強烈颱風將成為新常態？
摘錄自：天下雜誌 經濟學人電子報 2013/11/22
2013-11-19 Web only 作者：經濟學人
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| 圖片來源：flickr.com/photos/gsfc/ |
海燕是否為史上最強的颱風？氣象學家永遠無法確知此事，因為可靠的紀錄只能追溯至過去數十年。不過，海燕一定可以名列強颱之林；除了破壞力和死亡人數之外，評估其強度的方式之一，即為與2005年襲擊紐奧良的卡崔娜相比。海燕最強之時，最高風速可能超過每小時300公里，而卡崔娜在襲擊陸地之時，估計風速約為每小時200公里。
不論海燕在歷史強颱中的精確排名為何，在全球暖化對熱帶暴風的影響上，它都引起了各界的討論，就和卡崔娜一樣。華沙氣候變遷高峰會的菲律賓代表薩諾（Naderev Sano）十分確信此事，認為所有懷疑之人都該造訪他的家鄉；他說，「我們現在看見的趨勢就是，破壞力更強的暴風將成為新常態。」
但還有其他因素讓情況更為複雜。若高低大氣層風速差異過大，熱帶氣旋就無法形成；氣候模型指出，氣溫升高會讓這種風速差異更常見，至少在北大西洋是如此。政府間氣候變遷專門委員會（IPCC）認為，氣旋出現的頻率會持平或下滑，平均強度則會增加。
預測是如此，但現有的證據相當混亂。氣象紀錄品質參差不齊；熱帶暴風的強度差異極大，也讓尋找趨勢十分困難。最低氣壓為衡量強度的另一個標準，而1979年的強颱狄普，是目前最低氣壓的紀錄保持者。不過，主要溫室氣體二氧化碳的水準，1979年僅337ppm，2012年時則為394ppm。IPCC的結論是，雖然目前已有證據顯示，過去40年裡，大西洋的颶風變得更多更強，但成因並無共識。以全球來說，也看不出熱帶暴風頻率或強度增加的趨勢。此外，由於海燕這樣的強颱十分少見，就算真有特定趨勢，也得花上很長的時間才會浮現。（黃維德譯）
©The Economist Newspaper Limited 2013
The Economist
Cyclones
and climate change
The new
normal?
By The Economist
From The Economist
Published: November 19, 2013
Nov 16th 2013 |From the print edition
Physics suggests that storms will get worse as the planet warms. But it
is too early to tell if it is actually happening.
WAS typhoon Haiyan the strongest recorded storm to make landfall?
Meteorologists will never know. Reliable records go back only a few decades.
But it is surely one of them. Besides the devastation and the death toll, one
way to assess its potency is to compare it with Katrina, the hurricane that
devastated New Orleans in 2005. At its most intense, Haiyan's peak wind speeds
were probably greater than 300kph (190mph). The best estimate for Katrina, when
it hit land, is around 200kph.
Regardless of its precise position in the historical hierarchy,
Haiyan—like Katrina—has provoked discussion about the effects of global warming
on tropical storms. Naderev Sano, the Philippines' representative at a climate
summit in Warsaw, was unequivocal, daring doubters to visit his homeland.
"The trend we now see is that more destructive storms will be the new
norm," he said.
In theory, a warmer world should indeed produce more potent cyclones.
Such storms are fuelled by evaporation from the ocean. Warmer water means
faster evaporation, which means more energy to power the storm. A warmer
atmosphere can hold more moisture, which means more rain.
But other factors complicate things. Tropical cyclones cannot form when
wind speeds in the upper and lower atmosphere differ too much. Climate models
suggest, in the North Atlantic at least, that such divergent winds may be more
common in a warmer world. The Intergovernmental Panel on Climate Change (IPCC)
reckons that the frequency of cyclones will stay the same or decrease while
their average intensity goes up.
That is the forecast. But the evidence so far is messy. Meteorological
records are of uneven quality, and tropical storms vary widely in intensity,
which makes spotting trends tricky. One potent storm from 1979, Typhoon Tip,
holds the record for the lowest atmospheric pressure recorded, another measure
of a storm's intensity. Yet levels of carbon dioxide, the main greenhouse gas,
were only 337 parts per million (ppm) in 1979, compared with 394ppm in 2012.
The IPCC concludes that, although there is good evidence for more and stronger
Atlantic hurricanes over the past 40 years, there is no consensus on the cause
of them. Worldwide, there is no trend in either the frequency or the intensity
of tropical storms. And, given the rarity of such storms as Typhoon Haiyan, it
will take a long time for any trend to become apparent.
©The Economist Newspaper Limited 2013
