Air Pollution as a Climate Forcing: A Workshop
Day 1 Presentations
Efficacy of Air Pollution Climate Forcings
Larissa Nazarenko*+, James Hansen*, Makiko Sato*+, Reto Ruedy*°
* NASA Goddard Institute for Space Studies, New York, NY, U.S.A.
+ Center for Climate System Research, Columbia University Earth Institute, New York, NY, U.S.A.
° SGT, Inc., Goddard Institute for Space Studies, New York, NY, U.S.A.
Climate "forcings", i.e., the change of planetary energy balance caused by an imposed change such as increased CO2 or changed solar irradiance, are assumed to provide a useful measure of the expected global climate response. However, Hansen et al. (1997) found that calculated forcings by absorbing aerosols and ozone do not provide a good indication of the climate response calculated by a general circulation model with idealized ("Wonderland") geography. Because these forcings are prominent among "air pollution", we have made new calculations of the climate response to several air pollution climate forcings using recent versions of the GISS climate model with realistic geography.
We calculate the equilibrium climate response to black carbon (BC) and sulfate aerosols, ozone (O3) and methane (CH4) climate forcings, and we define a global mean efficacy of each forcing relative to the response by an equivalent carbon dioxide (CO2) forcing. The results reaffirm that the efficacy of a given forcing, i.e., its effectiveness in producing a global temperature change, depends on the geographical distribution of the forcing agent and on its vertical profile in the atmosphere. However, the results also show that the air pollution forcings have an efficacy in the same ballpark as that of CO2.
Figure 1 shows the global distribution of the radiative forcing for (a) doubled CO2, (b) change in tropospheric ozone from pre-industrial time to 1980 based on Wang and Jacob (1998), (c) five times the BC change between 1950 and 1990 based on Koch (2001), (d) five times the sulfate change between 1950 and 1990 based on Koch (2001). Figure 2 shows the corresponding changes of surface air temperature for the last 150 years of 250-year simulations with the GISS global climate model.
Anthropogenic tropospheric O3 with geographical distribution based on simulations of Wang and Jacob (1998) yields a radiative forcing (flux change at the tropopause) of 0.38 W/m2 and equilibrium global warming of 0.21°C, corresponding to an efficacy ~90% of that for CO2 (accounting for the decrease of climate sensitivity with increasing magnitude of the forcing). The BC change between 1950 and 1990 simulated by Koch (2001) has a forcing of 0.14 W/m2 and yields an equilibrium global warming of 0.06°C, an efficacy of ~70%. However, the BC aerosols simulated by Koch (2001) are distributed rather high in the troposphere; if the BC instead falls off above the surface with a 1 km scale height, the global warming increases to 0.12°C, which is a forcing efficacy of ~140%. The change of sulfate aerosols calculated by Koch (2001) yields a forcing of -0.34 W/m2 from which we calculate a global cooling of 0.25°C for a forcing efficacy of ~100%. [This is calculated relative to negative change of CO2, because the climate is more sensitive to a negative forcing than to a positive forcing of equal magnitude.]
One reason that some of these pollutants have a global mean efficacy less than unity is that the constituent perturbations, and thus their forcings, are concentrated at middle and low latitudes. Hansen et al. (1997) found that high latitude forcings are more effective in altering global mean temperature than low latitude forcings, because they bring ice and snow feedbacks into play more strongly and because the more stable temperature profile at high latitudes enhances the surface temperature response there.
Methane, because of its long atmospheric lifetime, causes a forcing that is more globally uniform, similar to the forcing distribution for CO2. If the effects of CH4 on stratospheric H2O and tropospheric O3 are included, the efficacy of CH4 as a climate forcing is about 140%. The high efficacy of CH4 as a climate forcing makes it a very attractive target for efforts to slow global warming.
- Hansen, J., M. Sato and R. Ruedy, Radiative forcing and climate response, J. Geophys. Res., 102, 6831-6864, 1997.
- Koch, D., Transport and direct radiative forcing of carbonaceous and sulfate aerosols in the GISS GCM, J. Geophys. Res., 106, 20311-20332, 2001.
- Wang, Y. and D.J. Jacob, Anthropogenic forcing on tropospheric ozone and OH since pre-industrial times, J. Geophys. Res., 103, 31123-31135, 1998.