The Long-Term Carbon Cycle, Phanerozoic CO2 and Paleoclimate

Educators and policymakers often focus solely on recent climate history and computer projections of the future to establish the relationship between atmospheric carbon dioxide and global warming. In contrast, geoscientists can correlate atmospheric CO2 and average Earth temperature backwards through geologic time. By adjusting a climate sensitivity parameter that is embedded within a model of the Phanerozoic long-term carbon-cycle, we have been able to approximately match proxy estimates of paleo-CO2. As a result we have independently confirmed the predicted sensitivity of Earth's climate to a doubling of CO2 (~3°C).

The long-term carbon cycle operates over thousands to billions of years, and involves transfer between rocks and the superficial reservoirs of the oceans, atmosphere, biosphere and soils. A computer model GEOCARB has been constructed to quantify carbon transfer rates over geological time within the long-term cycle. Major processes considered are CO2 uptake via the weathering of Ca and Mg silicates, the deposition and burial of carbon as sedimentary Ca and Mg carbonates, basalt seawater reaction, volcanic, metamorphic and diagenetic degassing of CO2, and the oxidative weathering, thermal degassing and sedimentary burial of organic matter. Factors affecting weathering rate include tectonic uplift and physical erosion, the evolution of land plants, terrestrial volcanism, and changes in climate due to a combination of continental drift, changing CO2 (greenhouse feedback) and solar evolution. Organic carbon cycling is tracked through the use of carbon isotopic data and degassing is tied to seafloor spreading rate and the formation of deep-sea carbonate sediments.

Noteworthy to modern global warming is that the burning of fossil fuels is a large acceleration of the rate of oxidation of ancient sedimentary organic matter which in the long-term carbon cycle occurs hundreds of times more slowly by weathering or thermal decomposition.