Policy-makers often suggest that climate model projections remain too uncertain to base policy on, frequently blaming the quality of the modeling science. But an under-appreciated fact is that policy-makers are themselves injecting roughly the same amount of uncertainty into the models. (Adam Sobel helped me fully appreciate this fact.)
The range of projections for a single emission scenario in 2100 (in the IPCC graph below, one of the grey bars on the right) is probably about 2.5 degrees Celsius. But the range of average projections across the various emissions scenarios (compare the colored mean-stripe across the grey bars) is about 2 degrees Celsius. The climate modelers are "responsible" for reducing the uncertainty of a projection for a given emissions scenario, but the policy-makers are "responsible" for determining which emissions scenario we're on. Thus, about half of our uncertainty regarding climate-induced changes by 2100 is due to our inability to choose a policy trajectory.