Knutti and Plattner (2012) wholly mischaracterize the "warming discrepancy" that we presented in our paper (Schwartz et al., 2010). Briefly, we noted that the calculated increase in global temperature due to long-lived greenhouse gases (LLGHGs) alone greatly exceeds the observed warming. We then examined possible causes of this discrepancy, importantly, thermal disequilibrium, forcing by aerosols, and uncertainty in climate sensitivity. We showed that the warming discrepancy can be resolved in a multiplicity of ways and that the way in which the discrepancy is resolved has major implications for understanding of and developing policy responses to human induced climate change. KP state that if the causes of the discrepancy "are properly taken into account, there is no discrepancy between predicted and observed warming." It is just this false sense of confidence in climate models, arising out of their concordance with observations, that we sought to avoid by not including these causes in calculating the expected warming. We stand by the key conclusions of our paper: 1) that there is substantial uncertainty in how to resolve the discrepancy between the observed increase in GMST and that expected from LLGHGs alone, 2) that the present uncertainty in climate sensitivity precludes determination even of the sign of the amount of future CO2 emissions that would be allowed so as not to exceed a given increase in GMST, and 3) that the only realistic way to reduce these uncertainties is to greatly reduce the uncertainty in aerosol forcing.
Knutti, Reto, Gian-Kasper Plattner, 2012: Comments on "Why Hasn't Earth Warmed as Much as Expected?" J. Climate 25, 2192-2199 (2012); doi: http://dx.doi.org/10.1175/2011JCLI4038.1
Schwartz S. E., Charlson R. J., Kahn R. A., Ogren, J. A., and Rodhe H. (2010) Why Hasn't Earth Warmed as Much as Expected? J. Climate 23, 2453-2464 (2010); doi: 10.1175/2009JCLI3461.1.
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