2014/3/16 「北極圈暖化真是氣候異常元兇？」

北極圈暖化真是氣候異常元兇？

摘錄自：天下雜誌 經濟學人電子報                        2014/3/14

2014-02-26 Web only 作者：經濟學人

圖片來源：flickr.com/photos/ooocha/

氣候變遷應該是在數十年間緩緩顯現，但對北極來說並非如此；北極圈的溫度較長期平均高出了攝氏2度，北冰洋上層的溫度也比過去2000年來得高。部分專家預測，最快在2020年代北冰洋就會出現首次完全沒有冰的夏天。

北極圈氣候快速變動，可能會連帶影響其他地區。羅格斯大學的大氣學家法蘭西斯（Jennifer Francis）的研究主題，正是此事是否已經開始發生。英美兩國的運動人士都將今年冬季的劇烈氣候歸疚於氣候變遷，但在氣候學上，情況從來不是那麼單純。法蘭西斯博士能做的只是提出理論，解釋為何北極圈變暖可能會使中緯度的氣候更加劇烈，部分證據亦顯示此事已經開始發生。

法蘭西斯博士的理論與高空的高速氣流有關。高速氣流產生的部分原因，即為北極圈與中緯度之間的溫差；由於北極圈暖化得比較快，差異亦隨之縮小；這理當會減弱高速氣流的強度，讓它更加變化無常。

高速氣流減弱遊移，會使它推動的鋒面減緩，讓氣候模式更具持續性，造成熱浪、酷寒和乾罕。理論如此，大部分氣候模型亦同意此看法。法蘭西斯博士亦指出，近年來，北極與中緯度之間的大氣厚度差異（大氣溫度差異的間接衡量手段）開始下滑，而且與高速氣流風速呈現相關性；此外，溫差最小的地方，似乎也高速氣流最常遊移地方。

不過，大氣厚度與風速之間的關係，一直到1990年代中期才變得明顯，只有不到20年的資料。另一個問題在於，法蘭西斯博士的理論認為，全球暖化會增加阻塞模式，但氣候模型一致認為全球暖化會讓它減少。

美國今年的嚴冬確實與高速氣流減弱有關，但法蘭西斯博士指出，英國的強風和降雨是另一回事，而且今年北大西洋的高速氣流其實比正常情況更強。這正是氣候科學最讓人挫折的地方之一；氣候本身就會變化，需要許多年的資料才能找出明確趨勢。沒有人確切知道，高速氣流減弱是否造成英美的劇烈氣候，但如果法蘭西斯博士是對的，再次出現如此嚴冬的機會將越來越高。（黃維德譯）

©The Economist Newspaper Limited 2014

經濟學人英文原文

The Economist

Climate change

Jet set

By The Economist

From The Economist

Published: February 26, 2014

Feb 22nd 2014 | CHICAGO | From the print edition

Is polar warming to blame for America's and Britain's bad winter weather?

CLIMATE change is supposed to unfold slowly, over decades. But that is not true up in the great white north, as those attending this year's meeting of the American Association for the Advancement of Science were reminded in the session on climate change in the Arctic. Temperatures there are 2°C higher than their long-term average (around twice the increase in the rest of the world), and the upper layers of parts of the Arctic Ocean are hotter than they have been for at least 2,000 years. Summer sea ice has been vanishing faster than even the gloomiest researchers thought likely, with some now predicting the first completely ice-free summer as soon as the 2020s.

The Arctic is not, though, isolated from the rest of the world; rapid changes there could have knock-on effects elsewhere. Whether or not that is happening was a question addressed by Jennifer Francis, an atmospheric scientist at Rutgers University. It is a topical subject. Along with much of the rest of America, Chicago endured a fierce and prolonged cold snap in January, in which temperatures fell to -27°C, the lowest since 1884. Meanwhile, Brits at the conference were fleeing a country that had been soaked by the heaviest winter rains in two and a half centuries, and battered by a seemingly endless succession of Atlantic storms and gales.

Campaigners in both countries have been quick to blame climate change for the rotten weather. But things are rarely so straightforward in climatology. The best Dr Francis could offer was a theory as to why a warmer Arctic might be expected to lead to wilder weather in mid-latitudes, and some tentative but suggestive evidence that this is already happening.

Her idea rests on the jet stream, a powerful, persistent, high-altitude "river of air" which flows around the world from west to east, affecting the weather as it goes. The jet stream is driven in part by the temperature difference between cold Arctic air and the warmer air of middle latitudes. Because the Arctic is warming more rapidly than the rest of the planet, that difference is shrinking. This ought to produce a less potent jet stream. And a less potent jet stream is a more wayward one.

In the same way that a mountain torrent runs straight down steep slopes, while a slower river meanders across its plain, a weak, slow jet stream should swing north and south more, taking its weather with it. That meandering would also mean any weather fronts it propelled moved more slowly than normal, leading to more persistent weather patterns. And it would increase the likelihood of eddies splitting from the jet stream and sitting in the sky blocking atmospheric movement. This would further increase the persistence of weather patterns, and cause heatwaves, cold snaps and droughts.

That, at least, is the theory. And climate models (mostly) agree with it. But models are not evidence, and so Dr Francis has gone out looking for some. She showed her audience several suggestive charts which measured how the difference between the thickness of the atmosphere at the poles and at mid latitudes (a proxy for temperature differences, since warm air takes up more room than cool air) has fallen in recent years. That correlates nicely with another set of data showing wind speeds in the jet stream, with the bits of the world that have the smallest temperature gradient showing the biggest drop in speeds. The places with the smallest temperature gradient also seem to be where the jet stream meanders most.

A set of observations that mostly match what a sensible-sounding theory predicts may seem a strong case. But not everyone is convinced. One problem is that the changes in atmospheric thickness and wind speed Dr Francis observed have become apparent only since the mid-1990s, which gives fewer than 20 years of data to work with. Another difficulty concerns the blocking patterns. Dr Francis's theory suggests there will be more of them in future, as the world warms. But climate models unanimously disagree, saying that global warming will produce fewer of them. Someone has to be wrong.

Trying to apply the theory to this year's batch of wild weather causes even more headaches. America's cold snap was indeed produced partly by a wayward jet stream. But Dr Francis herself points out that Britain's gales and rain are a different story, and that this year's North Atlantic jet stream has in fact been stronger than normal. That illustrates one of the chief frustrations of climate science. The weather is naturally changeable, which means many years of data are needed to extract a clear trend from among all the noise. Were the American freeze and the British floods caused by a weaker jet stream? No one can say for sure. But if Dr Francis and her colleagues are right, the odds on a repeat performance will get shorter every year.

©The Economist Newspaper Limited 2014