[climate models] systematically over-estimate the sensitivity of climate to carbon dioxide ... and modelers exclude forcings and feedbacks that run counter to their mission
GCMs systematically over-estimate the sensitivity of climate to carbon dioxide (CO2), many known forcings and feedbacks are poorly modeled, and modelers exclude forcings and feedbacks that run counter to their mission to find a human influence on climate. Doubling the concentration of atmospheric CO2 from its pre-industrial level, in the absence of other forcings and feedbacks, would likely cause a warming of ~0.3°C to 1.1°C, almost 50 percent of which must already have occurred.