Climate scientists struggle mightily to explain the hiatus: Increasing volcanic activity and decreased solar radiation explains half, internal variability explains the other half?

Royal Society meeting discusses IPCC fifth assessment report

[Mark Walport, chief scientific advisor to the UK government] "many of the climate sceptics communicate very well indeed. The public can only judge on the quality of the communication."
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The global warming "pause", or perhaps more accurately the not-so-global-only-surface-temperature-warming hiatus, was discussed by Jochem Marotzke of the Max Planck Institute for Meteorology, who was part of the IPCC team assembling scientific evidence on trends in temperature over the last ten to fifteen years.
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Marotzke reckons one of the factors that's added to the attention is that CMIP5 climate models reproduce the long-term global mean surface temperature over the twentieth century but don't reproduce the warming hiatus in the last ten to fifteen years. However, he explained, models have tended to show temperatures higher than observations in the past too.

So what's the cause? Marotzke said the hiatus is a northern-winter phenomenon, found in December, January and February. The spatial footprint is "not entirely clear" but one team reckons there's a cold centre over Eurasia.

A number of explanations have been posed - stratospheric water vapour, stratospheric or volcanic aerosols, the currently downward phase of the solar cycle, an increase in deep-ocean heat uptake, and low sea surface temperatures in the tropical eastern Pacific.

It seems that a decrease in radiative forcing caused by two natural factors over the period in question - an increase in volcanic activity, and hence more reflective particles in the atmosphere, and a downwards trend in solar radiation - account for around half the observed surface temperature trend. And the rest is ascribable to internal variability.
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"The recent surface warming hiatus poses a fascinating scientific challenge," said Marotzke. "It forces us to think across the entire climate system and possibly to rethink some of our basic concepts, for example how we formulate the energy budget and feedback." That said, "the hiatus is largely irrelevant to the physics (although not the communications) of anthropogenic climate change", Marotzke continued. "The timescale is too short and so dominated by natural - forced and internal - fluctuations."