Guest Post by Jos de Laat (Jos), Ph.D, Royal Netherlands Meteorological Institute.
Following the ongoing debate on this weblog with Andy Lacis I could not resist weighing in on the recent discussion. Although Dr. Lacis notes that “aerosols are the really big uncertainty”, later on it is suggested that the aerosol direct and indirect effects nevertheless are still “well known”. It is a sort of ambiguity that I have ran across very often, and from what I can determine it appears that this ambiguity has everything to do with aerosols acting as a “turning knob” to constrain climate models in their reproduction of 20th century temperature change.
“Recently I have been focusing in my work a little bit more on aerosols as well as climate modeling. I have started working on some interesting new aspects of aerosols (GRL paper in press: http://www.agu.org/journals/pip/gl/2010GL045171-pip.pdf), have been contributing to a more philosophical paper about the assessment of climate models, and I’m currently writing a book chapter about aerosols. While gathering information for this book I came across the 2009 CCSP report on aerosols. It contains some interesting statements that I think are worth mentioning.”
For example, the executive summary states that (boldface added):
“Calculated change of surface temperature due to forcing by anthropogenic aerosols was reported in IPCC AR4 based on results from more than 20 … modeling groups. Despite a wide range of climate sensitivity … exhibited by the models, they all yield a global average temperature change very similar to that observed over the past century. This agreement across models appears to be a consequence of the use of very different aerosols forcing values which compensate for the range of climate sensitivity. For example, the direct cooling effect of sulfate aerosol varied by a factor of six among the models. An ever greater disparity was seen in the model treatment of black carbon and organic carbon. … For those models that include the indirect effect, the aerosol effect on cloud brightness (reflectivity) varied by up to a factor of nine. Therefore, THE FACT THAT MODELS HAVE REPRODUCED THE GLOBAL TEMPERATURE IN THE PAST DOES NOT IMPLY THAT THEIR FUTURE FORECASTS ARE ACCURATE.”
“On a global average basis, the sum of direct and indirect forcing by anthropogenic aerosols at the top of the atmosphere is almost certainly negative (a cooling influence), and thus almost certainly offsets a FRACTION of the positive (warming) due to anthropogenic greenhouse gases. However, because of the spatial and temporal non-uniformity of the aerosol RF, and likely differences in the effects of shortwave and longwave forcings, THE NET EFFECT ON EARTH’S CLIMATE IS NOT SIMPLY A FRACTIONAL OFFSET TO THE EFFECTS OF FORCING BY ANTHROPOGENIC GREENHOUSE GASES.”
“Although the nature and geographical distribution of forcings by greenhouse gases and aerosols are quite different, it is often assumed that to first approximation these forcings on global mean surface temperature are additive, so that the negative forcing by anthropogenic aerosols has partly offset the positive forcing by incremental greenhouse gas increases over the industrial period. … … However, since aerosol forcing is much more pronounced on regional scales than on the global scale because of the highly variable aerosol distributions, IT WOULD BE INSUFFICIENT OR EVEN MISLEADING TO PLACE TOO MUCH EMPHASIS ON THE GLOBAL AVERAGE. Also, aerosol RF at the surface is stronger than at the TOA, exerting large impacts within the atmosphere to alter the atmospheric circulation patterns and water cycle. THEREFORE, IMPACTS OF AEROSOLS ON CLIMATE SHOULD BE ASSESSED BEYOND THE LIMITED ASPECT OF GLOBALLY AVERAGE RADIATIVE FORCING AT TOA.”
Hence, aerosols remain a big enigma and will continue to do so until – as the report notes – “a firmer estimate of the radiative forcing by aerosols, as well as climate sensitivity, is available”. To what extent this can be achieved in the near future probably depends on the continuation of the current remote sensing capacity like for example assembled with the A-train satellites. Unfortunately, and despite its tremendous success, there are no immediate plans to continue/upgrade/replace most of the A-train missions beyond the current orbiting satellites. Hence, it is not inconceivable that the uncertainties with regard to the aerosol radiative forcing and thus constrains on the climate sensitivity will remain for a long time.