Carbon cycling in a warming world: a deglacial test case

对未来气候变化的预测要求我们了解碳与其主要储存库之间的相互作用，包括海洋、陆地和大气之间的大规模自然交换。研究还表明，模拟碳循环的不确定性与模拟物理气候系统的不确定性同样重要。因此，至关重要的是，我们要发展一个更好的定量理解。今天，碳循环的自然组成部分已经受到了显著的干扰，因此在人类干预之前研究自然过程是对我们知识和理解的一个重要的额外考验。当气候系统在与人类相关的时间尺度上发生突然扰动时，情况尤其如此。一个主要目标是从大约2万年前的冰川期过渡出来。在这一转变过程中，全球气温上升了约3- 5摄氏度，海平面上升了120米，大气pCO 2增加了约100 ppmV。重要的是，这些变化中的很大一部分发生在快速事件中，这些事件覆盖了长期的冰川消退过渡。我们仍然不完全了解这段时间大气中的二氧化碳水平是如何增加的，尽管改变海洋，陆地和大气之间碳平衡的过程肯定是至关重要的。事实上，解释大气中二氧化碳含量大幅上升的主要假设是，深海中的碳释放，这些碳是有机物质在降雨到海洋深处时腐烂积累起来的。与此同时，陆地生物圈的变化被认为是通过包括永久冻土融化在内的过程造成的。生物生产力与碳库之间的碳交换之间的这种联系是一个及时的主题，因为人们对人为碳排放的命运感到关切，包括海洋酸化和深海长期过量储存碳的建议。然而，最近的研究提供的数据与这种碳储存和交换的观点相矛盾。有证据表明，气候触发了地质碳的释放。固体地球含有大量的碳，因此释放的微小变化可能对碳预算很重要。如果地质碳的释放确实对气候系统的变化敏感，那么在未来的气候情景下也可能触发这一过程。限制过去释放的地质碳的潜在影响有可能为预测未来变化提供关键的新信息。跨学科项目小组提出了一项雄心勃勃的方案，将实地工作、地球化学分析和综合建模结合起来，对用于解释冰期结束时二氧化碳水平上升的相互竞争的假设进行具体检验。我们将特别关注使用放射性碳来帮助诊断碳的不同来源。新的数据将基于对从这些试验的关键地点收集的精确定年的深海珊瑚骨骼进行的地球化学分析。我们将在东赤道太平洋进行第一次遥控潜水器研究考察，专门从假定发生地质碳释放的深度系统收集珊瑚。我们的第二个重点将是北大西洋（样本在手），那里海洋环流的变化和与孤立盆地的联系可能在深海释放碳方面发挥了关键作用。我们将把这些数据与现有知识结合起来，并将它们与气候建模（包括中等复杂性模型和最先进的大气环流模型）结合使用，以探索在变暖的世界中，哪些过程对地球系统最重要。我们也将很好地制定一个广泛的综合过程中控制碳循环在最近的地球历史上最明显的快速变暖时期。

Projections of future climate change require us to understand the interactions between carbon and its main reservoirs, including the large natural exchanges between ocean, land and atmosphere. Studies have also shown that uncertainties in modelling the carbon cycle are as important as uncertainties in modelling the physical climate system. Thus it is vital that we develop a better quantitative understanding. The natural component of the carbon cycle has already been significantly perturbed today, so studying the natural processes before human interventions is an important additional test of our knowledge and understanding. This is particularly true when there were abrupt perturbations on the climate system that occurred on human-relevant timescales. A prime target is the transition out of the glacial period, some 20,000 years ago. The transition saw global temperatures increase by ~ 3-5C, sea level rise by 120m and atmospheric pCO2 increase by ~100ppmV. Importantly, a large portion of these changes occurred in rapid events which overprinted the long term deglacial transition. We still do not fully understand how CO2 levels in the atmosphere increased over this time although processes that alter the balance of carbon between the ocean, land and the atmosphere are certainly critical. Indeed, the leading hypothesis to explain the major rise in atmospheric CO2 calls upon release of carbon from the deep ocean that accumulated from the decay of organic matter as it rains down through to the ocean's depths. At the same time changes in the terrestrial biosphere are thought to have contributed through processes including permafrost melting. This link between biological productivity and the exchange of carbon between its reservoirs is a timely topic given concern over the fate of anthropogenic carbon emissions - including ocean acidification and suggestions of long-term excess storage of carbon in the deep sea. However recent studies have provided data that contradict this view of carbon storage and exchange. Evidence has pointed towards climatically-triggered release of geologically-held carbon. The solid Earth contains vast reservoirs of carbon, so small changes in release have the potential to be important to the carbon budget. If release of geologic carbon is indeed sensitive to changes in the climate system, then this process may also be triggered under future climate scenarios. Constraining the potential impact of geologic carbon released in the past has the potential to provide critical new information in projecting future change. The interdisciplinary project team proposes an ambitious programme combining field work, geochemical analyses and integrated modelling to provide specific tests of the competing hypotheses that have been used to explain the rising CO2 levels at the end of the glacial period. We will particularly focus on using radiocarbon to help diagnose the distinct sources of carbon. New data will be based on geochemical analyses on precisely dated deep-sea coral skeletons collected from locations which are key to these tests. We will mount the first Remotely Operated Vehicle research expedition to the Eastern Equatorial Pacific dedicated to systematic collections of corals from depths where release of geological carbon is hypothesised to have occurred. Our second focus will be on the North Atlantic (samples in hand) where changes in ocean circulation and linkages to isolated basins may have played a key role in releasing carbon from the deep ocean. We will integrate these data with existing knowledge and use them in conjunction with climate modelling (including intermediate complexity models and state of the art General Circulation Models) to explore which processes are most important to the Earth system in a warming world. We will also be well placed to formulate a broad synthesis of the processes controlling the carbon cycle during the most pronounced rapid warming period of recent Earth history.