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

April 29, 2008

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 Hawaii / Carnegie Institution study published in the advance online edition of Nature Geoscience 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.

Ken Caldeira of the Carnegie Institution’s Department of Global Ecology and Richard Zeebe of the University of Hawaii 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 in soils 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 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 probably 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.

“The system is finely in tune,” says Caldeira. “That one or two percent imbalance works out to an average imbalance in natural carbon dioxide emissions that is thousands of times smaller than our current emissions from industry and the destruction of forests.”

Previous researchers had suggested that such a system existed, but Caldeira and Zeebe’s study provides the first observational evidence supporting the theory, and confirms its role in stabilizing 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.

“The imbalance in the carbon cycle that we are creating with our emissions is huge compared to the kinds of imbalances seen over the time of the glacial ice core records,” says Caldeira. “We are emitting CO2 far too fast to expect mother nature to mop up our mess anytime soon. Continued burning of coal, oil and gas will result in long-term changes to our climate and to ocean chemistry, lasting many thousands of years.”

Source: Carnegie Institution

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1.9 / 5 (13) Apr 29, 2008
The pre-human carbon cycle was "finely tuned" eh? Guess they forgot about 600 million years ago just before the Cambrian explosion that the entire Earth was an ice cube and it took a whopping 13% of our atmosphere as CO2 to unthaw it.

It subsequently returned to "normal" levels through natural means. If the Earth can clean out that much CO2 on its own I have little doubt the few parts per million we're putting into the system will be a problem.
1.8 / 5 (11) Apr 30, 2008
Modernmystic: I would like to know what where the timescales when that happened? I bet it was at least thousands of years into it and back. Now in few hundred years we are overstepping natural sources hundred fold. That can not happen without severe reactions globally. One example of human induced environment change with radical outcome: Easter island. They chopped all their trees and their economy did collapse. We better not to duplicate that globally.
1.5 / 5 (11) Apr 30, 2008
Great variations in CO2 content over millenia does not point to finely tuned ballance.CO2 can "migrate" in ice core samples,thus evening out over time,to give a false smooth curve.Ice core values do not match chemicaly measured values even in recent times.It's not disolved CO2 in ice that is measured but very small bubbles of atmosphere.Google "180 years of atmospheric CO2" for some interesting reading perhaps to make you rethink a few accepted graphs of CO2.
1.9 / 5 (13) Apr 30, 2008
PPihkala just google "snowball earth" and it will give you the whole story. And no we're not overstepping natural sources 100 fold. We put in about 6 giga tons per year, three of which are sequestered naturally. The Earth puts in 150 giga tons per year, your facts are wrong.

Also not to mention CO2 is a VERY small contributor to the overall greenhouse effect. Water vapor puts all other GHGs combined to shame.
2.7 / 5 (7) May 01, 2008
CO2 spikes every 80k years in the Antarctic ice cores. Human activity is irrelevant.
1.7 / 5 (9) May 01, 2008
Theses comments show the difficulty of a complete scientific proof of CO2 warming. All the critics have some truth. Please read http://www.biokur...omment_E&E-on_Beck_Meijer_update.doc
Nevertheless we cannot burn all the fossil fuel, because it served to make the oxygen that we breathe over millions of years! Thus we must stop burning in less than one century, unless we increase photosyntesis in the same ratio (2 to 3 times), even if CO2 is not warming.
We are changing the albedo of the earth, with farming and towns, a more dramatic efficient warming than CO2.
125 thousand years ago , the temperature was 2 to 3°C higher, with 3 to 5 meters higher seas, without more CO2 and humans.
It could happen again without CO2 and would be catastrophic.
Thus we must use free solar energy as soon as possible anc control the earth albedo.

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