One of the most important validations for a state-of-art Earth System Model (ESM) with respect to climate changes is the simulation of the climate evolution and abrupt climate change events since the Last Glacial Maximum (LGM), such as the Heinrich Event 1 (H1), the Bølling-Allerød warming (BA), the Younger Dryas cooling (YD) and the abrupt onsets and collapses of the global monsoons and associated ecosystems. However, one great challenge for model validation is that ESMs usually do not directly simulate geochemical variables that can be compared directly with past proxy records. Here, we propose to meet this challenge by developing the simulation capability of major isotopes in a state-of-art ESM, the Community Earth System Model (CESM), and to make direct model-data comparison by comparing the model directly against proxy climate records, with the focus on the transient climate evolution and abrupt climate changes. Specifically, the major objectives are:

1. To enhance the CESM (called iCESM herein to denote the isotope-enabled version) with the capability of simulating key isotopes and geotracers, notably δ18O, δD, Pa/Th, δ14C, and δ13C
2. To perform transient simulations (iTRACE21) to test the iCESM directly against proxy records of the last 21,000 years, especially on major abrupt events, such as the H1, BA, YD, 8.2 ka event and onset/collapse of monsoon-ecosystem systems
3. To understand the mechanisms for the abrupt changes in the CESM and to perform direct model data comparisons of isotopic geotracer 'proxy' records.

Our work will significantly enhance the CESM with the state-of-art isotope tracer capability such that it will be among the first such state-of-art ESMs in the world. Our transient experiments, iTRACE21, represent the first set of transient simulations of the last de-glaciation of physical as well as isotope variables in a state-of-art ESM. These experiments will provide an unprecedented opportunity for model validation through direct model-data comparison of climate evolution and abrupt changes in terms of proxy variables. Thus, this study will help us greatly enhance the credibility of CESM and its projections of future climate changes. This proposal will be lead by Professor Zhengyu Liu, University of Wisconsin-Madison and Dr. Bette Otto-Bliesner, NCAR. NCAR's role will be to provide the expertise in climate modeling of past atmospheric dynamics, atmospheric tracers, and ocean geochemistry and will include the expertise of co-PIs Andrew Gettelman and Synte Peacock. A project scientist will be charged with diagnosing and validating the science of the isotope-enabled CESM simulations. NCAR will also be responsible for the software engineering development in CESM, which will be lead by co-PI Marianna Vertenstein. The University of Wisconsin PI will provide the expertise in climate modeling of past ocean dynamics. Two graduate students at the University of Wisconsin are expected to be involved in the project.

This study will help validate the state-of-art CESM for its capability to simulate the most dramatic abrupt climate change since the LGM. It will significantly improve our understanding of the mechanisms of abrupt climate change not only for the past and present, but also for the future. It will greatly advance our understanding of climate feedbacks. It will help us evaluate the projected climate change and potential abrupt changes in the future. It will provide an unprecedented state-of-art model simulation of continuous climate evolution since the LGM for direct data-model comparison for the entire climate community.