國際團隊的分析結果縮小了氣候敏感度的範圍
如果二氧化碳的濃度變成工業革命之前的二倍並維持下去，全世界的暖化幅度最終可能為2.3到4.5°C，而非之前預估的1.5到4.5°C。
對於氣候敏感度截至目前為止最尖端的全面分析結果，讓我們可以比以往更有信心的評估地球氣候對二氧化碳的敏感程度。
在評估大氣裡的二氧化碳濃度變成工業革命前的二倍，全球氣溫最終的反應會是如何的時候，四十多年來估計出來的可能範圍一直停滯在1.5到4.5°C而沒有進展。
這項由世界氣候研究計畫(World Climate Research Programme，WCRP))委任的最新研究，四年多來的成果發表在一篇165頁由同儕審查的期刊論文當中。他們發現氣候敏感度實際上不太可能位在1.5到4.5°C 當中最低的區間。分析結果指出如果大氣裡的二氧化碳濃度變成工業命前的二倍並維持下去，全世界最終的暖化程度可能會是2.3到4.5°C。研究發現暖化程度維持在2°C以下的機率不到5%，超過4.5°C的機率為6到18%。
地球的氣溫已經比工業革命前高出了大約1.2°C。如果溫室氣體的排放速度仍然不減，預估二氧化碳的濃度會在未來60年到80年之間變成目前的兩倍。
主要作者Steven
Sherwood教授表示：「自從1979年美國國家科學研究委員會的論文首度提出氣候敏感度為1.5到4.5°C以來(Charney et al,.)，將此範圍縮小就一直是相當大的挑戰。即使是IPCC最新的報告援用的仍然是同一數值。」Sherwood是這項新南威爾斯大學雪梨校區與澳洲極端氣候卓越研究中心合作進行的計畫總主持人。
許多不同氣候領域的研究人員攜手組成了國際團隊使這項研究得以進行。利用工業革命以降的溫度記錄、從古氣候紀錄評估史前時代的溫度、運用衛星的觀測數據，並透過精密的電腦模型檢驗氣候系統當中的物理因子如何互相影響，研究團隊相較於以往的研究取得了更多條獨立證據，並把它們結合起來而得到這項成果。
團隊運用嚴格的統計方法結合這些線索來找出不同結果之間彼此重疊的地方，這讓他們可以交集出氣候敏感度的最佳預測值。團隊發現各項證據的新進展使得它們可以互相驗證而讓結果更加可信。他們謹慎地納入了其他觀點或假設、以及「未知的未知數」，得出暖化的可能範圍為2.3到4.5°C；如果是用較為直接的計算方式，則得出的可能暖化幅度可以進一步縮小到2.6到3.9°C。
愛丁堡大學的Gabi Hegerl是共同作者，他說：「這篇論文將我們對於氣候敏感度的知識給整合起來，包括了大氣作用的測量結果、歷史上的暖化事件以及過去的冷暖氣候。運用統計方法統整出來的估計值，證實了氣候敏感度不大可能會是IPCC提出的範圍下限，但是上限卻是可能的。結果可以讓模擬未來氣候的模型更加可信。」
「過程中一個很重要的部分是確保每條線索大致上是獨立的，」Sherwood教授表示。「你可以把這想像成下列情形的數學版本：確認分別從兩個人身上聽到的謠言是否為同一個來源傳出來的，或是某個案件的兩個目擊者其中之一是否有聽過另一位的說法而受到影響。」
接著團隊還更進一步地找出要在何種條件之下，才能讓氣候敏感度位在最有可能的範圍之外。
之前認為的氣候敏感度或許能低到1.5到2°C左右，但研究人員表示這只有在資料分析中出現數個預期之外且無關的錯誤時才有可能出現(像是雲的行為和海洋的長期暖化模式出現預料之外的發展)，支持了他們對於低氣候敏感度是極為不可能的判斷。
二氧化碳變成工業革命前兩倍時氣溫會上升超過4.5°C則因為另外一種情形而變得不太可能，不過氣溫大幅上升的可能性還是比非常低的敏感度來得更高。
即使有種種限制，這項由WCRP發起、長達三年的研究計畫還是透過重複檢查每一步、仔細檢驗物理過程、並瞭解估計值的發生條件，才終於讓這個懸宕40年的問題有了確實進展。
澳洲國立大學的教授Eelco Rohling是共同作者，他說：「這些結果證明了跨領域的研究人員加上慢工細活的科學方法是相當重要的，也完美地呈現出不同國家之間的合作如何解開最難解的問題。」
「如果不同國家的決策者也能像研究人員一樣聚焦在問題上並擁有合作精神，那我們未來才有希望防止全球暖化最糟的情況發生。」
International analysis narrows range of climate’s
sensitivity to CO2
If atmospheric carbon dioxide levels double from their pre-industrial
levels and are maintained, the world would probably experience eventual warming
from 2.3 – 4.5°C, rather than the previously estimated
1.5 – 4.5°C range.
