Background
The authors write that "consistent with our understanding of the records of past forcings," climate scientists associated with phase 3 of the Paleoclimate Modeling Intercomparison Project (PMIP3) and phase 5 of the Coupled Model Intercomparison project (CMIP5) proposed that "modeling groups perform the 'Last Millennium' simulation (LM; 850-1850 Common Era) with the same models and at the same resolutions as simulations being done to simulate the twentieth century and into the future," in order to allow for "an evaluation of the capability of models to capture observed variability on multidecadal and longer time scales."

What was done
In response to this proposal, Landrum et al. conducted just such a study of the Community Climate System Model, version 4 (CCSM4)," comparing its LM simulations to data-based reconstructions of LM temperature, the hydrologic cycle, and modes of climate variability."

What was learned
The seven scientists say that "the CCSM4 LM simulation reproduces many large-scale climate patterns suggested by historical and proxy-data records." However, they also report that (1) "the LM simulation does not reproduce La Niņa-like cooling in the eastern Pacific Ocean during the Medieval Climate Anomaly [MCA] relative to the Little Ice Age [LIA], as has been suggested by proxy reconstructions," that (2) in response to large volcanic eruptions, the CCSM4 simulates cooling "two to three times larger than the Northern Hemisphere summer anomalies estimated from tree-ring or multiproxy reconstructions," that (3) "patterns of simulated precipitation change for the Asian monsoon to large volcanic eruptions have nearly opposite anomalies from those reconstructed from tree-ring chronologies," and that (4) "we do not find a persistent positive NAO [North Atlantic Oscillation] or a prolonged period of negative PDO [Pacific Decadal Oscillation] during the MCA," such as is "suggested by the proxy reconstructions" of MacDonald and Case (2005) and Trouet et al. (2009).

What it means
Noting that some of the detected model deficiencies were also found to be operative in "LM simulations with an earlier version of CCSM," Landrum et al. provide further evidence for the "meet the new models, same as the old models" malady, which seems to literally be plaguing the climate-change prognosticators of today.

References
MacDonald, G.M. and Case, R.A. 2005. Variations in the Pacific decadal oscillation over the past millennium. Geophysical Research Letters 32: 10.1029/2005GL022478.

Trouet, V., Esper, J., Graham, N.E., Baker, A., Scourse, J.D. and Frank, D.C. 2009. Persistent positive North Atlantic oscillation mode dominated the medieval climate anomaly. Science 324: 10.1126/science.1166349.