冰河工程可以限制海平面的上升幅度，前提是我們控制住二氧化碳的排放量
根據發表在歐洲地質學會期刊《冰圈》(The
Cryosphere)的新研究，以延緩冰河融化為目標的工程計畫能減緩冰層崩解以及海平面上升的速度。雖然跟目前大型土木工程規模差不多的人為干預方法只有30%的機率成功延緩冰層崩解，但把計畫規模加大可以提高成功機率。不過，研究作者Michael
Wolovick和John
Moore提醒若要阻止氣候變遷及其造成的劇烈後果，最重要的仍然是減少溫室氣體排放量。
圖中棕色為岩盤、淺藍為冰層、紫色為漂浮的冰棚、灰色為人造海脊、藍色紅色分別為寒冷和溫暖的海水。a)初始情況：溫暖的海水會從下方融化冰棚。b)冰層因海水而變的不穩定：當基線(ground
line，冰層與冰棚交界)後退至向內陸傾斜的斜坡，會有更多溫暖海水流入使融化速度加快，冰層流速隨之提高。c)建造人工海脊：人工海脊可以擋住溫暖的海水，降低融化速度，使得冰棚變厚並前進。d)減緩冰層崩解：變厚的冰棚前進至人工海脊時會被擋住，有助於支撐冰層並讓冰層的流速變慢。圖片來源：Wolovick
& Moore, The Cryosphere, 2018
Moore身兼中國北京師範大學的科學家與芬蘭拉普蘭大學的氣候變遷教授，他說：「規劃地球工程通常代表要去認真考慮那些異想天開的事物。」「地球工程」時常用來稱呼對抗氣候變遷的大規模人為干預手段。不過Wolovick和Moore說與其嘗試對抗氣候整體，也許我們運用的策略可以聚焦在降低氣候變遷最嚴重的後果之一：海平面上升。
他們的「異想天開」方法便是冰河工程：在冰河流入海洋形成冰棚的地方改變此處的海底地形，藉此阻止冰河更進一步融化。世上某些冰河正快速後退，比方說南極西部和英國差不多大的特懷特
(Thwaites)冰川。美國普林斯頓大學地質科學系的研究員Wolovick解釋：「特懷特冰川很可能會讓(南極西部)冰層大量崩解，使全球海平面上升3公尺左右。」對於世上居住在海岸地區的數百萬人來說這會造成重大影響。
Wolovick和Moore提出除了透過傳統的海岸防護降低海平面上升帶來的影響，從源頭著手以阻止海水上漲的冰河工程也是種可行的替代方案或者可以並行。Wolovick說：「(我們)最重要的研究成果是顯示出干預冰層的方法要有成效，所需的規模大多是在人類可以達成的範圍之內。」
研究團隊探討兩種冰河地球工程的設計。第一個想法是在海底建造一道牆，阻止溫暖的海水到達容易融化的冰棚底部。另一種比較簡單的設計是在海床建造一連串的人造石丘或柱子，它們的功用不是攔阻溫暖的海水，而是支撐並阻擋冰河前進，幫助冰河重新增厚。Wolovick說：「在這兩個範例中我們假想的建物都非常簡單，不過是把沙子或石頭堆在海床上。」
特懷特冰川的寬度為80至100公里，是世界上最寬的冰川之一。科學家預估它是未來造成海平面上升的最大單一來源。研究團隊運用電腦模擬在逐漸暖化的世界中，把這兩種設計用在特懷特冰川的成效。在這篇發表於《冰圈》的研究中，作者寫道：「如果(冰河工程)在此處能發揮效用，那麼在其他挑戰性比較低的冰川上應該也能成功。」
研究顯示即使是比較簡單的設計也能延緩海平面上升的速度，為沿海社區爭取更多時間來適應海水上漲。此方法為在海床上分別建造許多300公尺高的石丘或石柱，依據施工材料的強度會用到0.1至1.5立方公里的粒料――大約是開鑿並建立埃及蘇伊士運河(1立方公里)或者是創造杜拜棕櫚島(0.3立方公里)時所用的材料體積。模擬結果指出這種規模最小的干預方式有30%的機率，可以防止西南極冰層在可預見的未來發生無法停止的崩解。
在海底建牆則是較為複雜的方法，規模超過人類目前執行過的工程計畫，不過它也有更高的機率成功阻止冰層在接下來的1000年(模擬的時間長度)中發生無法停止的崩解，同時也有更高的機會讓冰層體積重新增加。一道小型水下長城可以阻擋50%的溫暖海水到達冰棚底部，使其延緩冰層崩解的成功機率到達70%；而更高大的長城則有更高的成功機率甚至完全阻止冰層崩解。
儘管模擬得到的結果相當樂觀，兩位科學家表示他們不鼓勵人們在短時間之內就開始執行這些野心勃勃的計畫。雖然最簡單的設計跟現有工程計畫的規模相去不遠，但施工地點會是世上最嚴苛的環境之一，因此施工細節還是需要進一步地規畫才行。不過，團隊的目標是要探討冰河工程在理論層面是否可行，並讓科學界仔細思考這些設計然後加以改良。
Wolovick說：「我們這些科學家都能體認我們的重要職責之一，是盡快得出人類社會將來面臨的海平面上升幅度有多大，速度又有多快。然而，我們主張科學家也有職責想出方法來保護冰層迅速崩解對社會造成的威脅。」
即使Wolovick和Moore透過冰層物理模型得出冰河工程可以延緩冰層崩解，他們堅決在對抗氣候變遷時最優先的仍是減少溫室氣體排放。他們說：「有些不誠實的社會團體可能會試著利用我們的研究來質疑降低溫室氣體排放的必要性。在此聲明我們的研究不管在任何方面都不支持這種解讀方式。」
