Collaborative Research: P2C2--Fingerprinting Forced and Unforced Variability in Holocene Paleoclimate Record

区分自然气候变率和强迫气候变率对解释过去的变化和对未来作出准确预测都具有重大意义。气候模式和古气候观测在年代际和更长时间尺度上的变率大小不一致。这给平衡气候敏感性（由于大气二氧化碳浓度加倍而导致的温度升高）的估计和模式对长期气候变率的预估造成了不确定性。调和这一矛盾的关键在于理解古气候记录中强迫变率和非强迫变率的相对重要性。如果古气候重建是正确的，那么必须确定缺失的物理现象，无论是对外部强迫（例如火山喷发、太阳变率、温室气体和气溶胶浓度的变化）缺乏敏感性，还是对内部变率（气候系统的内部机制）的描述不足。本项目基于全新世古气候记录的不同时空统计，提出了一个强迫和非强迫气候变率指纹图谱框架。具体而言，研究人员将利用全球环流模式（GCMs）模拟和基本物理原理相结合，开发强迫和非强迫气候变率的时空指纹，并将其应用于全新世气候变率的多代理古气候记录。研究人员将从gcm开始构建指纹，它提供了气候系统的完整代表，并有能力分解强迫和非强迫变率。代理系统模型（生物或化学机制过程的正演模型，通过该模型将气候变率记录在诸如珊瑚或冰芯等气候档案中）将用于评估这些指纹如何在稀疏、嘈杂且可能改变气候信号统计的代理记录中表达。鉴于有充分证据表明gcm在模拟观测值和代用物的低频变率方面存在不足，基于gcm的指纹图谱将被物理统计随机模型补充。潜在的更广泛的影响包括：(1)通过建立气候系统中控制温度、水文和水文气候代用物响应的单一连贯的因子图，加深对全新世气候变率和模式预估的理解；(2)改进古气候记录低频气候变率的物理解释。这项研究将有可能阐明全新世气候变率的性质和来源，以及相关的物理机制。这对解释最近的观测记录以及对气候预测的推断具有重大意义。本项目开发的统计框架、模型模拟和简单的动力学模型将作为跨地球科学和所有地质时间尺度的气候相关研究的概念框架。本项目将为两名本科生和两名研究生提供科学培训和专业发展。本科生将参与研究，并通过研究人员的记录来证明其出版。模型模拟的输出、使用公开可用的社区地球系统模型（CESM）设置模拟的代码，以及使用公开可用的数据和模型输出再现诊断的代码都将公开提供。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。

Distinguishing between natural and forced climate variability has large implications for both interpreting past variations and for making accurate predictions of the future. Climate models and paleoclimate observations disagree about the magnitude of variability at interdecadal-and-longer time scales. This creates uncertainty in estimates of equilibrium climate sensitivity (temperature increase that result from a doubling of atmospheric carbon dioxide concentration) and in model projections of long-term climate variability. The key to reconciling this contradiction lies in understanding the relative importance of forced and unforced variability in the paleoclimate records. If the paleoclimate reconstructions are correct, then the missing physics must be identified, whether it be a lack of sensitivity to external forcing (e.g. volcanic eruption, solar variability, changing concentrations of greenhouse gases and aerosols), or an underrepresentation of internal variability (internal mechanisms within the climate system). This project proposes a framework for fingerprinting forced and unforced climate variability in Holocene paleoclimate records, based on their different spatial and temporal statistics. Specifically, the researchers will develop spatio-temporal fingerprints for forced and unforced climate variability, using a combination of Global Circulation Models (GCMs) simulations and fundamental physical principles, and apply them to the multiproxy paleoclimate record of Holocene climate variability. The researchers will build the fingerprints starting from GCMs, which provide a complete representation of the climate system and have the ability to disaggregate forced and unforced variability. The proxy-system models (forward modelling of mechanistic processes either biological or chemical, by which climate variability is recorded in a climate archive such as corals or ice cores) will be used to evaluate how these fingerprints are expressed in proxy records that are sparse, noisy, and can alter the statistics of climate signals. Given well-documented deficiencies in the ability of GCMs to simulate low-frequency variability in both observations and proxies, the GCM-based fingerprints will be complemented with physical-statistical stochastic models.The potential Broader Impacts include a greater understanding of Holocene climate variability and modeled projections by (1) building a single coherent picture of the factors controlling the response of temperature, hydrology, and hydroclimate proxies in the climate system; and (2) improving the physical interpretation of the low-frequency climate variability recorded in paleoclimate records. This research will potentially illuminate both the nature and sources of climate variability over the Holocene, and the physical mechanisms responsible. This has large implications for interpreting the recent observational record, and for inferences of climate projections. The statistical frameworks, model simulations, and simple dynamical models developed in this project will serve as a conceptual framework for climate-related research across the Earth Sciences and over all geologic timescales.The project will provide scientific training and professional development for two undergraduate and two graduate students. The undergraduate students will be involved in the research and its publication as demonstrated by the track record of the researchers. Output from the model simulations, Code for setting up the simulations with the publicly available Community Earth System Model (CESM), and Code for reproducing the diagnostics using the publicly available data and model output will be all made publicly available.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.