證據顯示過往大氣二氧化碳濃度跟現在一樣時，海平面也跟著大幅上升

原文網址：https://news.unm.edu/news/scientists-discover-evidence-for-past-high-level-sea-rise-at-current-atmospheric-carbon-dioxide-levels#

證據顯示過往大氣二氧化碳濃度跟現在一樣時，海平面也跟著大幅上升

從馬約卡島洞穴中得到400萬年前的地質證據，讓我們能從全新的角度了解全球海平面的上升幅度。

位在圖片中間像是一團泡沫的鐘乳石即為洞穴灰華的潛流增生。由於其生長位置剛好為海平面，因此可以精確指出大約350萬年前的海平面高度。圖片來源：UNM Newsroom

距今300萬年以前，地球的氣溫比前工業時代(pre-industrial era)還高了2到3℃。一組國際科學家團隊研究了一座臨海洞窟中的洞穴灰華，裡頭保存的證據顯示當時海平面比現在還高了16公尺。他們的發現在了解並預測目前氣候暖化會造成海平面以何種步調上升時，具有相當重要的意義。

這群科學家包括新墨西哥大學的教授Yemane Asmerom與資深研究科學家Victor Polyak，另外還有南佛羅里達大學、巴利亞利群島大學和哥倫比亞大學的科學家。結果發表於今日(8/30)發行的《自然》(Nature)期刊。科學家從地中海西部馬約卡島上的Artà洞穴採集沉積物，進行分析後得出過往的海平面高度。他們表示未來關於冰層穩定性、校對冰層模型、預期未來海平面上升的研究，可以用這項結果作為標準。

覆蓋在格陵蘭、南極這類地方的冰層融化時便會造成海平面上升。然而，暖化會讓海平面以多快的速度上升多少，仍是科學家在努力解答的問題。主要作者Oana Dumitru表示過往的氣候曾經在自然條件下比現在還要暖和，重建當時的冰層和海平面如何變化，可以讓我們透過全球尺度的實驗來探討這項問題。Dumitru大部分的定年工作是在新墨西哥大學就讀時，於Asmerom和Polyak的指導下完成。

「若要精確模擬暖化會造成海平面上升多少，就亟需過往海平面高度的實際測量結果。」Polyak表示，「這項研究對於上新世的海平面高度做出了十分可信的測量結果。」

新墨西哥大學地球科學系的教授Bogdan Onac說：「我們從過往暖期得到的知識可以用來校準冰層模型，之後可用於預測目前的全球暖化未來會對冰層造成什麼影響。」

此計畫著重於洞穴灰華上頭稱為潛流增生(phreatic overgrowth)的洞穴沉積物。每當海平面上升而淹進濱海洞窟時，在半鹹水和洞窟空氣的交界處就會形成這類沉積物。巴利亞利群島大學的教授Joan J. Fornós表示距離海岸不到100公尺的Artà洞穴，不論是現在或過往，內部的水面都跟海平面一致。

科學家在現今海平面上方22.5至32公尺處發現了6個地質層位，並加以分析跟解讀。他們謹慎地採集70個樣品之後，送往新墨西哥大學的放射源同位素實驗室，運用鈾鉛放射性定年法進行分析，結果顯示它們的年代介於距今440萬年至330萬年前，代表這些洞穴沉積物形成於上新世。

「洞穴科學家團隊努力尋找具有理想自然條件的洞窟，以及我們在新墨西哥大學實驗室花費數年發展出的技術在這裡合而為一。」Asmerom表示。「我們審慎地對儀器和技術進行投資，因而得到這些極具影響力的獲益。」

研究團隊的成員之一，哥倫比亞大學的助理教授Jacky Austermann說：「Artà洞的海平面變化有可能是因為冰層的融化增厚，或者是島嶼本身的抬升下沉。」她利用數學和統計模型仔細分析自上新世以來，島嶼可能抬升或下沉了多少，然後再把他們研究的地質層位所在高度扣除此結果。

上新世時他們特別感興趣的關鍵時期稱為中皮亞琴察期暖期(mid-Piacenzian Warm Period)，大約是326.4萬年至302.5萬年前，此時氣溫比前工業時代還高了2到3℃。Onac說：「這段期間是地球最近一次大氣二氧化碳濃度跟現在一樣高的時期，因此在面臨目前人為造成的暖化時，它具有未來地球樣貌會是如何的重要線索。」

研究發現這段期間全球平均海平面比現在高了16.2公尺(誤差範圍為5.6至19.2公尺)。科學家的結論認為這意味著就算大氣二氧化碳濃度保持在現今水準，全球平均海平面至少還是會上升到此高度。事實上，由於溫度升高造成海水體積增加，上升幅度恐怕還會更大。

Dumitru表示：「參照目前冰層的融化模式，海平面上升最有可能會源自於格陵蘭和西南極冰層的崩解。」

在距今大約400萬年前的上新世氣候最暖期(Pliocene Climatic Optimum)，全球均溫比前工業時代高出將近4°C，作者也測出此時海平面比現在還高了23.5公尺。Asmerom表示：「如果我們不積極主動地減少二氧化碳排放到大氣的量，這可能就是我們未來得面對的景象。」

研究團隊包括主要作者，南佛羅里達大學地球科學院的研究生Oana A. Dumitru與同單位的教授Bogdan Onac；哥倫比亞大學地球與環境科學系的教授Jacqueline Austermann；位在阿爾伯克基的新墨西哥大學地球與行星科學系的資深研究員Victor Polyak與教授Yemane Asmerom；還有位在馬約卡島的巴利亞利群島大學的副講師Joaquín Ginés和Angel Ginés。

這項研究源自於南佛羅里達大學和新墨西哥大學合作進行的美國國家科學基金會(NSF)計畫，部分源自南佛羅里達大學和巴利亞利群島大學簽訂的雙邊協議。經費來源為NSF和西班牙國家研究機構。

Evidence for past high-level sea rise at current atmospheric carbon dioxide levels

Mallorcan cave yields 4-million-year-old geologic evidence providing new insights into magnitude global sea level rise

An international team of scientists, studying evidence preserved in speleothems in a coastal cave, illustrate that more than three million years ago – a time in which the Earth was two to three degrees Celsius warmer than the pre-industrial era – sea level was as much as 16 meters higher than the present day. Their findings represent significant implications for understanding and predicting the pace of current-day sea level rise amid a warming climate.

