Carbon dioxide alone can’t explain the marked increase in global surface temperatures 55 million years ago, according to a letter published in the July 2009 issue of Nature Geoscience.
University of Hawaiʻi at Mānoa Associate Professor of Oceanography Richard Zeebe and two mainland colleagues evaluated data from core samples collected in deep-sea drilling expeditions around the globe. The sediments hold clues to the amount of carbon in the atmosphere in the past.
The researchers found that CO2 levels increased 70 percent over just a few thousand years during the Paleocene-Eocene thermal maximum. In applying carbon cycle models, however, the scientists found that the increase in CO2 levels account for less than half of the 5–9 degree Celsius climb in surface temperatures during the same period.
Some other process must have been at work, they surmise. For example, CO2-induced warming may have triggered an increase in the methane cycle that magnified the CO2 effect through a mechanism termed “feedback.”
While current warming is directly tied to carbon emissions, understanding of feedback and other contributing mechanisms is critical to accurately predicting their effect on future climate change, Zeebe says.