Over-dried Earth
The south-west of the United States, together with some 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 (take) 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 severity (see) on only four other occasions in the past 1,200 years.
Dr Williams studies the annual growth rings of 1,586 ancient trees, in order to reconstruct soil-moisture patterns going back to 800 A. D. During warm, wet years trees grow fast, producing wide rings. During cold, dry they grow more slowly, producing narrow rings. During a drought, a tree not grow much at all.
they describe in this week's Science, the team identified dozens of droughts over the centuries in question. But four stood out. They then took the average soil-moisture value for the current drought and compared it with sequential (连续的) 19-year averages with the previous four, one of them (last) nearly a century. This showed that the region is already drier than it was during the first three of the previous megadroughts, and is equivalent to the event of 1575-1603.
In a world human actions are driving temperatures up, Dr Parker and his colleagues wondered how much people are (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 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 (人类起源的) emissions had not increased the greenhouse-gas effect, the team found that a drought did indeed still influence the western reaches of North America during the first two decades of the 21st century. But this imaginary dry spell was considerably (severe) than the real one-ranking 11th rather than 2nd in the period under study (see chart).
