科學家探討125000年前的海平面得出了相當駭人的結果
地球的上個暖期發生在125000年前。新研究顯示當時的海平面比現在還高10公尺，這可以讓我們一窺如果目前的氣候變遷持續下去，可能會發生什麼景象。
考慮海平面上升時南極洲經常被視為「沉睡的巨人」，但新研究認為它其實是關鍵因子。圖片來源：Australian
Antarctic Division
末次間冰期持續了大約10000年。最近我們發表在《自然通訊》(Nature Communications)的論文顯示南極冰層融化，是這段期間海平面上升的主要因素。
海平面上升是氣候變遷帶來最嚴峻的挑戰之一，如果我們想要適應這項後果就需要十分可靠的預測。
預測海平面上升時南極洲經常被看作是「沉睡的巨人」，但這項研究顯示它事實上具有至關重要的地位。南極洲的冰層可以變化得相當快速，對於海邊的居民和建築來說可能會造成非常重大的影響。
來自過去的警訊
地球的循環包含了兩種狀態：一個是寒冷的冰河期，此時世上許多地方都會被遼闊的冰層覆蓋；另一個則是溫暖的間冰期，此時冰層融化使得海平面上升。
地球大概從10000年前進入了目前所處的間冰期，但是過去200年來人類把溫室氣體排放至到大氣當中，造成了比末次間冰期還要迅速也更加劇烈的氣候變遷。這意味著過去的海平面上升速率，只能讓我們對未來可能發生的情境做出最保守的預測。
我們檢視了125000年前至118000年前末次間冰期的數據，當時的溫度大概比現在高出1℃，跟預測中不久之後的未來差不多。
我們的研究顯示末次間冰期時由於冰層融化，使得全球海平面上升至比目前還高出10公尺。冰層融化始於南極洲，接著在幾千年之後輪到格陵蘭。
當時海平面的上升速度最高可以到每個世紀3公尺，比過去150年來觀察到的上漲幅度30公分左右還高出許多。
間冰期開始時，南大洋暖化造成南極洲的冰層率先減少。冰層融化產生的水改變了海洋環流，造成北極地區暖化，促使格陵蘭的冰層跟著融化。
剖析數據
目前的全球海平面上升速率估計為每年3厘米以上。科學家認為海平面會加速上升，依據我們排放出來的溫室氣體如何變化，預計海平面的總上升量到了2100年會比2000年高出70至100公分。
這類估計的依據通常是本世紀利用驗潮儀蒐集到的紀錄，以及自1990年代以來得到的衛星資料。
但這些估計通常沒有考慮到一個相當重要的自然作用：冰崖的不穩定性。由於這種作用無法利用短期的儀器紀錄觀測出來，所以觀察地質紀錄就變得相當重要。
冰層到達海洋時會飄浮起來成為冰棚，末端即稱作冰崖。冰崖長到相當高聳的時候穩定性會降低，可能在一夕之間迅速瓦解。
冰崖崩解會讓更多陸上的冰層流到海洋之中，結果便是造成全球海平面升高。某些模型曾試著納入冰崖的不穩定性，但得出來的結果卻頗有爭議。有趣的是，這些模型輸出結果中預測的海平面上升速率，卻跟我們最新觀察得到的末次間冰期數據相當類似。
我們的研究檢視了海平面總變化量的紀錄，就定義上來說包括了所有相關過程。
我們檢視了紅海的海洋沉積物中，浮游生物化石的化學性質如何變化，這是一種可以指示海平面變化的可靠指標。加上能夠指示南極和格陵蘭附近有多少融水輸入到海洋的證據，這份紀錄可以顯示出海平面上升的速度有多快，並且分辨出不同地方冰層的貢獻有多少。
展望未來
這份末次間冰期的紀錄令我們感到相當驚訝，因為它顯示出海平面能以相當快的速度，上升至比現今還高出許多的地方。因為末次間冰期的溫度跟我們預期中未來不久的溫度差不多，代表極區冰層融化對未來海平面造成的影響，可能會比目前認為的還要更加劇烈。
不過末次間冰期也無法完美反映出未來的景象。由於地球跟太陽之間的相對位置不同，當時進入地球的太陽輻射量比現在還高。此外，當時大氣中的二氧化碳濃度也只有280
ppm，而目前的則是410
ppm以上。
至關重要的是，末次間冰期時的兩極暖化並非同步發生，但是目前由溫室氣體造成的全球變遷，卻同時讓兩極的氣溫升高並造成冰層減少。這意味著如果當今的氣候變遷再不減緩，跟未來我們要面對的情況相比，過往地球海平面的劇烈上升不過是小巫見大巫。
Scientists looked at sea levels
125,000 years in the past. The results are terrifying
Sea levels rose 10 metres above present
levels during Earth’s last warm period 125,000 years ago, according to new
research that offers a glimpse of what may happen under our current climate
change trajectory.
