The Human Influence on Global Warming

We use an empirical model of global climate to quantify the role of human activity on global mean surface temperature (GMST) and project how GMST will rise over the next four decades. We show that humans have been responsible for 0.13±0.06°C/decade rise in GMST during the past three decades (1979 to 2010), which is considerably less than the rise in GMST attributed to humans over this same time period (0.22±0.08°C/decade) found within the Coupled Model Intercomparison Project Phase 5 (CMIP5) Global Climate Models (GCMs) used to project global warming in the IPCC AR5 reports. We will show that prior attempts to quantify the human influence on GMST over this time period (i.e., Foster and Rahmstorf, ERL, 2011) obtained an erroneously high value of 0.17±0.01°C/decade due to neglect of the influence on global climate of variations in the strength of the Atlantic Meridional Overturning Circulation. In the remainder of the presentation we focus on projections of GMST over the next four decades, using the rise in greenhouse gases (GHGs) described by the four Representative Concentration Pathway scenarios of IPCC AR5. These projections include detailed quantitative assessments of the sensitivity of global warming to radiative forcing of tropospheric aerosols and ocean heat export, resulting in a probability distribution function of future GMST for each RCP scenario. Of course GMST will rise over the next four decades unless anthropogenic release of GHGs is severely curtailed. Nonetheless, our empirically driven estimate of the future increase in GMST is considerably less than that calculated by the CMIP5 GCMs.