Shock: In a quote-rich David Appell article, Trenberth says "the IPCC has not paid enough attention to natural variability" and Ben Santer says "It’s certainly the case that we got some of the forcings wrong"

Wither Global Warming? Has It Slowed Down? | The Yale Forum on Climate Change & The Media

...Mathematics teaches us that 15 years is simply too short a period from which to draw statistically valid conclusions...atmospheric carbon dioxide levels are increasing exponentially
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These increases are certainly less than the warming rates of the 1980s and first half of the 1990s of about 0.15 to 0.20 C (.27 and .36 F respectively) and per decade. The earlier period may have provided an unrealistic view of the global warming signal, says Kevin Trenberth, climate scientist with the National Center for Atmospheric Research in Boulder, Co.

“One of the things emerging from several lines is that the IPCC has not paid enough attention to natural variability, on several time scales,” he says, especially El Niños and La Niñas, the Pacific Ocean phenomena that are not yet captured by climate models, and the longer term Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO) which have cycle lengths of about 60 years.
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“It was a time when natural variability and global warming were going in the same direction, so it was much easier to find global warming,” Trenberth says. “Now the PDO has gone in the other direction, so some counter-effects are masking some of the global warming manifestations right at the surface.”
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“It’s certainly the case that we got some of the forcings wrong,” [Ben Santer] says of the factors that specify the influence of any particular component of the atmosphere. “It’s likely we underestimated the true volcanic aerosol forcing, and may have underestimated the cooling effect of stratospheric ozone depletion.”
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The bottom line, Santer says, is “there are multiple, not mutually exclusive interpretations of modeled versus observed differences, and claiming that there is only one explanation is not scientifically accurate.”

“We study the signal. If others want to study the noise, let them.”
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Nonetheless, [Raymond Pierrehumbert] agrees that earlier warming may have been deceiving.
“I think it’s true that some rather sloppy discussion of the rapid warming from the 20th century has given people unrealistic expectations about the future course of warming.”
All the same, the warming effect of carbon dioxide is far down his list of topics that need further examination.
“Why would anyone seriously question greenhouse gases?” he asks. “They absolutely have a radiative effect, and no serious scientist thinks climate sensitivity could be much lower than 2 degrees Celsius based on the balance of the evidence.”
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There have been hiatus periods in the past — from about 1945 to 1975, and slow downs in 15-year warming rates around 1994-1995 — and there will likely be more in the future. Those times of hiatus are consistent with human-caused warming in the natural world, and they are no reason at all to be lulled into complacency.