高度還原地球氣候歷史的紀錄使得現今的氣候變化有了比較基準

 原文網址：https://news.ucsc.edu/2020/09/climate-variability.html

高度還原地球氣候歷史的紀錄使得現今的氣候變化有了比較基準

過去6600萬年的連續紀錄顯示自然情況下，地球繞日軌道改變而造成的氣候變化幅度，遠小於預計之中未來因為溫室氣體排放而造成的暖化幅度。

By Tim Stephens

氣候學家首度把過去6600萬年來地球的氣候變化彙整成一份高度還原的連續紀錄。這份紀錄顯示氣候有四種不同的狀態，研究人員將它們分別稱為熱室、溫室、涼室、冰室(Hothouse, Warmhouse, Coolhouse, Icehouse)。

全球平均氣溫過去6600萬年與未來的變化趨勢，其中顯示了不同的氣候狀態。圖片來源：Westerhold et al., CENOGRID

這些主要的氣候狀態可以維持數百萬年、有時甚至是數千萬年之久。在每段時期當中氣候仍會因為地球繞日軌道變化而有規律的改變，這類全球溫度變化跟不同氣候狀態之間的劇烈轉變相比之下還是小了許多。不過氣候在不同狀態時受到軌道影響而出現的變化也各自不同。

這項由大型國際團隊歷經數十年的研究結果刊登於九月十日的《科學》(Science)。研究的困難之處在於這些過去氣候變化的測量結果，其時間尺度要精細到足以看出是由地球軌道變化所造成。(軌道變化包括地球繞日軌道的偏心率，以及地球自轉軸的進動與歲差的變化)

共同作者James Zachos表示：「我們很久以前就已經知道由於地球軌道的變化會改變到達地表的太陽能有多少，因此決定了冰期與間冰期的循環步調。此外，天文學家也一直在把更早之前的軌道變化給計算出來。」Zachos是加州大學聖塔克魯茲分校地球與行星科學的特聘教授，也是海洋健康方面的Ida Benson Lynn教授。

「當我們把過去的氣候重建出來可以非常清楚地看到長期的變化歷程。雖然我們知道其中應該要有尺度更小、由軌道改變造成的規律變化，但長久以來科學家認為要把它們復原出來是不可能辦到的，」Zachos表示。「現在我們成功得到氣候的自然變化之後，就能看出由人為造成的預期變化要比它們大上許多。」

冰室地球

地球氣候在過去三百萬年來一直處於冰期與間冰期交替的冰室狀態。雖然現代人是在這段期間演化出來，但是溫室氣體排放以及其他人類活動正把地球推向溫室狀態，甚至是3400萬年的始新世結束之後就從未看見的熱室狀態。在始新世早期地球的極區完全沒有冰層，而全球的平均溫度則比現在高了9到14℃。

Zachos說：「IPCC預測照這樣下去2300年全球的氣溫可能會到達過去5000萬年來都從未見過的層級。」

在彙整這份新的氣候紀錄時，很重要的一部份是從國際海洋鑽探計畫(ODP，之後變成整合海洋鑽探計畫(IODP)，2013年由國際海洋發現計畫接續下去)取來的高品質沉積物岩芯。在這些海底沉積物當中保存的微小浮游生物(稱為有孔蟲)的殼體記錄了過去的氣候訊息。研究人員在分析沉積物岩芯之後，必須將沉積物紀錄的氣候變化和地球軌道的變化(稱為米蘭科維奇循環)匹配以得出「天文年代」(astrochronology)。

Zachos表示：「科學界在1990年代中期找出方法來把這種策略應用到更早以前的地質年代。」2001年發表在《科學》由他主持的研究顯示了大約在2500萬年前，漸新世與中新世交替之際的這五百萬年間氣候會隨著軌道改變而變化。

他說：「這改變了一切――我們知道若我們成功辦到這件事，那就可以繼續往回推，或許還能一路回推到6600萬年前，並且以軌道變化的尺度為脈絡來看待地球氣候史上的短期事件和重大轉變。」

沉積物岩芯

Zachos多年來和主要作者Thomas Westerhold合作，他所任職的德國不萊梅大學海洋環境科學中心儲藏了十分大量的沉積物岩芯。紀錄當中有關較老年代的新數據絕大部分都是由不萊梅大學以及Zachos在UCSC的實驗室產生。

