RATES OF CARBON EMISSION PAST, PRESENT, AND FUTURE: NO PALEO-ANALOGS FOR THE ANTHROPOCENE?

One of the goals of paleoenvironmental reconstruction is to explore the geologic past, that is, the history of the ailing patient, Earth, for analogs to what we project for the Anthropocene. Of particular interest are intervals in Earth history when we suspect that carbon (as carbon dioxide or methane) was added to the atmosphere from a source external to the biosphere, such as peat and coal, methane hydrates, or the mantle, at rates that substantially exceeded the background rates associated with a balanced global carbon cycle. We explore two intervals of Earth history for which the carbon isotope record indicates such a perturbation: the Paleocene-Eocene Thermal Maximum and the end-Permian mass extinction. Inversion of these records, using an Earth system model of intermediate complexity, allows us to assess the emission rates necessary to explain not only the isotope excursion but other geologic indicators of environmental change (e.g., temperature, carbonate compensation depth, and ocean saturation state). In both cases, using published isotope records and the best available geochronologies, the necessary peak carbon dioxide emission rates are on the order of 1-2 Pg C per year; if methane emissions are invoked, the rates are smaller. Current rates of carbon emission are several times larger than this, and projected to increase markedly in the coming century. With the caveat that the duration of the carbon isotope excursions is perhaps shorter than indicated by the geochronology, we conclude that no good analog for Anthropocene CO₂-driven climate change has been identified from Earth history.