分析遠達50億年前的氣候變遷

原文網址：www.sciencedaily.com/releases/2016/03/160316082942.htm

Climate variations analyzed five million years back in time

分析遠達50億年前的氣候變遷

When we talk about climate change today, we have to look at what the climate was previously like in order to recognise the natural variations and to be able to distinguish them from the human-induced changes. Researchers from the Niels Bohr Institute have analysed the natural climate variations over the last 12,000 years, during which we have had a warm interglacial period and they have looked back 5 million years to see the major features of the Earth's climate. The research shows that not only is the weather chaotic, but the Earth's climate is chaotic and can be difficult to predict. The results are published in the scientific journal, Nature Communications.

當我們討論眼下發生的氣候變遷時，我們必須要先研究過去的氣候是如何，這樣才能了解在自然情況下的氣候變化，而能夠與人類引發的變動之間做出區隔。波爾研究所的研究人員分析了過去12,000年以來，我們所處的溫暖間冰期中的自然氣候變化；他們同時也回顧了過去50億年以了解地球氣候的諸多重要特徵。研究顯示變化莫測的不只是天氣，地球氣候也是如此因而難以被人們預測。這項結果刊登於科學期刊《自然通訊》(Nature Communications)之上。

The Earth's climate system is characterised by complex interactions between the atmosphere, oceans, ice sheets, landmasses and the biosphere (parts of the world with plant and animal life). Astronomical factors also play a role in relation to the great changes like the shift between ice ages, which typically lasts about 100,000 years and interglacial periods, which typically last about 10-12,000 years.

地球氣候系統的重要特徵便是大氣、海洋、冰層、陸塊和生物圈(地球有動植物生活的部分)彼此之間複雜的交互作用。來自宇宙的因子也會參與一些重大的變動，像是冰期和間冰期的交替。前者通常會持續約100,000年，後者則通常歷時10至12,000年。

Climate repeats as fractals

氣候就像碎形(fractal)般不斷地重複

"You can look at the climate as fractals, that is, patterns or structures that repeat in smaller and smaller versions indefinitely. If you are talking about 100-year storms, are there then 100 years between them? -- Or do you suddenly find that there are three such storms over a short timespan? If you are talking about very hot summers, do they happen every tenth year or every fifth year? How large are the normal variations? -- We have now investigated this," explains Peter Ditlevsen, Associate Professor of Climate Physics at the Niels Bohr Institute at the University of Copenhagen. The research was done in collaboration with Zhi-Gang Shao from South China University, Guangzhou in Kina.

「你可以把氣候視作一種碎形，這種結構或模式會不斷地以越來越小的形式重複出現。如果你討論到百年強度的風暴，那麼在兩次事件之間的100年內還會再次出現這麼強的風暴嗎？或者你會突然發現在一段相當短的期間內就發生了三次強度相當的風暴？如果你提及了數個非常炎熱的夏季，那麼它們是每十年還五年就會發生一次嗎？『正常』的變化幅度究竟有多大？這是我們目前正在探討的。」哥本哈根大學波爾研究所的氣候物理學副教授 Peter Ditlevsen解釋。此研究在中國廣州南中國大學的 Zhi-Gang Shao協助下完成。

The researchers studied temperature measurements over the last 150 years, ice core data from Greenland from the interglacial period 12,000 years ago, for the ice age 120,000 years ago, ice core data from Antarctica, which goes back 800,000 years, as well as data from ocean sediment cores going back 5 million years.

研究人員探討了過去150年來的氣溫測量結果，和格陵蘭12,000年來的間冰期冰芯紀錄；而12萬年前的冰河期則以南極洲冰芯判讀，這最遠能追溯至80萬年前，最後還有可以回溯至50億年前的海洋沉積物岩芯資料。

"We only have about 150 years of direct measurements of temperature, so if, for example, we want to estimate how great of variations that can be expected over 100 years, we look at the temperature record for that period, but it cannot tell us what we can expect for the temperature record over 1000 years. But if we can determine the relationship between the variations in a given period, then we can make an estimate. These kinds of estimates are of great importance for safety assessments for structures and buildings that need to hold up well for a very long time, or for structures where severe weather could pose a security risk, such as drilling platforms or nuclear power plants. We have now studied this by analysing both direct and indirect measurements back in time," explains Peter Ditlevsen.

「我們僅僅擁有大約150年的氣溫直接測量紀錄，因此舉例來說，若我們想要預估在100年內氣溫的變化幅度可以有多大，我們就能參照這段時期的氣溫紀錄，但如果我們想要從1000年的氣溫資料中獲取任何訊息時，前述的百年紀錄就派不上用場了。然而若我們可以確知給定一段時間，當中的變化幅度就會有多大，那這種關係就可以讓我們作出預估。這種預測對於建築物或設施的安全評估來說相當重要，尤其是那些必須要長期維持在良好狀態，或是可能會面臨造成安全損害的惡劣天氣的建物或設施，像是鑽油平台或核電廠。我們現在利用分析過往的直接和間接測量記錄來研究這項議題。」 Peter Ditlevsen解釋。

The research shows that the natural variations over a given period of time depends on the length of this period in the very particular way that is characteristic for fractals. This knowledge tells us something about how big we should expect the 1000-year storm to be in relation to the 100-year storm and how big the 100-year storm is expected to be in relation to the 10-year storm. They have further discovered that there is a difference in the fractal behaviour in the ice age climate and in the current warm interglacial climate.

