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The South-West of the United States, together with adjacent parts of Mexico across the Rio Grande, is one of the driest parts of the North American continent.
美國西南部，以及橫跨格蘭德河的墨西哥相鄰部分是北美大陸最干燥的地區(qū)之一。
But, over the past two decades, even that expected dryness has been taken to the limit.
但是在過去20年里，即便是預期的干燥也已經達到了極限。
According to Park Williams, who works at Columbia University's Lamont-Doherty Earth Observatory,
在哥倫比亞大學拉蒙特都荷迪地球觀測站工作的Park Williams表示，
the current lack of rainfall in the area constitutes a megadrought of a magnitude seen on only four other occasions in the past 1,200 years.
目前該地降雨量的匱乏形成了百年不遇的干旱，在過去的1200年里，僅出現過4次這樣的場景。
Dr Williams and his collaborators studied the annual growth rings of 1,586 ancient trees from Oregon and Montana all the way down to the northern reaches of Mexico,
Williams博士和他的伙伴研究了從俄勒岡州和蒙大拿州一直到墨西哥北部的1586棵古樹的年輪，
in order to reconstruct soil-moisture patterns going back to 800AD. During warm, wet years trees grow fast, producing wide rings.
以重建公元800年的土壤水分模式。在溫暖、潮濕的年份，樹木生長迅速，年輪很寬。
During cold, dry ones they grow more slowly, producing narrow rings. During a drought, a tree might not grow much at all.
在寒冷、干燥的年份，它們生長緩慢，形成狹窄的年輪。在干旱期間，一棵樹可能根本長不了多少。
As they describe in this week's Science, the team identified dozens of droughts over the centuries in question.
正如他們在本周的《科學》雜志上所描述的那樣，該研究小組確定了幾個世紀以來的數十次干旱。
But four stood out: those of the late 800s, mid 1100s, 1200s and late 1500s.
但有4次脫穎而出：8世紀晚期、12世紀中期、13世紀中期和16世紀晚期。
They then took the mean soil-moisture value for the current drought (which has lasted 19 years so far)
然后他們計算了當前干旱（到目前為止已經持續(xù)了19年）的平均土壤含水量
and compared it with sequential 19-year averages from the previous four, one of which lasted nearly a century.
并將其與之前4次干旱的連續(xù)19年的平均值進行了比較，其中一次干旱持續(xù)了近一個世紀。
This showed that the region is already drier than it was during the first three of the previous megadroughts, and is on a par with the event of 1575-1603.
這表明該地區(qū)已經比前三次百年不遇的干旱時期更加干燥了，且與1575年到1603年的情況相當。
In a world where human actions are driving temperatures up, Dr Parker and his colleagues wondered how much people are to blame for this state of affairs.
在一個人類活動導致氣溫升高的世界里，Parker博士和他的同事想知道有多少人要為此狀況負責。
To estimate that, they turned to climate modelling.
為了估算出人數，他們求助于氣候模型。
Climate models are able to re-run the past with and without the warming effects of human activity,
氣候模型能夠在有和沒有人類活動的變暖影響的情況下重現過去，
offering a way to compare what actually happened with what might have done.
提供一條對比實際和預測的方法。
In their simulated world in which anthropogenic emissions had not ramped up the greenhouse-gas effect,
在他們模擬的世界中，人為排放并沒有增加溫室效應，
the team found that a drought did indeed still engulf the western reaches of North America during the first two decades of the 21st century.
該團隊發(fā)現，在21世紀的頭20年里，北美西部地區(qū)確實還在遭受旱災。
But this fictitious dry spell was considerably less severe than the real one—ranking 11th rather than 2nd in the period under study.
但在研究期間，此次虛構的干旱遠沒有真實情況那么嚴重——此次旱情僅排在第11位，而非第2位。
Worse still for those who live in the area, Dr Parker's research also confirmed suspicions that the area's water-management policies,
對于生活在這片地區(qū)的人而言，情況更糟糕，Parker博士的調查也證實，該地區(qū)的水資源管理政策
which govern abstraction rates from its rivers and aquifers,
（這些政策控制著河流和含水層的抽取率，
and which are mostly set on the assumption that 20th-century rainfall levels represent historical normality, are actually out of kilter.
并且大部分建立在‘20世紀的降雨量代表歷史正常水平’的假設上），實際上并不能起到什么作用。
He found that, compared with most of the previous millennium, the 20th century was a period of abundant moisture.
他發(fā)現，與前一個千年的大部分時間相比，20世紀是一個濕度充足的時期。
With nature providing less water than usual, and people taking more than seems wise, the future of the region looks parched indeed.
由于自然提供的水比平時少，且人們攝入的水異常得多，該地區(qū)的未來看起來確實很干旱。
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