Climate Change and Ocean Acidification: Lessons from the Past
Richard Zeebe, University of Hawaii, Honolulu, USA
Humans are continuing to add vast amounts of carbon dioxide (CO2) to the atmosphere through fossil fuel burning and other activities. Anthropogenic carbon emissions reached a record high in 2010 of 37 billion metric tons of CO2 per year — up by about 6% after a small drop in 2009 due to the global financial crisis. Until recently, much of the scientific and public discussion has been focused on the impacts of anthropogenic CO2 on climate, as increasing levels of CO2 in the atmosphere are causing changes in the radiative forcing of Earth’s climate. However, recent studies underline a second, major impact of CO2 emissions: ocean acidification. A large fraction of the CO2 is taken up by the oceans in a process that lowers ocean pH and carbonate mineral saturation state. This effect has potentially serious consequences for marine life, which are, however, difficult to predict. One approach to address the issue is to study the geologic record, which may provide clues about what the future holds for Earth’s climate, ocean chemistry and marine organisms. In this presentation, I will explain basic controls on climate and ocean chemistry on different timescales and will examine past climate aberrations and ocean acidification events focusing on the Paleocene-Eocene Thermal Maximum (~56 million years ago). The results allow evaluation of the current anthropogenic perturbation in the context of Earth’s history. It appears that during the Paleocene–Eocene Thermal Maximum processes and/or feedbacks other than direct CO2 forcing alone caused a substantial portion of the warming. These processes are hitherto unknown but need to be identified in order to evaluate their potential effect on future climate change. Regarding changes in ocean chemistry, I conclude that the ocean acidification event that humans are expected to cause is unprecedented in the geologic past, for which sufficiently well-preserved records are available.
Zeebe, R. E. History of seawater carbonate chemistry, atmospheric CO2, and ocean acidification. Annu. Rev. Earth Planet. Sci. 40, doi:10.1146/annurev-earth-042711-105521, 2012.
Zeebe, R. E., Zachos, J. C., and G. R. Dickens. Carbon dioxide forcing alone insufficient to explain Paleocene-Eocene Thermal Maximum warming. Nature Geoscience, doi:10.1038/NGEO578, 2009.
Zeebe, R. E., J. C. Zachos, K. Caldeira, and T. Tyrrell. Oceans: Carbon Emissions and Acidification. Science (Perspectives), 321, 51-52, doi:10.1126/science.1159124, 2008.