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Aerosols (General) -- Summary
What role do aerosols play in modulating earth's climate?  This question was addressed by Breon et al. (2002), who studied aerosol effects on cloud microphysics around the globe based on aerosol concentration and cloud droplet radii data obtained from the polarization and directionality of the earth reflectances (POLDER) instrument on the Advanced Earth-Observing Satellite (ADEOS), which began operation on 30 October 1996 and concluded on 30 June 1997.  The results of this study, in their words, "clearly demonstrate a significant impact of aerosols on cloud microphysics."  Specifically, they found that as aerosol concentrations increased, cloud droplet radii decreased, which phenomenon should have produced a cooling influence due to the greater albedo generally associated with smaller cloud droplets.  Breon et al. additionally determined that "the bulk of the aerosol load originates from slash-and-burn agriculture practices and from highly polluted areas," such that "a large fraction of the observed aerosol effect on clouds is probably of anthropogenic origin."  Although they were unable to quantify the degree of cooling provided by the aerosols they studied, they nevertheless demonstrated that this anthropogenic counterforce to the warming impetus provided by the ongoing rise in the air's CO2 content "is significant and occurs on a global scale."

In a study that focused on non-anthropogenic aerosols and did discuss magnitudes, Vogelmann et al. (2003) report that "mineral aerosols have complex, highly varied optical properties that, for equal loadings, can cause differences in the surface IR flux between 7 and 25 Wm-2 (Sokolik et al., 1998)," and that "only a few large-scale climate models currently consider aerosol IR effects (e.g., Tegen et al., 1996; Jacobson, 2001) despite their potentially large forcing."  In an attempt to persuade climate modelers to rectify this situation, they used high-resolution spectra to obtain the IR radiative forcing at the earth's surface for aerosols encountered in the outflow from northeastern Asia, based on measurements made by the Marine-Atmospheric Emitted Radiance Interferometer from the NOAA Ship Ronald H. Brown during the Aerosol Characterization Experiment-Asia.  As a result of this work, the five scientists determined that "daytime surface IR forcings are often a few Wm-2 and can reach almost 10 Wm-2 for large aerosol loadings."  These values, in their words, "are comparable to or larger than the 1 to 2 Wm-2 change in the globally averaged surface IR forcing caused by greenhouse gas increases since pre-industrial times" and "highlight the importance of aerosol IR forcing which should be included in climate model simulations."

In another study that addressed the magnitude of aerosol radiative effects, Chou et al. (2002) analyzed aerosol optical properties retrieved from the satellite-mounted Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and used them in conjunction with a radiative transfer model of the planet's atmosphere to calculate the climatic effects of aerosols over earth's major oceans.  In general, this effort revealed that "aerosols reduce the annual-mean net downward solar flux by 5.4 Wm-2 at the top of the atmosphere, and by 5.9 Wm-2 at the surface."  During the large Indonesian fires of September-December 1997, however, the radiative impetus for cooling at the top of the atmosphere was more than 10 Wm-2, while it was more than 25 Wm-2 at the surface of the sea in the vicinity of Indonesia.

These latter results are similar to those obtained earlier by Wild (1999), who used a comprehensive set of collocated surface and satellite observations to calculate the amount of solar radiation absorbed in the atmosphere over equatorial Africa and compared the results with the predictions of three general circulation models of the atmosphere.  This work revealed that the climate models did not properly account for spatial and temporal variations in atmospheric aerosol concentrations, leading them to predict regional and seasonal values of solar radiation absorption in the atmosphere with underestimation biases of up to 30 Wm-2.

By way of comparison, as noted in the study of Vogelmann et al., the globally averaged surface IR forcing caused by greenhouse gas increases since pre-industrial times is only 1 to 2 Wm-2.  Hence, it can be appreciated that over much of the planet's surface, the radiative cooling influence of atmospheric aerosols (many of which are produced by anthropogenic activities) must prevail, suggesting a probable net anthropogenic-induced climatic signal that must be very close to zero and nowhere near capable of producing what climate alarmists refer to as the unprecedented warming of the 20th century.  We thus conclude that the air temperature record on which they rely is either grossly in error or that the warming, if real, is due to something other than anthropogenic CO2 emissions.

Aerosol uncertainties and the problems they generate also figure prominently in the study of Anderson et al. (2003), who note there are two different ways by which the aerosol forcing of climate may be computed, as described in our Editorials of 14 Apr 2004 and 5 May 2004.  The first of these approaches is that of forward calculation, which is based, in their words, on "knowledge of the pertinent aerosol physics and chemistry."  The second approach is inverse calculation, based on "the total forcing required to match climate model simulations with observed temperature changes."

The first approach, which relies heavily on first principles, utilizes known physical and chemical laws and assumes nothing about the outcome of the calculation.  The second approach, in considerable contrast, is based on matching residuals, where the aerosol forcing is computed from what is required to match the calculated change in temperature with the observed change over some period of time.  Consequently, in the words of Anderson et al., "to the extent that climate models rely on the results of inverse calculations, the possibility of circular reasoning arises."

