气候变暖也许并不意味着厄尔尼诺现象会一直持续下去

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气候变暖也许并不意味着厄尔尼诺现象会一直持续下去

by Sid Perkins on 9 March 2011, 2:38 PM

作者：Sid Perkins

A warm period in Earth's past thought to be a possible analog to Earth's climate future wasn't stuck in a permanent El Niño, a new study suggests. Today's El Niños, which cause weather havoc by triggering floods in some regions and droughts in others, are characterized by warmer-than-normal sea-surface temperatures in the tropical Pacific.

地球历史上曾经出现过的某段暖温期，被认为是地球未来气候可能的写照。一项新的研究表明，这段暖温期并未陷入一场持续的厄尔尼诺现象当中。如今，厄尔尼诺现象在不同地域引发了许多洪水或干旱，其特征是热带太平洋的海面温度高于正常值。

Things were steamy back in the middle of the Pliocene epoch, which lasted from 2.8 million to 5.3 million years ago. Global temperatures back then averaged between 2°C and 3°C warmer than today. According to many climate models, that's where we're heading—Earth's temperature, they say, will reach those heights before the end of this century. So scientists often look to this so-called Pliocene warm period for hints about our future climate. At least one study, which used ocean-floor sediment samples to infer sea-surface temperatures in the tropical Pacific during the mid-Pliocene, suggests that the temperature difference between the eastern and western Pacific was much lower than it normally is today, a sign that the climate may have been stuck in a state that resembles today's El Niño.

当时间回到上新世中叶时（距今530万年开始,距今180万年结束），情况就变得难以捉摸了。当时地球的气温平均比现在高2至3摄氏度。根据众多的气候模型，地球的气温据说将在21世纪末之前达到上新世中叶时期的水平。因此科学家们经常在所谓上新世暖温期中寻找有关未来气候的线索。有一项研究采集了海床沉淀物用于推断热带太平洋区域上新世中叶的海表温度，该研究表明当时太平洋东、西侧温差显著低于现在的正常水平，这也许意味着地球气候曾经陷入了类似今天厄尔尼诺现象的状况之中。

But a new study in the 10 March issue of Nature suggests otherwise. Geochemist Tsuyoshi Watanabe of Hokkaido University in Sapporo, Japan, and his colleagues hunted for climate clues in corals that grew in the Philippines during the mid-Pliocene. They looked at how the ratios of oxygen isotopes locked in the corals' carbonate skeletons varied month by month as the colonies grew. Those ratios provide clues about the temperature and salinity of the water in which the corals were immersed, Watanabe says.

但在3月10日《自然》杂志刊发的一项最新研究却与上述研究得出了相反的结论。来自日本札幌北海道大学的地球化学家渡边刚和他的同事们在菲律宾的珊瑚中找到了上新世中叶的气候线索。他们研究了珊瑚碳酸盐骨骼中留存的氧同位素比例随珊瑚的生长逐月发生变化的情况。渡边刚表示，这些同位素比例为珊瑚生长环境的海水温度及盐度情况提供了线索。

The new study is the first to provide a window into the way the Pliocene climate varied from year to year, says paleoclimatologist David Lea of the University of California, Santa Barbara. Sediment cores, which formed the basis of the earlier studies, generally provide only a low-resolution look at climate over the long term, he notes, but corals chronicle their ever-changing environmental conditions in growth bands like tree rings, giving a high-resolution picture over short intervals.

来自圣巴巴拉市加州大学的古气象学家David Lea表示，该项研究为探寻上新世气候逐年变化的情况提供了一个突破口，在世界上实属首创。他还说，作为早期研究的基础，沉积物岩心仅能做到对气候进行长期的、低分辨率的解析，但珊瑚能记录下环境的不断变化，就像树木的年轮一样，这提高了气候研究的分辨率，并缩短了研究样本的时间间隔。

Besides finding season-to-season variations in oxygen isotopes, Watanabe and his colleagues discerned year-to-year variations. They found signs that in some years the waters were warmer or saltier than others, due to increased evaporation or reduced precipitation. During the 35-year interval chronicled in each fossil coral, variations in oxygen isotope ratios followed a 3½-year cycle. Today, El Niños occur every 3 to 7 years on average, says Watanabe. Whatever caused the ancient fluctuations in isotope ratios, the researchers contend, those variations are a sign that climate back then varied from year to year just as it does today.

除了比对氧同位素的季度变化之外，渡边刚及其同事们还比对了年度的变化。他们发现，某些年份的海水比其它年份更加温暖或盐度更高，其原因是蒸发量的增加或者降水量的减少。在所有珊瑚化石的35年生长片段中，氧同位素比例的变化呈现出每3.5年一次的周期性。渡边刚说，现在厄尔尼诺现象每隔3至7年发生一次。研究者们主张，不论是什么因素导致了氧同位素在古代的波动，这些波动确实证明了当时气候也曾逐年变化，就如同现在一样。

The team's findings are "pretty convincing evidence" that there were El Niño-type, year-to-year climate variations during the Pliocene, says isotopic geochemist Brad Linsley of the University at Albany, State University of New York. The variations in isotopic ratios take place on the right time scale, and they mirror those seen in modern corals, he notes.

来自奥尔巴尼市纽约州大学的地球同位素化学家 Brad Linsley表示，研究小组的发现为证明厄尔尼诺现象的曾经出现和上新世时期气候的逐年变化提供了“非常具有说服力的证据”。他说，同位素比例变化所发生的时间范围恰到好处，这与在现代珊瑚中的发现保持了一致。

Paleoclimatologist Ana Christina Ravelo of the University of California, Santa Cruz, agrees. "This is a really important result," she says, that provides "intriguing evidence of a possible El Niño-like variability from an era much warmer than now." Although computer models suggest that average climate conditions in the future will be much like today's El Niño, the climate at that time isn't stuck in one state year after year, she says: "They're all over the place with variability."

来自圣克鲁兹市加利福尼亚大学的古气象学家 Ana Christina Ravelo同意这样的观点。她说，“这是一个非常重要的结论”，而且为“在过去一个更加温暖的时期可能发生的类厄尔尼诺现象”提供了“有趣的证据”。虽然计算机模型显示未来的气候条件将更加贴近现在的厄尔尼诺现象，但上新世中叶的气候却并非是一成不变的， Ana说：“那时的气候充满了变数。”