NSF-SNSF: Past warm climates: Reconciling simulations and proxies

Purdue University, West Lafayette IN

Investigators

Abstract

The Miocene period (23 to 5 million years ago) had a climate much warmer than modern, with hotter temperatures, less ice at the poles, higher sea level, and greenhouse gas concentrations in the range likely over the next century. Models have had difficulty in reproducing these warm climates. Proxies for past climate are frequently used for comparing against climate models, but their interpretation may be complicated by poor understanding and representation of what environmental conditions they record. The project will evaluate the ability of climate models to predict past climate changes properly. This is important because these are the same models used to project future climate change. The project will be an international collaboration between a US modeling expert and a Swss geochemist expert. Broader impacts include support for a female early-career scientist and outreach activities in both Switzerland and the US. Climate models have difficulty simulating the weak meridional temperature gradients reconstructed from proxy records of past “greenhouse” climates. These models also normally fail to produce the right globally averaged temperatures when driven by CO2 concentrations reconstructed from proxies. The problem this poses is that: (a) either climate models are missing key physical processes that might be important for future climate prediction or (b) proxy interpretations incorporate deep, persistent biases. The goal of this study is to reconcile climate models and proxy interpretations for an extreme and well-characterized greenhouse climate in the Miocene period (23 to 5 million years ago). The project will use several methods: traditional paleoclimate proxy reconstruction and paleoclimate modeling methods as well as developing novel methods using proxy system models (PSMs). First, a PSM will be developed for coccolithophore-based CO2 and temperature (alkenone and clumped isotope) proxies, and then evaluated using preindustrial climate simulations and corresponding global proxy core top archives. Second, the PSM will be applied to the Miocene simulations. An initial and preliminary evaluation of whether the PSM reduced or improves model-data mismatch compared to traditional methods will be carried out. Third, new orbitally resolved records of phytoplankton isotope fractionation will be built to improve estimates of atmospheric CO2 levels. Additionally, new records of surface ocean temperature in high latitudes and tropical regions will be reconstructed using traditional and PSM methods to test if meridional temperature gradients were consistently flat in the Miocene. Fourth, a series of new Miocene simulations will be conducted using the Community Earth System Model Version 2 with estimated atmospheric CO2 levels, Antarctic ice sheet states and two eccentricity configurations. Finally, the simulated climates will be compared with the proxy results generated by the proxy system model for these Miocene climates. The effort will test the hypothesis that the apparent discrepancy between models and data in the Miocene (and perhaps more generally) is a matter of developing a better understanding of the paleoclimate proxies, not a failure of the climate models. If this hypothesis is invalidated the finger points directly at the climate models, which will be a more solid basis for future work. The results and methods developed in this work will be disseminated broadly and outreach efforts at the youth and K-12 levels will be made both by the Swiss and US teams. This collaborative U.S.-Swiss project is supported by the U.S. National Science Foundation (NSF) and the Swiss National Science Foundation (SNSF), where NSF funds the U.S. investigator and SNSF funds the partners in Switzerland. 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.