“This is a remarkable paper, and it is gratifying to see the GFDL group making a concerted effort to sort out the multitude of modes and time-scales of natural internal variability. For the first time, we have a coupled global climate model (AOGCM) that seems to be approaching a reasonable treatment of the ocean and the coupled modes between the ocean and atmosphere, including strong multi-decadal variability. The GFDL CM3 model is an outlier in this regard among CMIP5 models, apparently causing some to reject this model in their analyses. Personally, I think this model comes closest to a realistic treatment of internal variability. According to Laepple and Huybers (2014), AOGCM-simulated sea surface temperature variability is systematically smaller than instrumental and proxy-based estimates, increasingly with timescale, the discrepancy reaching two orders of magnitude for tropical variability at millennial timescales.
Further, the study actually addresses the implications of lower TCR values that have been determined from observational analyses using simple energy balance models. Lower TCR values, combined with a greater amplitude of natural internal variability, have substantial implications for attribution of 20th century warming as well as the projections of 21st century climate change.
The spike in global temperatures from the 2015/2016 El Nino has caused some to declare the warming slowdown to be over. This is not necessarily the case; we will need at least another 5 years of observations to determine whether the slowdown is over and warming resumes at a pace of at least 0.2°C/decade, or whether the slowdown will continue for another decade or two.” — Dr. Judith Curry