What was done
The author - Stewart W. Franks from the Centre for Environmental Dynamics of the University of Newcastle in New South Wales, Australia - reviews what is known about the abilities of atmospheric general circulation models (GCMs) to represent real-world processes of significance to climate change.  He also discusses the usefulness of GCMs in providing impact assessments of projected climate changes that are presumed to be driven by increases in atmospheric concentrations of greenhouse gases.  These analyses are conducted within the context of hydrological concerns and are contrasted with similar assessments that can be made on the basis of empirical correlations of climate with various solar phenomena.

What was learned
The author notes that "an obvious weak point of GCMs is their inability to model properly the physics of clouds."  In this regard, he correctly states that "various GCMs vary considerably, and often produce quite different results," which is not what one wants in attempting to anticipate the future.  Franks also correctly notes that other problems with state-of-the-art GCMs include "grossly uncertain land surface representations, the problem of sub-pixel heterogeneity, and, perhaps most seriously, the attendant problems of translating GCM output into hydrologically meaningful variables."  Finally, he concludes that "in a strict Popperian view of science, the testing of GCMs is inadequate, as model structures (i.e. different sets of hypotheses) are not rigorously evaluated."

To truly understand the potential impacts of climate change, Franks notes that "greater knowledge of natural climate variability is required."  In this context, he describes several recently documented climate shifts that have occurred at different points of the past century.  He then reviews what might have caused these decadal and multi-decadal changes and concludes there is a tremendous amount of evidence for "an external solar control," as summarized over the years by Reid (1991, 1997, 1999, 2000).

Franks' final conclusion is that the coherence displayed by temperature and solar irradiance trends over the 20th century strongly suggests that "documented hydrological changes in regional climates ... may be driven by solar-terrestrial interactions," implying that this linkage holds the key to making future hydrological forecasts.  "Until GCMs can elucidate the mechanisms of hydrological variability," Franks wisely warns, any of their projections of long-term future climate changes "must be viewed with obvious caution."

What it means
Today's most sophisticated climate models are still not up to the task required of them, i.e., correctly simulating earth's climate system and making valid climate change projections.

References
Reid, G.C.  1991.  Solar total irradiance variations and the global sea surface temperature record.  Journal of Geophysical Research 96: 2835-2844.

Reid, G.C.  1997.  Solar forcing of global climate change since the 17th century.  Climatic Change 37: 391-405.

Reid, G.C.  1999.  Solar variability and its implications for the human environment.  Journal of Atmospheric and Solar-Terrestrial Physics 61(1-2): 3-14.

Reid, G.C.  2000.  Solar variability and the Earth's climate: introduction and overview.  Space Science Reviews 94(1-2): 1-11.