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
the range of the previous IPCC projections.”


But is that so? Most temperature records show that since 1998
the models and the observed average global temperature trend have
parted ways. The temperatures in the models continue to rise, while
the real climate has refused to warm up much during the past 15

The IPCC report acknowledges that almost all of the “historical
simulations do not reproduce the observed recent warming hiatus.”
Not to worry, it assures us; 15-year pauses just happen, and you
can’t really expect the models to simulate these kind of random
natural fluctuations in the climate. Once this little slow-down
passes, “It is more likely than not that internal climate
variability in the near-term will enhance and not counteract the
surface warming expected to arise from the increasing anthropogenic
forcing.” In other words, when the warm-up resumes it will

John Christy, a climatologist at the University of Alabama in
Huntsville, has come to a different conclusion. Christy compared
the outputs of 73 climate models for the tropical troposphere used
by the IPCC in its latest report with satellite and weather balloon
temperature trends since 1979 until 2030. “The tropics is so
important because that is where models show the clearest and most
distinct signal of greenhouse warming—so that is where the
comparison should be made (rather than say for temperatures in
North Dakota),” Christy explains in an email. “Plus, the key cloud
and water vapor feedback processes occur in the tropics.” When it
comes to simulating the atmospheric temperature trends of the past
35 years, Christy found, all of the IPCC models are running hotter
than the actual climate.

Christy Models versus Temperatures

Even the IPCC report admits, “Most, though not all, of [the
climate models] overestimate the observed warming trend in the
tropical troposphere during the satellite period 1979–2012.”
Another graph from Christy, which simply compares the actual
average temperature trends with the IPCC model averages for the
tropics, makes the divergence starker.

Christy Simple Average Trends

To defend himself against any accusations of cherry-picking his
data, Christy notes that his “comparisons start in 1979, so these
are 35-year time series comparisons”—rather longer than the 15-year
periods whose importance the IPCC disputes.

Why the discrepancy between the IPCC and Christy results? As
Georgia Tech climatologist Judith Curry
, data don’t speak for themselves; researchers have to put
them into a context. And your choice of context—say, the year you
choose to begin with—can influence your conclusions considerably.
While there may be nothing technically wrong with the way the IPCC
chose to display the comparison between model data and observation
data, “Curry observes, it will mislead the public to infer that
climate models are
better than we thought
.” She adds, “What is wrong is the
failure of the IPCC to note the failure of nearly all climate model
simulations to reproduce a pause of 15+ years.”

The IPCC report does concede that “Almost all [climate model]
historical simulations do not reproduce the observed recent warming
hiatus.” It argues that the difference “could be caused by some
combination of (a) internal climate variability, (b) missing or
incorrect radiative forcing, and (c) model response error.” That is
to say, the projections are off owing to pesky natural climate
fluctuations, possible errors regarding how much warming a given
increase in greenhouse gases will produce, and/or boosting
temperature projections too high in response to given increases in
greenhouse gases.

The IPCC report also obliquely references an August study in

Nature Climate Change
that reported that the observed
rate of warming over the past 20 years was
actually half
of what a representative sample of the models
relied upon by the IPCC simulated. Looking at just the past 15
years, the models were four times hotter than the actual trend in
the average global temperature.

But the IPCC is confident that warming will soon resume at a
pretty fast clip too. Back in 2007, other modelers were similarly
confident about their predictions for future warming. Specifically,
at the United Nation’s annual climate change conference in Bali,
the U.K.’s Hadley Centre forecasted that between 2004 and 2014 the
global average temperature would
rise by around +0.3 degrees Celsius
. Instead, the Nature
Climate Change
article reports a trend over the last 15 years
of just +0.05 degrees Celsius per decade.