El Niño is now stronger than at any point in the last 1,000 years, study finds
El Niño is now stronger than at any point in the last 1,000 years, study finds
研究发现：厄尔尼诺现象强度已达过去1000年来的最高水平
Every few years, El Niño or its cool counterpart, La Niña, shifts rainfall across the tropics, dries out Australia, floods parts of Peru, and rearranges the weather on most of the planet. The two are the warm and cold swings of a single system, the El Niño–Southern Oscillation, or ENSO, and that system is the largest source of year-to-year climate variation on Earth. 每隔几年，厄尔尼诺现象或其对应的拉尼娜现象就会改变热带地区的降雨分布，导致澳大利亚干旱、秘鲁部分地区洪水泛滥，并重塑全球大部分地区的天气。这两种现象是同一系统——厄尔尼诺-南方涛动（ENSO）的冷暖波动，而该系统是地球年度气候变化的最大来源。
Its swings sit atop the steadily rising temperatures driven by global warming. But we didn’t know whether global warming was making it stronger. It’s possible that the added energy in the atmosphere and oceans was causing its swings to be more dramatic. 这种波动叠加在由全球变暖导致的持续升温之上。但我们此前并不清楚全球变暖是否正在使其变得更强。大气和海洋中增加的能量很可能导致其波动变得更加剧烈。
“We’ve been seeing some very strong El Niño events in the late 20th and early 21st century, and what we didn’t know was how unusual those are,” said Julie Cole, an environmental scientist at the University of Michigan. Because models disagree with one another and the instrumental data are too short to separate a trend, Cole’s team reconstructed a thousand-year-long ocean surface temperature record by analyzing fossilized Galápagos corals. 密歇根大学的环境科学家朱莉·科尔（Julie Cole）表示：“我们在20世纪末和21世纪初观察到了一些非常强烈的厄尔尼诺事件，但我们不知道这些事件有多么反常。”由于气候模型之间存在分歧，且仪器观测数据的时间跨度太短，无法区分出明确的趋势，科尔的团队通过分析加拉帕戈斯群岛的珊瑚化石，重建了长达一千年的海面温度记录。
It turns out that El Niño in the eastern Pacific has never been as violent as it is now. “The events don’t look anything like what we see in the pre-industrial era,” Cole said. “That was exciting. And a little surprising, actually.” 结果显示，东太平洋的厄尔尼诺现象从未像现在这样剧烈。科尔说：“这些事件与我们在前工业时代看到的情况完全不同。这令人兴奋，实际上也有些令人惊讶。”
Limestone thermometer
石灰岩温度计
The Galápagos sit in the bullseye of the eastern Pacific ENSO pattern, where the biggest events have their strongest impact. It is also one of the few options to look for fossilized corals in the region. “The Galápagos are located at 90 degrees west, and you have to go all the way to 160 degrees west to find another island in that near-equatorial zone,” Cole said. 加拉帕戈斯群岛位于东太平洋ENSO模式的中心地带，这里受重大气候事件的影响最为强烈。它也是该地区寻找珊瑚化石的少数几个选择之一。科尔说：“加拉帕戈斯群岛位于西经90度，你必须一直向西走到西经160度，才能在近赤道区域找到另一个岛屿。”
Corals there grow one to two centimeters a year, laying down a calcium carbonate skeleton with a chemistry that depends on the water it formed in. In these skeletons, Cole and her colleagues looked at the strontium-to-calcium ratio and at different isotopes of oxygen because both track temperature. “As temperatures get warmer, we get less strontium in the coral,” Cole said. 那里的珊瑚每年生长一到两厘米，形成碳酸钙骨架，其化学成分取决于它形成时的水体环境。科尔和她的同事们研究了这些骨架中的锶钙比以及不同的氧同位素，因为两者都能反映温度变化。科尔说：“随着温度升高，珊瑚中的锶含量会减少。”
Sampling the extracted cores of fossilized corals millimeter by millimeter yields more than a dozen measurements per year—a monthly temperature log for the life of the coral colony. Living colonies cover recent decades and can be checked against instrument data. The deeper history comes from dead coral heads, which can be dated using uranium-thorium methods to within a few years, even as far back as 500 years. 对提取的珊瑚化石核心进行毫米级的采样，每年可获得十多个测量数据，从而记录下珊瑚群落生命周期内的月度温度日志。活体珊瑚群落涵盖了近几十年，可以与仪器数据进行比对。更久远的历史则来自死亡的珊瑚头，利用铀钍测年法，可以将这些珊瑚的年代精确到几年之内，甚至可以追溯到500年前。
“We were also able to find corals that are dead and washed up on the beach,” Cole said. “They may be 100 years old, 200 years old, or 900 years old, or even older.” The result of sampling corals was a patchy but long temperature log—28 time-separated series from five islands, covering scattered years back to around 1100 CE. Set against that baseline, the last four decades look rather strange. “我们还能够找到死亡并被冲刷到海滩上的珊瑚，”科尔说，“它们可能已经有100年、200年、900年甚至更久远的历史了。”珊瑚采样最终形成了一份零散但漫长的温度日志——来自五个岛屿的28个时间序列，涵盖了从公元1100年左右至今的零星年份。以该基准线来看，过去四十年显得相当反常。
