Geological Factors and the Evolution of the Greenhouse Effect

For much of Earth history, atmospheric CO₂ is considered to have been the primary climate driver due to the greenhouse effect. Yet, there are times in Earth history when this relationship does not hold (e.g., Veizer et al., 2000). The breakdown of the CO₂-climate relationship on geological timescales begs the question: what is the linkage between geological factors and the greenhouse effect? The influence of geological factors on the greenhouse effect will be tested using a series of paleoclimate experiments simulated by the Fast Ocean-Atmosphere Model (FOAM), a coupled ocean-atmosphere general circulation model. These experiments will specifically test the role of continental area and distribution, orography, and atmospheric CO₂ level on the climate factors (e.g., cloud fraction and type, specific humidity, incoming and outgoing radiation) that control the greenhouse effect. Initial model results indicate that the greenhouse effect is sensitive to climatic differences arising from paleogeographic evolution. The global-average greenhouse effect is approximately 10% greater in a Cretaceous experiment than in a Triassic experiment with similar atmospheric CO₂ levels. This increase in greenhouse effect resulting from differences in paleogeography is greater than that resulting from a fourfold increase in Triassic atmospheric CO₂ levels. Preliminary model results indicate that factors other than CO₂ may have been important in the evolution of the Earth's greenhouse effect.