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ScienceWeek
ATMOSPHERIC PHYSICS: ON AEROSOL COOLING
The following points are made by Jim Coakley (Nature 2005 438:1091):
1) The Earth is warming because of rising concentrations of greenhouse gases in the atmosphere. But some of the human activities that produce those gases, including burning of fossil fuels and biomass, also produce hazes that partially offset the warming. The extent of this cooling influence is not known. But it occurs because human-generated haze particles scatter and absorb incoming sunlight, an effect known as "aerosol direct radiative forcing" that reflects solar radiation back into space.
2) New work [1] provides a new estimate of this aerosol forcing effect. The figure produced (0.8 +- 0.1 W m^(-2) of incident sunlight) is at the high end of the range (0.2-1.0 W m^(-2)) given in the previous report from the Intergovernmental Panel on Climate Change (IPCC)[2]. Notably, this new estimate comes with an uncertainty of only 10-15%, a far cry from the range cited in the IPCC assessment. For comparison, the warming effect from the build-up of the long-lived greenhouse gases carbon dioxide, methane, nitrous oxide and chlorofluorocarbons is 2.4 +- 0.2 W m^(-2).
3) Because of concerns about human health and acid rain, considerable efforts have been and are being made to improve air quality and reduce the amounts of anthropogenic aerosols that arise, for example, from power-plant and manufacturing emissions. So an implication of the figure obtained by Bellouin et al[1] is that such improvements will come at the price of substantial additional warming -- as much as half that already experienced from the build-up of greenhouse gases.
4) Estimating the effect of anthropogenic aerosols on climate is notoriously difficult. Computer models must predict the concentrations, and the physical and chemical make-up of the particles, and the consequent radiative properties. The variability of estimates for the aerosol direct radiative forcing in the IPCC report reflects the wide range of possible outcomes. To constrain this range, Bellouin et al[1] took advantage of new observations of aerosol properties from the Moderate Resolution Imaging Spectrometer (MODIS) instrument now flying on NASA's Terra and Aqua satellites.
5) A central concept in these studies is "aerosol optical depth", a measure of the attenuation of sunlight by particles that is proportional to the amount of aerosol. Over oceans, the MODIS observations separate the aerosol optical depths into the fractions contributed by small and large particles[3]. In situ measurements and surface-based observations of light attenuation and scattering led Bellouin et al[1] to propose using the separation of the optical depth into contributions made by small, anthropogenic particles and large, natural particles -- such as windblown dust and sea spray -- to estimate what fraction of aerosols is attributable to anthropogenic causes.[4,5]
References (abridged):
1. Bellouin, N. , Boucher, O. , Haywood, J. & Reddy, M. S. Nature 438, 1138-1141 (2005)
2. Ramaswamy, V. et al. (eds) Climate Change 2001: The Scientific Basis. Contribution of WG1 to the Third Assessment Report of the IPCC (Cambridge Univ. Press, 2001)
3. Remer, L. A. et al. J. Atmos. Sci. 62, 947-973 (2005)
4. Dubovik, O. et al. J. Atmos. Sci. 59, 590-608 (2002)
5. Loeb, N. G. & Manalo-Smith, N. J. Clim. 18, 3506-3526 (2005)
Nature http://www.nature.com/nature
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Related Material:
ON AEROSOLS AND CLOUD MICROPHYSICS
Notes by ScienceWeek:
The term "aerosol" refers to a dispersion in which a finely divided solid is suspended in air and the particles are of colloidal dimensions. The term "colloidal dimensions" refers to the range approximately 1 nanometer to 100 nanometers in diameter.
The following points are made by F-M. Breon et al (Science 2002 295:834):
1) Aerosols may reduce the degree of Earth global warming resulting from the increase of greenhouse gases in the atmosphere (1,2). They directly impact the radiative balance of Earth through a net increase of its albedo, particularly over the oceans (3,4). Aerosols can also act as cloud condensation nuclei, increasing the number of droplets in clouds, which tends to decrease the mean droplet size and may increase the cloud albedo (5), depending on the aerosol absorption and cloud optical thickness. This process, referred to as the "Twomey effect" or the "first indirect" aerosol radiative forcing, has a net cooling effect on climate. A direct demonstration of the aerosol effect on cloud albedo was provided by the observation of lines of larger reflectance in cloud fields identified as tracks of ship exhaust. Indirect observations of this effect can also be made by comparing cloud droplet size and aerosol concentration. Cloud droplet effective radii were derived by using global scale advanced very high resolution radiometer measurements. The results of a global application indicate a contrast in cloud droplet size of about 2 microns over land and ocean surfaces, as well as a hemispheric contrast of 1 micron, both of which support the Twomey hypothesis. Similar patterns of the aerosol optical thickness and the cloud droplet effective radius, derived from advanced very high resolution radiometer measurements, have been observed over the oceans. Cases of reduced droplet radii and suppression of rain -- the second indirect aerosol effect-- in areas of high aerosol load were identified on satellite imagery. Furthermore, several in situ measurements have shown a relationship between the aerosol concentration and the cloud droplet size distribution.
