The burning of fossil fuels is injecting CO2 into the biosphere (atmosphere/ocean/biomass) at rates that may meet or exceed any that have occurred in Earth history. There is no doubt that this is a significant perturbation to the carbon cycle. The carbon cycle has already responded: about half of the carbon emitted has been taken up by the ocean (demonstrably driving down ocean pH and carbonate saturation states) and terrestrial biota. Projections into the future demonstrate that the ability of these sinks to accommodate fossil fuel CO2 will be reduced, leading to an increase in the airborne fraction of CO2.
Fossil fuel burning is an external forcing of the carbon cycle. In contrast, the carbon cycle variations on, for example, glacial/interglacial timescales are internal responses (feedbacks) to externally driven variations in insolation (Milankovitch cycles). In feedback loops, cause and effect become meaningless: causes become effects and vice versa. Thus, the search for leads and lags between temperature and CO2 in ice-core records is of limited utility in determining the extent to which CO2 is a “climate driver.” More promising is the investigation of the Paleocene-Eocene Thermal Maximum (PETM) “supergreenhouse” event 55 million years ago. The combination of a sizeable negative carbon isotope excursion, marked global warming with polar amplification, and extensive seafloor carbonate dissolution provides a strong constraint on the source, magnitude, and rate of carbon addition that drove this climate perturbation. Based on numerical modeling results using an Earth system model of intermediate complexity, the source was fossil carbon, emitted over a few thousand years, with a total magnitude of ~7000 Pg. In other words, the rate of CO2 emission was somewhat slower, but the magnitude somewhat larger, than the projected rate of fossil-fuel burning under “business-as-usual” scenarios. Sensitivity analysis reveals that the rate of addition is critical to the oceanic response: slow additions (less than 1 Pg carbon per year) lead to deep-ocean acidification, but surface waters remain supersaturated with respect to CaCO3. Faster additions (including current and projected fossil-fuel burning rates) lead to surface-water acidification as well. Thus, the rather muted biotic response to the PETM (benthic foraminiferal extinctions, ecosystem migrations on land) may underestimate the biotic response to future fossil-fuel burning, both on land and in the ocean.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009