This research investigates the mechanisms of climate changes during the last deglaciation (21,000 to 11,000 years ago) using a state-of-the-art isotope-enabled Earth System Model (ESM). The major scientific questions to be addressed in this project are: (1) What are the relationships between the isotopic composition of water and climate changes over the globe? (2) How does the climate system respond to changing climate forcings of the last deglaciation, and what is the contribution from each forcing: insolation, atmospheric greenhouse gases, continental ice sheets, and meltwater? The simulations will allow the study of the continuous evolution and abrupt changes of climate from interannual to orbital time scales. The type of transient simulation marks a transformative breakthrough in model-data comparison, allowing for direct data and model time series comparisons, and will have a significant impact on future paleoclimate studies using both models and observations. The proposed simulations will have a ground breaking impact on research in the paleoclimate community, providing model data for a number of other community projects. It will lay a foundation for a systematic test of future generations of ESMs with coupled cryosphere and biogeochemical models.
Specifically, the new version of NCAR's community ESM (CESM), enabled with water isotope tracers, will be run in fully-coupled mode for the period 21,000 to 11,000 years ago. The main simulation will be forced by prescribed changes in insolation, atmospheric greenhouse gas concentrations, continental ice sheets, sea level, and meltwater, while sensitivity simulations will investigate the effect of the different forcings separately. This simulation will be run at a higher resolution (2 degrees in the atmosphere and 1 degree in the ocean) than the previous generation transient model run with CCSM3 (3 degrees). Furthermore, stable water isotope tracers have been included in the atmosphere, land-surface, ocean, and sea-ice models and will be tested first for the Last Glacial Maximum (LGM, 21,000 years ago) and present climate time slices before being run in transient mode for the last deglaciation. One central task of the proposed transient simulations is to explore a new paradigm of model-data comparison in close collaboration with the data community, focusing on direct time series comparisons of isotopes. The simulation of isotope time series will improve paleoclimate model-data comparison significantly, because it will reduce the uncertainties arising from both the inference of isotopes and the absolute chronology of the proxy records. The project will provide the basis for graduate research for students from the University of Wisconsin and Oregon State University.
