日本研究人员通过对珊瑚化石的分析发现，厄尔尼诺现象在约350万年前就已经出现，这是目前发现的全球最古老的证明厄尔尼诺现象的化石。
日本北海道大学与国立科学博物馆和产业技术综合研究所的联合研究小组，对在菲律宾吕宋岛的上新世温暖期（约460万年前至约300万年前）地层中发现的一种微孔珊瑚化石进行了分析。研究小组根据同一地层出现的浮游植物化石，断定珊瑚化石的年代在约350万年前。
珊瑚在生长过程中，会如同树木的年轮一样，每年都会产生一道横线，而作为珊瑚主要成分的碳酸钙中碳同位素的比例会随着海水温度和盐分的变化而变化。研究小组以此为线索，根据化石上的横线数目，分析了约70年间海水水温的变化，结果发现每隔三、四年会出现水温大幅下降和降水减少的现象。
化石显示的水温变化与现在出现厄尔尼诺现象时菲律宾附近的气候很相似。研究人员说，化石表明当时的厄尔尼诺现象是持续发生的，并且周期比现在的要短。有关论文发表在3月9日的英国《自然》杂志网络版上。
厄尔尼诺现象指南美秘鲁近海的海水温度升高，导致全球气象异常的现象。研究小组认为，当时的气温比现在高2至3摄氏度，如果当前全球变暖的趋势持续下去，厄尔尼诺现象很可能周期出现。（生物谷Bioon.com）
生物谷推荐原文出处：
Nature doi:10.1038/nature09777
Permanent El Niño during the Pliocene warm period not supported by coral evidence
Tsuyoshi Watanabe,1 Atsushi Suzuki,2 Shoshiro Minobe,1 Tatsunori Kawashima,1 Koji Kameo,3 Kayo Minoshima,2 Yolanda M. Aguilar,4 Ryoji Wani,5 Hodaka Kawahata,6 Kohki Sowa,1 Takaya Nagai1 & Tomoki Kase7
The El Ni?o/Southern Oscillation (ENSO) system during the Pliocene warm period (PWP; 3–5 million years ago) may have existed in a permanent El Ni?o state with a sharply reduced zonal sea surface temperature (SST) gradient in the equatorial Pacific Ocean1. This suggests that during the PWP, when global mean temperatures and atmospheric carbon dioxide concentrations were similar to those projected for near-term climate change2, ENSO variability—and related global climate teleconnections—could have been radically different from that today. Yet, owing to a lack of observational evidence on seasonal and interannual SST variability from crucial low-latitude sites, this fundamental climate characteristic of the PWP remains controversial1, 3, 4, 5, 6, 7, 8, 9, 10. Here we show that permanent El Ni?o conditions did not exist during the PWP. Our spectral analysis of the δ18O SST and salinity proxy, extracted from two 35-year, monthly resolved PWP Porites corals in the Philippines, reveals variability that is similar to present ENSO variation. Although our fossil corals cannot be directly compared with modern ENSO records, two lines of evidence suggest that Philippine corals are appropriate ENSO proxies. First, δ18O anomalies from a nearby live Porites coral are correlated with modern records of ENSO variability. Second, negative-δ18O events in the fossil corals closely resemble the decreases in δ18O seen in the live coral during El Ni?o events. Prior research advocating a permanent El Ni?o state may have been limited by the coarse resolution of many SST proxies, whereas our coral-based analysis identifies climate variability at the temporal scale required to resolve ENSO structure firmly.
