死海底下潛藏著大乾旱的警訊

原文網址：www.sciencedaily.com/releases/2017/03/170322143139.htm

死海底下潛藏著大乾旱的警訊

古代沉積物訴說的故事有朝一日可能再度發生

在死海底下將近1000英尺之處，科學家發現的證據顯示在過去的溫暖時期，中東遭受的旱災規模之大是人類歷史紀錄中從未遇到的，這可能是對現今狀況的警訊。深厚的結晶鹽層指出大約12,000年前，雨量急遽下降至現今水平的五分之一，並在10,000年前再度發生。今日此區域再度因氣溫暖化而越趨乾燥，而科學家表示未來情況還會變得更糟。這項發現或許會讓科學家重新思考，在這塊處於乾渴且紛亂的世界中，未來情形會惡化到什麼程度。

「所有的觀測結果都呈現出此區域是受現今氣候變遷影響最為劇烈的地區之一，而科學家預測未來還會變得更加乾燥。我們的研究結果顯示在自然情況下，此區域甚至可以變得比我們所有模型的預測結果還要乾燥許多。」主要作者，哥倫比亞大學拉蒙特-多爾蒂地球觀測站的地球化學家Yael Kiro表示。這項發現甫刊登於期刊《地球與行星科學通訊》(Earth and Planetary Science Letters)的線上預告版中。

位於內陸的死海跨越了以色列、約旦及巴勒斯坦地區，是地球陸地上的最低點。它現今的湖岸位於海平面之下1300英尺，而其湖床深度還要再往下900英尺。約旦河是死海的主要水源，流域除了上述地區外還經過了敘利亞和黎巴嫩。由於死海是河水的終點，使得湖水的鹽分極高；它在聖經中的希伯來文名稱為Yam ha-Melah，意謂著「鹽之海」。最近幾年，它的湖面以每年數英尺的速度下降。然而，主因不僅僅只有炙熱且乾燥的氣候，此區域急速增加的人口使得對水的需求更甚以往。人們大量抽取約旦海流域的河水，到達死海的水就變得更少，導致死海的蒸發量超出注入其中的水量。

聯合國糧食及農業組織估計此區域大部分的每人可用水量只有世界平均的十分之一。自1950年起雨量已經減少了大約百分之十，而目前的氣候模型顯示本世紀還會再下跌百分之20，於此同時人口仍會持續增長。以色列正利用淡化地中海海水的方式來迎合自身需求，但位處內陸且較貧窮的約旦和巴勒斯坦地區顯然極需更多淡水。在隔壁的敘利亞，一般認為可能受到氣候變遷助長，從1998歷時至2012年的破紀錄乾旱，協助點燃了持續至今的內戰。這場內戰迄今已奪走了超過500,000條人命，鄰近的國家也受到波及。

2010年，來自六個國家的科學家在湖床最深處往下方鑽探了1500英尺，取出的沉積物剖面紀錄了過去20萬年以來的區域氣候，是目前中東地區最長的氣候檔案(他們連續鑽探了40個日夜，或許是為了致敬導致聖經大洪水的大雨)。岩芯具有交錯出現的泥層以及結晶鹽層，前者為在濕潤時期隨逕流沖入死海，後者則是在乾燥時期隨湖水下降而蒸發出來。從這根岩芯一眼就能清楚看出此區曾經歷漫長且嚴峻的乾燥時期，但直到現在才對它進行極為詳細的分析。

新研究顯示鹽份累積的速度相當快―在許多段落預估可以達到每年半英吋。研究人員標出兩段特別驚人的時期：大約在岩芯一半以下的地方他們找到了一段厚達300英尺的鹽層，顯示死海湖面曾長期低於現今的水準。這發生於115,000至130,000年前兩段冰河期的中間，此時由於地球軌道的改變造成溫度比20世紀高上四度―等同預測中21世紀末會到達的溫度。當副極地地區的冰河再次擴張，中東的氣候變得較冷且雨量變多，死海又重新充滿湖水。岩芯顯示正好在6000至10000年前，也就是上次冰河期過後，湖面經歷了另一次類似的下降過程，此時的溫度可能比現今還低一些。

鹽晶中微小氣泡的化學成分可以讓科學家推測這些時期的雨量和逕流模式。他們計算出當時死海的逕流跟現今比起一般來說少了百分之50至70，這讓本世紀的預測結果相形失色不少。在最極端的時期逕流甚至少了百分之80，並整整持續了數十年到數百年之久。雨量減少或許跟大氣環流模式的大區域變化有關。如同今日似乎正在發生的情況，從地中海來到此區的暴雨減少許多；接著就跟現在一樣，較高的氣溫使得更多水氣從陸上蒸發。

為了減緩持續增加的水資源短缺壓力，約旦預計在明年開始建造一條運河，把紅海的水運來進行海水淡化；殘留的鹵水則會注入死海，或許可以讓其水位維持現況。但此計畫引來眾多非議，因為這可能會同時劇烈改變兩座湖泊的環境，還會留給其餘大部分區域不宜使用的水。

「由於人類用盡了死海的淡水水源，使得當下死海正逐步邁向死亡。」共同作者，拉蒙特-多爾蒂地球觀測站的地球化學家Steven Goldstein說。他也曾參與監督2010年的鑽探工作。「我們的研究顯示在過去沒有任何人類介入的情況下，淡水曾經幾乎不再注入死海。這意味著如果現今的氣候更加暖化，河水可能會再次停止流動，使得數百萬人遭受影響。」

Under the Dead Sea, warnings of dire drought

Ancient sediments tell a story that could be repeated

Nearly 1,000 feet below the bed of the Dead Sea, scientists have found evidence that during past warm periods, the Mideast has suffered drought on scales never recorded by humans -- a possible warning for current times. Thick layers of crystalline salt show that rainfall plummeted to as little as a fifth of modern levels some 120,000 years ago, and again about 10,000 years ago. Today, the region is drying again as climate warms, and scientists say it will get worse. The new findings may cause them to rethink how much worse, in this already thirsty and volatile part of the world.

