劍橋大學的研究人員設計並進行的這項試驗，對象包括了世界各地的數個研究團隊，他們利用同樣的原始樹輪數據來重建過去兩千年的溫度變化。
保存在冰島的樹木準化石。圖片來源：Hrafn Óskarsson
雖然每個團隊的重建結果都清楚顯示，近代由人為氣候變遷造成的暖化是過去兩千年前所未見的，但是變異度、振幅與敏感度都有顯著的差異，這可以歸因於個別結果在重建過程中由研究人員所下的決策。
主持這項研究的是劍橋大學的教授Ulf Büntgen，他說這項結果「對於科學研究的真實性以及透明度來說相當重要――我們信任我們的數據，同時也坦承所有氣侯學家在建立模型或是重建過去氣候的時候，都必須下定各自的決策。」
結果發表在期刊《科學通訊》(Nature Communications)。研究人員提出為了增加氣候重建結果的可信度，氣候團隊應該同時進行幾種重建方法，才能看到整體的情況。
研究人員以年為解析度來重建過去氣候的主要方法，便是利用從樹輪中得到的資訊：在熱帶地區以外的樹木會精確地以年為單位形成生長輪，它們就像指紋一樣道道分明。每道年輪都可以告訴我們對應生長季的某些氣候條件。將不同年代的許多樹木得到的數據結合起來，科學家就能一路重建到數百年前，甚至數千年前的氣候條件。
重建過去的氣候條件有其用意，科學家可以依循過去的自然界變化來得知目前的氣候條件或是預測未來。不過重建氣候的難處在於我們沒有時光機，也就是說沒有辦法驗證結果是否正確。
「雖然樹輪裡的資訊不會改變，但是人類卻會製造變化：他們可能會用不同的技術或是選用數據裡的不同區段來進行重建，」任職於劍橋大學地理學系的Büntgen表示。他也是捷克布爾諾捷克全球研究中心的兼任科學家。「任何重建結果當中，不確定性的範圍都是一個問題：也就是你對某個結果有多肯定。之前有很多研究的重點放在試著透過統計方法把不確定性定量化，但是卻沒有人研究決策所扮演的角色。」
「真相只有一個並非是實際的狀況――我們所做的每個決策或多或少都會有主觀的成分。科學家並不是機器人，這也不是我們樂見的，但是知道他們在哪些地方做出決策以及這麼做對結果的影響，則是相當重要的一件事。」
Büntgen和同僚設計了一個實驗來測試決策對氣候重建有什麼樣的影響。他們寄給世界各地的15個研究團隊同一份原始樹輪數據，接著要求他們以此為基礎，盡可能地建立最好的大尺度氣候模型，來重建過去兩千年北半球的夏季氣溫。
「除此之外他們怎麼做都行――雖然這樣的實驗聽起來沒什麼意義，不過之前完全沒有人做過，」Büntgen表示。
每個團隊得出的重建結果都不同，原因出在他們過程中所下的決策，也就是他們選擇的數據或是使用的方法。舉例來說：一個團隊可能採用器測目標數據(instrumental target data)裡六七八月的部分，但是另一團隊則可能只有採用七月和八月的平均。
重建結果的差異主要體現在數據的振幅：確切來說為中世紀暖期有多熱，或是大型火山爆發之後某個夏季有多冷。
Büntgen強調每個重建結果顯示出來的趨勢整體來看都是一樣的：西元3世紀，以及10到12世紀之間為暖期；西元6、15和19世紀的一連串大型火山爆發事件之後的夏季都有突然變冷的情形；最後，它們都顯示了近代從20和21世紀開始的全球暖化，程度是過去2000年前所未見的。
「你可能會認為如果一開始用的數據都是一樣的，那麼最後得出的結果也會是相同的，但是氣候重建的過程並非如此，」Büntgen表示。「所有的重建結果都指出了同樣的趨勢，任意兩者之間也沒有相悖的情形，但是彼此之間確實存在著差異，這只能歸因於過程中所下的決策。」
那麼未來出現某個氣候重建結果的時候，我們如何知道是否該相信它？在這個專業頻繁受到質疑，甚至是被完全摒棄的時代，我們該如何確信什麼是真的？其中一個答案或許是：將每個有進行決策的地方標註出來，然後考慮不同的選項之後進行多次的重建。理所當然地，這代表氣候學家的工作會變得更加繁重，但是在辨認決策對結果會造成什麼影響的時候，這種檢驗方法或許極具價值。
另一種讓氣候重建結果更加有說服力的方法則是讓不同團隊互相合作，將所有人的結果綜合起來成一個整體來看待。「幾乎在所有的科學領域當中，任何一篇研究或者結果都會告訴你它的想法，」他說。「但是當你觀看科學證據的全體，透過彼此之間所有的細微差異以及不確定性，反而可以得到更加清晰的整副圖像。」
Experiment evaluates the effect of human decisions on climate reconstructions
The experiment, designed and run by researchers from the University of Cambridge, had multiple research groups from around the world use the same raw tree-ring data to reconstruct temperature changes over the past 2,000 years.
