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Modelling Aerosol-Cloud Interactions ... Correctly

Rosenfeld, D., Sherwood, S., Wood, R. and Donner, L. 2014. Climate effects of aerosol-cloud interactions. Science 343: 379-380.
In a recent Perspective piece in Science, Rosenfeld et al. (2014) write that "aerosols counteract part of the warming effects of greenhouse gases, mostly by increasing the amount of sunlight reflected back to space." However, they note "the ways in which aerosols affect climate through their interaction with clouds are complex and incompletely captured by climate models." And as a result, they indicate "the radiative forcing (that is, the perturbation to Earth's energy budget) caused by human activities is highly uncertain, making it difficult to predict the extent of global warming," citing Anderson et al. (2003) and Stocker et al. (2013).

So what's new?

For one thing, the four researchers report "recent advances have revealed a much more complicated picture of aerosol-cloud interactions than considered previously," and they say "further progress is hampered by limited observational capabilities and coarse-resolution models." For another, they acknowledge "little is known about the unperturbed aerosol level that existed in the preindustrial era," noting "this reference level is very important for estimating the radiative forcing from aerosols," citing Carslaw et al. (2013) in Nature.

Also holding up progress is the fact, as Rosenfeld et al. put it, that "understanding of the formation of ice and its interactions with liquid droplets is even more limited, mainly due to poor ability to measure the ice-nucleating activity of aerosols and the subsequent ice-forming processes in clouds." And in this regard they say "improved observational tests are essential for validating the results of simulations and ensuring that modeling developments are on the right track." But they state that what they call "a major challenge" in this area is the fact that "the most important aerosol nucleation region is at the bottom of a cloud, which is obscured by the rest of the cloud if measured from above."

And so it is no surprise that Rosenfeld et al. conclude "fully resolved, global, multiyear simulations are not likely to become feasible for many decades." Yes, that's right - many decades. And this analysis of the situation makes one wonder if the policy cart is being pushed way before the horse, when it comes to pushing for such drastic CO2 emission reductions that are continually promoted in order to prevent what may eventually be found to actually be no problem at all.

Additional References
Anderson, T.L., Charlson, R.J., Schwartz, S.E., Knutti, R., Boucher, O., Rodhe, H. and Heintzenberg, J. 2013. Climate forcing by aerosols - a hazy picture. Science 300: 1103-1104.

Carslaw, K.S., Lee, L.A., Reddington, C.L., Pringle, K.J., Rap, A., Forster, P.M., Mann, G.W., Spracklen, D.V., Woodhouse, M.T., Regayre, L.A. and Pierce, J.R. 2013. Nature 503: 67-71.

Stocker, T.F., Qin, D., Plattner, G.K., Tignor, M.M.B., Allen, S.K., Boschung, J., Nauels, A., Xia, Y., Bex, V. and Midgley, P.M. (Eds.). Summary for Policymakers. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, UK.

Archived 11 March 2014