Background
The authors write that "even in the absence of external forcing, climate models often exhibit long-term trends that cannot be attributed to natural variability," and they say that "this so-called climate drift arises for various reasons," such as "deficiencies in either the model representation of the real world or the procedure used to initialize the model." They note, however, that "significant efforts by the climate modeling community have gone into reducing climate drift." Nevertheless, they say that "climate drift still persists."

What was done
In light of the latter unfortunate fact - i.e., that climate drift still persists - Sen Gupta et al. "quantify the size of drift relative to twentieth-century trends in climate models taking part in the Coupled Model Intercomparison Project phase 3 (CMIP3)," which they say "was used to inform the Intergovernmental Panel on Climate Change (IPCC) Forth Assessment Report (AR4)."

What was learned
The seven Australian scientists determined that below 1-2 km in the deep ocean, or for depth-integrated properties, drift generally dominates over any forced trend. In fact, they report that drift in sea level can be large enough to actually reverse the sign of the forced change, "both regionally and in some models for the global average." In addition, because surface drift is spatially heterogeneous, they say that "the regional importance of drift for individual models can be much larger than the global figures suggest." As an example, they note that "a typical error in calculating a regional forced sea surface temperature trend in the Bjerknes Center for Climate Research Bergen Climate Model, version 2.0 (BCM2.0), CSIRO Mk3.0, and GISS-EH models without accounting for drift would be 30% to 40%." And because this is an average value, still larger errors would be expected at some locations.

What it means
Although providing some suggestions for chipping away at these problems, Sen Gupta et al. write that "in the absence of a clear direction forward to alleviate climate drift in the near term, it seems important to keep open the question of flux adjustment within climate models that suffer from considerable drift." However, they indicate that "flux adjustments are nonphysical and therefore inherently undesirable," and they say that "they may also fundamentally alter the evolution of a transient climate response," citing the work of Neelin and Dijkstra (1995) and Tziperman (2000), all of which pretty much places climate modelers somewhere between the proverbial rock and a hard place.

References
Neelin, J.D. and Dijkstra, H.A. 1995. Ocean-atmosphere interaction and the tropical climatology. Part I: The dangers of flux correction. Journal of Climate 8: 1325-1342.

Tziperman, E. 2000. Uncertainties in thermohaline circulation response to greenhouse warming. Geophysical Research Letters 27: 3077-3080.