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《自然》及子刊综览
来源:中国科学报      作者:张章      时间:2017-02-26 19:52      关键词:

《自然》

一条简单规则预测间冰期发生时间

《自然》发表的一篇论文报告称,使用一条简单的规则就可以预测间冰期的发生时间。间冰期指两次冰期之间出现的相对温暖的时期。研究显示,这条规则不需要获取气候或大气数据,就可以成功预测过去几百万年里的每一个间冰期。

北半球冰盖的扩张和收缩周期受地球轨道和地轴倾斜角度的微小变化驱动。在260万年前到100万年前之间,这些变化导致每4.1万年左右出现一次间冰期;但是,在过去100万年里,间冰期出现的频率下降,约为每10万年出现一次。虽然过去的模型已经能够再现这一模式,但是许多都要求有大量的信息输入,如二氧化碳浓度,而且没有一个能够充分解释过去几百万年里间冰期出现的模式和时间。

英国伦敦大学学院的Chronis Tzedakis及同事开发出一个统计模型,仅使用北纬65°地区夏半年太阳日平均辐射量以及自前一次冰消期以来经过的时间,就能准确预测过去几百万年里的每一个完整的冰消期。

在《自然》的另一篇论文里,美国威斯康星大学麦迪逊分校的Stephen Meyers及同事证实了地球轨道变化混乱的理论,暗示在约5000万年前,间冰期的出现时间可能是不可预测的,除非可以发现确凿的年代校正点。作者在研究当今科罗拉多州的古代地质构造时发现了这样的一个校正点,并总结表明在8500-8700万年前,地球和火星轨道之间的相互作用发生了变化。这项发现或有助于进一步精确重建气候与天文因素在悠久的地球历史中的关联。

在相应的新闻与观点文章中,Didier Paillard写道:“这些行星如此难测,而气候和沉积过程响应天文作用的方式看起来如此简单,说来不免有些‘讽刺’。”

《自然》

拥有7颗行星的太阳系外恒星系

近期,研究者发现有七颗地球大小的系外行星围绕一颗邻近的恒星运转。《自然》在线发表的一篇论文指出,靠里的六颗行星位于宜居带中,表面温度可能在0到100摄氏度之间。

2016年5月,比利时列日大学Michael Gillon及同事发现有三颗系外行星飞凌超冷矮星TRAPPIST-1。TRAPPIST-1距太阳只有12秒差距(约39光年)之遥。受这一发现激励,作者从地面和太空对该恒星展开了监测,因而发现了这一行星系中的另外四颗行星。初始质量估计表明,六颗内行星质量与地球类似,可能是由岩石组成的。

Gillon研究组强调,要详细表征这些行星,研究者还需要进一步观测,尤其是最靠外的第七颗行星——研究者尚不了解它的公转周期,以及它与内行星的相互作用。

《自然—通讯》

复杂性可能影响金融体系稳定性

《自然—通讯》发表的一项研究报告,增强银行间互动的市场整合和市场多元化过程或许会使金融系统趋向不稳定。此前,人们认为这些过程会增强金融稳定性,但这项新研究提出,银行金融网络中正在浮现的复杂循环结构或许会快速放大金融困境。

监管机构通过压力测试来评估全球金融系统的稳定性,这种测试通常孤立地研究各个机构。 然而,银行网络日益相连的性质可能会导致困境传播和放大—— 一种无法在个别机构层面上预测的集体效应。

瑞士苏黎世大学Marco Bardoscia和同事使用数学建模方法和来自50家银行资产负债表的原始数据,表明金融系统复杂性增加或许不能增强其内在稳定性。他们发现,在金融机构及其相互间合约形成的网络中,一些特定的特征可能会导致不稳定性。 通过扩大参与金融系统的银行数量,并增强它们之间的联系,个别机构(如银行间借贷、共同持有资产等)成为了多层合同中的一部分,因而有可能扩大金融冲击的影响。 作者将这些发现与对生态系统的观测结果做了比较,在生态系统中,复杂性增加(物种间相互作用增加,曾被认为能增强稳定性)也有可能破坏稳定性。

虽然预测银行体系潜在压力的测试愈发复杂,但它们通常都孤立地研究金融系统。作者提出,他们的方法考虑了金融困境在银行间合同网络中传播的后果,因此更为灵活。

(张章/编译 更多信息请访问www.naturechina.com/st)


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