Modernized ENSO
现代化的ENSO
Temperature variability since 1984 runs 36.5 percent higher than it was during the preindustrial stretch from 1000 to 1850 CE and is still 16.2 percent higher than during the period from 1851 to 1982. All three intervals are statistically distinct, and the trend only goes up. Those figures, Cole explained, mean that events that once ran a degree or two above normal in the Galápagos now run three or four. 自1984年以来的温度波动比公元1000年至1850年的前工业时期高出36.5%，且比1851年至1982年期间仍高出16.2%。这三个时间段在统计学上截然不同，且趋势一直在上升。科尔解释说，这些数字意味着，曾经在加拉帕戈斯群岛比正常水平高出一两度的事件，现在已经高出三四度。
The corals also record a shift in skewness—whether the swings were lopsided in the warm or cold direction. “The skewness turned positive,” Cole said, “and the positive skewness indicates that it’s the warmer events that got stronger.” The extra variability in our times is due to El Niño specifically. What’s more, nothing comparable shows up elsewhere in the record. Neither the Little Ice Age nor the Medieval warm period left such a distinct fingerprint on ENSO variability. 珊瑚还记录了偏度（skewness）的变化——即波动是偏向温暖还是寒冷方向。“偏度变成了正值，”科尔说，“正偏度表明是更温暖的事件变得更强了。”我们这个时代额外的波动正是由厄尔尼诺现象引起的。此外，在记录的其他地方没有出现类似的情况。无论是小冰河时期还是中世纪暖期，都没有在ENSO波动中留下如此明显的印记。
However, Cole and her colleagues did not yet have evidence clearly indicating that human activity was to blame. That came when scientists fed coral records into multiple climate models. 然而，科尔和她的同事们当时还没有明确的证据表明人类活动是罪魁祸首。这一证据是在科学家将珊瑚记录输入多个气候模型后得出的。
Beating the models at their own noise
在模型自身的噪音中胜出
ENSO fluctuates on its own; quiet decades are followed by wild ones, with no external push required. To understand whether the recent decades were really special or we just happened to catch the cycle in a more energetic period, the team had to find some point of reference for their findings. This proved tricky. ENSO本身就会波动；平静的几十年后可能会紧接着狂暴的几十年，无需外部推动。为了弄清楚最近几十年是否真的特殊，还是我们恰好处于该周期中能量更强的时期，研究团队必须为他们的发现找到参考点。这被证明很棘手。
“We don’t have a direct source of observational data to compare our records to in the eastern Pacific,” Cole said. “If we had another long record, we wouldn’t be publishing this in Science.” As a workaround, Cole and her colleagues used simulations in several climate models running through the conditions of the last millennium but incorporating only natural factors like volcanoes or solar variability—changes in greenhouse gas concentrations were left out. “在东太平洋，我们没有直接的观测数据源来对比我们的记录，”科尔说，“如果我们有另一份长期记录，我们就不会在《科学》杂志上发表这篇文章了。”作为一种变通方法，科尔和她的同事们在几个气候模型中进行了模拟，运行了过去一千年的条件，但仅纳入了火山或太阳变化等自然因素——排除了温室气体浓度的变化。
This allowed them to ask how often a climate untouched by any human impact produces a swing this large on its own. “We tried to see if there was some evidence for this in climate models, and we found none,” Cole said. The actual increase retrieved from coral records sits outside the models’ internal variability at a confidence level approaching 99 percent. The team interpreted this as evidence that what’s currently happening with ENSO would not be happening without us. 这使他们能够探究在没有任何人类影响的情况下，气候本身产生如此大波动的频率。“我们试图在气候模型中寻找证据，但一无所获，”科尔说。从珊瑚记录中提取的实际增长超出了模型内部的波动范围，置信度接近99%。研究团队将其解读为证据，表明如果不是因为人类，目前ENSO所发生的情况就不会发生。
The picture in the central Pacific, though, is murkier. Line Islands corals show a similar mean increase but far wider scatter, which the researchers partly attribute to these records’ reliance on oxygen isotopes alone. Heavy El Niño rainfall there can account for half or more of the signal by changing the oxygen levels in water. But an ambiguous central Pacific coral record is also what climate models predict. A forced, human-caused increase should appear first in the east. “The faster emergence in the eastern Pacific is exactly consistent with the models,” Cole said. 然而，中太平洋的情况则较为模糊。莱恩群岛（Line Islands）的珊瑚显示出类似的平均增长，但离散度要大得多，研究人员将其部分归因于这些记录仅依赖氧同位素。当地强烈的厄尔尼诺降雨通过改变水中的氧含量，可以解释一半甚至更多的信号。但中太平洋珊瑚记录的模糊性也正是气候模型所预测的。由人类活动引起的强制性增长应该首先出现在东部。“东太平洋更快的显现速度与模型完全一致，”科尔说。