變動的氣候―過去與未來
研究人員比較冰河期和間冰期的全球氣溫變化程度
若想知道未來氣候會如何變化，就必須先回首過去。透過觀察數千年以前發生的氣候變遷，我們可以更加改進對未來氣候的預測。研究人員比較冰芯樣本和海洋沉積物的各個層位，可以推導出地球的平均氣溫如何隨著時間變化以及變動程度有多大諸如此類的氣候因子。從2萬1000年前的末次冰河期高峰到現在我們所處的間冰期，地球整體的暖化程度平均而言為5℃。有鑒於全球暖化會持續至未來，對現今地球的全體居民來說，知道將來的溫度上升會穩定成長或者突然飆高是相當重要的一件事。此外，對於氣候變遷的調適措施來說極端事件的頻率是規劃時必要的基準值，因為當對象為防洪措施、運輸系統和建築材料時，我們要防範的不僅是「平均」變化，還必須考慮到最糟的情況。
德國波茨坦的阿爾弗雷德˙魏格納極地與海洋研究所(AWI)中的亥姆霍兹青年研究員小組ECUS的氣候學家，探討隨著地球從末次冰河期進入到現今間冰期，全球氣溫的變化幅度是如何跟著改變。目前為止科學家推測氣溫的變化程度在末次冰河期期間較為劇烈，而現今間冰期大部分時候的溫度變化都比較和緩。此解釋的基礎為格陵蘭中部冰芯的水同位素資料。
由Kira
Rehfeld博士和Thomas
Laepple博士領導的團隊比較了格陵蘭(冰芯)的數據、採集自世界各地數個海域的沉積物數據，還有從南極取得的冰芯樣品。他們證實冰河期溫度劇烈波動的現象在全世界的表現絕非一致，而是各地皆有不同。比方說，末次冰河期高峰時熱帶的溫度變化比現今還要強烈三倍，而格陵蘭的冰芯則顯示當地的變動幅度比現今還要強個70倍。「格陵蘭的冰芯無疑是瞭解過往氣候的重要鑰匙。話雖如此，我們的研究證明從格陵蘭冰芯得出的結論並非總是能代表整個世界。」主持青年研究員小組的Laepple如此解釋。他同時也是歐洲研究委員會資助的青年研究小組SPACE的主持人。
第一作者Kira
Rehfeld和其同僚的成果為首次彙整並比較總計來自99個研究地點的不同類型氣候資料。在氣侯學界，冰芯通常被視為黃金標準，因為它的分層相當分明且連續，不像從海床取出的沉積物時常受到構造運動、洋流或海洋生物影響導致層狀構造毀損。AWI的研究人員設計出某些運算方法使他們在計算不同類型的古氣候資料時，可以估計各類資料的不確定因素和可能誤差來源，並將這些因子納入他們的分析當中。Laepple表示：「如此這般，我們可以比較地球歷史上不同時期的沉積物樣品和冰芯。」
在冰河期因為被冰層覆蓋的極區和熱帶地區的溫差更為巨大，造成冷暖氣團之間的能量流動更為活躍，使得冰河期的溫度變化比較強烈。「若我們依循此想法的邏輯便可以得到此結論：隨著全球暖化加劇，溫度變化幅度會持續減小。」Rehfeld表示這單純是因為北地暖化後跟熱帶之間的溫差會縮小。「然而，我們資料的時間單位一格便涵蓋數個世紀至數千年――也就是我們的視野無法聚焦至數年之間，代表在論及決定天氣的極端事件時，我們只能透過間接方法歸納出結論。」目前在英國南極調查局進行研究的氣候學者Rehfeld解釋。
進行氣候模擬的研究人員先前於2014年提出一套假說，來解釋在溫暖氣候條件下氣溫變動幅度會降低的機制。但直到現在Rehfeld、Laepple
和其同僚才首度利用過往的全球氣候資料來佐證這套理論。AWI的研究人員接著描述了他們的下個目標：「我們的計劃是詳細研究短期的氣溫變化在過去是如何隨著時間改變，以及和長期氣候變遷的關係為何。為了達成這項目標，我們需要更為可靠的氣候紀錄，並增進我們對它們運作方式的理解。」要把古氣候紀錄的準確度提升至可以同時反映極端事件的水準，可能會是未來數年最大的挑戰之一。
Climate variability – past and future
Researchers compare global temperature
variability in glacial and interglacial periods
If you want to know how the climate will change in
the future, you need to look at the past. By looking at the climate changes
that took place thousands of years ago, we can improve predictions for future
climate. Comparing layers in the ice-core samples and ocean sediments has
allowed researchers to deduce e.g. how the average temperature on Earth has
changed over time, and also how great the variability was. From the height of
the last glacial period 21,000 years ago to our current interglacial period,
the Earth has warmed by an average of five degrees Celsius. In view of future
global warming, it’s vital for today’s global population to know whether
temperatures will rise steadily, or whether there will be sudden, major
fluctuations. The frequency of extreme events represents an essential benchmark
for climate change adaptation measures, since, when it comes to flood
protection, transport and building materials, we need to be prepared for the
worst-case scenario, and not just for “average” changes.
