The need for mass balance and feedback in the geochemical carbon cycle
Robert A. Berner & Ken Caldeira
Carbon input and uptake exist currently in a very fine balance known as the carbon cycle. Previous papers have suggested that the increased weathering related to warming is irrelevant to this balance, and proposed that CO2 levels are stabilized in the long term by an increase in mountain uplift that would eventually match increased CO2 degassing. It is shown here that in order to avoid drastic changes to atmospheric concentration, a rise in CO2 must prompt a negative feedback to counteract its effects. Increased weathering caused by warming is sufficiently large enough to do so.
On a multimillion-year time scale, the level of atmospheric carbon dioxide, and consequently the atmospheric greenhouse effect, is affected largely by the geochemical, or long-term, carbon cycle. This cycle (for a review, see Holland, 1978; Berner, 1991) involves the exchange of carbon dioxide between rocks, on the one hand, and the surficial reservoir, consisting of the combined atmosphere, oceans, biosphere, plus soils, on the other. Carbon dioxide is exchanged with the surficial system via weathering of silicates and organic matter on the continents, the burial of carbonates (derived from silicate weathering) and organic matter in sediments, and the thermal breakdown of carbonates and organic matter at depth. It has been suggested (Edmond et al., 1995; see also Edmond and Huh, 1997; Bickle, 1996) that in the geochemical carbon cycle there is no necessity for a close balance between atmospheric inputs and outputs of CO2, and therefore no need for a strongly coupled feedback to stabilize CO2 level in the atmosphere. On the basis of their study of the chemistry of major rivers, Edmond et al. (1995) concluded that climate has little effect on global weathering rate and criticized the idea that long-term stabilization of CO2 and climate comes about by increased chemical weathering rate that accompanies global greenhouse warming. In place of a climate-weathering feedback, Edmond et al. (1995) and Bickle (1996) assumed that increases in global degassing are eventually matched by increases in CO2 uptake via enhanced weathering accompanying mountain uplift. We show here that in this case, atmospheric CO2 would vary untenably as a result of large imbalances in the carbon cycle.