Smoke gets in Eli's ears
There have been interesting developments in the big smoke. To start with something old, Gunnar Myhre has narrowed the uncertainty for directs aerosol forcing from -0.9 to -0.1 W/m2 to -0.3 + 0.2 W/m2. This means that not only is the negative direct aerosol forcing less than previously thought but that the range is much smaller.
In the Intergovernmental Panel on Climate Change Fourth AssessmentReport, the direct aerosol effect is reported to have a radiative forcing estimate of –0.5 Watt per square meter (W m–2), offsetting the warming from CO2 by almost one-third. The uncertainty,however, ranges from –0.9 to –0.1 W m–2, which is largely due to differences between estimates from global aerosol models and observation-based estimates, with the latter tending to have stronger (more negative) radiative forcing. This study demonstrates consistency between a global aerosol model and adjustment to an observation-based method, producing a global and annual mean radiative forcing that is weaker than –0.5 W m–2, with a best estimate of –0.3 W m–2. The physical explanation for the earlier discrepancy is that the relative increase in anthropogenic black carbon(absorbing aerosols) is much larger than the overall increase in the anthropogenic abundance of aerosols.Myhre compares results from observation based estimates of aerosol optical depth and those derived from modeling. The observation based estimates start with measured aerosol optical depth (AOD) or radiative emission. Estimates are made of the human contribution to the scattering. In the second approach aerosol models are used to estimate how humans have changed the amount and type of aerosol with radiative transfer models that calculate the forcing. Previously there was a significant difference between the two. A key assumption for the observational models has been that the nature of pre-industrial aerosols was the same as today but the amount has simply increased. Myhre observes that the black carbon component has grown roughly twice as fast as the other anthropogenic aerosols. When the observational models are corrected for this, they come into agreement with the model based estimates.
The much stronger increase in the anthropogenic fraction of absorption AOD than total AOD will to some extent be model-dependent but will be robust because of the much stronger growth in the BC emissions than the growth in the rest of the aerosols and their precursors. This suggests that the global annual average single scattering albedo of the aerosols has been reduced because of human activity. The global mean annual average single scattering albedo computed in the model for all aerosols at 5 µm is 0.986 at preindustrial conditions and 0.970 at present-day conditions. Thus, aerosol in present times is approximately twice as absorbing as that in preindustrial conditions.The net effect would be to slightly lower the estimated greenhouse gas forcing since a higher aerosol forcing would have to be counteracted by a larger CO2 forcing. Watch the bounding ball.