More on Carbon Sinks

Sarmiento and Gruber reply: We appreciate the opportunity McGuire and Argyle have given us to clarify two distinctions underlying our assertion ¹ that CO₂ accounts for more than half the increase in direct radiative trapping. The first is between “natural” greenhouse gases and the “increase” in trapping that has occurred since preindustrial times. The preindustrial (natural) forcing is about 60 times greater than the anthropogenic perturbation, and water vapor accounts for well over half of that. ² Our assertion applies to the perturbation in the radiative balance.

Second, climate scientists draw a distinction between radiative forcing and feedback effects. The increase in atmospheric water vapor that occurs as Earth warms is a feedback response to anthropogenic radiative forcing. Model simulations suggest that such feedback will increase global warming by two to three times more than would occur if the water vapor were held constant at its preindustrial concentration. ³ That feedback effect is a dominant one in determining how much warming will occur. However, the fundamental driver of the warming is the increase in greenhouse gases due to human activities. There are more recent measurements of the greenhouse gases’ absorption properties than those that McGuire and Argyle refer to, but further measurements are still needed, particularly regarding the optical properties of clouds and water vapor (see the article by Thomas Ackerman and Gerald Stokes, Physics Today, January 2003, page 38 ). We emphasize, however, that these are unlikely to lead to a significant change in climate sensitivity estimates. ¹

The letter from Pitter, Finnegan, and Hinsvark raises some points regarding the removal of CO₂ from the atmosphere by rainfall and by chemical reactions that convert CO₂ to formate ions and formaldehyde on ice crystals in clouds. Dissolution of CO₂ in rain removes an estimated 0.08 petagrams of carbon per year from the atmosphere. ⁴ That amount is negligible compared to the atmosphere-ocean and land-air anthropogenic CO₂ fluxes of about 2 Pg C/yr, and also compared to interannual atmospheric variability, which is several Pg C/yr. Besides, most of this flux (~0.06 Pg C/yr) is “natural” and thus has no impact on the anthropogenic transient. Rainfall also contains a large amount of dissolved organic carbon (DOC), ⁴ about 0.4 Pg C/yr. The role of this in anthropogenic carbon removal is difficult to assess because identifying the sources of the organic carbon is difficult. Formaldehyde accounts for only 3% of the DOC, and formic acid is only one of a long list of organic acids that together account for 40% of the DOC. ⁴ We thus believe that direct removal of CO₂ in the atmosphere by the reactions that Pitter and colleagues propose is unlikely to contribute significantly to either the anthropogenic perturbation or inter-annual variability.