Royer et al. (2004) illustrated the strength of model-data comparisons in their analysis of the role of CO2 in driving climate during the past 600 Ma. How CO2 functions as a greenhouse gas is now well established through observations, experiments, and modeling of the present day atmosphere as well as the analysis of climatic proxies and CO2 in the Quaternary geohistorical record. However, substantiating CO2’s role in the regulation of pre-Quaternary climate is difficult because of the lack of direct information on the composition of the atmosphere through most of geologic time. Geochemical models, based primarily on sedimentary weathering rates, have been devised to predict the evolution of CO2 through geologic time (e.g., Berner, 2004; Berner and Kothavala, 2001). These predictions can be evaluated by comparing the model results to estimates of past CO2 based on geochemical and paleobotanical indicators in the rock record. Such indicators are said to be “proxy indicators,” in that they substitute for direct measurements. The development and calibration of these and other proxy indicators depends on observations and experiments with modern systems.
Royer et al.’s (2004) review of proxy records indicates a consistent relationship between various CO2 indicators, geologic evidence of glaciations, and CO2 predictions from geochemical models. Accordingly, model-proxy comparisons support the hypothesis that CO2 has acted as the primary driver of climate during the past 600 Ma, identifies key time intervals in which proxy indicators and model predictions do not agree, and evaluates the extent to which proxy indicators differ.