Comparison of CMIP6 Historical Simulations and Future Projected Warming to an Empirical Model of Global Climate

The sixth phase of the Coupled Model Intercomparison Project (CMIP6) is the latest modeling effort for General Circulation Models (GCMs) to simulate past and project future climate change. First, we will analyze near surface air temperature output from the historical CMIP6 simulations for comparison to observational records of global mean surface temperature (GMST) to ascertain how well these GCMs can reproduce the historical temperature record. The metric we will use to determine a model’s reproducibility of the observed temperature rise is the attributable anthropogenic warming rate (AAWR), or the human contribution to the rise of GMST. AAWR is found by regressing the anomaly of GMST from CMIP6 models against the total solar irradiance and stratospheric optical depth records used to drive these historical runs. Internal GCM components that drive GMST, such as ENSO, will cancel since so many model realizations are examined. AAWR from the climate record is found by using an Empirical Model of Global Climate (EM-GC) to account for the natural influences on GMST due to ENSO, the 11-year solar cycle, major volcanic eruptions, as well as the anthropogenic influence of greenhouse gases and aerosols, and the efficiency of ocean heat uptake, as described by Hope et al., 2017 (DOI: 10.1007/978-3-319-46939-3_2). Preliminary analysis shows that, over the past three decades, the human component of global warming inferred from CMIP6 output tends to be larger than the human component of global warming inferred from the climate record. We will also examine projections of future temperature anomalies for specific Shared Socioeconomic Pathway (SSP) scenarios from the CMIP6 models and compare these to projections calculated by the EM-GC. We will conclude by assessing the probability of achieving the target (1.5°C) and upper limit (2.0°C) of the Paris Climate Agreement for specific SSP scenarios, based upon CMIP6 and EM-GC projections.