Background
The authors write that "representing clouds and cloud climate feedback in global climate models (GCMs) remains a pressing challenge," but one that is needed to be overcome in order "to reduce and quantify uncertainties associated with climate change projections." And two of the primary parameters that must be accurately modeled in this regard are cloud ice water content (CIWC) and cloud ice water path (CIWP).

What was done
Li et al. performed, in their words, "an observationally based evaluation of the cloud ice water content and path of present-day GCMs, notably 20th century CMIP5 simulations," after which they compared the results to two recent reanalyses. This they did using "three different CloudSat + CALIPSO ice water products and two methods to remove the contribution from the convective core ice mass and/or precipitating cloud hydrometeors with variable sizes and falling speeds so that a robust observational estimate can be obtained for model evaluations."

What was learned
Unfortunately, the eleven U.S. scientists report that "for annual mean CIWP, there are factors of 2-10 in the differences between observations and models for a majority of the GCMs and for a number of regions," additionally noting that "systematic biases in CIWC vertical structure occur below the mid-troposphere where the models overestimate CIWC." And in light of these and other shortcomings they identify, they ultimately conclude that "neither the CMIP5 ensemble mean nor any individual model performs particularly well," adding that "there are still a number of models that exhibit very large biases," and this "despite the availability of relevant observations." What is more, even in cases where they indicate "the models may be providing roughly the correct radiative energy budget," they state that "many are accomplishing it by means of unrealistic cloud characteristics of cloud ice mass at a minimum, which in turn likely indicates unrealistic cloud particle sizes and cloud cover."

What it means
In response to the proverbial Are we there yet? question, the answer with regard to properly modeling earth's climate system and accurately explaining what makes it do what it does, would appear to be a resounding No! So far from success are we, in fact, that Li et al. conclude that "cloud feedback will undoubtedly still represent a key uncertainty in [even] the next IPCC assessment report [italics added]." And in that prediction they are probably correct.