Our paper, published today in Nature Communications, shows that melting ice from Antarctica was
the main driver of sea level rise in the last interglacial period, which lasted
about 10,000 years.
Rising sea levels are one of the biggest challenges
to humanity posed by climate change, and sound predictions are crucial if we
are to adapt.
This research shows that Antarctica, long thought to
be the “sleeping giant” of sea level rise, is actually a key player. Its ice
sheets can change quickly, and in ways that could have huge implications for
coastal communities and infrastructure in future.
A warning from
the past
Earth’s cycles consist of both cold glacial periods -
or ice ages - when large parts of the world are covered in large ice sheets,
and warmer interglacial periods when the ice thaws and sea levels rise.
The Earth is presently in an interglacial period
which began about 10,000 years ago. But greenhouse gas emissions over the past
200 years have caused climate changes that are faster and more extreme than
experienced during the last interglacial. This means past rates of sea level
rise provide only low-end predictions of what might happen in future.
We examined data from the last interglacial, which
occurred 125,000 to 118,000 years ago. Temperatures were up to 1℃ higher than today - similar to those
projected for the near future.
Our research reveals that ice melt in the last
interglacial period caused global seas to rise about 10 metres above the
present level. The ice melted first in Antarctica, then a few thousand years
later in Greenland.
Sea levels rose at up to 3 metres per century, far
exceeding the roughly 0.3-metre rise observed over the past 150 years.
The early ice loss in Antarctica occurred when the
Southern Ocean warmed at the start of the interglacial. This meltwater changed
the way Earth’s oceans circulated, which caused warming in the northern polar
region and triggered ice melt in Greenland.
Understanding the
data
Global average sea level is currently estimated to be
rising at more than 3 millimetres a year. This rate is projected to increase
and total sea-level rise by 2100 (relative to 2000) is projected to reach
70-100 centimetres, depending on which greenhouse gas emissions pathway we
follow.
Such projections usually rely on records gathered
this century from tide gauges, and since the 1990s from satellite data.
Most of these projections do not account for a key
natural process - ice-cliff instability - which is not observed in the short
instrumental record. This is why geological observations are vital.
When ice reaches the ocean, it becomes a floating
ice-shelf which ends in an ice-cliff. When these cliffs get very large, they
become unstable and can rapidly collapse.
This collapse increases the discharge of land ice
into the ocean. The end result is global sea-level rise. A few models have
attempted to include ice-cliff instability, but the results are contentious.
Outputs from these models do, however, predict rates of sea-level rise that are
intriguingly similar to our newly observed last interglacial data.
Our work examines records of total sea-level change,
which by definition includes all relevant natural processes.
We examined chemical changes in fossil plankton
shells in marine sediments from the Red Sea, which reliably relate to changes
in sea level. Together with evidence of meltwater input around Antarctica and
Greenland, this record reveals how rapidly sea level rose, and distinguishes
between different ice sheet contributions.
Looking to the
future
What is striking about the last interglacial record
is how high and quickly sea level rose above present levels. Temperatures
during the last interglacial were similar to those projected for the near
future, which means melting polar ice sheets will likely affect future sea levels
far more dramatically than anticipated to date.
The last interglacial is not a perfect scenario for
the future. Incoming solar radiation was higher than today because of
differences in Earth’s position relative to the Sun. Carbon dioxide levels were
only 280 parts per million, compared with more than 410 parts per million
today.
Crucially, warming between the two poles in the last
interglacial did not happen simultaneously. But under today’s
greenhouse-gas-driven climate change, warming and ice loss are happening in
both regions at the same time. This means that if climate change continues
unabated, Earth’s past dramatic sea level rise could be a small taste of what’s
to come.
原始論文：Eelco
J. Rohling, Fiona D. Hibbert, Katharine M. Grant, Eirik V. Galaasen, Nil
Irvalı, Helga F. Kleiven, Gianluca Marino, Ulysses Ninnemann, Andrew P.
Roberts, Yair Rosenthal, Hartmut Schulz, Felicity H. Williams & Jimin Yu. Asynchronous Antarctic and Greenland
ice-volume contributions to the last interglacial sea-level highstand. Nature Communicatio, 2019;
doi:10.1038/s41467-019-12874-3
引用自：The
Conversation. “Scientists looked at sea levels 125,000 years in the past. The
results are terrifying.”
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