冰河工程只能限制海平面的上升幅度，但是減少溫室氣體的排放還能降低其他氣候變遷帶來的災害，像是海洋酸化、洪水、乾旱與熱浪。此外，團隊也指出暖化程度越高，冰河工程計畫的可行性和成功機率就越低。畢竟，他們提出的水下設施只能夠保護冰棚底部，但無法阻止溫暖的空氣從上方一點一滴地蠶食冰層。
Wolovick總結：「我們排放越多碳，冰層的體積長期下來只會跟現在越差越多。」
Glacial engineering could limit
sea-level rise, if we get our emissions under control
Targeted engineering projects to hold off
glacier melting could slow down the collapse of ice sheets and limit sea-level
rise, according to a new study published in the European Geosciences Union
journal The Cryosphere. While an
intervention similar in size to existing large civil engineering projects could
only have a 30% chance of success, a larger project would have better odds of
holding off ice-sheet collapse. But study authors Michael Wolovick and John
Moore caution that reducing emissions still remains key to stopping climate
change and its dramatic effects.
“Doing geoengineering means often considering the
unthinkable,” says Moore, a scientist at Beijing Normal University, China, and
a professor of climate change at the University of Lapland, Finland. The term
‘geoengineering’ is usually applied to large-scale interventions to combat
climate change. But instead of trying to change the entire climate, Wolovick
and Moore say we could apply a more targeted approach to limit one of the most
drastic consequences of climate change: sea-level rise.
Their “unthinkable” idea is glacial geoengineering:
making changes to the geometry of the seafloor near glaciers that flow into the
ocean, forming an ice shelf, to prevent them from melting further. Some
glaciers, such as the Britain- or Florida-sized Thwaites ice stream in West
Antarctica, are retreating fast. “Thwaites could easily trigger a runaway [West
Antarctic] ice sheet collapse that would ultimately raise global sea level by
about 3 metres,” explains Wolovick, a researcher at Princeton University’s
Department of Geosciences, US. This could have dramatic effects to the millions
of people living in the world’s coastal areas.
