A talk by Jim Hansen and coworkers on the role of non-CO2 pollutants, given at the Climate Change Conference in Copenhagen, sounds yet again optimistic that we can avoid climate destabilization. Non CO2 forcings take a surprisingly large part in the total amount of climate forcings, but they can be reduced with appropriate policies. The main problem remains to be CO2 emissions from fossil fuels. Therefore, Jim Hansen again calls for strict and drastic policies to drastically reduce CO2 emissions, especially by quickly moving away from coal-fired power plants that cannot sequester CO2.

We note that it will be exceedingly difficult to determine the aerosol climate forcing relative to pre-industrial climate.  However, for policy purposes it may be sufficient to start with the present situation and consider climate forcing changes relative to today.  The planet’s present energy imbalance, at least to first order, determines the change of climate forcings needed to stabilize climate.  Climate models, using typical presumed scenarios of climate forcings for the past century, suggest that the planet should be out of energy balance by +0.75 ± 0.25 W/m2, but observations of ocean heat content change (averaged over the 11-year solar cycle) suggest an imbalance of only +0.5 ± 0.25 W/m2 (absorbed solar energy exceeding heat radiation to space). (more…)