科學家警告人類已經越過了九項氣候臨界點
在十年前定義出來的氣候臨界點中，頂尖科學家團隊警告人類已經越過了其中的一半以上。
一架飛機掠過阿拉斯加的蘭格爾－聖伊萊亞斯國家公園的冰河上方。圖片來源：Frans
Lanting/Nat Geo Image Collection
這代表亞馬遜雨林和南極洲與格陵蘭的巨厚冰層可能即將消失，目前進行的測量顯示它們出現前所未見變化的時間比預期中還早了許多。
由於全球暖化而引發的這一連串變化可能會危及人類文明的存亡。
越來越多證據指出這些事件的發生機率以及彼此之間的關聯程度比過去認為的還高，這可能會引發骨牌效應。
在發表於期刊《自然》(Nature)的文章中，科學家呼籲人類要立即採取行動來減少溫室氣體的排放量，防止氣候跨過關鍵的臨界點。他們警告最糟的情況是地球變成「暖房」這種不適合居住的星球。
「十年前我們定義出一系列地球系統臨界點的候選名單，而我們看到的證據顯示目前已經達成其中的一半以上。」主要作者Tim
Lenton博士表示。他是艾希特大學全球系統研究所的主任。
「在迅速、無法回復的變化造成的威脅與日俱增的情況下，袖手旁觀是不負責任的態度。現在的局勢已經相當危急，而我們需要做出緊急應變。」
波次坦氣候衝擊研究院的主任Johan
Rockström是研究共同作者，他說：「原因不只是人類對地球造成的壓力不斷升高至前所未見的地步。」
「經過更多的研究之後，我們也必須承認之前低估了地球自身把全球暖化變得更加嚴重，導致一連串無法回復的變化迅速發生的風險。」
「在全球氣溫已經升高1°C的當下，這些是我們開始見到的現象。」
「從科學上來看，這是相當強烈的證據足以宣布地球已經進入緊急狀態。全世界需要立即採取行動，加快腳步讓世界可以在一個穩定的星球上持續發展。」
在這篇評論中作者提出了一道公式來計算地球處在多危急的狀態：利用風險高低乘上緊急程度。
臨界點帶來的風險就目前來看比先前預估的高出許多，而緊急程度則代表採取行動的速度要多快才能降低風險。
人類不太可能在2050年以前終止基於化石燃料的經濟活動，但在目前溫度已經高出工業革命前1.1℃的狀況下，地球到了2040年大概就會跨越1.5℃的警戒線。作者的結論認為光憑此點就可以斷定地球已經處於緊急狀態。
九項已經達成的臨界點為：
1. 北極海冰
2. 格陵蘭冰層
3. 北方針葉林
4. 永凍層
5. 大西洋經向翻轉環流
6. 亞馬遜雨林
7. 熱帶海洋珊瑚
8. 南極西部冰層
9. 南極東部的部分冰層
格陵蘭、南極西部和南極東部一部分的大型冰層崩解會讓全世界的海平面永久上升10公尺左右。
減少碳排放可以延緩此過程，使得低海拔地區的人口有更多時間可以遷離。
雨林、永凍層和北方針葉林則是生物圈臨界點的其中幾項，如果越過這些臨界點就會有更多的溫室氣體釋放出來而加重暖化。
儘管大多數國家都有簽署巴黎協定，承諾要將全球暖化控制在2°C以下，但以目前各國承諾的排放量來說，就算他們真能達到還是會讓暖化達到3°C。
雖然各個臨界點未來的達成情形以及它們之間的交互作用難以預測，這些科學家仍然主張：「如果一連串會造成破壞的不可逆變化有可能發生，而且不能排除發生在全球的可能性，那麼這對文明來說就是攸關存亡的威脅。」
「就算進行再多的成本效益分析都沒有任何幫助。我們需要改變處理氣候問題的方法。」
Lenton教授補充：「我們可能已經跨過了一連串臨界點會交互作用而接連發生的界線。」
「但是減少碳排放可以降低它們進行的速度，進而削減它們造成的風險。」
作者說雖然從數百萬年來看全球溫度本來就會有所波動，但是人類目前使大氣二氧化碳濃度與全球溫度升高的速度是末次冰期結束時的數十倍，因而「強行」推動了這個系統。
Nine climate tipping points now
“active”, warn scientists
More than half of the climate tipping
points identified a decade ago are now “active”, a group of leading scientists
have warned.
