Climatologists are nothing if not persistent. A new paper by a large team of climate scientists, among them Susan Solomon and Ben Santer, reckons it has found the reason for the model-observations divergence. I think this brings the tally of explanations to over 40 now. The correct answer, it seems, is that aerosol cooling, in particular that due to volcanoes, has been severely underestimated:
Understanding the cooling effect of recent volcanoes is of particular interest in the context of the post-2000 slowing of the rate of global warming. Satellite observations of aerosol optical depth (AOD) above 15 km have demonstrated that small-magnitude volcanic eruptions substantially perturb incoming solar radiation. Here we use lidar, AERONET and balloon-borne observations to provide evidence that currently available satellite databases neglect substantial amounts of volcanic aerosol between the tropopause and 15 km at mid to high latitudes, and therefore underestimate total radiative forcing resulting from the recent eruptions. Incorporating these estimates into a simple climate model, we determine the global volcanic aerosol forcing since 2000 to be −0.19 ± 0.09 Wm−2. This translates into an estimated global cooling of 0.05 to 0.12 °C. We conclude that recent volcanic events are responsible for more post-2000 cooling than is implied by satellite databases that neglect volcanic aerosol effects below 15 km.
I'm not sure what they've actually done, since the paper is paywalled. No doubt the credibility of their claims will be examined over the next couple of days.