Instead of, or in addition to, limiting the effects
of rising seas through traditional coastal protection, using glacier
geoengineering to stop the flood at the source could be a viable option, as
Wolovick and Moore show. “The most important result [of our study] is that a
meaningful ice sheet intervention is broadly within the order of magnitude of
plausible human achievements,” says Wolovick.
The team looked into two glacial-geoengineering
designs. One idea would be to build a wall underwater to block warm water
reaching an ice shelf’s base, which is very sensitive to melting. A simpler
design consists of constructing artificial mounds or columns on the seafloor:
they wouldn’t block warm water but could support and hold back the glacier,
helping it regrow. “In either case, we were imagining very simple structures,
simply piles of sand or gravel on the ocean floor,” says Wolovick.
The team ran computer models where they applied these
designs to Thwaites Glacier in a warming world. Thwaites is projected to be the
largest individual source of future sea-level rise and, at 80 to 100 km wide,
it’s one of the widest glaciers in the world. “If [glacial geoengineering]
works there then we would expect it to work on less challenging glaciers as
well,” the authors write in The
Cryosphere study.
The research shows that even the simpler design could
slow down the rate of sea-level rise, giving more time to coastal societies to
adapt to rising waters. The smallest intervention has a 30% probability of
preventing a runaway collapse of the West Antarctic Ice Sheet for the
foreseeable future, according to the models. This intervention would consist of
building isolated 300-metre-high mounds or columns on the seafloor using
between 0.1 and 1.5 cubic kilometres of aggregate, depending on the strength of
the material. This is similar to the amount of material that was excavated to
build the Suez Canal in Egypt (1 cubic kilometre) or used in Dubai’s Palm
Islands (0.3 cubic kilometres).
A more sophisticated project, going beyond the scale
humanity has attempted so far, would have higher chances of success in avoiding
a runaway ice-sheet collapse within the next 1000 years (the time the
simulations run for), as well as better odds of causing the ice sheet to regain
mass. A small underwater wall blocking about 50% of warm water from reaching
the ice shelf base could have 70% chance of succeeding, while larger walls
would be even more likely to delay or even stop ice-sheet collapse.
Despite the encouraging results, the scientists say
they don’t advocate starting these ambitious projects any time soon. While the
simplest design would be similar in scale to existing engineering projects, it
would be built in one of Earth’s harshest environments. So, the engineering
details still need to be worked out. Nonetheless, the team wanted to see
whether glacial geoengineering could work in theory, and wanted to get the
scientific community to think about, and improve on, the designs.
“We all understand that we have an urgent
professional obligation to determine how much sea level rise society should
expect, and how fast that sea level rise is likely to come. However, we would
argue that there is also an obligation to try to come up with ways that society
could protect itself against a rapid ice-sheet collapse,” says Wolovick.
Ice physics shows glacial geoengineering could work
to hold off ice-sheet collapse, but both Wolovick and Moore are adamant that
reducing greenhouse-gas emissions remains a priority in the fight against
climate change. “There are dishonest elements of society that will try to use
our research to argue against the necessity of emissions’ reductions. Our
research does not in any way support that interpretation,” they say.
Engineering glaciers would only limit sea-level rise,
while reducing emissions could also limit other harmful consequences of climate
change, such as ocean acidification, floods, droughts and heat waves. In
addition, the team points out that more warming would mean glacial engineering
projects would become less feasible and would have lower chances of success.
After all, their underwater structures might protect the bottom of the ice
shelves, but wouldn’t prevent warm air from eating away the ice at the top.
“The more carbon we emit, the less likely it becomes
that the ice sheets will survive in the long term at anything close to their
present volume,” Wolovick concludes.
原始論文：Michael J. Wolovick, John C. Moore. Stopping the flood: could we use
targeted geoengineering to mitigate sea level rise? The Cryosphere, 2018;
12 (9): 2955 DOI: 10.5194/tc-12-2955-2018
引用自：European Geosciences Union. "Glacial engineering could
limit sea-level rise, if we get our emissions under control."
沒有留言:
張貼留言