預測未來氣候的關鍵在於瞭解過去

原文網址：https://news.arizona.edu/story/past-key-predicting-future-climate-scientists-say

預測未來氣候的關鍵在於瞭解過去

By Daniel Stolte

在評估人為排放的溫室氣體造成的影響、預估未來氣候的情境以及提出如何降低影響的政策時，世界各地的專家都會用到氣候模型這種工具。最近在發表於期刊《科學》(Science)的評論性文章中，一群氣候專家提出了充分的理由，呼籲在研發氣候模型時，應該把古氣候的數據也納入考量。

燃燒化石燃料而造成的未來劇烈氣候變遷，海平面上升不過是預期的其中一項後果而已。圖片來源：Pete Linforth/Pixabay

世界各地都有運用數值模型來預測未來氣候變遷的研究單位。最近一組氣候學家的國際團隊建議這些單位在評估並描述它們的模型表現時，內容應該要有模擬過去氣候的結果。

「我們主張研發氣候模型的社群必須關注過去的氣候，並且在預測未來時主動將其納入考量。」亞利桑那大學地球科學系的副教授Jessica Tierney表示。她是這篇期刊《科學》的評論性文章的主要作者。「如果你的模型可以準確模擬過去的氣候，那它在預測未來的情境時可能也會表現得出色許多。」

這篇《科學》的文章作者表示，隨著我們對地球許久以前――最遠到達人類出現以前的數千萬年――的氣候有更多品質更好的資訊，在瞭解溫室氣體含量對氣候系統中的關鍵因子會有什麼樣的影響時，過去的氣候在這方面能提供的見解也越來越多。人類對於氣候的歷史紀錄通常只能追溯到一兩百年前，在地球氣候的歷史當中只是一眨眼的時間而已。然而，古氣候紀錄能涵蓋的氣候條件範圍卻多出許多，因此它能提供給氣候模型的資訊是歷史數據無法達成的。在地球的過去，有許多時期的溫度、降雨模式和冰層分布範圍的差異都相當大。

「我們應該要利用過去的氣候來評價氣候模型並加以微調，」Tierney表示。「放眼過去、探討未來可以幫助我們在預估溫度、冰層和水循環時減少相關的不確定性。」

一般而言，氣候科學家評價它們的模型會用到過去的氣象紀錄，像是衛星的測量結果、海水溫度、風速、雲量……等參數。接著模型的演算法會不斷進行校正與微調，直到預測結果跟觀測到的氣候紀錄互相吻合。如果電腦模型可以根據過去某個時段的觀測結果，精確地重現出當時的氣候，那麼氣候科學家就會認為該模型適合用來預測未來的氣候，且結果具有合理的準確度。

「我們發現許多模型在重現人類歷史上的氣候表現得相當良好，但要重現地球的地質歷史就沒那麼好了。」

Tierney表示造成這項區別的原因之一，是不同模型會運用不同方法來計算雲的影響，這是模擬氣候時最困難的其中一道問題。這類差異會造成模型彼此之間得到的「氣候敏感度」出現分歧，此參數是氣候學家用來測量當溫室氣體排放量變成兩倍，地球氣候的反應會有多強烈。

政府間氣候變化專門委員會(IPCC)正在用最新一代的模型來撰寫下次報告，其中幾個模型得出的氣候敏感度比之前的版本還高。Tierney解釋：

「這代表如果你把二氧化碳排放量加倍，它們得出的全球暖化幅度會比之前的結果還要嚴重。問題來了：我們對於這些非常敏感的新模型有多少信心？」

IPCC的報告通常每八年發表一次，在每次報告之間氣候模型都會依據最新的研究數據來更新。

「在模型越來越複雜的同時，理論上來說也會變得更好，但這個的意義是什麼？」Tierney表示。「你想要知道未來會發生什麼事，所以你會希望模型對於二氧化碳變高時產生的反應是能夠信賴的。」

人類消耗化石燃料的結果正把地球推向史上從未見過的溫暖狀態，雖然氣候科學界對於這點看法一致，但是不同模型的預測結果卻天差地遠。其中有些預測本世紀末的溫度上升幅度會高達6℃。

Tierney表示儘管地球曾經經歷過二氧化碳濃度比目前的400 ppm左右還要高出許多的時期，但是人類造成的溫室氣體排放速度，卻是地質紀錄中任何一段時間都無法匹敵的。

作者在這篇文章中把氣候模型用在地質記錄中幾段已知的極端氣候。Tierney表示最近一次可以讓我們一窺未來樣貌的溫暖氣候，大約發生在五千萬年前的始新世。當時全球的二氧化碳濃度高達1000 ppm，冰層也完全消失。

Tierney說：「如果我們再不減少二氧化碳的排放量，那麼到了2100年大氣中的二氧化碳濃度就會越來越像始新世。」

文章作者對於氣候變遷的討論一路追溯至9000萬年前左右，恐龍還在地球上漫步時的白堊紀。當時的紀錄顯示氣候還能更加溫暖――Tierney將其描述為「更加驚悚」的情況――二氧化碳濃度高達2000 ppm，海洋就像裝滿溫水的浴缸一樣。

「關鍵是二氧化碳，」Tierney說。「每當我們在地質紀錄中看到氣候溫暖的證據，同時二氧化碳也會相當高。」

作者表示有些模型比其他的更能重現出地質紀錄中看到的氣候，這強調出我們需要用古氣候來測試氣候模型。特別是像始新世等過去的溫暖氣候，凸顯了在二氧化碳增加的情況下，雲對溫度升高的重要性。

「我們呼籲氣候科學界應該要盡早用古氣候來測試模型，也就是開發時就要這麼做，而不是跟目前的做法一樣，大都是在完成後才進行，」Tierney說。「像雲這種看似微不足道的事物其實對地球的能量平衡有重大影響，因此也會影響到你的模型對2100年的溫度預測結果。」

 

Past is key to predicting future climate

In a review paper published in the journal Science, a group of climate experts makes the case for including paleoclimate data in the development of climate models. Such models are used globally to assess the impacts of human-caused greenhouse gas emissions, predict scenarios for future climate and propose strategies for mitigation.

