Wednesday, January 16, 2013

Settled science update: New paper finds aerosols cause 24 times more cooling than estimated by the IPCC

A new paper published in Atmospheric Chemistry & Physics finds that aerosols have a very significant cooling effect upon the Earth surface that is 24 times greater than estimated by the IPCC. According to the authors, the global, annual aerosol radiative effect is -12 Watts per square meter at the Earth surface, or 24 times more negative than "the current estimate of the global mean radiative forcing due to all anthropogenic aerosols given by the Intergovernmental Panel on Climate Change of (−0.5 ± 0.4) W m−2." Meanwhile, as Tom Nelson notes today, "consensus" climate scientists are saying "the impact of aerosol emissions could end up being either significantly positive or negative" and “It’s not over yet.”

By way of comparison, the IPCC alleges doubled CO2 levels would cause 1 Wm-2 positive forcing at the surface, much more than offset by an aerosol negative forcing of -12 Wm-2 at the surface. 

Super-settled science update: "the impact of aerosol emissions could end up being either significantly positive or negative"

Soot a major contributor to climate change : Nature News & Comment
Fahey also acknowledges the remaining uncertainties in the study. Although black carbon contributes to warming, he says, the impact of aerosol emissions could end up being either significantly positive or negative. “It’s not over yet.”

Atmos. Chem. Phys., 13, 393-410, 2013

Regional and monthly and clear-sky aerosol direct radiative effect (and forcing) derived from the GlobAEROSOL-AATSR satellite aerosol product

G. E. Thomas1, N. Chalmers2, B. Harris2, R. G. Grainger1, and E. J. Highwood2
1Atmospheric, Oceanic and Planetary Physics, University of Oxford, Oxford, UK
2Department of Meteorology, University of Reading, Reading, UK

 Abstract. Using the GlobAEROSOL-AATSR dataset, estimates of the instantaneous, clear-sky, direct aerosol radiative effect and radiative forcing have been produced for the year 2006. Aerosol Robotic Network sun-photometer measurements have been used to characterise the random and systematic error in the GlobAEROSOL product for 22 regions covering the globe. Representative aerosol properties for each region were derived from the results of a wide range of literature sources and, along with the de-biased GlobAEROSOL AODs, were used to drive an offline version of the Met Office unified model radiation scheme. In addition to the mean AOD, best-estimate run of the radiation scheme, a range of additional calculations were done to propagate uncertainty estimates in the AOD, optical properties, surface albedo and errors due to the temporal and spatial averaging of the AOD fields. This analysis produced monthly, regional estimates of the clear-sky aerosol radiative effect and its uncertainty, which were combined to produce annual, global mean values of (−6.7 ± 3.9) W m−2 at the top of atmosphere (TOA) and (−12 ± 6) W m−2 at the surface. These results were then used to give estimates of regional, clear-sky aerosol direct radiative forcing, using modelled pre-industrial AOD fields for the year 1750 calculated for the AEROCOM PRE experiment. However, as it was not possible to quantify the uncertainty in the pre-industrial aerosol loading, these figures can only be taken as indicative and their uncertainties as lower bounds on the likely errors. Although the uncertainty on aerosol radiative effect presented here is considerably larger than most previous estimates, the explicit inclusion of the major sources of error in the calculations suggest that they are closer to the true constraint on this figure from similar methodologies, and point to the need for more, improved estimates of both global aerosol loading and aerosol optical properties.

 Final Revised Paper (PDF, 3887 KB)   Discussion Paper (ACPD)   


  1. Hi MS,

    How does a claim that the aerosol effect could be 20 times stronger than the IPCC thinks help if you do not believe that CO2 is causing a large warming? You need the net aerosol effect to be zero or causing warming don't you to support your position don't you?

    1. Alex,

      1. The main point of this post is to show that the vast uncertainties in climate science (including aerosols, clouds, solar amplification, ocean oscillations, and a host of other issues) are being hidden from the debate under a facade of the "IPCC consensus"

      2. The alleged climate effects of CO2 & GHGs are independent of aerosol effects, and thus a marked increase in aerosol cooling effects does not imply that the warming effect of GHGs is greater than assumed by the IPCC

  2. Hi MS,

    Sure, but there is no controversy over radiative forcing itself or the fact that a doubling of CO2 causes a forcing of about 3.7 W/m^2. So it's common sense that if aerosols are causing a massive -12 W/m^2 of cooling - of magnitude greater than 3 doublings of CO2! - something very very big must be making up for the difference. Personally, I will be hoping that the answer isn't very, very strong positive feedbacks!

    Alternatively, let's just hope this result is completely wrong. :-)