A direct link between ocean circulation and abrupt climate change?

通过研究在格陵兰冰盖上钻取的冰芯，科学家们发现了一种令人惊讶的可能性，即在短短10到20年内，气候可能会变化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变的可能原因和未来发生的可能性。气候突变振荡，称为Dansgaard-Oeschger （D-O）事件，在3万至6万年前（ka）的间隔内频繁发生。大西洋海洋环流的重组（所谓的大西洋经向翻转环流，AMOC）被认为在控制D-O事件的突发性方面发挥了主导作用。AMOC的特点是温暖的含盐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。正是暖流向高纬度地区的运动以及随后向大气释放的热量帮助调节了北大西洋的气候。在模拟研究中，AMOC的变化与气候变化有关。然而，尽管进行了深入的研究，我们仍然缺乏明确的物理证据来证明深海环流在气候突变中有系统和直接的作用。在这里，我们建议在非常高的时间分辨率下检查海洋沉积物，以测试过去气候突变期间深层环流变化的参与。NADW （AMOC的深水分支）的流动沿着北美东部大陆边缘形成了一个强烈的深层流。我们的目标是通过评估海底沉积物粒度的变化来重建洋流强度的变化：很简单，更快的洋流会导致沉积物的分选，导致更大的平均粒度。以往的研究表明，冷间隔与北大西洋北部深水形成减少和NADW浅滩化有关。我们将通过在1.8公里和3公里深度使用沉积物岩心检测较强浅层水流的间隔来检验这一假设。以前的研究表明，我们选择的岩心位置和采样间隔将产生分辨率为~100年的记录，因此能够重建多百年尺度的D-O振荡。我们从较早的时间间隔获得的初步研究数据为我们选择的方法提供了信心，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变的理解中一个明显的空白。验证气候突变与海洋环流变化直接相关的假设，并进一步研究不同冷事件之间的差异，将是古气候学领域的重要进展。这项研究的结果将对气候建模者等直接有益，因此在增强我们对未来气候突变的预测知识方面具有重要意义。我们的研究将直接解决NERC当前的挑战，即在其地球系统科学主题中“量化驱动地球系统的力量和反馈”，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地球系统的运行？”）。

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?).