Good point and a good paper. And just to emphasize, climate models (and
climatologists) don't make "predictions," they make "projections." That
is, they assume a certain scenario for the future -- CO2 emissions, land
use changes, etc. (For example, the IPCC Assessment Reports offers
dozens of scenarios.) Obviously though, no assumption or scenario can be
completely true, because the future emissions of CO2, land use, etc. are
subject to many factors that differ from a mathematical simplification.
Volcanoes go off at random times. Recessions happen. Some people switch
to coal. So you can only look at these projections withing a certain
uncertainty band. And the major point it, *all* scenarios lead to a
dangerous future. I think it's true that no climate model has ever been
constructed which _did not_ predict a warmer future.