美国西南部曾经历“百年不遇干旱”

There's an old saying that if you don't like the weather in New Mexico, wait five minutes. Maybe it should be amended¹ to 10,000 years, according to new research. In a letter published recently in the journal Nature, Los Alamos National Laboratory researchers and an international team of scientists report that the Southwest region of the United States undergoes "megadroughts"—warmer, more arid²（干旱的） periods lasting³ hundreds of years or longer. More significantly, a portion of the research indicates that an ancient period of warming may be analogous⁴ to natural present-day climate conditions. If so, a cooler, wetter period may be in store for the region, unless it is thwarted⁵（挫败，反对） by increased concentrations of greenhouse gasses in the atmosphere that could warm the planet.

Using a long core of sediments⁷ obtained in 2004 from beneath a dry lakebed located on the Valles Caldera National Preserve—an 86,000-acre grassland⁸ located on the site of a dormant⁹（休眠的，静止的） prehistoric¹⁰ volcano about 20 miles west of Los Alamos—the researchers were able to peer back in time into the climate as it existed between 360,000 and 550,000 years ago. Layers in the 260-foot-long sediment⁶ core were easily distinguishable, and were bounded by distinct layers of volcanic¹¹ ash that allowed for very accurate dating. Researchers looked at chemical constituents¹² trapped within the layers as well as plant and pollen¹³ debris¹⁴ to characterize the climate conditions of the time.

The sediment layers from beneath the South Mountain Lake covered two "interglacial" periods. Such periods are significant because they represent a time between ice ages when warmer temperatures mimicked¹⁵ present-day temperatures. The ancient interglacial period, known as Marine¹⁶ Isotope¹⁷ Stage 11—MIS 11 for short—lasted about 50,000 years and included, among several periods of climatic variation, one definitive¹⁸ megadrought period followed by a cooler period. Prior to the study, detailed¹⁹ data about MIS 11 had been scarce（缺乏的） because most of the information was gathered from Antarctic ice cores or marine sediments. The terrestrial record obtained beneath South Mountain Lake revealed a bounty²⁰ of information that nicely details the ancient climate.

The oldest warm period in MIS 11 appears somewhat analogous to the present-day Holocene interglacial period, which has been ongoing²¹ for about the past 10,000 years. During MIS 11, the ancient climate warmed dramatically by about 14 degrees Fahrenheit²². This warming in the wake of a preceding period of cold gave rise to an abundance of plant life and seasonally²³ wet conditions. As warming continued, grasses and shrubs²⁴ died off and lakes dried up. The ensuing（接着发生的） drought lasted thousands of years before ending abruptly²⁵ with a cooler, wetter period.

The research could indicate that the Southwest, having been through a present-day drought period that included the historic Dust Bowl, might be due for a change unless increased concentrations of greenhouse gasses interfere²⁶.

"Results from this study have implications for the development of models that could predict future megadroughts and other climate conditions in the Southwestern United States," said Jeffrey Heikoop, leader of the Los Alamos study team.