Background
The authors write that "the Coupled Model Intercomparison Project Phase 5 (CMIP5) has devised an innovative experimental design to assess the predictability and prediction skill on decadal time scales of state-of-the-art climate models, in support of the Intergovernmental Panel on Climate Change (IPCC) 5th Assessment Report," citing Taylor et al. (2012). To date, however, they say that decadal predictions from different CMIP5 models "have not been evaluated and compared using the same evaluation matrix," a problem which they resolve for seven of the models with their new study.

What was done
Kim et al. assessed the CMIP5 decadal hindcast-forecast simulations of seven state-of-the-art ocean-atmosphere coupled models for situations where "each decadal prediction consists of simulations over a 10-year period each of which are initialized every five years from climate states of 1960/1961 to 2005/2006."

What was learned
In regard to problems they uncovered with the models via this methodology, the three US researchers report that "most of the models overestimate trends," whereby they "predict less warming or even cooling in the earlier decades compared to observations and too much warming in recent decades." They also state that "low prediction skill is found over the equatorial and North Pacific Ocean," and that "the predictive skill for the Pacific Decadal Oscillation index is relatively low for the entire period."

What it means
In introducing their study, Kim et al. write that "the prediction of decadal climate variability against a background of global warming is one of the most important and challenging tasks in climate science." And in light of their findings, it would appear that the CMIP5 models they tested are still not fully up to accomplishing this "most important and challenging task."

Reference
Taylor, K.E., Stouffer, R.J. and Meehl, G.A. 2012. An overview of CMIP5 and the experiment design. Bulletin of the American Meteorological Society 93: 485-498.