地球科學家畫出過去6600萬年大氣二氧化碳的變化過程

 原文網址：https://attheu.utah.edu/science-technology/geoscientists-map-changes-in-atmospheric-co2-over-past-66-million-years/

By Brian Maffly

由於人類過去兩個世紀大量燃燒化石燃料，現今大氣裡的二氧化碳含量至少是數百萬年來的最高峰。

這張圖顯示了從工業革命前至6500萬年前，整個新生代大氣二氧化碳濃度的變化過程(單位為百萬分之一(ppm))。科學家解譯在地質紀錄中的代用指標來進行估算，不同顏色的條帶代表透過不同的代用指標數據，獨立重建出來的全球平均溫度。虛線為今日的二氧化碳濃度420 ppm。圖片來源：Gabe Bowen, University of Utah

但是這種溫室氣體目前在大氣中的濃度——419 ppm(百萬分之一)在地球歷史上算高還低？

一個國際科學團隊為了釐清這道問題，檢視了地質紀錄中各式各樣可以提供古代大氣成分線索的標記，其中猶他大學的地質學家做出了重要貢獻。他們的初步研究成果本周發表於期刊《科學》，(Science)內容重建了從6600萬年前恐龍消失、哺乳類崛起的新生代開始以來，二氧化碳濃度的變化過程。

猶他大學的地質學教授Gabe Bowen是此研究的通訊作者之一。他說冰河含有的氣泡可以做為直接證據，讓科學家了解80萬年前至今的大氣二氧化碳濃度，但是這種紀錄並無法延伸到非常久以前的地質歷史。

「一旦拿不到冰芯就沒有直接證據，再也無法取得可供分析的大氣氣體樣品，」Bowen表示。「這時你就得依賴間接證據，我們稱其為『代用指標』(proxy)。但因為它們是間接的，所以使用上相當不容易。」

地質紀錄中的「代用指標」

這些代用指標包括礦物的同位素組成、葉片化石的型態，以及其他可以反映大氣化學的地質證據。其中一個代用指標源自於此篇新研究的共同作者之一，猶他大學的地質學家Thure Cerling所做出的基礎發現。他過往的研究確立了古代土壤的碳同位素可以指示過去的二氧化碳含量。

然而這些代用指標的強度不一，而且大部分只能涵蓋過去的一小片段。因此由哥倫比亞大學的氣候學家Bärbel Hönisch統籌的研究團隊「新生代二氧化碳代用指標整合計畫」(Cenozoic CO₂ Proxy Integration Project, CenCO2PIP)的目標便是要評估、分類並整合現有的代用指標，進而製作出高度還原的大氣二氧化碳紀錄

「這是在解讀過去6600萬年來二氧化碳如何變化的研究中，採納的線索最多且統計方法最為完善的成果之一，」共同作者，猶他大學Bowen實驗室的博士後研究員Dustin Harper表示。「新研究的重點之一是我們成功結合了不同類型的沉積物紀錄中的多種代用指標，不論它們是來自陸地或海洋。過去從來沒有研究可以做到這種地步。」

這項新研究是來自16國的90多名科學家齊心協力的成果。由多個單位核准的數十件補助案構成了該團隊的經費，他們希望最終能把二氧化碳的紀錄一路重建至距今5億4千萬年前，複雜生命誕生的時刻。

過去地球曾經比現在更加溫暖，二氧化碳的含量也高出許多。然而，目前的數值419 ppm在紀錄中仍然是道陡峭的高峰且有可能造成危險，而在近代地質史也是前所未見的。

「從8百萬年前到現在，二氧化碳含量曾經比現在高的可能性大概為5%，」Bowen表示。「但我們認為要確實看到跟現在差不多的二氧化碳含量，必須要回溯到1400萬年前。」

換句話說，人類活動在幾個世代之間就讓大氣的成分產生了巨變。結果便是全球出現越來越多氣候系統受到擾亂的警訊，像是危及性命的熱浪、劇烈的風暴、長期乾旱與海洋酸化……等現象。

對於地質時間中的二氧化碳變化歷程有詳實的理解，在解釋並探討地質歷史上各式各樣的事件時也是必須的。大氣二氧化碳含量與氣候的變化可能促成了生物大滅絕，卻也讓演化有所革新。