This threatens the loss of the Amazon rainforest and
the great ice sheets of Antarctica and Greenland, which are currently
undergoing measurable and unprecedented changes much earlier than expected.
This “cascade” of changes sparked by global warming
could threaten the existence of human civilisations.
Evidence is mounting that these events are more
likely and more interconnected than was previously thought, leading to a possible
domino effect.
In an article in the journal Nature, the scientists call for urgent action to reduce greenhouse
gas emissions to prevent key tipping points, warning of a worst-case scenario
of a “hothouse”, less habitable planet.
“A decade ago we identified a suite of potential
tipping points in the Earth system, now we see evidence that over half of them
have been activated,” said lead author Professor Tim Lenton, director of the
Global Systems Institute at the University of Exeter.
“The growing threat of rapid, irreversible changes
means it is no longer responsible to wait and see. The situation is urgent and
we need an emergency response.”
Co-author Johan Rockström, director of the Potsdam
Institute for Climate Impact Research, said: “It is not only human pressures on
Earth that continue rising to unprecedented levels.
“It is also that as science advances, we must admit
that we have underestimated the risks of unleashing irreversible changes, where
the planet self-amplifies global warming.
“This is what we now start seeing, already at 1°C
global warming.
“Scientifically, this provides strong evidence for
declaring a state of planetary emergency, to unleash world action that
accelerates the path towards a world that can continue evolving on a stable
planet.”
In the commentary, the authors propose a formal way
to calculate a planetary emergency as risk multiplied by urgency.
Tipping point risks are now much higher than earlier
estimates, while urgency relates to how fast it takes to act to reduce risk.
Exiting the fossil fuel economy is unlikely before
2050, but with temperature already at 1.1°C above pre-industrial temperature,
it is likely Earth will cross the 1.5°C guardrail by 2040. The authors conclude
this alone defines an emergency.
Nine active tipping points:
1. Arctic
sea ice
2. Greenland
ice sheet
3. Boreal
forests
4. Permafrost
5. Atlantic
Meridional Overturning Circulation
6. Amazon
rainforest
7. Warm-water
corals
8. West
Antarctic Ice Sheet
9. Parts
of East Antarctica
The collapse of major ice sheets on Greenland, West
Antarctica and part of East Antarctica would commit the world to around 10
metres of irreversible sea-level rise.
Reducing emissions could slow this process, allowing
more time for low-lying populations to move.
The rainforests, permafrost and boreal forests are
examples of biosphere tipping points that if crossed result in the release of
additional greenhouse gases amplifying warming.
Despite most countries having signed the Paris
Agreement, pledging to keep global warming well below 2°C, current national
emissions pledges – even if they are met – would lead to 3°C of warming.
Although future tipping points and the interplay
between them is difficult to predict, the scientists argue: “If damaging
tipping cascades can occur and a global tipping cannot be ruled out, then this
is an existential threat to civilization.
“No amount of economic cost–benefit analysis is going
to help us. We need to change our approach to the climate problem.”
Professor Lenton added: “We might already have
crossed the threshold for a cascade of inter-related tipping points.
“However, the rate at which they progress, and
therefore the risk they pose, can be reduced by cutting our emissions.”
Though global temperatures have fluctuated over
millions of years, the authors say humans are now “forcing the system”, with atmospheric
carbon dioxide concentration and global temperature increasing at rates that
are an order of magnitude higher than at the end of the last ice age.
原始論文：Timothy M. Lenton, Johan Rockström, Owen
Gaffney, Stefan Rahmstorf, Katherine Richardson, Will Steffen, Hans Joachim
Schellnhuber. Climate
tipping points — too risky to bet against. Nature, 2019; 575
(7784): 592 DOI: 10.1038/d41586-019-03595-0
引用自：University of Exeter. "Nine climate tipping
points now 'active,' warn scientists."
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