Computational assessment of a proposed technique for global warming mitigation via albedo-enhancement of marine stratocumulus clouds

Abstract

A simplified version of the model of marine stratocumulus clouds developed by Bower, Jones and Choularton [Bower, K.N., Jones, A., and Choularton, T.W., 1999. A modeling study of aerosol processing by stratocumulus clouds and its impact on GCM parameterisations of cloud and aerosol. Atmospheric Research, Vol. 50, Nos. 3–4, The Great Dun Fell Experiment, 1995-special issue, 317–344.] was used to examine the sensitivity of the albedo-enhancement global warming mitigation scheme proposed by Latham [Latham, J., 1990. Control of global warming? Nature 347, 339–340; Latham, J., 2002. Amelioration of global warming by controlled enhancement of the albedo and longevity of low-level maritime clouds. Atmos. Sci. Letters (doi:10.1006/Asle.2002.0048).] to the cloud and environmental aerosol characteristics, as well as those of the seawater aerosol of salt-mass m s and number concentration Δ N, which–under the scheme–are advertently introduced into the clouds. Values of albedo-change Δ A and droplet number concentration N d were calculated for a wide range of values of m s, Δ N, updraught speed W, cloud thickness Δ Z and cloud-base temperature T B: for three measured aerosol spectra, corresponding to ambient air of negligible, moderate and high levels of pollution. Our choices of parameter value ranges were determined by the extent of their applicability to the mitigation scheme, whose current formulation is still somewhat preliminary, thus rendering unwarranted in this study the utilisation of refinements incorporated into other stratocumulus models. In agreement with earlier studies: (1) Δ A was found to be very sensitive to Δ N and (within certain constraints) insensitive to changes in m s, W, Δ Z and T B; (2) Δ A was greatest for clouds formed in pure air and least for highly polluted air. In many situations considered to be within the ambit of the mitigation scheme, the calculated Δ A values exceeded those estimated by earlier workers as being necessary to produce a cooling sufficient to compensate, globally, for the warming resulting from a doubling of the atmospheric carbon dioxide concentration. Our calculations provide quantitative support for the physical viability of the mitigation scheme and offer new insights into its technological requirements.