Jorge Sarmiento and Nicolas Gruber (Physics Today, August 2002, page 30) mention that carbon dioxide has a relatively long residence time in the atmosphere because it is non-reactive there. However, that statement could be misleading. The residence time of a gas at equilibrium in a reservoir is τ = M/F, where M is the total mass of the gas in the reservoir, and F is either the rate of release plus formation or the rate of removal plus decomposition. 1 Thus, the residence time of CO2 should not be considered as an indication of its atmospheric nonreactivity.

Since the authors neglect atmospheric reactions of CO2, the article ascribes all sinks of atmospheric CO2 to either terrestrial or oceanic sources. We suggest that the models should also include atmospheric reactions.

Atmospheric CO2 and included carbonate aerosols form carbonic acid in cloud droplets in the atmosphere. The carbonic acid undergoes dissociation to bicarbonate and carbonate ions sufficiently rapidly to influence rainwater pH. That natural mechanism removes atmospheric CO2 from the atmosphere. The magnitude of that gaseous scavenging process is not characterized well enough for a quantitative estimate of the magnitude of the sink, but large volumes of air flow through clouds and are processed by precipitating clouds during their life cycle. Land and water sinks for atmospheric CO2 provide surfaces that interact only with the air in the immediate proximity.

Although atmospheric CO2 has been regarded by many as chemically nonreactive, formate ions and formaldehyde are produced by its reduction in single ice crystals in mixed clouds—those having regions that contain both liquid and solid phases—when the cloud droplets contain low concentrations of sodium chloride. 2 It is assumed, but not determined, that methyl alcohol is also a product of the reduction process. A number of laboratory cloud chamber and field experiments have demonstrated that growing ice crystals containing low concentrations of calcium carbonate or magnesium carbonate derived from terrestrial dust also absorb and reduce CO2. This class of chemical reactions occurs at growing ice interfaces in the presence of appropriate solutes in the system. The presence of formate ions in natural precipitation, which is normally attributed to the oxidation of methane by hydroxyl free radicals, may actually be the result of reduction of CO2.

In their figure 3, Sarmiento and Gruber show a strong correlation of incidences of El Niño years with higher CO2 accumulation rates in the atmosphere. The authors suggest that the correlation may be the result of terrestrial vegetation’s response to climatic variability. The presence of the growing ice phase in the atmosphere is highly variable and may contribute to the observed correlation shown. During El Niño years, there is a significant reduction in oceanic precipitation over a significant portion of the globe’s tropical oceans, thus reducing both the incidence of maritime clouds and the opportunity for CO2-reducing reactions in the growing ice phase of those clouds.

Sarmiento and Gruber mention the uncertainty associated with CO2 uptake in the Southern Ocean. At 60° S, oceans exist at all longitudes; the climatic high frequency of storms at that latitude is reflected in the rising air in the Ferrell Cell (the region near 60 S) of the general circulation. Consequently, this region contains both maritime clouds and storms, in which cloud glaciation occurs and frequently leads to precipitation. Thus, the uncertainty of results that Sarmiento and Gruber find in the region near 60° S is probably partially due to weather patterns leading to chemical reduction and removal of carbon dioxide in precipitation.

Before reading the article, we believed that the atmospheric reactions to remove CO2 could not compare in magnitude to the estimated oceanic and terrestrial sinks. Inclusion of atmospheric reactions that chemically reduce CO2 in mixed clouds, followed by removal of the resulting products in precipitation, might advance the modeling technique presented in Sarmiento and Gruber’s article.

1.
C. E.
Junge
,
Air Chemistry and Radioactivity
,
Academic Press
,
New York
(
1963
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2.
W. G.
Finnegan
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R. L.
Pitter
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B. A.
Hinsvark
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J. Colloid Interface Sci.
242
,
373
(
2001
) .