What to Make of a Warming Plateau
全球暖化減緩 原因何在？
By Justin Gillis

As unlikely as this may sound, we have lucked out in recent years when it comes to global warming.
聽起來不太可能，但說到全球暖化，近年來我們還真是走運。

The rise in the earth’s surface temperature has been markedly slower over the last 15 years than in the 20 years before that. And that lull in warming has occurred even as greenhouse gases have accumulated in the atmosphere at a record pace.
最近十五年來地表溫度的上升遠比之前的廿年間緩慢。雖然大氣層中溫室氣體累積的速度刷新紀錄，暖化現象卻減弱了。

The slowdown is a bit of a mystery to climate scientists. True, the basic theory that predicts a warming of the planet in response to human emissions does not suggest that warming should be smooth and continuous. To the contrary, in a climate system still dominated by natural variability, there is every reason to think the warming will proceed in fits and starts.
對氣候學家來說，暖化的減緩有點是個謎。誠然，根據人類排放量預測地球暖化的基本理論，並不認定暖化就會平穩而持續地進行下去。在氣候系統仍舊受到自然變率的強烈影響的前提下，我們反而有充分理由相信暖化將會斷斷續續地進行。（譯註：「自然變率」說認為氣候系統是一個強烈的非線性系統，本身包含極大的自然變率（Natural Variability）。即使沒有外力因素影響，氣候系統本身也會一下子暖，一下子冷，也會忽乾忽濕，全球暖化現象也許只不過是這種自然變率的表現而已。）

But given how much is at stake, climate science would like to understand what is going on. They admit that they do not, even though some potential mechanisms of the slowdown have been suggested. The situation highlights important gaps in our knowledge of the climate system, some of which cannot be closed until we get better measurements from high in space and deep in the ocean.
由於茲事體大，氣候學家想要了解究竟是怎麼一回事。儘管氣候暖化減緩的某些可能機制已經為人所知，氣候學家坦承仍不了解箇中堂奧。此一景況凸顯出人類對於氣候系統的知識存有重大缺口，除非我們能夠從太空和深海獲得較佳的測量結果，否則有些缺口將永遠存在。

Those dismissive of climate-change concerns have made much of this plateau. They typically argue that “global warming stopped 15 years ago”, and then assert that this disproves the notion that greenhouse gases are causing warming.
認為擔心氣候變遷問題有如杞人憂天的人，已經就暖化減緩的現象大作文章。典型論調是「全球暖化在15年前就已停止」，更據此反駁溫室氣體造成暖化的觀念。

Rarely do they mention that most of the warmest years in the historical record have occurred recently. Moreover, their claim depends on careful selection of the starting and ending points. The starting point is almost always 1998, a particularly warm year because of a strong El Niño weather pattern.
這群人很少提到的是，歷史紀錄上最溫暖的幾年，絕大多數都出現在最近這些年裡。除此之外，這群人的主張建立在精心挑選的時間起訖點上。起算點幾乎總是1998年，這一年因為有強烈的聖嬰現象而特別暖和。

Scientists reject such a selective use of numbers. When they calculate the long-term temperature trends for the earth, they conclude that it continues to warm through time. Despite the recent lull, it is unknown whether the pace of that warming has undergone any lasting shift.
這種對數字年分的選擇性取用，科學家不採。他們計算地球氣溫的長期趨勢後，結論是地球仍在繼續暖化。雖然最近有所緩和，但地球暖化的速度是否已經產生持久性的轉變，仍屬未知。

What to make of it all?
如何了解這是怎麼一回事？

We certainly cannot conclude, as some people want to, that carbon dioxide is not actually a greenhouse gas. More than a century of research thoroughly disproves that claim.
我們當然不能如同某些人所想的，下結論說二氧化碳其實不是溫室氣體。超過百年的研究已徹底反駁此一說法。

