A Rapid Rise in Greenhouse Gas Concentrations 55 Million Years Ago: Lessons for the Future

James C. Zachos

Professor of Earth Sciences, Earth Sciences Department,
University of California, Santa Cruz

Since the onset of the industrial revolution, civilization has pumped roughly 300 Gt of carbon dioxide (CO₂) into the atmosphere. Less than half has actually accumulated in the atmosphere, in large part, because of absorption by the ocean. While this has helped to slow the rise of atmospheric CO₂, as well as the rate of global warming, the dissolution of CO₂ in the ocean has begun to lower the pH of the upper ocean. Numerical models suggest that, as we continue to add carbon to the atmosphere over the next several centuries, acidification will eventually propagate deeper into the ocean. As a buffer, calcite on the seafloor will begin to dissolve, thereby slowing the drop in pH and allowing the ocean to absorb more carbon. However, because of the rapid rate of change, the pH will drop enough to slow the absorption process as well as potentially impact organisms that secrete carbonate tests. Moreover, assuming the entire inventory of fossil fuel (~4000 Gt) is combusted, permanent sequestration of the anthropogenic carbon and complete restoration of ocean carbonate chemistry will require tens of thousands of years.

This extreme perturbation to the carbon cycle is not unprecedented. Evidence exists in the geologic record of a similar event approximately 55 million years ago at the boundary between the Paleocene and Eocene epochs. This event is characterized by an extreme, but short-lived rise in global temperature, and by a major change in ocean carbon chemistry. The latter has been attributed to the rapid decomposition of marine hydrates and release of as much as 2000 Gt of methane, a phenomenon that in itself may have been triggered by gradual warming. In theory, the subsequent oxidation and uptake of this carbon by the ocean should have acidified the ocean in much the same way that anthropogenic CO₂ is beginning to alter modern ocean chemistry.

To better characterize the evolution of this unique greenhouse event and its impacts on ocean chemistry, several coring expeditions to the Pacific and Atlantic were undertaken by the Ocean Drilling Program. The primary objective was to recover an array of sedimentary sections spanning the Paleocene-Eocene boundary that would provide a 3 dimensional perspective on changes in ocean chemistry. In this presentation, I will discuss how data collected during these expeditions have provided new insight into several critical aspects of this event including the total mass of carbon released, the extent of ocean acidification, and the rate at which the carbon was sequestered. I will also discuss potential implications for the future.

About the Lecturer

James Zachos is an oceanographer. His research focuses primarily on ocean history and past climate change. He is the co-director of the UCSC Stable Isotope Laboratory, was the founding director of the Center for Dynamics and Evolution of the Land-Sea Interface (CDELSI) at UCSC, and is a member of the Canadian Institute for Advanced Research (CIAR), Earth System Evolution Program. He is also the lead investigator of a multi-institution project that is investigating the Biocomplexity of the Paleocene-Eocene Boundary (BIOPE).

More Information

www.es.ucsc.edu/~jzachos/

James C. Zachos