Westerhold負責的步驟相當重要：在從世界各處取得沉積物岩芯之後，他們必須把還原出來的氣候紀錄重疊的部份給銜接在一起。Zachos表示：「要把氣候紀錄剪接成這麼長的超長膠捲(megasplice)是個相當冗長的過程，而且我們也想從不同的沉積物岩芯中得出相同的紀錄來證明訊號確實為真，因此這是需要各國科學家彼此合作才能完成的大工程。」

既然研究人員已經彙整出過去6600萬年由天文年代定出來的連續氣候紀錄，他們便能看出在不同的變數之下，像是溫室氣體的含量、極區冰層的覆蓋面積不同時，軌道的變動會對氣候產生的影響。

Zachos加以解釋：「在溫室氣體含量極高、冰層完全消失的世界中，就能完全屏除跟冰層有關的回饋作用，以及這些作用對氣候動力學造成的影響。」

溫室氣體含量

過去6600萬年氣候的主要轉變多數都和溫室氣體含量變化有關。以Zachos做過許多研究的古新世―始新世氣候最暖期(PETM)為例，研究結果顯示這段全球迅速暖化而讓氣候變成熱室狀態的時期，原因和大量的碳釋放到大氣有關。同樣的，在始新世晚期隨著大氣中的二氧化碳含量逐漸降低，冰層開始在南極形成、氣候也轉變成涼室狀態。

Zachos表示：「氣候在接近這些轉變時期會變得不穩定，而我們看到軌道作用力決定了氣候的反應，因此我們想要更加瞭解其中關係。」

他接著表示新的氣候紀錄對許多研究領域來說提供了一個相當重要的基礎。這不只可以用來驗證氣候模型，也可以讓地球物理學家研究地球動力學的許多面向，而古生物學家也可以藉此來探討環境變化如何驅策物種演化。

Zachos表示：「這不只是地球科學的重大進展，同時也是國際海洋鑽探計畫留給世人的重要財產。」

共同作者南安普敦大學的Steven Bohaty和艾希特大學的Kate Littler都曾和Zachos在加州大學聖塔克魯茲分校共事。其他作者則包含來自世界各地十幾座研究機構的科學家。研究經費來自德國研究基金會、英國自然環境研究委員會、歐盟的展望2020計畫、中國國家科學基金會、荷蘭地球系統科學中心以及美國國家科學基金會。

 

High-fidelity record of Earth’s climate history puts current changes in context

A continuous record of the past 66 million years shows natural climate variability due to changes in Earth’s orbit around the sun is much smaller than projected future warming due to greenhouse gas emissions

For the first time, climate scientists have compiled a continuous, high-fidelity record of variations in Earth’s climate extending 66 million years into the past. The record reveals four distinctive climate states, which the researchers dubbed Hothouse, Warmhouse, Coolhouse, and Icehouse.

These major climate states persisted for millions and sometimes tens of millions of years, and within each one the climate shows rhythmic variations corresponding to changes in Earth’s orbit around the sun. But each climate state has a distinctive response to orbital variations, which drive relatively small changes in global temperatures compared with the dramatic shifts between different climate states.

The new findings, published September 10 in Science, are the result of decades of work and a large international collaboration. The challenge was to determine past climate variations on a time scale fine enough to see the variability attributable to orbital variations (in the eccentricity of Earth’s orbit around the sun and the precession and tilt of its rotational axis).

“We’ve known for a long time that the glacial-interglacial cycles are paced by changes in Earth’s orbit, which alter the amount of solar energy reaching Earth’s surface, and astronomers have been computing these orbital variations back in time,” explained coauthor James Zachos, distinguished professor of Earth and planetary sciences and Ida Benson Lynn Professor of Ocean Health at UC Santa Cruz.

“As we reconstructed past climates, we could see long-term coarse changes quite well. We also knew there should be finer-scale rhythmic variability due to orbital variations, but for a long time it was considered impossible to recover that signal,” Zachos said. “Now that we have succeeded in capturing the natural climate variability, we can see that the projected anthropogenic warming will be much greater than that.”

Icehouse

For the past 3 million years, Earth’s climate has been in an Icehouse state characterized by alternating glacial and interglacial periods. Modern humans evolved during this time, but greenhouse gas emissions and other human activities are now driving the planet toward the Warmhouse and Hothouse climate states not seen since the Eocene epoch, which ended about 34 million years ago. During the early Eocene, there were no polar ice caps, and average global temperatures were 9 to 14 degrees Celsius higher than today.