The most advanced and
comprehensive analysis of climate sensitivity yet undertaken has revealed with
more confidence than ever before how sensitive the Earth’s climate is to carbon
dioxide.
For more than 40 years, the
estimated likely range of the eventual global temperature response to a
doubling of atmospheric carbon dioxide compared to preindustrial levels has
stubbornly remained at 1.5°C – 4.5°C.
This new research, revealed
in a 165 page, peer-reviewed journal article commissioned by the World Climate Research Programme
(WCRP) written over four years, finds that the true climate sensitivity is
unlikely to be in the lowest part of the 1.5-4.5°C range. The analysis
indicates that if atmospheric carbon dioxide levels double from their
pre-industrial levels and are maintained, the world would probably experience
eventual warming from 2.3 – 4.5°C. The researchers found there would be
less than 5% chance of staying below 2°C and a 6-18% chance of exceeding 4.5°C.
With the Earth’s
temperature already at around 1.2°C above preindustrial levels, if greenhouse
gas emissions trajectories continue unabated the world can expect to see a
doubling of carbon dioxide in the next 60-80 years.
“Narrowing the range of
climate sensitivity has been a major challenge since the seminal US National
Research Council paper came up with a 1.5 – 4.5°C range in 1979 (Charney
et al). That same range was still quoted in the most recent IPCC report,” said
lead author Prof Steven Sherwood, a chief investigator in University of New
South Wales, Sydney, with the ARC Centre of Excellence for Climate
Extremes.
The research was only made
possible by bringing together an international team of researchers from a wide
range of climate disciplines. Using temperature records since the industrial
revolution, paleoclimate records to estimate prehistoric temperatures, satellite
observations and detailed models that examine the physics of interactions
within the climate system the team were able to combine more independent lines
of evidence than any previous study to get their results.
These lines of evidence
were then combined in a statistically rigorous way allowing the team to find
where the results overlapped. This allowed them to converge on the best
estimate of climate sensitivity. The team found that, with new developments,
the various lines of evidence corroborated one another leading to more
confidence in the result. The 2.3-4.5°C range accounts cautiously for
alternative views or assumptions and “unknown unknowns,” with a more
straightforward calculation yielding an even narrower 2.6-3.9°C likely range.
“This paper brings together
what we know about climate sensitivity from measurements of atmospheric
processes, the historical warming, and warm and cold climates of the past.
Statistically combined, these estimates make it improbable that climate
sensitivity is at the low end of the IPCC range and confirm the upper range.
This adds to the credibility of climate model simulations of future climate,”
said co-author Gabi Hegerl from the University of Edinburgh.
“An important part of the
process was to ensure that the lines of evidence were more or less
independent,” said Prof Sherwood. “You can think of it as the mathematical
version of trying to determine if a rumour you hear separately from two people
could have sprung from the same source; or if one of two eyewitnesses to a
crime has been influenced by hearing the story of the other one.”
The researchers then went
another step further and identified the conditions that would be required for
the climate sensitivity to lie outside this most likely range.
The researchers show that
low climate sensitivities previously thought to be plausible, around 1.5-2C,
could only occur if there were multiple unexpected and unconnected errors in
the data analysis (for example unexpected cloud behaviour and patterns of
long-term ocean warming), underlying their judgment that these low values are
now extremely unlikely.
A different set of
circumstances make it unlikely that global temperatures would rise more than
4.5°C for a doubling of carbon dioxide from pre-industrial times, although
these higher temperature responses are still more likely than very low
sensitivities.
Even with this
qualification, the three-year long research process initiated by the WCRP with
double-checking at every step, a detailed examination of the physical processes
and an understanding of the conditions required for the estimate has finally
consolidated an advance on the 40-year-problem.
“These results are a
testament to the importance of cross-disciplinary research along with slow,
careful science and perfectly highlight how international co-operation can
unpick our most vexing problems,” said co-author Prof Eelco Rohling from the
Australian National University.
“If international
policymakers can find the same focus and spirit of co-operation as these
researchers then it will give us hope that we can forestall the worst of global
warming."
原始論文：S. Sherwood, M. J. Webb, J. D. Annan, K. C.
Armour, P. M. Forster, J. C. Hargreaves, G. Hegerl, S. A. Klein, K. D. Marvel,
E. J. Rohling, M. Watanabe, T. Andrews, P. Braconnot, C. S. Bretherton, G. L.
Foster, Z. Hausfather, A. S. von der Heydt, R. Knutti, T. Mauritsen, J. R.
Norris, C. Proistosescu, M. Rugenstein, G. A. Schmidt, K. B. Tokarska, M. D.
Zelinka. An
assessment of Earth's climate sensitivity using multiple lines of evidence. Reviews of Geophysics,
2020; DOI: 10.1029/2019RG000678
引用自：University of New South Wales.
"International analysis narrows range of climate's sensitivity to CO2”
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