Penner and her colleagues found faults in the techniques that satellite estimates use to find the difference between cloud drop concentrations today and before the Industrial Revolution.
“We found that using satellite data to try to infer how much radiation is reflected today compared to the amount reflected in the pollution-free pre-industrial atmosphere is very inaccurate,” Penner said. “If one uses the relationship between aerosol optical depth — essentially a measure of the thickness of the aerosols — and droplet number from satellites, then one can get the wrong answer by a factor of three to six.”
These findings are a step toward generating better models, and Penner said that will be the next phase of this research.
Note — this is not saying that the aerosol forcing is underestimated by a factor of 3 to 6. The paper’s abstract says it’s more like 25% to 35%, which is still uncomfortably high, as it implies an even bigger whiplash from our ongoing attempts to scrub the sulfur out of coal plant emissions.
You can get the paper here.
As a quick reminder, here’s the relevant chart from the latest IPCC report:
Notice the two blue bars near the bottom (“Total Aerosol”), and how large their error bars are. And also note that if you sum the aerosol effects they offset a large portion of the CO2 forcing (about 1.3 of the 1.6 Watts / m2). Add in that 25% to 35% correction mentioned above, and suddenly aerosols are offsetting all of the CO2 forcing and a smidge more.
As “it’s worse than we thought” news goes, this is truly nasty, and I certainly hope it proves to be wrong.