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。北极气候系统的快速变化发生在相对较近的过去，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。本研究的重点是之间的过渡全新世热最大值（HTM）和新冰川作用的开始，并在随后发生的全新世晚期的阶梯状的变化。千年尺度的冷却趋势，其次是高温天气与减少北方半球夏季日照驱动地球的缓慢变化？s轨道。尽管有接近线性的强迫，从HTM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，将使用地理上分布的代用气候记录网络来研究变化的空间和时间模式，并量化这些过渡期间的变化幅度。该合作项目的研究人员将利用湖泊沉积物产生过去8000年的13个新的高分辨率代用气候记录。研究地点形成两个重点区域（东白令海和西北大西洋），通常与北极涛动（AO）的表面温度表达的节点相吻合。这一努力将使过去两千年的高分辨率湖泊记录数量增加近一倍，并将产生一些捕获HTM的第一批高分辨率记录。在热季期间，北冰洋上的夏季海冰覆盖面积可能是目前间冰期最小的;当然，它比过去100年的任何时候都要小，因此提供了一个机会来研究一个类似于未来世纪预测的温暖时期。这项研究的重点是湖泊，因为湖泊是最广泛分布的代理气候记录的来源，一直延伸到冰后期。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。该项目产生的代用记录将使用传统和新兴技术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。大多数湖泊以前已经取过芯，显示出产生高质量代用记录的潜力。五个湖泊含有层状沉积物，每年解决的记录，其他人有高沉积速率（0.5毫米yr-1）的子十年分辨率的气候转变。在古气候重建的信心将得到支持的多代理的方法，并复制湖泊记录在每个重点地区，将被用来区分流域规模的阈值从区域规模的气候变化。该项目建立在正在进行的气候模拟实验，使用NCAR？的气候系统模式（CCSM 3），研究北极系统的火山活动和太阳变化的敏感性。一个新的数据模型的比较，这项研究将测试是否在北极系统最突出的变化，在过去的8万年，重建从代理记录，可以解释一个合理的组合系统组件的条件与长期的火山活动相一致。与NCAR合作者进行的实验将侧重于北极系统的要素（例如，AO和海冰的范围），参与突然转变，并可能引起未来的非线性变化。

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.