Ocean Heat Uptake of the Last Twenty Thousand Years

当地球接收到的能量与释放到太空的能量不平衡时，全球变冷或变暖就会发生。由于在任何给定的时间内大量的能量来来去去，这个预算中的不平衡很难被发现。海洋温度是地球能量收支变化的最可靠指标，因为地球主要将多余的能量储存在海洋中。此外，储存能量的增加导致海洋温度上升。海洋储存更多热量的速率是海洋热吸收率，这是衡量气候变化的关键指标。在过去的几十年里，海洋吸收热量的速度正在加快，但我们不知道地球过去是否有过这样的先例。这项研究将重建过去两万年的海洋热吸收，以便将正在发生的变化纳入背景。在过去的两万年里，气候发生了巨大的变化，包括向南延伸到现在美国大部分地区的大冰原的解体。这一时期也发生了大规模的变暖事件，海洋热吸收可以与现在相媲美。海洋热吸收的重建可以通过提高我们对气候如何响应大气二氧化碳变化的理解而造福社会。此外，该项目将量化深海和冰盖影响21世纪气候的潜力。该项目包括通过支持一名博士后研究人员和将方法纳入麻省理工学院/世卫组织联合项目研究生课程来培训下一代海洋科学家的计划。所有研究成果都将通过国家环境信息中心和GitHub的档案公开和免费获取。该项目旨在通过将古海洋温度数据同化到描述年代际、百年和千年气候变化时间尺度的海洋环流模型中，重建过去2万年的全球海洋热吸收。关键的方法学问题是，对海洋热吸收的古温度观测能有多好地反演？该方法将利用已知的气候幂律，即低频温度变化比高频温度变化包含更多的能量。这些幂律允许填补古数据中的空白，并进行统计上严格的不确定性分析。改进的海洋热吸收重建将解决由于观测系统的时间演变组成、1955年以前观测的稀疏性和深海的预期变率所造成的隐藏的系统误差。由此产生的海洋热吸收演变与代理和仪器数据相关联，包括最后一次消冰期、全新世中期气候最佳期、中世纪气候异常期、小冰期和工业时代，这有助于比较过去和现在的海洋条件。对末次冰期极大期（2万年前）以来海洋热吸收的研究将使我们对变暖速率有一个更广泛的认识，新的观测证据表明，在某些时间间隔内，去冰期海洋热吸收与现在相当。该项目通过对过去2万年的海洋热吸收进行动态信息重建，包括不确定性估计，使更广泛的科学界受益。海洋热吸收是诊断平衡和瞬态气候敏感性所需的量，这是预测地表温度的两个关键气候指标。对不确定性进行量化是对未来气候情景进行更准确的风险评估的必要步骤，并与政策决定有关。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Global cooling or warming occurs when Earth has an imbalance between the energy received versus that released to space. Imbalances in this budget are difficult to detect due to the large amounts of energy that come and go at any given time. Ocean temperature is the most reliable indicator of changes in the planet’s energy budget because Earth primarily stores excess energy in the ocean. Furthermore, increases in stored energy cause ocean temperature to rise. The rate that the ocean stores more heat is the ocean heat uptake, a key measure of climate variability. Ocean heat uptake is accelerating over the last few decades, but it is unknown whether there is any precedent in Earth’s past. This study will reconstruct ocean heat uptake for the last twenty thousand years in order to put ongoing changes into context. The last twenty thousand years included enormous changes in climate, including the disintegration of large ice sheets that extended southward into much of the current United States. This time period also had large warming events with ocean heat uptake that could have rivaled the present day. The reconstruction of ocean heat uptake may benefit society by improving our understanding of how climate responds to changes in atmospheric carbon dioxide. In addition, this project will quantify the potential for the deep ocean and ice sheets to affect the climate of the 21st century. The project includes plans to train the next generation of ocean scientists through support for a postdoctoral researcher and integration of methods into a MIT/WHOI Joint Program graduatecourse. All research products will be publicly and freely accessible through archives at the National Centers for Environmental Information and GitHub.This project aims to reconstruct global ocean heat uptake for the last 20,000 years by assimilating paleoceanographic temperature data into an ocean circulation model that describes decadal, centennial, and millennial timescales of climate variability. The key methodological question is, how well can paleo-temperature observations be inverted for ocean heat uptake? The method will utilize known climate power laws that dictate that low-frequency temperature variations contain more energy than high frequencies. These power laws permit the infilling of gaps in paleo-data and a statistically-rigorous uncertainty analysis. Improved reconstructions of ocean heat uptake will address the hidden systematic errors due to the time-evolving composition of the observing system, the sparsity of observations before 1955, and the expected variability of the deep ocean. The resulting ocean heat uptake evolution connects through periods with both proxy and instrumental data, including the last deglaciation, the mid-Holocene climate optimum, the Medieval Climate Anomaly, the Little Ice Age, and the industrial era, which facilitates comparison between past and present ocean conditions. Examination of ocean heat uptake since the Last Glacial Maximum (20,000 years ago) will allow a wide perspective on warming rates, and new observational evidence suggests that deglacial ocean heat uptake rivaled the present day during some time intervals. This project benefits the broader science community by producing dynamically-informed reconstructions of ocean heat uptake for the last 20,000 years, including uncertainty estimates. Ocean heat uptake is a required quantity to diagnose equilibrium and transient climate sensitivity, two key climate metrics for predicting surface temperature. Quantification of uncertainties is a necessary step toward more accurate risk assessments of future climate scenarios and has relevance to policy decisions.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.