© The Author(s) 2017 51
R.J. Salawitch et al., Paris Climate Agreement: Beacon of Hope,
Springer Climate, DOI 10.1007/978-3-319-46939-3_2
Chapter 2
Forecasting Global Warming
Austin P. Hope, Timothy P. Canty, Ross J. Salawitch,
Walter R. Tribett, and Brian F. Bennett
Abstract This chapter provides an overview of the factors that will govern the rise
in global mean surface temperature (GMST) over the rest of this century. We evalu-
ate GMST using two approaches: analysis of archived output from atmospheric,
oceanic general circulation models (GCMs) and calculations conducted using a
computational framework developed by our group, termed the Empirical Model of
Global Climate (EM-GC). Comparison of the observed rise in GMST over the past
32 years with GCM output reveals these models tend to warm too quickly, on aver-
age by about a factor of two. Most GCMs likely represent climate feedback in a
manner that amplifies the radiative forcing of climate due to greenhouse gases
(GHGs) too strongly. The GCM-based forecast of GMST over the rest of the cen-
tury predicts neither the target (1.5 °C) nor upper limit (2.0 °C warming) of the Paris
Climate Agreement will be achieved if GHGs follow the trajectories of either the
Representative Concentration Pathway (RCP) 4.5 or 8.5 scenarios. Conversely,
forecasts of GMST conducted in the EM-GC framework indicate that if GHGs fol-
low the RCP 4.5 trajectory, there is a reasonably good probability (~75 %) the Paris
target of 1.5 °C warming will be achieved, and an excellent probability (>95 %)
global warming will remain below 2.0 °C. Uncertainty in the EM-GC forecast of
GMST is primarily caused by the ability to simulate past climate for various com-
binations of parameters that represent climate feedback and radiative forcing due to
aerosols, which provide disparate projections of future warming.
Keywords Global warming projections • Attributable Anthropogenic Warming
- Global warming hiatus • Climate feedback
2.1 Introduction
The objective of the Paris Agreement negotiated at the twenty-first session of the
Conference of the Parties of the United Nations Framework Convention on Climate
Change (UNFCCC) is to hold the increase in global mean surface temperature (GMST)
to well below 2 °C above pre-industrial levels and to pursue efforts to limit the increase
to 1.5 °C above pre-industrial levels. The rise in GMST relative to the pre-industrial
baseline, termed ΔT, is the primary focus throughout this book. We consider