"All the observations show this region is one of those most affected by modern climate change, and it's predicted to get dryer. What we showed is that even under natural conditions, it can become much drier than predicted by any of our models," said lead author Yael Kiro, a geochemist at Columbia University's Lamont-Doherty Earth Observatory. The findings were just published in an early online edition of the journal Earth and Planetary Science Letters.

The landlocked Dead Sea, straddling Israel, Jordan and Palestinian lands, is earth's lowest spot on land. Its current shoreline lies about 1,300 feet below sea level, and its floor extends down another 900 feet. Fed mainly by the Jordan River drainage, which extends also into Syria and Lebanon, it is a dead end for water, and so is extremely salty; its Biblical name in Hebrew is Y?m ha-Melah, the sea of salt. In recent years, its level has dropped about four feet a year. But hot, dry weather is not the main cause yet; rather, booming populations in the region need more water than ever, and people are sucking so much from the watershed, very little reaches the Dead Sea, where evaporation is outweighing input.

The U.N. Food and Agriculture Organization estimates that much of the region already has per capita water availability only a tenth of the world average. Rainfall has declined about 10 percent since 1950, and existing climate models say it could sink another 20 percent this century, even as population continues to grow. Israel is meeting demand by desalinating Mediterranean seawater, but poorer, landlocked Jordan and the Palestinian territories are desperate for more. In adjoining Syria, a record 1998-2012 drought likely stoked by climate change is believed to have helped spark the ongoing civil war, which has now claimed more than 500,000 lives and infected neighboring nations.

In 2010, scientists from a half-dozen nations drilled 1,500 feet into the deepest part of the seabed, bringing up a cross section of deposits recording 200,000 years of regional climate history -- the longest such archive in the Mideast. (Around-the-clock drilling went for 40 days and 40 nights -- perhaps a respectful bow to the rainfall of the Biblical Flood.) The cores revealed alternating layers of mud washed in with runoff during wet times, and crystallized salt, precipitated out during dry times when the water receded. This instantly made it clear that the region has suffered epic dry periods, but the core was not analyzed in great detail until now.

The new study shows that the salt accumulated rapidly-an estimated half-inch per year in many cases. The researchers spotted two striking periods. About halfway down they found salty layers some 300 feet thick, indicating a long-term drop below the sea's current level. This came in a period between ice ages, 115,000 to 130,000 years ago, when variations in Earth's orbit brought temperatures about 4 degrees hotter those of the 20th century -- equivalent to what is projected for the end of the 21st century. The lake refilled when glaciers readvanced in sub-polar regions and the Mideast climate cooled and became moister. The cores show a similar drop in lake level just 6,000 to 10,000 years ago, following the most recent ice age, when temperatures were probably a bit cooler than now.

The chemistry of tiny fluid bubbles within the salt allowed the researchers to extrapolate rainfall and runoff patterns of these periods. They calculated that runoff to the Dead Sea generally declined 50 to 70 percent compared to today, dwarfing current projections for this century. In the most extreme periods, it went down 80 percent, and this lasted for decades to centuries at a time. The declines are probably linked to broader shifts in atmospheric flow patterns. Storms coming in from the Mediterranean could have slackened, as they appear to be doing today; and then as now, higher temperatures increase evaporation of moisture from the land.

To alleviate growing water shortages, Jordan plans to break ground next year on a canal to bring in water from the Red Sea for desalination; leftover brine would be dumped into the Dead Sea, possibly stabilizing its level. But the project is controversial, because it could cause drastic environmental changes in both seas, and could still leave much of the rest of the region with inadequate water.

"The Dead Sea is wasting away today because humans are using up all its fresh water sources," said Steven Goldstein, a geochemist at Lamont-Doherty and coauthor of the paper who helped oversee the 2010 drilling. "Our study shows that in the past, without any human intervention, the fresh water nearly stopped flowing. This means that if it keeps getting hotter now, it could stop running again. This time, it would affect millions of people."

原始論文：Yael Kiro, Steven L. Goldstein, Javier Garcia-Veigas, Elan Levy, Yochanan Kushnir, Mordechai Stein, Boaz Lazar. Relationships between lake-level changes and water and salt budgets in the Dead Sea during extreme aridities in the Eastern Mediterranean. Earth and Planetary Science Letters, 2017; 464: 211 DOI: 10.1016/j.epsl.2017.01.043

引用自：Lamont-Doherty Earth Observatory, Columbia University. "Under the Dead Sea, warnings of dire drought: Ancient sediments tell a story that could be repeated." ScienceDaily. ScienceDaily, 22 March 2017.