“The IPCC projections for 2300 in the ‘business-as-usual’ scenario will potentially bring global temperature to a level the planet has not seen in 50 million years,” Zachos said.

Critical to compiling the new climate record was getting high-quality sediment cores from deep ocean basins through the international Ocean Drilling Program (ODP, later the Integrated Ocean Drilling Program, IODP, succeeded in 2013 by the International Ocean Discovery Program). Signatures of past climates are recorded in the shells of microscopic plankton (called foraminifera) preserved in the seafloor sediments. After analyzing the sediment cores, researchers then had to develop an “astrochronology” by matching the climate variations recorded in sediment layers with variations in Earth’s orbit (known as Milankovitch cycles).

“The community figured out how to extend this strategy to older time intervals in the mid-1990s,” said Zachos, who led a study published in 2001 in Science that showed the climate response to orbital variations for a 5-million-year period covering the transition from the Oligocene epoch to the Miocene, about 25 million years ago.

“That changed everything, because if we could do that, we knew we could go all the way back to maybe 66 million years ago and put these transient events and major transitions in Earth’s climate in the context of orbital-scale variations,” he said.

Sediment cores

Zachos has collaborated for years with lead author Thomas Westerhold at the University of Bremen Center for Marine Environmental Sciences (MARUM) in Germany, which houses a vast repository of sediment cores. The Bremen lab along with Zachos’s group at UCSC generated much of the new data for the older part of the record.

Westerhold oversaw a critical step, splicing together overlapping segments of the climate record obtained from sediment cores from different parts of the world. “It’s a tedious process to assemble this long megasplice of climate records, and we also wanted to replicate the records with separate sediment cores to verify the signals, so this was a big effort of the international community working together,” Zachos said.

Now that they have compiled a continuous, astronomically dated climate record of the past 66 million years, the researchers can see that the climate’s response to orbital variations depends on factors such as greenhouse gas levels and the extent of polar ice sheets.

“In an extreme greenhouse world with no ice, there won’t be any feedbacks involving the ice sheets, and that changes the dynamics of the climate,” Zachos explained.

Greenhouse gas levels

Most of the major climate transitions in the past 66 million years have been associated with changes in greenhouse gas levels. Zachos has done extensive research on the Paleocene-Eocene Thermal Maximum (PETM), for example, showing that this episode of rapid global warming, which drove the climate into a Hothouse state, was associated with a massive release of carbon into the atmosphere. Similarly, in the late Eocene, as atmospheric carbon dioxide levels were dropping, ice sheets began to form in Antarctica and the climate transitioned to a Coolhouse state.

“The climate can become unstable when it’s nearing one of these transitions, and we see more deterministic responses to orbital forcing, so that’s something we would like to better understand,” Zachos said.

The new climate record provides a valuable framework for many areas of research, he added. It is not only useful for testing climate models, but also for geophysicists studying different aspects of Earth dynamics and paleontologists studying how changing environments drive the evolution of species.

“It’s a significant advance in Earth science, and a major legacy of the international Ocean Drilling Program,” Zachos said.

Coauthors Steven Bohaty, now at the University of Southampton, and Kate Littler, now at the University of Exeter, both worked with Zachos at UC Santa Cruz. The paper’s coauthors also include researchers at more than a dozen institutions around the world. This work was funded by the German Research Foundation (DFG), Natural Environmental Research Council (NERC), European Union’s Horizon 2020 program, National Science Foundation of China, Netherlands Earth System Science Centre, and the U.S. National Science Foundation.

原始論文：Thomas Westerhold, Norbert Marwan, Anna Joy Drury, Diederik Liebrand, Claudia Agnini, Eleni Anagnostou, James S. K. Barnet, Steven M. Bohaty, David De Vleeschouwer, Fabio Florindo, Thomas Frederichs, David A. Hodell, Ann E. Holbourn, Dick Kroon, Vittoria Lauretano, Kate Littler, Lucas J. Lourens, Mitchell Lyle, Heiko Pälike, Ursula Röhl, Jun Tian, Roy H. Wilkens, Paul A. Wilson, James C. Zachos. An astronomically dated record of Earth’s climate and its predictability over the last 66 million years. Science, 2020 DOI: 10.1126/science.aba6853

引用自：University of California - Santa Cruz. "High-fidelity record of Earth's climate history puts current changes in context."