While each of the reconstructions clearly showed that recent warming due to anthropogenic climate change is unprecedented in the past two thousand years, there were notable differences in variance, amplitude and sensitivity, which can be attributed to decisions made by the researchers who built the individual reconstructions.
Professor Ulf Büntgen from the University of Cambridge, who led the research, said that the results are “important for transparency and truth – we believe in our data, and we’re being open about the decisions that any climate scientist has to make when building a reconstruction or model.”
To improve the reliability of climate reconstructions, the researchers suggest that teams make multiple reconstructions at once so that they can be seen as an ensemble. The results are reported in the journal Nature Communications.
Information from tree rings is the main way that researchers reconstruct past climate conditions at annual resolutions: as distinctive as a fingerprint, the rings formed in trees outside the tropics are annually precise growth layers. Each ring can tell us something about what conditions were like in a particular growing season, and by combining data from many trees of different ages, scientists are able to reconstruct past climate conditions going back hundreds and even thousands of years.
Reconstructions of past climate conditions are useful as they can place current climate conditions or future projections in the context of past natural variability. The challenge with a climate reconstruction is that – absent a time machine – there is no way to confirm it is correct.
“While the information contained in tree rings remains constant, humans are the variables: they may use different techniques or choose a different subset of data to build their reconstruction,” said Büntgen, who is based at Cambridge’s Department of Geography, and is also affiliated with the CzechGlobe Centre in Brno, Czech Republic. “With any reconstruction, there’s a question of uncertainty ranges: how certain you are about a certain result. A lot of work has gone into trying to quantify uncertainties in a statistical way, but what hasn’t been studied is the role of decision-making.
“It’s not the case that there is one single truth – every decision we make is subjective to a greater or lesser extent. Scientists aren’t robots, and we don’t want them to be, but it’s important to learn where the decisions are made and how they affect the outcome.”
Büntgen and his colleagues devised an experiment to test how decision-making affects climate reconstructions. They sent raw tree ring data to 15 research groups around the world and asked them to use it to develop the best possible large-scale climate reconstruction for summer temperatures in the Northern hemisphere over past 2000 years.
“Everything else was up to them – it may sound trivial, but this sort of experiment had never been done before,” said Büntgen.
Each of the groups came up with a different reconstruction, based on the decisions they made along the way: the data they chose or the techniques they used. For example, one group may have used instrumental target data from June, July and August, while another may have only used the mean of July and August only.
The main differences in the reconstructions were those of amplitude in the data: exactly how warm was the Medieval warming period, or how much cooler a particular summer was after a large volcanic eruption.
Büntgen stresses that each of the reconstructions showed the same overall trends: there were periods of warming in the 3rd century, as well as between the 10th and 12th century; they all showed abrupt summer cooling following clusters of large volcanic eruptions in the 6th, 15th and 19th century; and they all showed that the recent warming since the 20th and 21st century is unprecedented in the past 2000 years.
“You think if you have the start with the same data, you will end up with the same result, but climate reconstruction doesn’t work like that,” said Büntgen. “All the reconstructions point in the same direction, and none of the results oppose one another, but there are differences, which must be attributed to decision-making.”
So, how will we know whether to trust a particular climate reconstruction in future? In a time where experts are routinely challenged, or dismissed entirely, how can we be sure of what is true? One answer may be to note each point where a decision is made, consider the various options, and produce multiple reconstructions. This would of course mean more work for climate scientists, but it could be a valuable check to acknowledge how decisions affect outcomes.
Another way to make climate reconstructions more robust is for groups to collaborate and view all their reconstructions together, as an ensemble. “In almost any scientific field, you can point to a single study or result that tells you what to hear,” he said. “But when you look at the body of scientific evidence, with all its nuances and uncertainties, you get a clearer overall picture.”
原始論文：Ulf Büntgen et al. The influence of decision-making in tree ring-based climate reconstructions. Nature Communications, 2021 DOI: 10.1038/s41467-021-23627-6
引用自：University of Cambridge. “Experiment evaluates the effect of human decisions on climate reconstructions.”
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