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Sarmiento and Gruber reply: We appreciate the opportunity McGuire and Argyle have given us to clarify two distinctions underlying our assertion 1 1. Our assertion is supported by V. Ramaswamy et al. , in Climate Change 2001: The Scientific Basis, J. T. Houghton et al. , eds., Cambridge U. Press, New York (2001), p. 349. 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 2. R. E. Dickinson, R. J. Cicerone, Nature 319, 109 (1986) https://doi.org/10.1038/319109a0. 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 3. I. M. Held, B. J. Soden, Annu. Rev. Energy Environ. 25, 441 (2000) https://doi.org/10.1146/annurev.energy.25.1.441. 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 1. Our assertion is supported by V. Ramaswamy et al. , in Climate Change 2001: The Scientific Basis, J. T. Houghton et al. , eds., Cambridge U. Press, New York (2001), p. 349. 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 4. J. D. Willey, R. J. Kieber, M. S. Eyman, G. B. Avery Jr, Global Biogeochem. Cycles 14, 139 (2000) https://doi.org/10.1029/1999GB900036. 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 4. J. D. Willey, R. J. Kieber, M. S. Eyman, G. B. Avery Jr, Global Biogeochem. Cycles 14, 139 (2000) https://doi.org/10.1029/1999GB900036. 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 4. J. D. Willey, R. J. Kieber, M. S. Eyman, G. B. Avery Jr, Global Biogeochem. Cycles 14, 139 (2000) https://doi.org/10.1029/1999GB900036. 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.REFERENCESSection:ChooseTop of pageREFERENCES <<1. Our assertion is supported by V. Ramaswamy et al. , in Climate Change 2001: The Scientific Basis, J. T. Houghton et al. , eds., Cambridge U. Press, New York (2001), p. 349. Google Scholar2. R. E. Dickinson, R. J. Cicerone, Nature 319, 109 (1986) https://doi.org/10.1038/319109a0. Google ScholarCrossref, CAS3. I. M. Held, B. J. Soden, Annu. Rev. Energy Environ. 25, 441 (2000) https://doi.org/10.1146/annurev.energy.25.1.441. Google ScholarCrossref4. J. D. Willey, R. J. Kieber, M. S. Eyman, G. B. Avery Jr, Global Biogeochem. Cycles 14, 139 (2000) https://doi.org/10.1029/1999GB900036. Google ScholarCrossref, CAS© 2003 American Institute of Physics.