影響了太陽能如何分配至地球的各個角落。結果可能會讓某些循環中北半球的夏季氣溫變低而催生冰河期；但在之後的循環夏季氣溫可能會升高使得冰層融化。

軌道循環會受到許多因素影響而放大它們的效應。其中之一便是大氣中的二氧化碳濃度。

由於MPT發生時軌道循環的特性沒有出現明顯變化，因此科學家長久以來都在試著找出MPT發生的原因為何。

為了進行這項研究，Kender博士和他的團隊聯合國際海洋發現計畫在白令海鑽取深海沉積物，接著測量其中的殼體化石與沉積物的化學性質。

團隊藉此重建出海洋水體隨著時間的詳細變化。他們發現在冰河期這段期間，白令海峽關閉造成副極地北大西洋海水的分層現象變得更加明顯。

他們主張分層現象減少了大氣中的二氧化碳，進而造成全球冷化。

Kender博士進一步解釋：「今日海水結冰所形成的低溫海水大多會通過白令海峽，往北流入北極海。大約一百萬年前因為冰河擴張、海平面下降，造成白令海峽關閉而把低溫海水留在白令海。低溫水體的範圍擴大可能堵住了富含二氧化碳的深層水體形成湧升流，使海洋可以封存更多大氣中的二氧化碳。連帶產生的冷化效應也許使地球對軌道循環更加敏感，造成自此之後的氣候特徵具有更為寒冷且為時更長的冰河期。」

「我們的發現強調出瞭解高緯度海洋現在與未來的變化是相當重要的，因為長期來看這些區域對於大氣中的二氧化碳封存或排放具有相當的重要性。」

Research sheds new light on what drove last, long-term global climate shift

The quest to discover what drove the last, long-term global climate shift on Earth, which took place around a million years ago, has taken a new, revealing twist.

A team of researchers led by Dr Sev Kender from the University of Exeter, have found a fascinating new insight into the causes of the Mid-Pleistocene Transition (MPT) - the phenomenon whereby the planet experienced longer, intensified cycles of extreme cold conditions.

While the causes of the MPT are not fully known, one of the most prominent theories suggests it may have been driven by reductions in glacial CO₂ emissions.

Now, Dr Kender and his team have discovered that the closure of the Bering Strait during this period due to glaciation could have led the North Pacific to become stratified – or divided into distinct layers – causing CO₂ to be removed from the atmosphere. This would, they suggest, have caused global cooling.

The team believe the latest discovery could provide a pivotal new understanding of how the MPT occurred, but also give a fresh insight into the driving factors behind global climate changes.

The research is published in Nature Communications on December 19^th 2018.

Dr Kender, a co-author on the study from the Camborne School of Mines, based at the University of Exeter’s Penryn Campus in Cornwall said: ”The subarctic North Pacific is composed of some of the oldest water on Earth, which has been separated from the atmosphere for such a long time that a high concentration of dissolved CO₂ has built up at depth. When this water upwells to the surface, some of the CO₂ is released. This is thought to be an important process in geological time, causing some of the global warming that followed past glaciations.

“We took deep sediment cores from the bottom of the Bering Sea that gave us an archive of the history of the region. By studying the chemistry of sediment and fossil shells from marine protists called foraminifera, we reconstructed plankton productivity, and surface and bottom water masses. We were also able to better date the sediments so that we could compare changes in the Bering Sea to other global changes at that time.

“We discovered that the Bering Sea region became more stratified during the MPT with an expanded intermediate-depth watermass, such that one of the important contributors to global warming – the upwelling of the subarctic North Pacific – was effectively curtailed.”

The Earth’s climate has always been subjected to significant changes, and over the past 600,000 years and more it has commonly oscillated between warm periods, similar today, and colder, ‘glacial’ periods when large swathes of continents are blanketed under several kilometres of ice.

These regular, natural changes in the Earth’s climate are governed by changes in how the Earth orbits around the sun, and variations in the tilt of its axis caused by gravitational interactions with other planets.

These changes, known as orbital cycles, can affect how solar energy is dispersed across the planet. Some orbital cycles can, therefore, lead to colder summers in the Northern Hemisphere which can trigger the start of glaciations, while later cycles can bring warmer summers, causing the ice to melt.,

These cycles can be influenced by a host of factors that can amplify their effect. One of which is CO₂ levels in the atmosphere.

As the MPT occurred during a period when there were no apparent changes in the nature of the orbit cycles, scientists have long been attempting to discover what drove the changes to take place.

For this research, Dr Kender and his team drilled for deep-sea sediment in the Bering Sea, in conjunction with the International Ocean Discovery Program, and measured the chemistry of the fossil shells and sediments.

The team were able to create a detailed reconstruction of oceanic water masses through time – and found that the closure of the Baring Strait caused the subarctic North Pacific became stratified during this period of glaciation.

This stratification, that argue, would have removed CO₂ from the atmosphere and caused global cooling.

Dr Kender added: “Today much of the cold water produced by sea ice action flows northward into the Arctic Ocean through the Bering Strait. As glaciers grew and sea levels fell around 1 million years ago, the Bering Strait would have closed, retaining colder water within the Bering Sea. This expanded watermass appears to have stifled the upwelling of deep CO₂-rich water and allowed the ocean to sequester more CO₂ out of the atmosphere. The associated cooling effect would have changed the sensitivity of Earth to orbital cycles, causing colder and longer glaciations that characterise climate ever since.

“Our findings highlight the importance of understanding present and future changes to the high latitude oceans, as these regions are so important for long term sequestration or release of atmospheric CO₂.”

原始論文：Sev Kender, Ana Christina Ravelo, Savannah Worne, George E. A. Swann, Melanie J. Leng, Hirofumi Asahi, Julia Becker, Henrieka Detlef, Ivano W. Aiello, Dyke Andreasen, Ian R. Hall. Closure of the Bering Strait caused Mid-Pleistocene Transition cooling. Nature Communications, 2018; 9 (1) DOI: 10.1038/s41467-018-07828-0

引用自：University of Exeter. "New light shed on what drove last, long-term global climate shift."