Reconstructing Earth Energy Imbalance

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The net energy balance at the top of Earth’s atmosphere is critical for many important problems in climate science (e.g., climate sensitivity to changing greenhouse gas concentrations and modes of interaction between the atmosphere and ocean). Despite this central role, Earth’s energy budget has only been well observed for about the past 20 years. This short period of direct observation leaves a limited sample for evaluating important questions about climate variability, especially for distinguishing between variability that is externally forced versus internal to the climate system. While these questions can be approached using climate models, paleoclimate proxies provide the only encoding of climate information before the instrumental era, and thus are essential for quantifying climate variability, and particularly Earth’s energy imbalance on interannual, decadal and longer timescales. This project proposes to reconstruct Earth’s energy budget for the past 1000 years using paleoclimate data assimilation (PDA) method. This approach combines information from paleoclimate proxies with the physical constraints of climate models to reconstruct climate fields that are not directly observed by the proxies. The proposed research takes advantages of recent numerical simulations and preliminary-proof-of concept calculations to achieve this reconstruction.Specifically, the researchers will use an offline PDA, where the stationary statistics of long climate model simulations are used to supply physical constraints, and online PDA, where computationally efficient emulators of the climate models add dynamical “memory” to the reconstructions from proxies at earlier times. The researchers aim to construct outgoing shortwave and longwave radiation at the top of the atmosphere, and storage in terms of ocean heat content. The reconstructions will then be used to address a hypothesis on the relationship between spatial patterns of sea-surface temperature variability and energy imbalance. This project will provide a proxy-constrained estimate of Earth’s energy imbalance, both in ocean heat content and in the top-of-atmosphere radiative imbalance. A hierarchy of validation experiments, using satellite measurements, instrumental measurements, and historical simulations will provide confidence bounds on these estimates. The proposed research directly addresses important open problems in climate science, including pre-industrial energy imbalance needed for climate sensitivity, and the contribution of patterns of Pacific temperature variability to the planetary energy imbalance. The potential Broader Impacts include a gridded data product synthesizing paleoclimate proxy and model data for the top-of-atmosphere energy imbalance. Such a gridded millennial scale reconstruction will be made publicly available to the wide paleoclimate community for a widespread climate research, including detection and attribution-studies. This work will support the scientific training and professional development of one graduate student at the University of Washington.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.