2) The authors report a study in which aerosol concentration and cloud droplet radii derived from space-borne measurements are used to explore the effect of aerosols on cloud microphysics. Cloud droplet size is found to be largest (14 microns) over remote tropical oceans and smallest (6 microns) over highly polluted continental areas. Small droplets are also present in clouds downwind of continents. By using estimates of droplet radii coupled with aerosol load, a statistical mean relationship is derived. The cloud droplet size appears to be better correlated with an aerosol index that is representative of the aerosol column number under some assumptions than with the aerosol optical thickness. This study reveals that the effect of aerosols on cloud microphysics is significant and occurs on a global scale.
3) The authors conclude: Whether the observed impact on cloud microphysics is of anthropogenic origin is a question of importance. The satellite measurement cannot unambiguously distinguish natural and human-generated aerosols. However, the analysis of the spatial and temporal patterns in the aerosol index monthly maps strongly suggests that the bulk of the aerosol load originates from slash-and-burn agriculture practices and from highly polluted areas (25). A large fraction of the observed aerosol effect on clouds is probably an anthropogenic impact.
References (abridged):
1. J. E. Penner, et al., Bull. Am. Meteorol. Soc. 75, 375 (1994)
2. R. J. Charlson, et al., Science 255, 423 (1992)
3. J. Haywood, V. Ramaswamy, B. Soden, Science 283, 1299 (1999)
4. O. Boucher and D. Tanre, Geophys Res. Lett. 27, 1103 (2000)
5. S. Twomey, J. Atmos. Sci. 34, 1149 (1977)
Science http://www.sciencemag.org
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Related Material:
ANTHROPOGENIC ATMOSPHERIC AEROSOLS AND GLOBAL CLIMATE CHANGE
The following points are made by S.E. Schwartz and P.R. Buseck (Science 2000 288:989):
1) Most considerations of global climate change caused by human activities have focused on the warming influence of greenhouse gases. However, aerosols are another important atmospheric constituent that influences climate and that has been affected by human activities. In general, aerosol particles increase scattering and absorption of shortwave (solar) radiation, increase cloud reflectance, enhance cloud lifetimes, and suppress precipitation. These phenomena are all thought to exert a cooling influence on climate. Recent data indicate that anthropogenic aerosols reduce cloud droplet size and suppress precipitation downward of major urban areas and industrial facilities, which is consistent with earlier hypotheses.
2) The influences of aerosols on climate are more complex than those of greenhouse gases. Bulk aerosol composition is highly variable spatially and temporally because of different sources and production mechanisms and short atmospheric residence times (from less than a day to more than a month). Particles sizes range from nanometers to microns, and within the same size class, particles can exhibit widely different compositions and morphologies, with different constituents present within the same particle (e.g., 10 nanometer carbon spherules can be found embedded within much larger sulfate particles). The inhomogeneities in properties and geographical distribution of aerosols make it difficult to characterize their influences on climate and to represent these influences in models.
3) Recent analysis of the consequence of absorption of shortwave radiation by aerosols indicates that the heating of the atmosphere can evaporate clouds. Clouds exert both cooling and warming influences on climate: cooling in the shortwave (because of their reflectance), and warming in the longwave (because of absorption and re-emission of thermal infrared radiation). The shortwave component dominates, so a reduction in cloud coverage would result in a net warming influence.
4) The authors conclude: "Recent studies demonstrate both the importance of aerosol effects on climate and the complexity of aerosol-cloud interactions. Unfortunately for those would like a quick and accurate assessment of anthropogenic climate forcing over the industrial period, the studies also demonstrate that there is much to be learned before such an assessment can confidently be given."
Science http://www.sciencemag.org
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