Abstract
We use the Empirical Model of Global Climate (EM-GC) to show that human
activity has been responsible for ~0.14 °C/decade
(range: 0.08 to 0.20) of warming from 1979 to 2010. This EM-GC based
quantification of Attributable Anthropogenic Warming Rate (AAWR) is
constrained by the observed global mean surface temperature and ocean
heat content records; the largest contribution to the uncertainty in our
estimate of AAWR is imprecise knowledge of the radiative forcing due to
tropospheric aerosols (AER RF). Our value of AAWR is noticeably lower
than the mean value from the IPCC 2013 models, 0.22 °C/decade (range:
0.08 to 0.32) with no overlap of interquartile ranges. We also compute
probabilistic forecasts of the rise in GMST where again the largest
source of uncertainty is AER RF, and cast results in terms of the
likelihood of achieving either 1.5 °C or 2.0 °C warmings relative to
pre-industrial. We show that the likelihoods of limiting global warming
to 2°C are 92%, 50%, and 20% if greenhouse gases follow the RCP 2.6,
4.5, and 6.0 scenarios; the likelihoods of limiting warming to 1.5°C
drop to 67%, 10%, and 0.1% for these same three RCPs. Warming
forecasts based upon our EM-GC are more optimistic than found by CMIP5
GCMs, following how many GCMs exhibit faster warming than inferred from
the recent climate record. Our EM-GC forecasts show that aggressive
controls on emissions of both CO2 and
CH4 starting this decade are needed to limit global
warming to 1.5°C with high probability.