The scientists, including Professor Yemane Asmerom and Sr. Research Scientist Victor Polyak from The University of New Mexico, the University of South Florida, Universitat de les Illes Balears and Columbia University, published their findings in today’s edition of the journal Nature. The analysis of deposits from Artà Cave on the island of Mallorca in the western Mediterranean Sea produced sea levels that serve as a target for future studies of ice sheet stability, ice sheet model calibrations and projections of future sea-level rise, the scientists said.

Sea level rises as a result of melting ice sheets, such as those that cover Greenland and Antarctica. However, how much and how fast sea level will rise during warming is a question scientists have worked to answer. Reconstructing ice sheet and sea-level changes during past periods when climate was naturally warmer than today, provides an Earth’s scale laboratory experiment to study this question according to USF Ph.D. student Oana Dumitru, the lead author, who did much of her dating work at UNM under the guidance of Asmerom and Polyak.

“Constraining models for sea-level rise due to increased warming critically depends on actual measurements of past sea level,” said Polyak. “This study provides very robust measurements of sea-level heights during the Pliocene.”

“We can use knowledge gained from past warm periods to tune ice sheet models that are then used to predict future ice sheet response to current global warming,” said USF Department of Geosciences Professor Bogdan Onac.

The project focused on cave deposits known as phreatic overgrowths on speleothems. The deposits form in coastal caves at the interface between brackish water and cave air each time the ancient caves were flooded by rising sea levels. In Artà Cave, which is located within 100 meters of the coast, the water table is - and was in the past - coincident with sea level, says Professor Joan J. Fornós of Universitat de les Illes Balears.

The scientists discovered, analyzed, and interpreted six of the geologic formations found at elevations of 22.5 to 32 meters above present sea level. Careful sampling and laboratory analyses of 70 samples resulted in ages ranging from 4.4 to 3.3 million years old BP (Before Present), indicating that the cave deposits formed during the Pliocene epoch. The ages were determined using uranium-lead radiometric dating in UNM’s Radiogenic Isotope Laboratory.

“This was a unique convergence between an ideally-suited natural setting worked out by the team of cave scientists and the technical developments we have achieved over the years in our lab at The University of New Mexico,” said Asmerom. “Judicious investments in instrumentation and techniques result in these kinds of high-impact dividends.”

“Sea level changes at Artà Cave can be caused by the melting and growing of ice sheets or by uplift or subsidence of the island itself,” said Columbia University Assistant Professor Jacky Austermann, a member of the research team. She used numerical and statistical models to carefully analyze how much uplift or subsidence might have happened since the Pliocene and subtracted this from the elevation of the formations they investigated.

One key interval of particular interest during the Pliocene is the mid-Piacenzian Warm Period – some 3.264 to 3.025 million years ago - when temperatures were 2 to 3º Celsius higher than pre-industrial levels. “The interval also marks the last time the Earth’s atmospheric CO₂ was as high as today, providing important clues about what the future holds in the face of current anthropogenic warming,” Onac says.

This study found that during this period, global mean sea level was as high as 16.2 meters (with an uncertainty range of 5.6 to 19.2 meters) above present. This means that even if atmospheric CO₂ stabilizes around current levels, the global mean sea level would still likely rise at least that high, if not higher, the scientists concluded. In fact, it is likely to rise higher because of the increase in the volume of the oceans due to rising temperature. 

“Considering the present-day melt patterns, this extent of sea-level rise would most likely be caused by a collapse of both Greenland and the West Antarctic ice sheets,” Dumitru said.

The authors also measured sea level at 23.5 meters higher than present about four million years ago during the Pliocene Climatic Optimum, when global mean temperatures were up to 4°C higher than pre-industrial levels. “This is a possible scenario if active and aggressive reduction in greenhouse gases into the atmosphere is not undertaken,” Asmerom said.

The research team included graduate student and lead author Oana A. Dumitru and Professor Bogdan Onac, both at the School of Geosciences, University of Southern Florida, Professor Jacqueline Austermann, Department of Earth and Environmental Sciences, Columbia University, Senior Research Scientist Victor Polyak and Professor Yemane Asmerom at the Department of Earth and Planetary Sciences, University of New Mexico in Albuquerque, and Professor Joan Fornós, Associate Lecturers Joaquín Ginés and Angel Ginés at the Universitat de les Illes Balears in Mallorca.

This research is the result of a collaborative National Science Foundation (NSF) project between the University of South Florida and the University of New Mexico and part of the bilateral agreement between USF and UIB, and has been funded by NSF and the Spanish State Research Agency.

原始論文：Oana A. Dumitru, Jacqueline Austermann, Victor J. Polyak, Joan J. Fornós, Yemane Asmerom, Joaquín Ginés, Angel Ginés & Bogdan P. Onac. Constraints on global mean sea level during Pliocene warmth. Nature, 2019 DOI: 10.1038/s41586-019-1543-2

引用自：University of New Mexico. "Evidence for past high-level sea rise: Mallorcan cave yields 4-million-year-old geologic evidence providing new insights into magnitude global sea level rise."