CLIMATE SCIENCE AND ITS CRITICS - The Climate Casino - page 298

temperature trends are like investigators using only eyewitness reports
and ignoring fi ngerprints, video surveillance cameras, social media,
and DNA-based evidence. 14 Yet the contrarians continue to repeat their
claims using outmoded techniques and data.
A second argument is that the climate models are exaggerating the
extent of warming. The sixteen scientists wrote, “The lack of warming
for more than a decade—indeed, the smaller-than-predicted warming
over the 22 years since the UN's Intergovernmental Panel on Climate
Change (IPCC) began issuing projections—suggests that computer mod-
els have greatly exaggerated how much warming additional CO 2 can
cause.”
What is the evidence on the performance of climate models? Do
they predict the historical trend accurately? Statisticians routinely ad-
dress this kind of question. The standard approach is to perform an ex-
periment in which modelers put the changes in CO 2 concentrations and
other climate infl uences in a climate model and estimate the resulting
temperature path (“with GHGs”); and then modelers calculate what
would happen in the counterfactual situation where the only changes
were due to natural sources, for example, the sun and volcanoes, with no
human-induced changes (“without GHGs”). They then compare the ac-
tual temperature increases, the model predictions with all sources (with
GHGs), and the model predictions with natural sources alone (without
GHGs).
This experiment has been performed many times using climate
models. 15 The experiments showed that the projections of climate mod-
els are consistent with recorded temperature trends over recent decades
only if human impacts are included. The divergent trend is especially
pronounced after 1980. By 2010, calculations using natural sources
alone underpredict the actual temperature increases by about 1°C, while
the calculations including human sources track the actual temperature
trend closely.
In reviewing the results, the IPCC report concluded, “No climate
model using natural forcings [i.e., natural warming factors] alone has
reproduced the observed global warming trend in the second half of the
twentieth century.” 16