Quantification of Global Warming: Critical Evaluation of GCMs

We quantify the effect of human activity on global mean surface temperature (GMST) using two approaches: calculations conducted using an empirical model of global climate (EM-GC) developed by our research group (Canty et al., ACPD, 2013), and analysis of archived output from the Coupled Model Intercomparison Project Phase 5 (CMIP5) general circulation models (GCMs). Our focus is on quantification of attributable anthropogenic warming rate (AAWR) over 1979 to 2010, equilibrium climate sensitivity, and transient response of climate to cumulative carbon emission (TCRE). It has been documented that the CMIP5 GCMs tend to warm too quickly, as shown for instance by Figure 11.25, WGI, IPCC (2013). Here, we examine AAWR, ECS, and TCRE in the context of the behavior of these GCMs. We show AAWR inferred from the climate record over the past 32 years using our EM-GC is about a factor of two less than the GCM-based multi-model-mean for AAWR. The CMIP5 GCMs tend to warm about a factor of two too quickly, on average, most likely because most models represent climate feedback in a manner that too strongly amplifies the radiative forcing of climate due to greenhouse gases. We show the inferred ECS from the climate record inferred using the EM-GC framework is much smaller than that associated with CMIP5 GCMs; we also illustrate how this comparison is complicated by uncertainties in the ocean heat record. Finally, we compare TCRE inferred using the EM-GC to the GCM-based estimate published in IPCC (2013). Differences will be assessed in terms of how much more carbon can be emitted to the atmosphere, before either the target (1.5 °C) or upper limit (2.0 °C warming) of the Paris Climate Agreement will be crossed.