舉例來說，新生代期間二氧化碳含量的長期低落以及連帶造成的氣候冷化，可能促使了植物的生理變化、物種之間的競爭並使優勢種改變，進而影響哺乳類的演化過程。

「因此在理解當今所見的物種與生態系如何出現以及它們未來可能的遭遇時，對於過去二氧化碳的變化趨勢要有更加精進的了解是不可或缺的，」這篇研究寫道。

 

Geoscientists map changes in atmospheric CO₂ over past 66 million years

Today atmospheric carbon dioxide is at its highest level in at least several million years thanks to widespread combustion of fossil fuels by humans over the past couple centuries.

But where does 419 parts per million (ppm)—the current concentration of the greenhouse gas in the atmosphere—fit in Earth’s history?

That’s a question an international community of scientists, featuring key contributions by University of Utah geologists, is sorting out by examining a plethora of markers in the geologic record that offer clues about the contents of ancient atmospheres. Their initial study was published this week in the journal Science, reconstructing CO₂ concentrations going back through the Cenozoic, the era that began with the demise dinosaurs and rise of mammals 66 million years ago.

Glaciers contain air bubbles, providing scientists direct evidence of CO₂ levels going back 800,000 years, according to U geology professor Gabe Bowen, one of the study’s corresponding authors. But this record does not extend very deep into the geological past.

“Once you lose the ice cores, you lose direct evidence. You no longer have samples of atmospheric gas that you can analyze,” Bowen said. “So you have to rely on indirect evidence, what we call proxies. And those proxies are tough to work with because they are indirect.”

“Proxies” in the geologic record

These proxies include isotopes in minerals, the morphology of fossilized leaves and other lines of geological evidence that reflect atmospheric chemistry. One of the proxies stems from the foundational discoveries of U geologist Thure Cerling, himself a co-author on the new study, whose past research determined carbon isotopes in ancient soils are indicative of past CO₂ levels.

But the strengths of these proxies vary and most cover narrow slices of the past. The research team, called the Cenozoic CO₂ Proxy Integration Project, or CenCO2PIP, and organized by Columbia University climate scientist Bärbel Hönisch, set out to evaluate, categorize and integrate available proxies to create a high-fidelity record of atmospheric CO₂.

“This represents some of the most inclusive and statistically refined approaches to interpreting CO₂ over the last 66 million years,” said co-author Dustin Harper, a U postdoctoral researcher in Bowen’s lab. “Some of the new takeaways are we’re able to combine multiple proxies from different archives of sediment, whether that’s in the ocean or on land, and that really hasn’t been done at this scale.”

The new research is a community effort involving some 90 scientists from 16 countries. Funded by dozens of grants from multiple agencies, the group hopes to eventually reconstruct the CO₂ record back 540 million years to the dawn of complex life.

At the start of the Industrial Revolution–when humans began burning to coal, then oil and gas to fuel their economies–atmospheric CO₂ was around 280 ppm. The heat-trapping gas is released into the air when these fossil fuels burn.

工業革命之初大氣二氧化碳濃度大約為280ppm。此時人類為了經濟發展而開始燃燒煤炭，接著是石油和天然氣。當這些化石燃料被燃燒，可以把熱困住的二氧化碳就會釋放到大氣當中。

Looking forward, concentrations are expected to climb up to 600 to 1,000 ppm by the year 2100, depending on the rate of future emissions. It is not clear exactly how these future levels will influence the climate.

放眼未來，依據人類之後的排放速率，二氧化碳的濃度預估2100年會攀升到600到1000 ppm。科學家還不清楚未來這樣的濃度對氣候有什麼影響。

But having a reliable map of past CO₂ levels could help scientists more accurately predict what future climates may look like, according to U biology professor William Anderegg, director the U’s Wilkes Center for Climate & Policy.

然而，猶他大學的生物學教授，也是該大學Wilkes氣候與政策中心的主任William Anderegg表示，若能如實還原過去二氧化碳濃度的變化歷程，就能幫助科學家更準確地預測未來氣候的樣貌。

“This is an incredibly important synthesis and has implications for future climate change as well, particularly the key processes and components of the Earth system that we need to understand to project the speed and magnitude of climate change,” Anderegg said.

