Does this all have either an implication for the global climate sensitivity (how much warming would result from a doubling of CO2) or the scenarios used by IPCC to project climate changes out to 2100? This is where I have to disagree most strongly with the commentary in the program. First, if we were trying to estimate climate sensitivity purely from the response over the 20th century, we would need to know a number of things quite exactly: chiefly the magnitude of all the relevant forcings. However, the uncertainties in the different aerosol effects in particular, preclude an accurate determination from the instrumental period alone. While it is true that, holding everything else equal, an increase in how much cooling was associated with aerosols would lead to an increase in the estimate of climate sensitivity, the error bars are too large for this to be much of a constraint. The estimate of 3+/-1 deg C (for doubled CO2) based on paleo-data and model studies is therefore still valid, even after this program.
7/29/08
