What was done
The authors analyzed a comprehensive observational dataset of solar radiation fluxes measured at 720 sites at the earth's surface and the corresponding top of the atmosphere to assess the true amount of solar radiation absorbed within the atmosphere, comparing the results to the modeled solar radiation absorption in four general circulation models (GCMs) of the atmosphere.

What was learned
The authors found that "GCM atmospheres are generally too transparent for solar radiation," as they typically calculate a mean global value of around 70 Wm^-2 for the solar radiation absorbed in the atmosphere, when, in reality, surface and satellite measurements suggest the amount of solar radiation absorbed in the atmosphere should be 10 - 20 Wm^-2 higher.

What it means
The discrepancy between true and modeled atmospheric solar radiation absorption discovered by the authors is rather substantial, representing, in the mean, an error of close to 20%.  If the GCMs suffer from such a large degree of error in the conduct of so basic a task as the calculation of the amount of incoming sunlight captured by the atmosphere, one can only wonder what errors the models may make in their assessments and representations of more complex quantities and processes.  Could it be that these errors are more than substantial?