「對未來的氣候變遷來說，這篇綜合性的研究也相當重要且有所啟發，特別是在預估氣候變遷的速度與規模時，可以幫助我們了解地球系統中的關鍵部位與作用，」Anderegg表示。

Today’s 419 ppm is the highest CO₂ in 14 million years

今日二氧化碳的含量419 ppm是過去1400萬年的最高峰

At times in the past when Earth was a far warmer place, levels of CO₂ were much higher than now. Still, the 419 ppm recorded today represents a steep and perhaps dangerous spike and is unprecedented in recent geologic history.

“By 8 million years before present, there’s maybe a 5% chance that CO₂ levels were higher than today,” Bowen said, “but really we have to go back 14 million years before we see levels we think were like today.”

In other words, human activity has significantly altered the atmosphere within the span of a few generations. As a result, climate systems around the globe are showing alarming signs of disruption, such as powerful storms, prolonged drought, deadly heat waves and ocean acidification.

A solid understanding of atmospheric CO₂ variation through geological time is also essential to deciphering and learning from various features of Earth’s history. Changes in atmospheric CO₂ and climate likely contributed to mass extinctions, as well as evolutionary innovations.

During the Cenozoic, for example, long-term declines in CO₂ and associated climate cooling may have driven changes to plant physiology, species competition and dominance, which in turn impacted mammalian evolution.

“A more refined understanding of past trends in CO₂ is therefore central to understanding how modern species and ecosystems arose and may fare in the future,” the study states.

原始論文：Bärbel Hönisch, Dana L. Royer, Daniel O. Breecker, Pratigya J. Polissar, Gabriel J. Bowen, Michael J. Henehan, Ying Cui, Margret Steinthorsdottir, Jennifer C. McElwain, Matthew J. Kohn, Ann Pearson, Samuel R. Phelps, Kevin T. Uno, Andy Ridgwell, Eleni Anagnostou, Jacqueline Austermann, Marcus P. S. Badger, Richard S. Barclay, Peter K. Bijl, Thomas B. Chalk, Christopher R. Scotese, Elwyn de la Vega, Robert M. DeConto, Kelsey A. Dyez, Vicki Ferrini, Peter J. Franks, Claudia F. Giulivi, Marcus Gutjahr, Dustin T. Harper, Laura L. Haynes, Matthew Huber, Kathryn E. Snell, Benjamin A. Keisling, Wilfried Konrad, Tim K. Lowenstein, Alberto Malinverno, Maxence Guillermic, Luz María Mejía, Joseph N. Milligan, John J. Morton, Lee Nordt, Ross Whiteford, Anita Roth-Nebelsick, Jeremy K. C. Rugenstein, Morgan F. Schaller, Nathan D. Sheldon, Sindia Sosdian, Elise B. Wilkes, Caitlyn R. Witkowski, Yi Ge Zhang, Lloyd Anderson, David J. Beerling, Clara Bolton, Thure E. Cerling, Jennifer M. Cotton, Jiawei Da, Douglas D. Ekart, Gavin L. Foster, David R. Greenwood, Ethan G. Hyland, Elliot A. Jagniecki, John P. Jasper, Jennifer B. Kowalczyk, Lutz Kunzmann, Wolfram M. Kürschner, Charles E. Lawrence, Caroline H. Lear, Miguel A. Martínez-Botí, Daniel P. Maxbauer, Paolo Montagna, B. David A. Naafs, James W. B. Rae, Markus Raitzsch, Gregory J. Retallack, Simon J. Ring, Osamu Seki, Julio Sepúlveda, Ashish Sinha, Tekie F. Tesfamichael, Aradhna Tripati, Johan van der Burgh, Jimin Yu, James C. Zachos, Laiming Zhang. Toward a Cenozoic history of atmospheric CO 2. Science, 2023; 382 (6675) DOI: 10.1126/science.adi5177

引用至：University of Utah. "Geoscientists map changes in atmospheric carbon dioxide over past 66 million years."