Before fossil fuels, Earth’s minerals kept CO2 in check

University of Hawaiʻi at Mānoa
Richard Zeebe, (808) 956-6473
School of Ocean and Earth Science and Technology
Posted: Apr 28, 2008

HONOLULU - Over millions of years carbon dioxide levels in the atmosphere have been moderated by a finely-tuned natural feedback system— a system that human emissions have recently overwhelmed. A joint University of Hawaiʻi at Mānoa / Carnegie Institution study is the feature cover story in the May journal Nature Geoscience. The study links the pre-human stability to connections between carbon dioxide in the atmosphere and the breakdown of minerals in the Earth‘s crust. While the process occurs far too slowly to have halted the historical buildup of carbon dioxide from human sources, the finding gives scientists new insights into the complexities of the carbon cycle.

Richard Zeebe of the University of Hawaiʻi at Mānoa and Ken Caldeira of the Carnegie Institution‘s Department of Global Ecology studied levels of carbon dioxide in the atmosphere over the past 610,000 years using data from gas bubbles trapped in Antarctic ice cores. They used these records, plus geochemical data from ocean sediments, to model how carbon dioxide released into the atmosphere by volcanoes and other natural sources is ultimately recycled via carbon-bearing minerals back into the crust.

When carbon dioxide levels in the atmosphere rise, the chemical reactions that break down silicate minerals at the Earth‘s surface are accelerated. Among the products of these reactions are calcium ions, which dissolve in water and are washed to the ocean by rivers. Marine organisms such as calcareous plankton and mollusks combine the calcium ions with dissolved carbon dioxide to make their shells (calcium carbonate), which removes both calcium and carbon dioxide from the ocean, restoring the balance.

The researchers found that over hundreds of thousands of years the equilibrium between carbon dioxide input and removal was never more than one to two percent out of balance, a strong indication of a natural feedback system. This natural feedback acts as a thermostat which is critical for the long-term stability of climate. During Earth‘s history it has likely helped to prevent runaway greenhouse and icehouse conditions over time scales of millions to billions of years — a prerequisite for sustaining liquid water on Earth‘s surface.

"Before anthropogenic emissions were added to the equation, the system was nicely balanced," says Zeebe. "But this has changed. The average man-made increase in atmospheric CO2 from fossil fuel burning and deforestation over the past 200 years is about 14,000 times faster than the long-term average change over the past 610,000 years."

Previous researchers had suggested that such a system existed, but Zeebe and Caldeira‘s study provides the first observational evidence supporting the theory, and confirms its role in balancing the carbon cycle. But because it operates over such a long time scale—the time scale over which landscapes are eroded and washed to the sea—this geological feedback system offers little comfort with respect to the current climate crisis.

Carbon dioxide is added naturally to the atmosphere and oceans from volcanoes and hydrothermal vents at a rate of about 0.1 billion tons of carbon each year. Human industrial activity and destruction of forests is adding carbon about 100 times faster, approximately 10 billion tons of carbon each year. "We are currently releasing fossil fuels from long-term reservoirs that usually store carbon over millions of years," says Zeebe. "The natural long-term feedbacks which remove carbon from the atmosphere are way too slow to keep up with the pace at which we release it. We cannot expect that those feedbacks will protect us from climate change and ocean acidification in the near future. This will take tens of thousands to hundreds of thousands of years. "

Zeebe, Richard E., and Ken Caldeira. Close mass balance of long-term carbon fluxes from ice-core CO2 and ocean chemistry records. Nature Geoscience, Advance Online Publication, April 27, 2008.

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