Background
In an Hydrology and Earth System Sciences "Opinion" article, the authors write that "despite considerable progress in recent years, output of both global and regional circulation models is still afflicted with biases to a degree that precludes its direct use, especially in climate change impact studies," noting that "this is well known, and to overcome this problem, bias correction (BC, i.e., the correction of model output towards observations in a post-processing step) has now become a standard procedure in climate change impact studies."

What was done
Ehret et al. present "a brief overview of state-of-the-art bias correction methods, discuss the related assumptions and implications, draw conclusions on the validity of bias correction and propose ways to cope with biased output of circulation models."

What was learned
The five German researchers state that (1) "BC methods often impair the advantages of circulation models by altering spatiotemporal field consistency, relations among variables and by violating conservation principles," that (2) "currently used BC methods largely neglect feedback mechanisms, that (3) "it is unclear whether they are time-invariant under climate change conditions," that (4) "applying BC increases agreement of climate model output with observations in hindcasts and hence narrows the uncertainty range of simulations and predictions," but that (5) this is often done "without providing a satisfactory physical justification," which sleight of hand "is in most cases not transparent to the end user."

What it means
Ehret et al. argue that this set of negative consequences of bias correction "hides rather than reduces uncertainty," which they suggest may lead to avoidable forejudging of end users and decision makers." And they thus conclude that BC is often "not a valid procedure."