研究顯示在一段時間內的自然變動幅度取決於這段時間的長短，這種關係跟碎形的特徵十分相似。這項知識告訴我們千年強度的風暴會有多強，跟百年強度的風暴規模之間有所關聯；而百年強度的風暴會有多強，又跟十年強度的風暴規模之間有所關聯。他們更進一步的發現冰河期氣候的碎形行為，與目前溫暖間冰期氣候的碎形行為兩者之間有所不同。

Abrupt climate fluctuations during the ice age

冰河期的急遽氣候波動

"We can see that the climate during an ice age has much greater fluctuations than the climate during an interglacial period. There has been speculation that the reason could be astronomical variations, but we can now rule this out as the large fluctuation during the ice age behave in the same 'fractal' way as the other natural fluctuations across the globe," Peter Ditlevsen.

「我們可以看到冰河期時的氣候波動與間冰期的相比大了許多。之前有人猜測原因可能跟天文因子的變化有關，但我們現在可以排除這個可能性，因為冰河期內發生的大型波動，就『碎形』行為來說跟全球其他自然因子的變動一模一樣。」 Peter Ditlevsen說。

The astronomical factors that affect the Earth's climate are that the other planets in the solar system pull on the Earth because of their gravity. This affects the Earth's orbit around the sun, which varies from being almost circular to being more elliptical and this affects solar radiation on Earth. The gravity of the other planets also affects the Earth's rotation on its axis. The Earth's axis fluctuates between having a tilt of 22 degrees and 24 degrees and when the tilt is 24 degrees, there is a larger difference between summer and winter and this has an influence on the violent shifts in climate between ice ages and interglacial periods.

會影響地球氣候的天文因素像是地球會因為太陽系其他行星的重力而受到牽引，這會影響地球環繞太陽的軌道，使得它會在近乎圓形與傾向橢圓之間來回擺盪，進一步地影響地球受到的日照量。其他行星的重力也會施加在地軸上而影響地球自轉。地球自轉軸的角度會在22至24度之間擺動，當地軸傾斜24度時，夏季和冬季之間的差異便會增加，這造成的劇烈氣候變化使得冰河期和間冰期交替出現。

The abrupt climate changes during the ice age could be triggered by several mechanisms that have affected the powerful ocean current, the Gulf Stream, which transports warm water from the equator north to the Atlantic, where it is cooled and sinks down into the cold ocean water under the ice to the bottom and is pushed back to the south. This water pump can be put out of action or weakened by changes in the freshwater pressure, the ice sheet breaking up or shifting sea ice and this results in the increasing climatic variability.

冰河期時的急遽氣候變遷可能是因為數種機制作用在重要的海洋環流上而導致。這種環流譬如像是墨西哥灣流，它會將赤道溫暖的海水往北運輸至大西洋，在北大西洋海水會冷卻而下沉至冰層下的冰冷海底，並逐漸被推送回南邊。這種海水幫浦機制可能會因一些變動而弱化甚至停擺，像是淡水水壓、冰層崩解或是海冰移動，這會導致氣候變化幅度增加。

Natural and human-induced climate changes

自然與人為導致的氣候變遷

The climate during the warm interglacial periods is more stable than the climate of ice age climate.

在溫暖間冰期時的氣候較冰河氣的氣候要更加穩定。

"In fact, we see that the ice age climate is what we call 'multifractal', which is a characteristic that you see in very chaotic systems, while the interglacial climate is 'monofractal'. This means that the ratio between the extremes in the climate over different time periods behaves like the ratio between the more normal ratios of different timescales," explains Peter Ditlevsen

「事實上我們發現冰河期的氣候呈現出『多重碎形』(multifractal)，你可以在非常混亂的系統中發現這種特徵；而間冰期氣候則呈現『單一碎形』(monofractal)，這代表間冰期不同時段內極端氣候的比例，跟不同時間尺度下的正常比例之間更加相似。」Peter Ditlevsen解釋。

This new characteristic of the climate will make it easier for climate researchers to differentiate between natural and human-induced climate changes, because it can be expected that the human-induced climate changes will not behave in the same way as the natural fluctuations.

這項新發現的氣候特性可以讓氣候研究人員更輕易地分辨出自然與人為導致的氣候變遷，因為可以預期人類導致的氣候變遷的行為模式會跟自然波動之間有所差異。

"The differences we find between the two climate states also suggest that if we shift the system too much, we could enter a different system, which could lead to greater fluctuations. We have to go very far back into the geological history of the Earth to find a climate that is as warm as what we are heading towards. Even though we do not know the climate variations in detail so far back, we know that there were abrupt climate shifts in the warm climate back then," points out Peter Ditlevsen.

「我們發現的這兩個氣候狀態之間的差異也意味著如果這個系統被我們過度改變，我們就有可能進入一個截然不同的系統，而可能導致更大的變化發生。我們必須要探究地球地質史至十分久遠之處，才能找到與我們即將邁入的氣候同樣溫暖者。即便我們還不清楚那麼久以前氣候變化的細節，我們確實知道在過去的溫暖氣候下曾發生急劇的氣候變化。」 Peter Ditlevsen指出這點。

引用自：University of Copenhagen - Niels Bohr Institute. "Climate variations analyzed five million years back in time." ScienceDaily. ScienceDaily, 16 March 2016.