So which approach do climate models typically employ?  "Unfortunately," according to Anderson et al., "virtually all climate model studies that have included anthropogenic aerosol forcing as a driver of climate change have used only aerosol forcing values that are consistent with the inverse approach."

How significant is this choice?  Anderson et al. report that the negative forcing of anthropogenic aerosols derived by forward calculation is "considerably greater" than that derived by inverse calculation, so much so, in fact, that if forward calculation is employed, the results "differ greatly" and "even the sign of the total forcing is in question," which implies that "natural variability (that is, variability not forced by anthropogenic emissions) is much larger than climate models currently indicate."  The bottom line, in the words of Anderson et al., is that "inferences about the causes of surface warming over the industrial period and about climate sensitivity may therefore be in error."

Schwartz (2004) also addressed the subject of uncertainty as it applies to the role of aerosols in climate models.  Noting that the National Research Council (1979) concluded that "climate sensitivity [to CO2 doubling] is likely to be in the range 1.5-4.5°C" and that "remarkably, despite some two decades of intervening work, neither the central value nor the uncertainty range has changed," he opined that this continuing uncertainty "precludes meaningful model evaluation by comparison with observed global temperature change or empirical determination of climate sensitivity," and that it "raises questions regarding claims of having reproduced observed large-scale changes in surface temperature over the 20th century."

Schwartz thus contends that climate model predictions of CO2-induced global warming "are limited at present by uncertainty in radiative forcing of climate change over the industrial period, which is dominated by uncertainty in forcing by aerosols," and that if this situation is not improved, "it is likely that in another 20 years it will still not be possible to specify the climate sensitivity with [an] uncertainty range appreciably narrower than it is at present."  Indeed, he says "the need for reducing the uncertainty from its present estimated value by at least a factor of 3 and perhaps a factor of 10 or more seems inescapable [all our italics] if the uncertainty in climate sensitivity is to be reduced to an extent where it becomes useful for formulating policy to deal with global change," which surely suggests that even the best climate models of the day are wholly inadequate for this purpose.

As evidenced by these few examples, uncertainties related to aerosols have plagued climate modelers since the beginning of the enterprise; and in an attempt to put these problems into a broader context by estimating the change in the sum of what they consider to be all the major climate forcings that have occurred over the last 150 years, Hansen et al. (1998) considered well-mixed greenhouse gases (CO2, CH4, N2O and CFCs), tropospheric ozone, stratospheric ozone, tropospheric aerosols, forced cloud changes, vegetation and other planetary surface alterations, solar variability, and volcanic aerosols.  Their analysis revealed so many uncertainties in the various forcings that they ended up concluding "the forcings that drive long-term climate change are not known with an accuracy sufficient to define future climate change," which is an amazing acknowledgement with incredible implications about the wisdom (or worse) of those who would reformulate the way the world works, i.e., global energy policy, on the basis of what current climate models predict.

References
Anderson, T.L., Charlson, R.J., Schwartz, S.E., Knutti, R., Boucher, O., Rodhe, H. and Heintzenberg, J.  2003.  Climate forcing by aerosols - a hazy picture.  Science 300: 1103-1104.

Breon, F.-M., Tanre, D. and Generoso, S.  2002.  Aerosol effect on cloud droplet size monitored from satellite.  Science 295: 834-838.

Chou, M-D., Chan, P-K. and Wang, M.  2002.  Aerosol radiative forcing derived from SeaWiFS-retrieved aerosol optical properties.  Journal of the Atmospheric Sciences 59: 748-757.

Hansen, J.E., Sato, M., Lacis, A., Ruedy, R., Tegan, I. and Matthews, E.  1998.  Climate forcings in the industrial era.  Proceedings of the National Academy of Sciences, U.S.A. 95: 12,753-12,758.

Jacobson, M.Z.  2001.  Global direct radiative forcing due to multicomponent anthropogenic and natural aerosols.  Journal of Geophysical Research 106: 1551-1568.

National Research Council, 1979.  Carbon Dioxide and Climate: A Scientific Assessment.  National Academy of Sciences, Washington, DC, USA.

Schwartz, S.E.  2004.  Uncertainty requirements in radiative forcing of climate.  Journal of the Air & Waste Management Association 54: 1351-1359.

Sokolik, I.N., Toon, O.B. and Bergstrom, R.W.  1998.  Modeling the radiative characteristics of airborne mineral aerosols at infrared wavelengths.  Journal of Geophysical Research 103: 8813-8826.

Tegen, I., Lacis, A.A. and Fung, I.  1996.  The influence on climate forcing of mineral aerosols from disturbed soils.  Nature 380: 419-422.

Vogelmann, A.M., Flatau, P.J., Szczodrak, M., Markowicz, K.M. and Minnett, P.J.  2003.  Geophysical Research Letters 30: 10.1029/2002GL016829.

Wild, M.  1999.  Discrepancies between model-calculated and observed shortwave atmospheric absorption in areas with high aerosol loadings.  Journal of Geophysical Research 104: 27,361-27,371.

Last updated 20 July 2005