Climate researchers from the Helmholtz Young
Investigators Group ECUS at the Alfred Wegener Institute, Helmholtz Centre for
Polar and Marine Research (AWI) in Potsdam have now investigated how
temperature variability changed as the Earth warmed from the last glacial
period to the current interglacial period. To date it has been assumed that
temperatures varied greatly during the last glacial, while the current
interglacial was largely characterised by small temperature variations. This
interpretation was based on water isotope data from central Greenland ice
cores.
The team, led by Dr Kira Rehfeld and Dr Thomas
Laepple, compared the Greenland data with that from sediments collected in
several ocean regions around the globe, as well as from ice-core samples
gathered in the Antarctic. They demonstrate that the phenomenon of major
temperature fluctuations during glacial periods has by no means manifested
uniformly worldwide, but has instead varied from region to region. For
instance, in the Tropics the temperature variations were three times as intense
as today at the height of the last glacial, whereas the ice cores from
Greenland indicate variations that were 70 times as intense. “The ice cores
from Greenland are without a doubt an important key to understanding the
climate of the past. That being said, our study confirms that the conclusions
regarding Greenland aren’t always representative of the entire world,” explains
Young Investigators Group leader Laepple, who also heads the ERC-funded Young
Investigators Group SPACE.
The achievement of first author Kira Rehfeld and her
colleagues: they have for the first time gathered and compared data from
diverse climate archives and a total of 99 research sites. In the climate
research community, ice cores are generally considered the gold standard,
because their layers are highly consistent, unlike sediment layers from the
seafloor, which are frequently marred by tectonic shifts, currents or marine
organisms. The AWI researchers have devised mathematical methods that allow
them to estimate the uncertainties and potential sources of error while
assessing various paleoclimate archives, and to take these factors into account
in their analyses. “As such, we can compare the sediment samples with the ice
cores for various epochs in the planet’s history,” says Laepple.
The more intensive variations during glacial periods
are due to the greater difference in temperature between the ice-covered polar
regions and the Tropics, which produced a more dynamic exchange of warm and
cold air masses. “If we then follow that idea to its logical conclusion, it
tells us the variations will continue to lessen as global warming progresses,”
says Rehfeld – simply because the difference in temperature between the warming
North and the Tropics will decline. “However, our data covers timeframes
spanning centuries and millennia – we can’t zoom in on just a handful of years,
which means we can only draw indirect conclusions regarding the extreme events
that shape weather,” explains climate researcher Rehfeld, who is currently
pursuing research with the British Antarctic Survey (BAS).
Climate modellers had previously postulated the
mechanism of reduced variability under warmer climatic conditions in 2014. Yet
with their analysis, Rehfeld, Laepple and colleagues are the first to reinforce
this theory with global climate data from the past. The AWI researchers
describe their next endeavour as follows: “We plan to investigate in detail the
changes in short-term variations in the past and their relation to long-term
climate changes. To do so, we need reliable climate archives, and to improve
our understanding of how they work.” Increasing the accuracy to a level at
which paleo-archives can also reflect extreme events will likely be one of the
greatest challenges for the years to come.
原始論文：Kira
Rehfeld, Thomas Münch, Sze Ling Ho, Thomas Laepple. Global patterns of declining temperature variability from the Last
Glacial Maximum to the Holocene. Nature,
2018; DOI: 10.1038/nature25454
引用自：Alfred
Wegener Institute, Helmholtz Centre for Polar and Marine Research.
"Climate variability -- past and future."
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