A direct link between ocean circulation and abrupt climate change?

通过检查钻入格陵兰冰盖的冰芯，发现了令人惊讶的可能性，即气候可能在短短10-20年内变化多达10摄氏度。类似的未来变化可能对世界社会产生的巨大影响促使人们努力了解气候突然变化的可能原因和未来发生的可能性。气候突变被称为Dansgaard-Oeschger(D-O)事件，在距今3-6万年(Ka)的时间间隔内频繁发生。大西洋海洋环流的重组(所谓的大西洋经向翻转环流，AMOC)在控制D-O事件的突发性方面发挥了主导作用。AMOC的特点是温暖的咸水地表水向北流向北大西洋北部，在那里冷却并下沉，形成北大西洋深水(NADW)，作为深回流向南流动。正是温暖的水向高纬度移动，随后向大气释放热量，才有助于调节北大西洋的气候。AMOC的变化与模型研究中的气候变化有关。然而，尽管进行了密集的研究，我们仍然缺乏明确的物理证据来证明深海环流在突然的气候变化中起到了系统和直接的作用。在这里，我们建议以非常高的时间分辨率检查海洋沉积物，以测试过去突然气候变化期间深部环流变化的影响。NADW(AMOC的深水部分)的流动在北美东部大陆边缘的深处形成了一股强烈的洋流。我们的目标是通过评估海底沉积物粒度的变化来重建这种海流强度的变化：很简单，更快的水流导致沉积物分选，并导致更大的平均粒度。以前的研究表明，寒冷的间隔与北大西洋北部深水形成的减少和NADW的浅滩有关。我们将通过使用1.8公里和3公里深度的沉积物岩芯探测到更强的浅水水流间隔来检验这一假设。以前的工作已经证明，我们选择的核心站点和采样间隔将产生分辨率为~100年的记录，因此将能够重建数百年尺度的D-O振荡。我们从较早的时间间隔获得的初步研究数据提供了对我们选择的方法的信心，并产生了非常有希望的结果，表明海洋环流变化与气候之间存在紧密耦合。这项研究解决了我们对气候突然变化的理解中一个明显存在的差距。验证气候突变与海洋环流变化直接相关的假设，并进一步研究不同寒冷事件之间的差异，将代表着古气候学领域的重要进展。例如，这项研究的结果将直接有益于气候建模人员，因此将对增强我们对未来突然气候变化的预测知识具有重要意义。我们的研究将直接解决NERC在其地球系统科学主题中面临的“量化驱动地球系统的力量和反馈”的挑战，特别是挑战8和11(“海洋过程及其与地球系统的相互作用”和“过去环境的记录揭示了关于地球系统运行的什么？”)。

Summary The surprising possibility that climate may change by as much as 10 degrees C within just 10-20 years was discovered through examining ice-cores drilled into the Greenland ice sheet. The enormous impact that similar future changes could have on the world's society motivates efforts to understand the possible causes of abrupt climate change and the likelihood of future occurrences. Abrupt oscillations in climate, termed Dansgaard-Oeschger (D-O) events, occurred frequently during an interval 30 - 60 thousand years ago (ka). Reorganisations of the ocean circulation in the Atlantic (the so-called Atlantic meridional overturning circulation, AMOC) have been invoked to play a dominant role in governing the abrupt nature of D-O events. The AMOC is characterised by warm salty surface water flowing northwards to the northern North Atlantic, where it cools and sinks, forming North Atlantic deepwater (NADW) that flows southwards as a deep return flow. It is the movement of warm water to the high latitudes and the subsequent release of heat to the atmosphere that helps regulates the climate of the North Atlantic. Changes in the AMOC have been related to climate shifts in modelling studies. Yet despite intensive study, we still lack unequivocal physical evidence for a systematic and direct role of deep ocean circulation in abrupt climate change. Here we propose to examine ocean sediments at very high temporal resolution in order to test the involvement of deep circulation changes during past abrupt climate change. The flow of NADW (the deep water limb of the AMOC) forms an intense current at depth along the eastern continental margin of North America. We aim to reconstruct changes in the strength of this current by assessing changes in the grain size of seafloor sediments: very simply, faster current flow causes sorting of the sediment and leads to larger average grain size. Previous studies have suggested that cold intervals are related to reduced deep water formation in the northern North Atlantic and a shoaling of NADW. We will test this hypothesis by detecting intervals of stronger shallow flow using sediment cores at 1.8 km and 3 km depth. Previous work has demonstrated that our chosen core sites and sampling intervals will produce records with a resolution of ~100 years and will therefore be capable of reconstructing the multi-centennial scale D-O oscillations. Pilot study data that we have obtained from an earlier time interval provide confidence in our chosen methods and have yielded very promising results, suggesting a tight coupling between ocean circulation changes and climate. This study addresses a clearly identified gap in our understanding of abrupt climate change. Testing the hypothesis that abrupt climate change is directly related to changes in ocean circulation, and further investigating the differences between different cold events, will represent an important advance in the field of paleoclimatology. The results of this study will be of direct benefit to, for example, climate modellers, and hence will be of importance in enhancing our predictive knowledge of future abrupt climate change. Our study will address directly NERC's current challenge to 'Quantify forces and feedbacks that drive the Earth System' within its Earth System Science Theme, and specifically Challenges 8 and 11 ('Ocean processes and their interaction with the Earth System' and 'What do records of past environments reveal about the operation of the Earth System?).