Background
In setting the stage for their study, the authors note that the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC) concluded that climate projections based on models that consider both human and natural factors provide "credible quantitative estimates of future climate change." However, as they continue, mismatches between IPCC AR4 model ensembles and observations, especially the multi-decadal variability (MDV), "have cast shadows on the confidence of the model-based decadal projections of future climate," as has also been noted by Meehl et al. (2009), who indicate that considerably more work needs to be done in this important area.

What was done
In an exercise designed to illustrate the extent of this model failure, Fu et al. evaluated "many individual runs of AR4 models in the simulation of past global mean temperature," focusing on the performance of individual runs of models included in the Coupled Model Intercomparison Project phase three (CMIP3) in simulating the multi-decadal variability of the past global mean temperature.

What was learned
In the words of the three researchers, they determined that "most of the individual model runs fail to reproduce the MDV of past climate, which may have led to the overestimation of the projection of global warming for the next 40 years or so." More specifically, they note that simply taking into account the impact of the Atlantic Multi-decadal Oscillation or AMO, "the global average temperature could level off during the 2020s-2040s," such that the true temperature change between 2011 and 2050 "could be much smaller than the AR4 projection."

What it means
Apparently, the models upon which the IPCC bases its climate projections still fail to include the physics necessary to faithfully replicate even well-known features of earth's climate system, which should give one pause to wonder what else of a more stealth-type nature they might be missing.

Reference
Meehl, G.A., Goddard, L., Murphy, J., Stouffer, R.J., Boer, G., Danabasoglu, G., Dixon, K., Giorgetta, M.A., Greene, A.M., Hawkins, E., Hegerl, G., Karoly, D., Keenlyside, N., Kimoto, M., Kirtman, B., Navarra, A., Pulwarty, R.S., Smith, D., Stammer, D. and Stockdale, T. 2009. Decadal prediction: Can it be skillful? Bulletin of the American Meteorological Society 90: 1467-1485.