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.But, over the past two decades, even that expected dryness has been taken to the limit.According to Park Williams, who works at Columbia University's Lamont-Doherty Earth Observatory,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.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,in order to reconstruct soil-moisture patterns going back to 800AD. During warm, wet years trees grow fast, producing wide rings.
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美国西南部，以及横跨格兰德河的墨西哥相邻部分是北美大陆最干燥的地区之一。但是在过去20年里，即便是预期的干燥也已经达到了极限。在哥伦比亚大学拉蒙特都荷迪地球观测站工作的Park Williams表示，目前该地降雨量的匮乏形成了百年不遇的干旱，在过去的1200年里，仅出现过4次这样的场景。Williams博士和他的伙伴研究了从俄勒冈州和蒙大拿州一直到墨西哥北部的1586棵古树的年轮，以重建公元800年的土壤水分模式。在温暖、潮湿的年份，树木生长迅速，年轮很宽。

During cold, dry ones they grow more slowly, producing narrow rings. During a drought, a tree might not grow much at all.As they describe in this week's Science, the team identified dozens of droughts over the centuries in question.But four stood out: those of the late 800s, mid 1100s, 1200s and late 1500s.They then took the mean soil-moisture value for the current drought (which has lasted 19 years so far) and compared it with sequential 19-year averages from the previous four, one of which lasted nearly a century.
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在寒冷、干燥的年份，它们生长缓慢，形成狭窄的年轮。在干旱期间，一棵树可能根本长不了多少。正如他们在本周的《科学》杂志上所描述的那样，该研究小组确定了几个世纪以来的数十次干旱。但有4次脱颖而出：8世纪晚期、12世纪中期、13世纪中期和16世纪晚期。然后他们计算了当前干旱（到目前为止已经持续了19年）的平均土壤含水量并将其与之前4次干旱的连续19年的平均值进行了比较，其中一次干旱持续了近一个世纪。

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.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.To estimate that, they turned to climate modelling.Climate models are able to re-run the past with and without the warming effects of human activity,offering a way to compare what actually happened with what might have done.In their simulated world in which anthropogenic emissions had not ramped up the greenhouse-gas effect,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.
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这表明该地区已经比前三次百年不遇的干旱时期更加干燥了，且与1575年到1603年的情况相当。在一个人类活动导致气温升高的世界里，Parker博士和他的同事想知道有多少人要为此状况负责。为了估算出人数，他们求助于气候模型。气候模型能够在有和没有人类活动的变暖影响的情况下重现过去，提供一条对比实际和预测的方法。在他们模拟的世界中，人为排放并没有增加温室效应，该团队发现，在21世纪的头20年里，北美西部地区确实还在遭受旱灾。

But this fictitious dry spell was considerably less severe than the real one—ranking 11th rather than 2nd in the period under study.Worse still for those who live in the area, Dr Parker's research also confirmed suspicions that the area's water-management policies,which govern abstraction rates from its rivers and aquifers,and which are mostly set on the assumption that 20th-century rainfall levels represent historical normality, are actually out of kilter.He found that, compared with most of the previous millennium, the 20th century was a period of abundant moisture.With nature providing less water than usual, and people taking more than seems wise, the future of the region looks parched indeed.
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但在研究期间，此次虚构的干旱远没有真实情况那么严重——此次旱情仅排在第11位，而非第2位。对于生活在这片地区的人而言，情况更糟糕，Parker博士的调查也证实，该地区的水资源管理政策（这些政策控制着河流和含水层的抽取率，并且大部分建立在‘20世纪的降雨量代表历史正常水平’的假设上），实际上并不能起到什么作用。他发现，与前一个千年的大部分时间相比，20世纪是一个湿度充足的时期。由于自然提供的水比平时少，且人们摄入的水异常得多，该地区的未来看起来确实很干旱。

注释

1.adjacent 临近的
Tom's house is adjacent to the park.
汤姆的房子紧挨着公园。

2.reconstruct 重建
The structure was reconstructed inside the museum in 1971.
1971年，博物馆重建了该神庙。

3.considerably 相当地
The degree of secrecy naturally varies considerably.
保密程度自然差别很大。

4.assumption 假定
This assumption is wrong or greatly exaggerated.
这种假设不是错了，就是太夸张。

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