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

	FIGURE 1: Deep ocean drilling collects cores, such as the one above (A), which contains organic carbon-rich black mudstones deposited in the tropical North Atlantic Ocean ~ 95 million years ago. In the laboratory, the organic molecules within freeze-dried mudstone samples (B) are analyzed to obtain an estimate of the amount of carbon dioxide dissolved in ocean waters 95 million years ago. The lighter colored, carbonate fraction of the same sediment sample (C) contains abundant calcite fossils (D) of microscopic organisms known as foraminifera. The foraminiferal calcite is analyzed to provide an estimate of upper ocean water temperatures in the tropical North Atlantic 95 million years ago. When pCO2 and temperature are estimated from the same sediment sample, we can use these data to evaluate climate model performance.
	FIGURE 2: The colored curves in the graph below indicate the warming resulting from a quadrupling of atmospheric CO2 concentration in 6 coupled ocean-atmosphere models used in the IPCC 4th Assessment Report. The warming is calculated as the zonal annual mean surface temperature for a "4x CO2" case minus that for a "1x CO2" case. The red bars indicate the "target" warming needed to reproduce minimum and maximum upper ocean temperatures inferred for the late Turonian, a Cretaceous interval (~ 91 to 89.5 million years ago) when the CO2 concentration inferred from carbon molecules was between 1600 and 2100 ppm, or between 4.2 and 5.5 times the modern CO2 concentration of 380 ppm.

Karen L. Bice, Geology & Geophysics

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.