Beyond the instrumental record: Reconstructing Atlantic overturning over the past 7000 yrs (ReconAMOC)

大西洋经向翻转环流（AMOC）是所谓的“海洋传送带”的一部分，是地球气候系统的关键组成部分。它包括温暖的地表水向北输送到高纬度的北大西洋，在那里它们冷却（向大气释放热量），下沉并在深处流回南方。AMOC的变化被认为会改变全球温度和降水模式、区域海平面以及具有重要社会经济意义的海洋生态系统。人们对未来AMOC的强度和稳定性感到担忧。这是因为预测的海洋表面变暖和变新鲜可能会削弱有助于驱动AMOC的致密水的形成。早期的研究表明，AMOC可能有不同的稳定状态，这增加了AMOC可能迅速切换到较弱甚至“关闭”状态的可能性，从而对全球气候产生严重影响。在21世纪的典型情景下，IPCC模式并不预测AMOC会突然减弱；然而，也有人认为，目前的气候模型可能过于稳定。NERC和国际社会在监测AMOC方面投入了大量资金，包括自2004年以来实施的RAPID阵列和最近的OSNAP阵列。自2004年开始观测以来，AMOC的减弱速度比大多数模型预测的快10倍。然而，这种下降在多大程度上可归因于自然的多年代际变率是不确定的。RAPID阵列的有限时间跨度意味着我们无法在年际到年代际以上的时间尺度上了解AMOC变化的本质。因此，我们必须借助地质档案来重建仪器记录之外的AMOC变化。然而，没有现有的记录来提供最近在多年代际和更长时间尺度上的AMOC变化的观点。利用最近的新技术来限制过去的变化，再加上特殊的沉积物档案，ReconAMOC将在十年到百年的时间尺度上限制过去的AMOC变化，生成过去7000年的记录，这些记录将成为AMOC行为的基准约束。我们将把重点放在过去的7000年，因为当时的气候与现在没有太大的不同，残留的冰川冰盖已经融化，因此大西洋深处环流的主要特征与现代大致相似。ReconAMOC采用了两种方法，一种是利用AMOC特征的地下温度指纹，另一种是利用AMOC变化对深层西部边界电流的响应。我们已经根据仪器记录的变率验证了这些新的古气候方法，并通过广泛的试点研究证明了它们的适用性。因此，ReconAMOC是一个低风险但雄心勃勃的项目，汇集了一个国际合作者团队，这将满足广泛的气候科学家和建模者长期寻求和急需的需求。ReconAMOC将能够测试和改进AMOC的模式模拟，从而有助于评估AMOC对气候变化的脆弱性，并允许调查AMOC对气候系统其他组成部分的作用。ReconAMOC处理的主题是英国国家（例如在NERC战略中确定的战略科学主题和目标）和国际（例如CMIP6， IMAGESII， SCOR， PAGES， IODP和NSF）级别的关键研究目标。具体来说，ReconAMOC提案建立在NERC项目RAPID、RAPID- watch和RAPID-AMOC的基础上，其中AMOC的年际到多年代际变化是一个中心焦点，以及NERC项目ACSIS研究大西洋的年际到十年气候变化。

The Atlantic meridional overturning circulation (AMOC) - part of the so-called 'ocean conveyor belt' - is a key component of Earth's climate system. It involves the northward transport of warm surface waters to the high latitude North Atlantic, where they cool (releasing heat to the atmosphere), sink and flow back southwards at depth. Changes in the AMOC are thought to alter global temperature and precipitation patterns, regional sea-level, and socio-economically important marine ecosystems. There are concerns regarding the strength and stability of AMOC in the future. This is because predicted surface ocean warming and freshening could weaken the formation of dense water that helps drive the AMOC. Earlier research suggests that the AMOC may have different stable states, raising the possibility that the AMOC could rapidly switch to a weaker, or even an 'off', state, having a severe impact on global climate. IPCC models do not predict an abrupt weakening of the AMOC under typical 21st century scenarios; yet there are suggestions that current climate models may be excessively stable.NERC and the international community have invested heavily in monitoring the AMOC, including the implementation of the RAPID array since 2004 and more recently the OSNAP array. Since observations began in 2004, AMOC has weakened at a rate ten times faster than predicted by most models. Yet the extent to which this decline can be attributed to natural multi-decadal variability is uncertain. The limited time span of the RAPID array means we are unable to gain an understanding of the nature of AMOC variability on timescales longer than interannual-to-decadal. Therefore we must turn to geological archives to reconstruct AMOC changes beyond the instrumental record. Yet there are no existing records to provide perspective on recent AMOC variability at multi-decadal and longer timescales. Using recent, novel techniques to constrain past variability, coupled with exceptional sediment archives, ReconAMOC will constrain past AMOC variability on decadal to centennial timescales, generating records for the last 7000 years that will become benchmark constraints on AMOC behaviour. We will focus on the past 7000 years because the climate was not dramatically different to the present day, and remnant glacial ice sheets had melted away so that the major features of deep Atlantic circulation were broadly similar to modern. ReconAMOC deploys a twin approach that utilizes (i) the characteristic subsurface temperature AMOC fingerprint, and (ii) the deep western boundary current response to AMOC change. We have verified these new paleoclimate approaches against variability in the instrumental record and demonstrated their applicability through an extensive pilot study. ReconAMOC is therefore a low risk yet ambitious project, bringing together an international team of collaborators, that will meet a long-sought and much-needed requirement of a wide range of climate scientists and modellers. ReconAMOC will enable testing and improvement of model simulations of AMOC that help facilitate assessment of the vulnerability of the AMOC to climate change, and permit the investigation of the role of AMOC on other components of the climate system. The topics addressed by ReconAMOC are key research targets at national UK (e.g. identified strategic science themes and goals within the NERC strategy) and international (e.g. CMIP6, IMAGESII, SCOR, PAGES, IODP and NSF) levels. Specifically, the ReconAMOC proposal builds on the NERC programmes RAPID, RAPID-WATCH, and RAPID-AMOC, in which interannual to multi-decadal variability in the AMOC is a central focus, as well as NERC programme ACSIS examining interannual to decadal climate variability in the Atlantic.