An international team of climate scientists suggests that research centers around the world using numerical models to predict future climate change should include simulations of past climates in their evaluations and statements of their model performance.

"We urge the climate model developer community to pay attention to the past and actively involve it in predicting the future," said Jessica Tierney, an associate professor in the University of Arizona's Department of Geosciences and lead author of a new research review paper in the journal Science. "If your model can simulate past climates accurately, it likely will do a much better job at getting future scenarios right."

As more and better information becomes available about climates in Earth's distant history – reaching back many millions of years before humans existed – past climates become increasingly relevant for improving scientists' understanding of how key elements of the climate system are affected by greenhouse gas levels, according to the Science paper's authors. Unlike historic climate records, which typically only go back a century or two – a mere blink of an eye in the planet's climate history – paleoclimates cover a vastly broader range of climatic conditions that can inform climate models in ways historical data cannot. These periods in Earth's past span a large range of temperatures, precipitation patterns and ice sheet distribution.

"Past climates should be used to evaluate and fine-tune climate models," Tierney said. "Looking to the past to inform the future could help narrow uncertainties surrounding projections of changes in temperature, ice sheets and the water cycle."

Typically, climate scientists evaluate their models with data from historical weather records, such as satellite measurements, sea surface temperatures, wind speeds, cloud cover and other parameters. The model's algorithms are then adjusted and tuned until their predictions mesh with the observed climate records. If a computer simulation produces a historically accurate climate based on the observations made during that time, it is considered fit to predict future climate with reasonable accuracy.

"We find that many models perform very well with historic climates, but not so well with climates from the Earth's geological past," Tierney said.

One reason for the discrepancies are differences in how the models compute the effects of clouds, which is one of the great challenges in climate modeling, Tierney said. Such differences cause models to diverge from each other in terms of what climate scientists refer to as climate sensitivity – a measure of how strongly the Earth's climate responds to a doubling of greenhouse gas emissions.

Several of the latest generation models that are being used for the next report by the Intergovernmental Panel on Climate Change, or IPCC, have a higher climate sensitivity than previous iterations, Tierney explained.

"This means that if you double carbon dioxide emissions, they produce more global warming than their previous counterparts, so the question is: How much confidence do we have in these very sensitive new models?"

In between IPCC reports, which typically are released every eight years, climate models are being updated based on the latest research data.

"Models become more complex and, in theory, they get better, but what does that mean?" Tierney said. "You want to know what happens in the future, so you want to be able to trust the model with regard to what happens in response to higher levels of carbon dioxide."

While there is no debate in the climate science community about human fossil fuel consumption pushing Earth toward a warmer state for which there is no historical precedent, different models generate varying predictions. Some forecast an increase as large as 6 degrees Celsius by the end of the century.

Tierney said while Earth's atmosphere has experienced carbon dioxide concentrations much higher than today's level of about 400 parts per million, there is no time in the geological record that matches the speed at which humans are contributing to greenhouse gas emissions.

In the paper, the authors applied climate models to several known past climate extremes from the geological record. The most recent warm climate that may offer a glimpse into the future occurred about 50 million years ago during the Eocene epoch, Tierney said. Global carbon dioxide was at 1,000 parts per million at that time, and there were no large ice sheets.

"If we don't cut back emissions, we are headed for Eocene-like CO2 levels by 2100," Tierney said.

The paper's authors discuss climate changes all the way back to the Cretaceous period about 90 million years ago, when dinosaurs still roamed Earth. That period shows that the climate can get even warmer – a scenario that Tierney described as "even scarier," with carbon dioxide levels up to 2,000 parts per million and the oceans as warm as a bathtub.

"The key is CO₂," Tierney said. "Whenever we see evidence of warm climate in the geologic record, CO₂ is high as well."

Some models are much better than others at producing the climates seen in the geologic record, which underscores the need to test climate models against paleoclimates, the authors said. In particular, past warm climates such as the Eocene highlight the role that clouds play in contributing to warmer temperatures under increased carbon dioxide levels.

"We urge the climate community to test models on paleoclimates early on, while the models are being developed, rather than afterwards, which tends to be the current practice," Tierney said. "Seemingly small things like clouds affect the Earth's energy balance in major ways and can affect the temperatures your model produces for the year 2100."

原始文章：Jessica E. Tierney, Christopher J. Poulsen, Isabel P. Montañez, Tripti Bhattacharya, Ran Feng, Heather L. Ford, Bärbel Hönisch, Gordon N. Inglis, Sierra V. Petersen, Navjit Sagoo, Clay R. Tabor, Kaustubh Thirumalai, Jiang Zhu, Natalie J. Burls, Gavin L. Foster, Yves Goddéris, Brian T. Huber, Linda C. Ivany, Sandra Kirtland Turner, Daniel J. Lunt, Jennifer C. Mcelwain, Benjamin J. W. Mills, Bette L. Otto-Bliesner, Andy Ridgwell, Yi Ge Zhang. Past climates inform our future. Science, 2020 DOI: 10.1126/science.aay3701

引用自：University of Arizona. "Past is key to predicting future climate, scientists say."