氣候較溫暖時每個世紀的氣候也會越來越不穩定嗎？

原文網址：https://www.ucl.ac.uk/news/news-articles/1018/121018-climate-change-variation-century-to-century

氣候較溫暖時每個世紀的氣候也會越來越不穩定嗎？

根據倫敦大學學院最新的研究結果，地球在比現今間冰期(最近11,700年)還要溫暖的末次間冰期(129,000至116,000年前)期間，氣候在百年尺度下的不穩定性也比較高。

今日發表在《自然通訊》( Nature Communications)的這項研究經費來自自然環境研究委員會以及澳洲研究委員會。研究結果顯示以百年尺度來看，末次間冰期的南歐曾出現一連串乾旱事件，以及北大西洋低溫水體的擴張。

要預估人類持續排碳的情況下未來氣候的樣貌，瞭解氣候較溫暖時的自然變化是相當重要的。研究主要作者，倫敦大學學院地理系的Chronis Tzedakis教授表示：「末次間冰期在這方面具有密切關連，因為它可以讓我們理解在極為溫暖的時期氣候如何運作。」

末次間冰期有段時間北極的暖化情形十分嚴重，地表氣溫估計比工業革命前的還高了3-11℃，這跟預測中本世紀末高緯度地區的暖化情形相當。

末次間冰期的全球海平面估計比現在高了6-9公尺左右，其中有0.6-3.5公尺是由格陵蘭的冰層融化造成。

之前幾個北大西洋和歐洲的紀錄顯示末次間冰期的溫度雨量有百年尺度的變化，但這些氣候振盪的發生時間、程度、原因還是有很多地方無法定論。

進行這項新研究的科學家來自國際上十二所不同的機構，他們利用海洋與陸上的地質紀錄，配合氣候模擬實驗，製作出極為詳盡的時間表顯示末次間冰期北大西洋和南歐的海洋大氣變化。

為了解決比較不同環境形成的紀錄時產生的不確定性，主導古海洋地層分析的劍橋大學博士Luke Skinner表示：「我們從里斯本外海採集了一根海洋岩芯，分析此沉積物樣品裡的不同化石可以讓我們製作出地層學中的『羅塞塔石碑』。」

負責孢粉分析的倫敦大學學院地理系博士Vasiliki Margari表示：「這根海洋岩芯也具有太加斯河運至深海的孢粉，藉此可以直接比對陸上植被和北大西洋的變化關係。」

植被變化的主因是雨量變化。而植被變化又可以連結到義大利北部Corchia洞穴中石筍紀錄的雨水化學訊號變化。

領導團隊研究這座義大利洞窟的墨爾本大學博士Russell Drysdale表示：「運用鈾同位素衰變的放射性定年法讓Corchia的紀錄有詳細的年代背景，使其成為該時期最好的年表之一，因此相當重要。」

新南威爾斯大學的Laurie Menviel 和Andrea Taschetto博士進行了氣候模擬實驗，結果顯示這些變化的空間分布特徵和大西洋經向翻轉環流(Atlantic meridional overturning circulation)中斷的後果一致。

末次間冰期高緯度地區強烈暖化造成格陵蘭的冰層融化並流到海裡，可能促成了大西洋經向翻轉環流弱化以及觀察到的氣候變遷。

Tzedakis博士說：「雖然末次間冰期無法完全類比成未來人類引發的氣候變遷，但這段時期的概況顯示出未來的氣候以百年尺度來看會越來越不穩定，與冰層和海洋動力學的變化有關。」

「未來研究的重點應該放在更加確定末次間冰期格陵蘭冰層融化並流到海中的量，以及對海洋環流的確切影響。」

Does climate vary more from century to century when it is warmer?

Century-scale climate variability was enhanced when the Earth was warmer during the Last Interglacial period (129-116 thousand years ago) compared to the current interglacial (the last 11,700 years), according to a new UCL-led study.

The findings, published today in Nature Communications and funded by the Natural Environment Research Council (NERC) and the Australian Research Council (ARC), reveal that the Last Interglacial period was punctuated by a series of century-scale arid events in southern Europe and cold water-mass expansions in the North Atlantic. 

Assessing natural climate variability under relatively warm conditions is crucial to inform projections under future carbon emission scenarios.  Professor Chronis Tzedakis (UCL Geography), study lead author, said: “The Last Interglacial is particularly relevant because it provides insights into climate processes during a period of excess warmth.”

The Last Interglacial period contained an interval of intense Arctic warming, with surface air temperatures estimated at 3-11°C above pre-industrial, comparable to high-latitude warming scenarios for the end of this century.

Global sea-level during the Last Interglacial is estimated to have been ~6-9 m above present, with 0.6-3.5 m derived from melting of the Greenland Ice Sheet.

Previously, several North Atlantic and European records have detected century-scale changes in temperature and precipitation within the Last Interglacial, but there has been considerable uncertainty over the timing, extent and origin of these climate oscillations.

This new study by international researchers from twelve institutions used marine and terrestrial geological archives, coupled with climate model experiments, to create the most detailed timeline of ocean and atmosphere changes in the North Atlantic and southern Europe during the Last Interglacial.

To address the uncertainties in comparing records from different environments, researchers produced a “stratigraphic ‘rosetta stone’ by analysing different fossils from the same sediment samples in a marine core off Lisbon,” said Dr Luke Skinner (Cambridge University) who led the palaeoceanographic analyses.

“The marine core also contained pollen transported from the Tagus river into the deep sea, thus enabling a direct comparison of vegetation and North Atlantic ocean changes,” said Dr Vasiliki Margari (UCL Geography), who undertook the pollen analysis.

Changes in vegetation, primarily caused by variations in the amount of rainfall, were then linked to changes in the chemical signature of rainfall recorded in stalagmites from Corchia Cave in northern Italy.

“The Corchia record is particularly important because it is supported by very detailed radiometric dating using the decay of uranium isotopes, producing one of the best chronologies for this period available,” said Dr Russell Drysdale (University of Melbourne), who led the team studying the Italian cave.

Climate model experiments, undertaken by Dr Laurie Menviel and Dr Andrea Taschetto of the University of New South Wales Sydney, revealed that the spatial fingerprint of these changes was consistent with disruptions of the Atlantic meridional overturning circulation.

Greenland ice-melt and runoff as a result of strong high-latitude warming during the Last Interglacial may have contributed to the weakening of the Atlantic meridional overturning circulation and to the observed climate changes.

“Although not a strict analogue for future anthropogenically-driven changes, the profile of the Last Interglacial that emerges is one of enhanced century-scale climate instability, with implications for ice-sheet and ocean dynamics,” said Professor Tzedakis.

“Future research efforts should focus on constraining further the extent of melting and runoff from the Greenland ice-sheet and its effects on ocean circulation during the Last Interglacial.”

原始論文：P. C. Tzedakis, R. N. Drysdale, V. Margari, L. C. Skinner, L. Menviel, R. H. Rhodes, A. S. Taschetto, D. A. Hodell, S. J. Crowhurst, J. C. Hellstrom, A. E. Fallick, J. O. Grimalt, J. F. McManus, B. Martrat, Z. Mokeddem, F. Parrenin, E. Regattieri, K. Roe, G. Zanchetta. Enhanced climate instability in the North Atlantic and southern Europe during the Last Interglacial. Nature Communications, 2018; 9 (1) DOI: 10.1038/s41467-018-06683-3

引用自：University College London. "Does climate vary more from century to century when it is warmer?."

原文網址：https://www.ucl.ac.uk/news/news-articles/1018/121018-climate-change-variation-century-to-century