What was done
The author reviews the history of climate model predictions of greenhouse-gas-induced global warming and the uncertain role played by aerosols in this frustrating enterprise.

What was learned
Schwartz notes that the National Research Council (1979) concluded that "climate sensitivity [to CO2 doubling] is likely to be in the range 1.5-4.5°C," and that, "remarkably, despite some two decades of intervening work, neither the central value nor the uncertainty range has changed."  [If anything, we note, it has increased, as an even greater range (1.4-5.8°C) is bandied about nowadays.]  This continuing uncertainty, in his opinion, "precludes meaningful model evaluation by comparison with observed global temperature change or empirical determination of climate sensitivity," and "raises questions regarding claims of [the models] having reproduced observed large-scale changes in surface temperature over the 20th century."

What it means
Schwartz contends that climate model predictions of CO2-induced global warming "are limited at present by uncertainty in radiative forcing of climate change over the industrial period, which is dominated by uncertainty in forcing by aerosols," and that if this situation is not improved, "it is likely that in another 20 years it will still not be possible to specify the climate sensitivity with [an] uncertainty range appreciably narrower than it is at present."  Indeed, he states that "the need for reducing the uncertainty from its present estimated value by at least [our italics] a factor of 3 [our italics] and perhaps a factor of 10 [our italics] or more [our italics] seems inescapable [our italics] if the uncertainty in climate sensitivity is to be reduced to an extent where it becomes useful for formulating policy to deal with global change," which surely suggests that even the best climate models of the day are wholly inadequate for this purpose.

Reference
National Research Council, 1979.  Carbon Dioxide and Climate: A Scientific Assessment.  National Academy of Sciences, Washington, DC, USA.