G. Edward Birchfield Professor emeritus Ph.D., The University of Chicago, 1962 847-491-7460 edward@earth.northwestern.edu

Research Interests
The climate of the Tertiary-Pleistocene: a search for the causes of climate change using climate models and geophysical fluid dynamics.

Research Projects

The focus of my research is on causes of major climate changes in the Pleistocene and Tertiary. Based on several sources of paleoclimatic evidence there are at least three important time scales of interest. The first, is associated with effects of tectonic changes on the Earth's climate, for example, the raising of the Tibetan Plateau and the Western Cordillera.

Climate changes on the second time scale of 10⁴-10⁵ years are framed in the astronomical theory of the ice ages (Croll, Milankovitch). This theory asserts that insolation anomalies resulting from the perturbation of the Earth's orbit by the gravitational attraction of the other planets are responsible for the alternating glacial and interglacial climate experienced in the Pleistocene. Strong support for this theory has come from the high correlation of the dominant periods and phases in such insolation changes with the periods and phases of proxy continental ice volume records deduced from ¹⁸O variations in foraminifera shells in deep sea sediment cores.

Evidence for climatic change on the time scale of 10²-10³ years has come from analyses of ice cores from the polar ice caps. These data suggest that in the Pleistocene there were many transitions between full glacial and a less glacial- yet not interglacial- climate. These oscillations, superimposed on the longer trends of the Pleistocene, highlight the potential instability of the climate system and suggest that the climate of the Earth may exist in more than one quasi-equilibrium state. If the climate system potentially can have multi-equilibria, relatively slow processes, such as tectonic changes, may be responsible for rapid changes in climate.

We address these factors using computer models of components of the climate system. These models are based on careful parameterization of important physical processes in each component, such as the physics of the continental ice sheets, interaction between the ice sheets and the lithosphere-asthenosphere, the interaction of the ice sheets with the atmosphere and the ocean, and the interaction of the atmosphere and the ocean themselves. These model studies investigate certain components or processes, one by one, in order to assess their potential role in the overall climatic response. The procedure is to systematically perturb a process in the model and to examine the sensitivity of the model response to such perturbations. Such experiments give qualitative insight into interactions between components and lead to a better understanding of potential causes of climate change. The models can also suggest experiments for more sophisticated models or point to critical observations needed from the paleoclimatic record.

Bibliography 1990 to present

Birchfield, G.E., M. Wyant and H. Wang, A coupled ocean-atmosphere box model of the Atlantic Ocean: a bimodal climate response, J. Marine Systems, 1, 197-208, 1990.

Birchfield, G.E., H. Wang and M. Wyant, A bimodal climate response controlled by water vapor transport in a coupled ocean-atmosphere box model, Paleoceanography, 5, 383-395, 1990.

Birchfield, G.E., and W.S. Broecker, A salt oscillator in the glacial ocean? Part II: A 'scale analysis' model, Paleoceanography, 5, 835-843, 1990.

Wang, H., G.E. Birchfield, An energy-salinity balance climate mdoel: water vapor transport as a cause of changes in the global thermohaline circulation, J. Geophys. Res., 97, 1992, 2335-2346.

Birchfield, G.E., Comments on "Two stable equilibria of a coupled ocean-atmosphere model", by S. Manabe and R.J. Stouffer, J. Climate, 6, 1992, 175-177.

Birchfield, G.E. and M. Ghil, Climate evolution in the Pliocene-Pleistocene as seen in deep sea 18O records and in simulations: internal variability versus orbital forcing, J. Geophys.Res., 98, 1993, 10385-10399.

Wang, H., G.E. Birchfield, and J. Rich, Hydrological cycle scenarios, deep ocean circulation and century/millenium climate change: a simulation study using an ocean-atmosphere ice sheet model, In: Ice in the Climate System, edited by W. Richard Peltie, NATO-ARW Volume, Springer-Verlag, 1993, 237-255

Birchfield, G.E. and H. Wang, Century/Millennium oscillations of the ocean-atmosphere and continental ice sheets, J. Geophys. Res., 99, 1994, 12459-12470.