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Last Millennium Climate Simulations Still Falling Short of Reality
Landrum, L., Otto-Bliesner, B.L., Wahl, E.R., Conley, A., Lawrence, P.J., Rosenbloom, N. and Teng, H. 2013. Last millennium climate and its variability in CCSM4. Journal of Climate 26: 1085-1111.

"The purpose of this paper," in the words of its authors, "is to provide an overview of the Last Millennium [LM] simulation of the Community Climate System Model, version 4 (CCSM4)."

What was done
To accomplish their stated objective, Landrum et al. compared LM simulations to real-world "data reconstructions of temperature, the hydrologic cycle, and modes of climate variability."

What was learned
In addition to some successes of the CCSM4, the seven scientists report a number of negative findings, namely, that (1) "the LM simulation does not reproduce La Niña-like cooling in the eastern Pacific Ocean during the MCA [Medieval Climate Anomaly] relative to the LIA [Little Ice Age], as has been suggested by proxy reconstructions," that (2) "patterns of simulated precipitation change for the Asian monsoon to large volcanic eruptions have nearly opposite anomalies from those reconstructed from tree-ring chronologies [italics added]," that (3) "the model simulates cooling of ~1.0°-1.5°C after the large eruptions of the late thirteenth, mid-fifteenth, late eighteenth, and early nineteenth centuries, 2-3 times larger than the NH [Northern Hemisphere] summer anomalies estimated from tree-ring or multiproxy reconstructions [italics added]," that (4) "twentieth-century simulations indicate that the CCSM4 hemispheric response to volcanic eruptions is stronger than observed (Meehl et al., 2012)," and that (5) they "do not find a persistent positive NAO or a prolonged period of negative PDO during the MCA suggested by the proxy reconstructions (MacDonald and Case, 2005; Trouet et al., 2009)."

What it means
The several model failures listed above clearly demonstrate that current state-of-the-art climate models still have a long ways to go before their projections about the future can be given much credence.

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.

Meehl, G. A., Washington, W.M., Arblaster, J.M., Hu, A., Teng, H., Tebaldi, C., Sanderson, B.N., Lamarque, J.-F., Conley, A., Strand, W.G. and White III, J.B. 2012. Climate system response to external forcings and climate change projections in CCSM4. Journal of Climate 25: 3661-3683.

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: 78-80.

Reviewed 7 April 2013