紐時賞析／研究顯示 美西特大乾旱恐數十年都無解

一場持續近25年的特大幹旱導致美國西南部水源供應枯竭。圖／摘自Pixabay

The West’s Megadrought Might Not Let Up for Decades, Study Suggests

研究顯示 美西特大幹旱恐數十年都無解

A megadrought has sapped water supplies, ravaged farms and ranches, and fueled wildfires across the American Southwest for going on 25 years. Not in 12 centuries has the region been so dry for so long.

一場持續近25年的特大幹旱導致美國西南部水源供應枯竭，嚴重破壞農場和牧場，並助長野火蔓延。過去12個世紀以來，這個地區從未遭遇過持續這麼久的乾旱。

Now comes worse news: Relief might still be decades away.

現在傳來更壞消息：從乾旱解脫可能還要等數十年。

According to new findings published in the journal Nature Geoscience, the dry spell is no mere bout of bad luck.

刊登在期刊「自然地球科學」的新發現顯示，乾旱並非僅是偶然運氣不好那麼簡單。

Instead, it seems to be the result of a pattern of Pacific Ocean temperatures that is “stuck” because of global warming, said Victoria Todd, a doctoral student in paleoclimatology at the University of Texas at Austin who led the new research.

領導這項新研究的德州大學奧斯汀分校古氣候學博士生維多莉亞．託德表示，相反地，這似乎是全球暖化導致的太平洋溫度模式「停滯」的結果。

That means the drought could continue through 2050, perhaps even 2100 and beyond — effectively, Todd said, for as long as humans keep heating up the planet.

託德表示，這意味着乾旱可能持續至2050年，甚至2100年以後——實際上，只要人類持續使地球溫度升高，乾旱就會持續。

In their study, Todd and her colleagues set out to understand a different dry period in the region’s deep past. For clues, they looked to mud from the bottoms of two lakes in the Rocky Mountains: Stewart Bog in New Mexico and Hunters Lake in Colorado.

在他們的研究中，託德和她的同事着手去了解該地區遠古時期的另一段乾旱期。爲了尋找線索，他們研究洛磯山脈兩個湖泊底部的泥土：新墨西哥州的史都華沼澤和科羅拉多州的杭特斯湖。

The waxy coating on a plant’s leaves preserves a chemical signature of the rain and snow that the plant absorbs. So by analyzing the vegetal remains that had accumulated on the lake beds and become entombed in layers of sediment, Todd and her colleagues reconstructed how wet the Rockies had been over the past 14 millennia. They found that winters were dry for thousands of years in the middle of this period.

植物葉片上的蠟質外層保存植物吸收雨水和雪水的化學特徵。所以，透過分析湖牀堆積並被層層沉積物掩埋的植被殘骸，託德和她的同事重現過去1萬4000年間洛磯山脈的溼潤程度。他們發現，在這段期間之中，有數千年的冬季是乾旱的。

Todd and her colleagues ran computer simulations of the prehistoric climate during this warm time to see what might have led to such a severe drought in the Southwest. They found that the extra heat gave rise to something striking in the Pacific: a giant blob of warm water extending east from Japan and surrounded on three sides by cool water, including along the West Coast of the United States. The warm blob shifted the band of winds known as the jet stream and deflected storms away from the Southwest.

託德和她的同事使用電腦模擬這段溫暖時期的史前氣候，觀察導致美國西南部經歷如此嚴重乾旱的可能原因。他們發現，額外的熱量在太平洋引起驚人現象：一團巨大的溫暖海水從日本向東延伸，三面被冷水包圍，包括美國西海岸。這團溫暖海水改變被稱爲噴射氣流的風帶，使暴風雨偏離美國西南部。

In the warm world of 6,000 years ago, the blob stayed put, drying out the Southwest for thousands of years.

在6000年前那個溫暖的世界，這團溫暖海水停在原地，導致美國西南部乾旱數千年。

And, when Todd and her colleagues ran simulations of the present-day climate, they found that the blob might be stuck in place again — only this time, it appears to be because humans are changing the atmosphere by burning coal, oil and gas.

而且，託德和她的同事模擬當今氣候時，發現這團溫暖海水可能再次留在原地—只是這次，似乎是因爲人類燃燒煤、石油和天然氣改變了大氣層。

文／Raymond Zhong，譯／羅方妤

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