A Small Misconception

Abstract

Regarding “Carbon sink: A clue from Biosphere 2?” (R. L. Walford, Science 's Compass, 15 Jan. p. [330][1]), I want to clear up a small misconception that has been widely repeated in the press (and also in the Perspective by J. E. Cohen and D. Tilman, 15 Nov. 1996, p. [1150][2]), namely that “carbon dioxide [CO2] was combining with the cement of the structure, carrying oxygen along with it to form calcium carbonate… hence the fall in oxygen concentration.” The cement in Biosphere 2 did not cause the oxygen loss. It is true that the CO2 reacted with the cement and that CO2 contains oxygen atoms. But it is molecular oxygen (O2) rather than oxygen atoms that was being lost from the air in Biosphere 2. The amount of oxygen atoms present in the water in Biosphere 2 is about 200 times more than the amount of oxygen atoms present as O2, so the loss of oxygen atoms to the cement was insignificant. Rather, what caused the O2 loss was the excess of organic matter in the soil, which supported an imbalance of O2-consuming respiration over O2-producing photosynthesis. The reaction of CO2 with the cement only made it a little harder for us to find the true cause of O2 loss, by scrubbing from the air the telltale product of respiration, CO2. Walford correctly points out that concrete absorbs CO2, but he does not point out that the Ca(OH)2 responsible for this uptake was obtained by driving CO2 off of limestone. Because some of the CaO2 becomes silicate-bound and some remains unreacted, concrete manufacture is a net source rather than a net sink for CO2. Further, the contribution of concrete manufacture to global CO2 production is only about 0.2 gigaton of carbon (GiC), compared with 6.5 or so GiC produced by fossil fuel burning and to a continental sink of about 1.7 GiC (S. Fan et al. , Reports, 16 Oct. p. [442][3]). Hence, even if limestone were slaked at one region and the concrete were used in another, the impact on the distribution of CO2 in the global atmosphere would be negligible. [1]: /lookup/doi/10.1126/science.283.5400.327g [2]: /lookup/doi/10.1126/science.283.5405.1150 [3]: /lookup/doi/10.1126/science.282.5388.442