Background
The authors write that "climate models contain various uncertain parameters in the formulations of parameterizations for physical processes," and they say that "these parameters represent 'tunable knobs' that are typically adjusted to let the models reproduce realistic values of key-observed climate variables.".

What was done
In this study, Suzuki et al. examined "the validity of a tunable cloud parameter, the threshold particle radius triggering the warm rain formation in a climate model." And the model they chose for this purpose was the Geophysical Fluid Dynamics Laboratory (GFDL) Coupled Climate Model version 3 (CM3), because it is known that alternate values of that model's tunable cloud parameter that fall within its real-world range of uncertainty "have been shown to produce severely different historical temperature trends due to differing magnitudes of aerosol indirect forcing.

What was learned
The results of the three researchers' analysis indicated that "the simulated temperature trend best matches [the] observed trend when the model adopts the threshold radius that worst reproduces satellite-observed microphysical statistics and vice versa."

What it means
The three researchers state that "this inconsistency between the 'bottom-up' process-based constraint and the 'top-down' temperature trend constraint implies the presence of compensating errors in the model." And they note that "if this behavior is not a peculiarity of the GFDL CM3, the contradiction may be occurring in other climate models as well," which is not what one would want to find.