Collaborative Research: Nonlinearities in the Arctic climate system during the Holocene

该奖项是根据2009年美国复苏和再投资法案（公法111-5）资助的。北极气候系统最近发生的快速变化可以与气候模式的输出进行比较，以提高对非线性系统变化过程的理解。本文研究了全新世热极大期（HTM）和新冰期之间的过渡，以及随后在全新世晚期发生的阶梯状变化。HTM之后的千年尺度降温趋势与地球缓慢变化导致的北半球夏季日照减少相吻合。s轨道。尽管有接近线性的强迫作用，但从热热期到小冰期（公元1500-1900年）的过渡既不是渐进的，也不是均匀的。为了了解反馈和扰动如何导致快速变化，将使用地理分布的代用气候记录网络来研究变化的时空格局，并量化这些转变期间的变化幅度。这个合作项目的研究人员将利用湖泊沉积物生成过去8000年的13个新的高分辨率代用气候记录。研究地点形成了两个重点区域（东白令陆桥和西北大西洋），它们通常与北极涛动（AO）地表温度表达的节点一致。这项工作将使过去两千年的高分辨率湖泊记录的数量增加近一倍，并将产生第一批捕捉HTM的高分辨率记录。在HTM期间，北冰洋夏季海冰覆盖可能是目前间冰期最小的；当然，它的范围比过去100年中的任何时候都要小，因此提供了一个机会来研究类似于下一个世纪预测的温暖时期。这项研究的重点是湖泊，因为湖泊是代理气候记录的最广泛的来源，这些记录持续地延伸到冰期后。由于气候变化在北极被放大，保存在北极湖泊沉积物中的气候信号应该比其他地方更强。本项目生成的代理记录将使用传统的和新兴的技术来记录环境突变的时空模式，并得出过去夏季温度和水文气候变量的定量估计。大多数湖泊以前都有过取样，显示出产生高质量代理记录的潜力。其中五个湖泊含有层状沉积物，每年都有分解记录；其他地区在气候变迁的次年代际分辨率上具有较高的沉积速率（0.5毫米/年）。对古气候重建的信心将通过多代理方法和在每个重点区域复制湖泊记录来增强，这些记录将用于区分盆地尺度阈值和区域尺度气候变化。该项目建立在正在进行的使用NCAR？美国的气候系统模式（CCSM3）研究北极系统对火山活动和太阳变率的敏感性。为这项研究提出的一种新的数据模型比较将检验从代理记录重建的北极系统在过去8ka期间最显著的变化是否可以用与长期火山活动相一致的系统成分条件的合理组合来解释。与NCAR合作进行的实验将集中于北极系统的要素（例如，AO和海冰范围），这些要素参与了突变，并可能在未来引发非线性变化。

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). Abstract Rapid changes in the arctic climate system that occurred in the relatively recent past can be compared with the output of climate models to improve the understanding of the processes responsible for nonlinear system change. This study focuses on the transition between the Holocene thermal maximum (HTM) and the onset of Neoglaciation, and on the step-like changes that occurred subsequently during the late Holocene. The millennial-scale cooling trend that followed the HTM coincides with the decrease in Northern Hemisphere summer insolation driven by slow changes in Earth?s orbit. Despite the nearly linear forcing, the transition from the HTM to the Little Ice Age (1500-1900 AD) was neither gradual nor uniform. To understand how feedbacks and perturbations result in rapid changes, a geographically distributed network of proxy climate records will be used to study the spatial and temporal patterns of change, and to quantify the magnitude of change during these transitions. The researchers of this collaborative project will use lacustrine sediments to produce 13 new high-resolution proxy climate records of the past 8000 years. The study sites form two focus regions (eastern Beringia and the NW Atlantic) that generally coincide with the nodes of the surface temperature expression of the Arctic Oscillation (AO). This effort will nearly double the number of high-resolution lacustrine records that extend through the last two millennia, and will generate some of the first high resolution records that capture the HTM. During the HTM, summer sea-ice cover over the Arctic Ocean was likely the smallest of the present interglacial period; certainly it was less extensive than at any time in the past 100 years, and therefore affords an opportunity to investigate a period of warmth similar to what is projected during the coming century. This study focuses on lakes because lakes are the most widely distributed sources of proxy climate records that consistently extend through the post-glacial interval. Because climate change is amplified in the Arctic, the climate signal preserved in arctic lake sediments should be stronger than elsewhere. The proxy records generated in this project will use conventional and newly emerging techniques to document the spatio-temporal patterns of abrupt environmental changes, and to derive quantitative estimates of past summer temperature and hydroclimate variables. Most lakes have been cored previously and show potential for generating high-quality proxy records. Five of the lakes contain laminated sediment with annually resolved records; others have high sedimentation rates (0.5 mm yr-1) for sub-decadal resolution across the climate transitions. Confidence in the paleoclimate reconstructions will be bolstered by a multi-proxy approach, and by replicate lake records in each of the focus regions that will be used to distinguish basin-scale thresholds from regional-scale climate shifts. This project builds on on-going climate-modeling experiments that use NCAR?s Climate System Model (CCSM3) to study the sensitivities of the arctic system to volcanism and solar variability. A new data-model comparison proposed for this study will test whether the most prominent changes in the arctic system during the past 8 ka, as reconstructed from the proxy records, can be explained by a plausible combination of system-component conditions coincident with prolonged volcanism. The experiments, conducted with NCAR collaborators, will focus on the elements of the Arctic system (e.g., AO and extent of sea ice) that participate in abrupt transitions, and that might elicit nonlinear changes in the future.