In fact, scientists can calculate how much extra heat should be accumulating from the human-caused increases in greenhouse gases, and the energies involved are staggering. By a conservative estimate, current concentrations are trapping an extra amount of energy equivalent to 400,000 Hiroshima bombs exploding across the face of the earth every day.
其實，科學家可以算出，因人類活動而增加的溫室氣體使得大氣中多蓄積了多少的熱，其中的能量讓人瞠目結舌。依保守估計，大氣中因目前溫室氣體濃度而多蓄積的能量，相當於40萬枚投在廣島的原子彈每天在地表爆炸。

So the real question is where all that heat is going, if not to warm the surface. And a prime suspect is the deep ocean. Our measurements there are not good enough to confirm it absolutely, but a growing body of research suggests this may be an important part of the answer.
所以真正的問題是如果地球沒有暖化，這些熱都到哪兒去了？最有可能的地方是深海。人類的測量技術尚不足以完全確認，但愈來愈多的研究顯示深海可能與答案大有關聯。

Exactly why the ocean would have started to draw down extra heat in recent years is a mystery, and one we badly need to understand. But the main ideas have to do with possible shifts in winds and currents that are causing surface heat to be pulled down faster than before.
海洋為何開始吸收多餘的熱，確切原因仍是個謎，需要我們解謎。主要的想法認為，這與風和洋流方向的可能轉變有關，以致海洋吸收地表熱的速度比以前快。

The deep-ocean theory is one of a half-dozen explanations proffered for the warming plateau. The answer may turn out to be some mix of all of them.
科學家對地球暖化減緩提出的解釋大概有半打，深海說是其中之一。真正的答案或許會是集各家之說的結果。

In any event, computer forecasts of climate change suggest that pauses in warming lasting a couple of decades should not surprise us.
無論如何，氣候變遷的電腦預測顯示，暖化就算停滯數十年也不足為奇。

It turns out we had an earlier plateau in global warming, from roughly the 1950s to the 1970s, and scientists do not fully understand that one either. A lot of evidence suggests that sunlight-blocking pollution from dirty factories may have played a role, along with natural variability in ocean circulation. The pollution was ultimately reduced by stronger clean-air laws in the West.
結果是人類早些年就遇過地球暖化的渟滯期，差不多是在1950年代到1970年代，科學家也沒完全搞懂是怎麼回事。大量證據顯示，骯髒工廠排放的汙染氣體阻絕了陽光，加上洋流中的自然變率，可能與此有關。西方國家實施比較嚴格的空汙法後，終於減少了汙染。

Today, factory pollution from China and other developing countries could be playing a similar role in blocking some sunlight. We will not know for sure until we send up satellites that can make better measurements of particles in the air.
如今，中國大陸和其他開發中國家的工廠所排放的汙染，可能也阻絕了陽光。要到發射能更精準測量空氣中粒子的人造衛星之後，才能確認真正的原因。

What happened when the mid-20th-century lull came to an end? You guessed it: an extremely rapid warming of the planet.
20世紀中期的地球暖化減緩現象結束後，發生了甚麼事？你猜對了：地球急速暖化。

So, if past is prologue, this current plateau will end, too, and a new era of rapid global warming will begin. That will put extra energy and moisture into the atmosphere that can fuel weather extremes, like heat waves and torrential rains.
如果過去是今日序曲，這一波的減緩也會結束，新一波的地球急速暖化接菩登場。這將使得多餘的熱能和濕氣進入大氣層，可能讓極端氣候現象加劇，例如熱浪和暴雨。

We might one day find ourselves looking back on the crazy weather of the 2010s with a deep yearning for those halcyon days.
也許有一天，我們回首2010年代的瘋狂氣候時，會深深地渴望這一段氣候平穩的歲月。

原文參照：
http://www.nytimes.com/2013/06/11/science/earth/what-to-make-of-a-climate-change-plateau.html

2013-06-25聯合報/G9版/UNITEDDAILYNEWS張佑生譯 原文參見紐時週報十版右