Can We Trust the U.N.’s IPCC Climate Models?

On Monday, the U.N.’s Intergovernmental Panel on Climate Change
(IPCC) released the final draft of Climate Change 2013: The
Physical Sciences Basis
. The report’s
Summary for Policymakers
flatly states: “Warming of the climate
system is unequivocal, and since the 1950s, many of the observed
changes are unprecedented over decades to millennia. The atmosphere
and ocean have warmed, the amounts of snow and ice have diminished,
sea level has risen, and the concentrations of greenhouse gases
have increased.” Pretty much everyone concerned with this issue
agrees that those are the facts. But what is causing the planet to
warm up? Here is where it gets interesting.

The Summary for Policymakers declares it “extremely likely that
human influence has been the dominant cause of the observed warming
since the mid-20th century.” Whether that is so can be probed by
comparing actual observed temperature trends with the simulations
of the U.N.’s computer climate models, which assume that human
influences are driving climate change. According to the IPCC
researchers, “There is very high confidence that models
reproduce the general features of the global and annual mean
surface temperature changes over the historical period, including
the warming in the second half of the 20th century.” So far, so
good: Both the model’s projections and actual temperatures did rise
during the latter half of the 20th century.

As evidence that the models “reproduce the general features” of
actual temperature trends, the new report provides a handy graph
comparing projections made in the panel’s previous report with
three different temperature records. The report further states that
“the trend in globally-averaged surface temperatures falls within

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