Background
The authors write that "the last generation of models, those comprising [the] Climate Model Intercomparison Project III (CMIP3) archive, was unable to capture key statistics characterizing decadal to multidecadal (D2M) precipitation fluctuations," noting specifically that "CMIP3 simulations overestimated the magnitude of high frequency fluctuations and consequently underestimated the risk of future decadal-scale droughts." And they say that since "a new generation of coupled general circulation models (GCMs) has been developed and made publicly available as part of the Climate Model Intercomparison Project 5 (CMIP5) effort," it is critical "to evaluate the ability of these models to simulate realistic 20th century variability regionally and across a variety of timescales," which they thus go on to do.

What was done
Using gridded (2.5 x 2.5) version 4 reanalysis product data made available to them by the Global Precipitation Climatology Centre (Rudolf et al., 2005) - which spans the period January 1901 through December 2007 - Ault et al. assessed the magnitude of D2M variability in new CMIP5 simulations.

What was learned
The three U.S. researchers say their results suggest that "CMIP5 simulations of the historical era (1850-2005) underestimate the importance [of] D2M variability in several regions where such behavior is prominent and linked to drought," namely, "northern Africa (e.g., Giannini et al., 2008), Australia (Cai et al., 2009; Leblanc et al., 2012), western North America (Seager, 2007; Overpeck and Udall, 2010), and the Amazon (Marengo et al., 2011)."

What it means
Ault et al. state that "the mismatch between 20th century observations and simulations suggests that model projections of the future may not fully represent all sources of D2M variations," noting that "if observed estimates of decadal variance are accurate, then the current generation of models depict D2M precipitation fluctuations that are too weak, implying that model hindcasts and predictions may be unable to capture the full magnitude of realizable D2M fluctuations in hydroclimate," with the result that "the risk of prolonged droughts and pluvials in the future may be greater than portrayed by these models."

Oh, well. The climate modelers can always try again when CMIP7 rolls around.

References
Cai, W., Cowan, T., Briggs, P. and Raupach, M. 2009. Rising temperature depletes soil moisture and exacerbates severe drought conditions across southeast Australia. Geophysical Research Letters 36: 10.1029/2009GL040334.

Giannini, A., Biasutti, M., Held, I.M. and Sobel, A.H. 2008. A global perspective on African climate. Climatic Change 90: 359-383.

Leblanc, M., Tweed, S., Van Dijk, A. and Timbal, B. 2012. A review of historic and future hydrological changes in the Murray-Darling Basin. Global and Planetary Change 80-81: 226-246.

Marengo, J.A., Tomasella, J., Alves, L.M., Soares, W.R. and Rodriguez, D.A. 2011. The drought of 2010 in the context of historical droughts in the Amazon region. Geophysical Research Letters 38: 10.1029/2011GL047436.

Overpeck, J. and Udall, B. 2010. Dry times ahead. Science 328: 1642-1643.

Rudolf, B., Beck, C., Grieser, J. and Schneider, U. 2005. Global Precipitation Analysis Products of Global Precipitation Climatology Centre (GPCC). Technical Report. Dtsch. Wetterdienst, Offenbach, Germany.

Seager, R. 2007. The turn of the century North American drought: Global context, dynamics, and past analogs. Journal of Climate 20: 5527-5552.