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A Test of CO2 Sensitivity in a Coupled Atmosphere-Ocean Model with Implications for Future Climate Change Predictions

OCCI Funded Project: 2006

Proposed Research

Concerns over the magnitude of future global warming cause us to look back in Earth history to periods when atmospheric carbon dioxide concentrations were higher than present and the surface temperatures were much warmer than present. Using data from these past warm periods and computer climate models of the Earth’s climate system, we can test the “CO2 sensitivity” of climate models used to predict future climates. The most important question here is, “Does a realistic amount of warming occur in the models when CO2 is increased?”

In the paleoclimate model-data comparison approach, past surface temperatures and pCO2 are estimated from well-preserved geochemical proxies (see below, Figure 1). The inferred CO2 concentration is specified to the model, along with other realistic boundary conditions for the study interval, and the model is run to equilibrium. If the model CO2 sensitivity is adequate, then model-predicted temperatures should match the temperatures inferred from data to within the uncertainty in the data technique.

Previous such studies (Bice et al., 2006) using an atmospheric climate model with no dynamical ocean suggest that the model sensitivity to CO2 is far too low, grossly underestimating the amount of warming that will result with a similar increase in CO2 in the future (see below, Figure 2).

In this project, we are performing tests of model CO2 sensitivity using a global general circulation model with a fully dynamical ocean component, the Max Planck Institute for Meteorology coupled model ECHAM5/MPI-OM. We are assessing the MPI model CO2 sensitivity and experimenting with modifications to model parameterizations and resolution, following on leads that suggest changes that have significant positive impacts on the model CO2 sensitivity.

One possible cause of the discrepancy shown above between temperatures predicted by the models and temperatures inferred from geochemical data is that the model CO2 sensitivity is inadequate, thereby causing the model to underestimate the amount of past (and future) warmth.

IPCC model data output are archived by the Program for Climate Model Diagnosis and Intercomparison (PCMDI) at http://www-pcmdi.llnl.gov/ipcc/about_ipcc.php .

Cretaceous temperature estimates are from Bice et al., 2003 (for the southern hemisphere), Bice et al., 2006 (the tropics) and Jenkyns et al., 2004 (the Arctic ).

References
Bice, K. L., B. T. Huber, and R. D. Norris, 2003, Extreme polar warmth during the Cretaceous greenhouse?: Paradox of the Late Turonian ∂18O record at DSDP Site 511, Paleoceanography, 18, doi: 10.1029/2002PA000848.
Bice, K. L., D. Birgel, P. A. Meyers, K. A. Dahl, K. Hinrichs, and R. D. Norris (2006), A multiple proxy and model study of Cretaceous upper ocean temperatures and atmospheric CO2 concentrations, Paleoceanography, 21, PA2002, doi:10.1029/2005PA001203. Available online through the Woods Hole Open Access Server: https://darchive.mblwhoilibrary.org/handle/1912/846
Jenkyns, H. C., A. Forster, S. Schouten, and J. S. Damste, High temperatures in the Late Cretaceous Arctic Ocean, Nature, 432, 888-892, 2004.

Originally published: January 1, 2006