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)提供资金的。摘要北极气候系统在相对较近的过去发生的快速变化可以与气候模式的输出进行比较，以提高对造成非线性系统变化的过程的理解。本文主要研究了全新世最高温度(HTM)与新冰期开始之间的转换，以及随后在全新世晚期发生的阶梯式变化。在HTM之后的千年尺度的降温趋势与北半球夏季日照强度的减少是一致的，这是由于地球？S轨道的缓慢变化造成的。尽管有近乎线性的强迫，但从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理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。这个合作项目的研究人员将使用湖泊沉积物来制作过去8000年来13个新的高分辨率气候记录。研究地点形成了两个焦点区域(Beringia东部和大西洋西北部)，这两个区域与北极涛动(AO)地表温度表达的节点基本重合。这一努力将使跨越过去两千年的高分辨率湖泊记录的数量几乎翻一番，并将产生一些捕捉HTM的第一批高分辨率记录。在HTM期间，北冰洋上的夏季海冰覆盖面积可能是目前间冰期中最小的；当然，它比过去100年中的任何时候都要小，因此提供了一个机会来调查一个类似于下个世纪预测的暖期。这项研究的重点是湖泊，因为湖泊是分布最广的替代气候记录来源，这些记录一直延伸到冰川后的时间间隔。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。该项目产生的替代记录将使用传统和新出现的技术来记录环境突然变化的时空模式，并得出对过去夏季温度和水文气候变量的定量估计。大多数湖泊以前都曾进行过取心，并显示出产生高质量替代记录的潜力。其中五个湖泊含有年分辨率记录的层状沉积物；其他湖泊具有较高的沉积速率(0.5 mm yr-1)，在气候过渡期间具有亚十年分辨率。通过多代理方法以及复制每个重点区域的湖泊记录，将用来区分盆地规模的阈值和区域规模的气候变化，将增强人们对古气候重建的信心。这个项目建立在正在进行的气候模拟实验的基础上，这些实验使用美国国家航空航天局S气候系统模型(CCSM3)来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项新的数据模型比较将测试，根据代理记录重建的过去8 ka期间北极系统最显著的变化，是否可以用系统组件条件与持续的火山活动相吻合的合理组合来解释。这些实验是与NCAR的合作者一起进行的，将重点放在北极系统的要素(例如，北极涛动和海冰范围)上，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.