Department of Geography, University College London, London, UK.
Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA, USA.
Nature. 2018 Apr;556(7700):227-230. doi: 10.1038/s41586-018-0007-4. Epub 2018 Apr 11.
The Atlantic meridional overturning circulation (AMOC) is a system of ocean currents that has an essential role in Earth's climate, redistributing heat and influencing the carbon cycle. The AMOC has been shown to be weakening in recent years ; this decline may reflect decadal-scale variability in convection in the Labrador Sea, but short observational datasets preclude a longer-term perspective on the modern state and variability of Labrador Sea convection and the AMOC. Here we provide several lines of palaeo-oceanographic evidence that Labrador Sea deep convection and the AMOC have been anomalously weak over the past 150 years or so (since the end of the Little Ice Age, LIA, approximately AD 1850) compared with the preceding 1,500 years. Our palaeoclimate reconstructions indicate that the transition occurred either as a predominantly abrupt shift towards the end of the LIA, or as a more gradual, continued decline over the past 150 years; this ambiguity probably arises from non-AMOC influences on the various proxies or from the different sensitivities of these proxies to individual components of the AMOC. We suggest that enhanced freshwater fluxes from the Arctic and Nordic seas towards the end of the LIA-sourced from melting glaciers and thickened sea ice that developed earlier in the LIA-weakened Labrador Sea convection and the AMOC. The lack of a subsequent recovery may have resulted from hysteresis or from twentieth-century melting of the Greenland Ice Sheet . Our results suggest that recent decadal variability in Labrador Sea convection and the AMOC has occurred during an atypical, weak background state. Future work should aim to constrain the roles of internal climate variability and early anthropogenic forcing in the AMOC weakening described here.
北大西洋经向翻转环流（AMOC）是一种洋流系统，在地球气候中起着至关重要的作用，它重新分配热量并影响碳循环。近年来，AMOC 已经被证明正在减弱；这种下降可能反映了拉布拉多海对流的年代际可变性，但短期观测数据集排除了对拉布拉多海对流和 AMOC 的现代状态和可变性的更长期视角。在这里，我们提供了几条古海洋学证据，表明与过去 1500 年相比，拉布拉多海深海对流和 AMOC 在过去 150 年左右（自小冰期结束后，约公元 1850 年）一直异常薄弱。我们的古气候重建表明，这种转变要么是小冰期末期的主要突然转变，要么是过去 150 年持续逐渐下降；这种模糊性可能是由于非 AMOC 对各种代理的影响，或者是由于这些代理对 AMOC 各个组成部分的不同敏感性。我们认为，小冰期末期来自融冰川和更早形成的厚海冰的北极和北欧海域向海洋输送的淡水通量增加，削弱了拉布拉多海的对流和 AMOC。此后没有恢复可能是由于滞后或由于 20 世纪格陵兰冰盖的融化 。我们的结果表明，最近拉布拉多海对流和 AMOC 的年代际可变性发生在一个非典型的、较弱的背景状态下。未来的工作应该旨在确定内部气候变率和早期人为强迫在描述的 AMOC 减弱中的作用。
