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).

If all other forcings were fixed, a reduction of CO2 amount to 350 ppm would restore the planet’s energy balance, assuming that the present imbalance is 0.5 W/m2.  If fossil fuel emissions continue at anything
approaching “business-as-usual” scenarios, it is not feasible to restore planetary energy balance and stabilize climate.  However, stabilization of climate becomes a realistic objective if coal emissions are phased out and
unconventional fossil fuels (such as tar sands and oil shale) are not developed as substitutes for oil and gas as the oil and gas resources decline.  With these assumptions, the non-CO2 forcings become an important
factor in stabilizing climate.

Of course, all other forcings are not fixed, but with appropriate directed efforts it is realistic to keep the net future change of non-CO2 forcings near zero.  N2O will continue to increase, at least in the near future, but its growth could be slowed with improved fertilization techniques.  An N2O increase could be compensated by a decrease of CH4.  There is a realistic possibility of decreasing the source strength of CH4 emissions, and thus CH4 atmospheric amount.  However, if global warming continues, the CH4 source from melting of methane hydrates could increase.

Thus there is a coupling between the need to reduce CO2 and the possibility of reducing CH4.  Reflective aerosols are likely to decrease, thus adding a warming effect, but that warming effect may be compensated via an emphasis on reducing black soot aerosols.

The charts (from the powerpoint presentation) include an accurate status report on climate forcings by greenhouse gases.  Unfortunately, such data are not available for aerosols, but the NASA Glory mission, planned for launch late this year, promises to provide the first accurate global aerosol measurements.  Comments that accompany the powerpoint charts are included below.

Source: Jim Hansen

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