科學家終於解決了一項歷時已久的問題：在驅動全球冰期循環的作用中，地球軌道扮演什麼樣的腳色。
在今日發表於期刊《科學》(Science)的新研究，卡迪夫大學的團隊明確指出地球環繞太陽時自身的傾斜與擺動，對於過去大約二百萬年以來北半球的冰層消融過程有什麼影響。
科學家從很久以前就注意到北半球龐大的冰層有所消長，是由地球繞日軌道的幾何形狀變化所引起。
地球軌道的幾何形狀中有兩個要素可以影響冰層的融化：傾角與歲差。
傾角是地球環繞太陽時傾斜的角度，也是一年有不同季節的原因。
歲差則是地球旋轉時自身的擺動，就像一個稍微偏離中心轉動的陀螺。這種擺動所產生的角度意味著有時候靠太陽最近的是北半球，其他時候則是南半球；也就是說，其中一個半球的夏天會比另一半更加暖和，大概每過一萬年這種情形便會交換過來。
科學家確認過去大約一百萬年以來，傾角與歲差結合起來的效應，透過和氣候系統的複雜交互作用造成北半球的冰層有所消長，形成大約以十萬年為周期的冰期循環。
然而在早於一百萬年以前，稱為更新世早期的時代，冰期循環的週期卻只受控於傾角。當時的冰期循環幾乎都剛好以41000年為周期。
為什麼那段期間歲差在驅動冰期循環的作用當中沒有發揮顯著的影響？這項問題已經困惑科學家數十年之久。
卡迪夫大學的團隊在新研究揭露的新證據顯示，歲差在更新世早期其實具有一席之地。
他們的結果指出北半球因為歲差導致夏季較為高溫的時候冰層一定會融化，但是在早於一百萬年之前，這類事件的破壞性沒有那麼強，因此不會讓冰層完全崩解。
卡迪夫大學地球與環境科學院的教授Stephen
Barker是研究主要作者，他說：「更新世早期北半球的冰層跟現在比起來較小而且只分布在高緯度地區。由於高緯度地區傾角造成的效應會蓋過歲差，因此這也許能解釋為什麼我們花了如此長的時間，才找到證據指出更新世早期歲差也有發揮其作用。」
「這些發現是一項大型研究的最終成果，12多年來我們在實驗室費心處理將近10000個樣品，並且開發一連串新的分析方法。因為如此我們終於解決了這個古氣候學中歷時已久的問題，並且使我們對於氣候系統有更深的瞭解。」
「如果我們希望可以預測下個世紀或是更加長遠的氣候變遷，深入理解地球氣候動力學——即便是許久以前的氣候——都是非常重要的。雖然往後的氣候變遷可能是由人類所造成，但氣候系統總歸來說只有一個，而我們必須要加以理解才行。」
Scientists shine new light on role of
Earth’s orbit in the fate of ancient ice sheets
Scientists have finally put to bed a
long-standing question over the role of Earth’s orbit in driving global ice age
cycles.
In a new study published today in the journal Science, the team from Cardiff
University has been able to pinpoint exactly how the tilting and wobbling of
the Earth as it orbits around the Sun has influenced the melting of ice sheets
in the Northern Hemisphere over the past 2 million years or so.
Scientists have long been aware that the waxing and
waning of massive Northern Hemisphere ice sheets results from changes in the
geometry of Earth’s orbit around the Sun.
There are two aspects of the Earth’s geometry that
can influence the melting of ice sheets: obliquity and precession.
Obliquity is the angle of the Earth’s tilt as it
travels around the Sun and is the reason why we have different seasons.
Precession is how the Earth wobbles as it rotates,
much like a slightly off-centre spinning top. The angle of this wobble means
that sometimes the Northern Hemisphere is closest to the Sun and other times
the Southern Hemisphere is closest, meaning that roughly every 10,000 years one
hemisphere will have warmer summers compared to the other, before it switches.
Scientists have determined that over the past million
years or so, the combined effects of obliquity and precession on the waxing and
waning of Northern Hemisphere ice sheets has resulted, through complicated
interactions within the climate system, in ice age cycles lasting approximately
100,000 years.
However, before 1 million years ago, in a period
known as the early Pleistocene, the duration of ice age cycles was controlled
only by obliquity and these ice age cycles were almost exactly 41,000 years
long.
For decades, scientists have been puzzled as to why
precession did not play a more important part in driving ice age cycles during
this period.
In their new study, the Cardiff University team
reveal new evidence suggesting that precession did actually play a role during
the early Pleistocene.
Their results show that more intense summers, driven
by precession, have always caused Northern Hemisphere ice sheets to melt, but
before 1 million years ago, these events were less devastating and did not lead
to the complete collapse of ice sheets.
Lead author of the study Professor Stephen Barker,
from Cardiff University’s School of Earth and Environmental Sciences, said:
“Early Pleistocene ice sheets in the northern hemisphere were smaller than
their more recent counterparts, and limited to higher latitudes where the
effects of obliquity dominate over precession. This probably explains why it
has taken so long for us to find evidence of precession forcing during early
Pleistocene.
“These findings are the culmination of a major
effort, involving more than 12 years of painstaking work in the laboratory to process
nearly 10,000 samples and the development of a range of new analytical
approaches. Thanks to this we can finally put to rest a long-standing problem
in paleoclimatology and ultimately contribute to a better understanding of
Earth’s climate system.
“Improving our understanding of Earth’s climate
dynamics, even in the remote past, is crucial if we hope to predict changes
over the next century and beyond. Ongoing changes may be manmade, but there’s
only one climate system and we need to understand it.”
原始論文：Stephen
Barker, Aidan Starr, Jeroen van der Lubbe, Alice Doughty, Gregor Knorr, Stephen
Conn, Sian Lordsmith, Lindsey Owen, Alexandra Nederbragt, Sidney Hemming, Ian
Hall, Leah Levay, M. A. Berke, L. Brentegani, T. Caley, A. Cartagena-Sierra, C.
D. Charles, J. J. Coenen, J. G. Crespin, A. M. Franzese, J. Gruetzner, X. Han,
S. K. V. Hines, F. J. Jimenez Espejo, J. Just, A. Koutsodendris, K. Kubota, N.
Lathika, R. D. Norris, T. Periera dos Santos, R. Robinson, J. M. Rolison, M. H.
Simon, D. Tangunan, M. Yamane, H. Zhang. Persistent influence of
precession on northern ice sheet variability since the early Pleistocene. Science,
2022; 376 (6596): 961 DOI: 10.1126/science.abm4033
引用自：Cardiff University. "Scientists shine new
light on role of Earth's orbit in the fate of ancient ice sheets.”
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