West Africa is already living on the edge, and new research indicates that even worse droughts are possible than the one that devastated the region in the late 20th century.

西非已經生活在水深火熱邊緣，新研究顯示，20世紀末更嚴重的乾旱可能肆虐這個地區。

As many as 100,000 people died in the multi-decade Sahel drought that began in the 1960s, and a detailed look at lake sediments in Ghana indicates that such dry periods have occurred periodically, punctuated by occasional centurylong droughts.

1960年代開始，西非薩赫勒荒漠草原長達數十年的乾旱，造成多達10萬人死亡，再詳細檢視迦納湖泊的沉積物更顯示，這種乾旱期經常出現，偶爾還會發生世紀旱象。

And changing climate could result in even more such disastrous droughts, according to U.S. researchers who report their findings in April 17's edition of the journal Science.

據美國研究人員發表於4月17日《科學》期刊上的發現，氣候改變可能導致更多這類乾旱災難。

“Clearly, much of West Africa is already on the edge of sustainability, and the situation could become much more dire in the future with increased global warming,” said lead researcher Jonathan Overpeck of the University of Arizona.

亞利桑納大學首席研究員歐佛培克說：「顯然，非洲大半地區已經在永續生存邊緣，隨著全球暖化日益嚴重，未來情況可能更悲慘恐怖。」

Timothy M. Shanahan, an assistant professor of geosciences at the University of Texas at Austin, added: “Even more disconcerting is that it suggests the region is vulnerable to longer and more severe droughts.”

奧斯汀德州大學地球科學助理教授沙納罕又說：「更令人不安的是這顯示該地區可能會發生更嚴重、更長久的嚴重旱象。」

“What's really striking about droughts in this area is that they last such a long time,” he said. “You have droughts that last 30 to 60 years, and then some that last four times as long.”

他說：「更值得注意的是這個地區的乾旱動輒持續很長一段時間，有的乾旱長達30到60年之久，有些乾旱時間甚至長達四倍。」

The researchers looked at sediments from Lake Bosumtwi spanning about the last 3,000 years.“Lake Bosumtwi is really unique in that its one of the few locations in tropical West Africa where varves, annual sediment layers, are preserved. This allows us to look at changes in climate at very high resolution,” said Shanahan.

研究人員檢視迦納的博蘇姆推湖過去3,000年來的沉積物，沙納罕說：「博蘇姆推湖的確很獨特，因為它是熱帶西非少數年融積層及每年沉積層都妥善保存的地點之一，這容許我們以極高的解析度來看氣候變化。」

As global warming progresses, the increases in temperature may make the normal climate pattern more extreme, producing even more severe and prolonged droughts than those of the past, Overpeck said.“We also know that global warming will make these droughts a lot hotter. This could be devastating,” he said.

歐佛培克說，隨全球暖化日益嚴重，氣溫升高可能使正常的氣候形態更走極端，產生比過去嚴重持久的乾旱。他說：「我們也知道全球暖化會使旱象更酷熱，這可能是毀滅性的。」

Shanahan noted in a teleconference that there have been six large, century-scale droughts in the last 2,700 years in the region, including two“really large ones” in the last 1,000 years.

沙納罕在一項視訊會議上表示，西非地區過去2,700年來，總共發生六次大世紀規模的旱災，包括兩次「真正特大旱災」是發生在過去1,000年內。

In the two most recent large droughts the lake level dropped about 81 feet（25 meters） and 97 feet （30 meters） respectively, while in the Sahel drought it dropped just over 16 feet （5 meters）, Shanahan said.

沙納罕並指出，在最近兩次大旱災中，湖水水平面分別驟降了約81呎（25公尺）和97呎（30公尺），至於薩赫勒旱災中，水平面下降了16呎（5公尺）。

The researchers were able to tell wet from dry years by analyzing the amounts of different forms of oxygen in the lake sediment.

研究人員從分析湖泊沉積物不同形式的含氧量，就足以分辨是乾旱年還是雨水較多的濕潤年。

In addition, they found remains of a partially submerged forest, which grew during a centurylong drought only a few hundred years ago when the lake was much lower, added Shanahan.

沙納罕又說，此外，他們發現部分被淹沒的森林殘跡，是一、兩百年前發生世紀乾旱時湖泊水平面低得多時所長出來的。

The team found a correlation between wet and dry periods and a shift between warmer and cooler ocean water called the Atlantic Multidecadal Oscillation. The oscillation has never been confirmed over long time periods, but computer simulations and several data sets, including tree-ring variations from sites around the West Atlantic, have hinted at the scenario.“More and more, it's starting to look like the AMO is a big player affecting climate change around the Northern Hemisphere, including drought variability over Western Africa and western North America,” noted Overpeck.

這個研究團隊發現濕潤期和乾旱期之間交互作用，而比較溫暖和寒冷的海水也交互更迭，就叫做「大西洋數百年周期振盪」。這種周期振盪有很長一段時間並未獲得證實，但電腦模擬和幾套資料，包括西非周遭樹木年輪的變化都暗指這種場景。「大西洋數百年周期振盪是日益影響北半球氣候變化，包括西非和北美洲西岸旱象變化的重要因素，」歐佛培克說。

“Temperatures are rising because of human causes and that will have a knock-on effect by creating more evaporation from the soil and vegetation,”Overpeck said. The research was supported by the National Science Foundation.

歐佛培克說：「氣溫上升是因人為因素，並會從泥土和植被中蒸發更多水氣而產生連鎖反應。」這項研究得到國家科學基金會贊助。

This undated handout photo provided by The Journal Science shows a large tropical tree in Lake Bosumtwi, Ghana. （AP）

圖（美聯社）為迦納博蘇姆推湖的檔案照，由《科學》期刊提供，拍攝日期不詳。