Background
The authors write that "clouds are the primary modulators of the Earth's radiation budget" and that they therefore constitute "the main source of uncertainty in model estimates of climate sensitivity," citing Randall et al. (2007); and as a result of this fact, they say that the modeling of cloud properties represents "a major limitation to the reliability of climate change projections," citing Dufresne and Bony (2008). Faced with this problem, Cesana and Chepfer thus indicate that in order "to improve the reliability of climate change projections, it is therefore imperative to improve the representation of cloud processes in models." But how much improving do the models need?

What was done
In broaching this important question, the two French researchers compare the most recent cloud representations of five of the climate models involved in the Coupled Model Intercomparison Project Phase 5 (CMIP5) effort that is describe by Taylor et al. (2012) with real-world satellite-derived observations obtained from the GCM-Oriented CALIPSO Cloud Product (GOCCP), which is described by Chepfer et al. (2010). And what did they learn from this exercise?

What was learned
In the words of Cesana and Chepfer: (1) "low- and mid-level altitude clouds are underestimated by all the models (except in the Arctic)," (2) "high altitude cloud cover is overestimated by some models," (3) "some models shift the altitude of the clouds along the ITCZ by 2 km (higher or lower) compared to observations," (4) "the models hardly reproduce the cloud free subsidence branch of the Hadley cells," (5) "the high-level cloud cover is often too large," (6) "in the tropics, the low-level cloud cover (29% in CALIPSO-GOCCP) is underestimated by all models in subsidence regions (16% to 25%)" and (7) "the pronounced seasonal cycle observed in low-level Arctic clouds is hardly simulated by some models."

What it means
If a climate modeler asks you are we there yet? - in terms of state-of-the-art models being good enough to predict the climatic future of the planet to a degree deemed adequate to inform both domestic and foreign policy on preferred energy sources of the future - you need to tell him or her no way!

References
Chepfer, H., Bony, S., Winker, D., Cesana, G., Dufresne, J.L., Minnis, P., Stubenrauch, C.J. and Zeng, S. 2010. The GCM Oriented CALIPSO Cloud Product (CALIPSO-GOCCP). Journal of Geophysical Research 115: 10.1029/2009JD012251.

Dufresne, J.-L. and Bony, S. 2008. An assessment of the primary sources of spread of global warming estimates from coupled atmosphere-ocean models. Journal of Climate 21: 5135-5144.

Randall, D.A., Wood, R.A., Bony, S., Coleman, R., Fichefet, T., Fyfe, J., Kattsov, V., Pitman, A., Shukla, J., Srinivasan, J., Stouffer, R.J., Sumi, A. and Taylor, K.E. 2007. Chapter 8: Climate Models and Their Evaluation. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom.

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