Reply to ‘‘Comments on ‘Why Hasn’t Earth Warmed as Much as Expected?’’’

Knutti and Plattner (2012, hereinafter KP) wholly mischaracterize the ‘‘warming discrepancy’’ that we presented in our paper (Schwartz et al. 2010, hereinafter S10). 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 the way in which the discrepancy is resolved has major implications for the understanding of and development of 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. In addition, KP dispute our conclusion that for the present best estimate of climate sensitivity, emissions of greenhouse gases (GHGs) would need to be abruptly halted to avoid an increase in global temperature that exceeds 2 K above preindustrial levels. We concede that our use of the terms ‘‘equilibrium’’ and ‘‘stabilization’’ may have led to some confusion. We clarify here that the focus of our calculation was on allowable CO2 emissions on the decadal time scale such that the global mean surface temperature (GMST) not exceed a given increase above its preindustrial value, not on the ultimate stabilization of global temperature. The essential differences between the scenario that we presented in S10 and those examined by KP deal with forcings over the time period in which the climate system responds to cessation of emissions. The model calculation presented by KP shows an increase in GMST to nearly 2 K above its preindustrial value following cessation of emissions of CO2 and associated aerosols and aerosol precursors. This result in fact supports the conclusion reached in our paper that if Earth’s equilibrium climate sensitivity is either at or near the present Intergovernmental Panel on Climate Change (IPCC) best estimate, such a reduction in CO2 emissions would be necessary to avoid committing the planet to such a temperature increase. Corresponding author address: Stephen E. Schwartz, Brookhaven National Laboratory, Upton, NY 11973. E-mail: [email protected] 2200 J O U R N A L O F C L I M A T E VOLUME 25