A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change (1996) / Chapter Skim
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1 CLIMATE FORCING BY AEROSOLS
1 CLIMATE FORCING BY AEROSOLS
Pages 7-34
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From page 7... ...
Climate forcings are changes in the energy balance of the Earth that are imposed upon it; forcings are calculated or measured in units of heat flux watts per square meter (W mob.
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From page 8... ...
Further, the inclusion of aerosols and their effects in climate models is highly simplified and may contain errors. An incorrect or uncertain calculation of forcing by anthropogenic aerosols could significantly alter the understanding of climate response because of the requirement that the climate response to any given forcing should not be much different from that allowed by comparison to historical data.
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From page 9... ...
Figure 1.1 charts the growth of Northern Hemisphere sulfur emissions during the past century and, for comparison, provides estimates of Northern Hemisphere natural fluxes. [Southern Hemisphere anthropogenic emissions are perhaps 10 percent of those in the Northern Hemisphere.]
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From page 10... ...
wrote the following: Recent work has suggested that anthropogenic aerosols, especially sulfates, may exert a substantial radiative forcing of climate, comparable in magnitude, but opposite in sign, to the forcing from anthropogenic greenhouse gases. They may also play an important secondary role in climate as a source of cloud condensation nuclei.
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From page 11... ...
Finally, the nature of climate forcing by different classes of anthropogenic aerosols is discussed. AEROSOL RADIATIVE FORCING OF CLIMATE Climate forcings are changes imposed on the planetary heat balance that alter global temperature (Hansen and Lacis, 1990; Hansen et al., 1993a)
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From page 12... ...
As a consequence, the number concentration of cloud droplets, which is governed, in part, by the number concentration of aerosol particles in the pre-cloud, is also increased. An increased number concentration of cloud droplets leads, in turn, to enhanced multiple scattering of light within clouds and to an increase in the optical depth and albedo of the cloud.
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From page 13... ...
There are a number of independent lines of evidence supporting the hypothesis that anthropogenic aerosols cause substantial climate forcing that is of a magnitude comparable to that of GHGs, but opposite in sign. The following lists some of this evidence, subdivided into direct and indirect r rorclngs.
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From page 16... ...
Regional-scale optical depth estimates coupled to a regional-scale atmospheric sulfur model for the eastern United States (Ball and Robinson, 1982) were suggested to produce an annual average loss of solar irradiance of 7.5 percent from sulfate and other anthropogenic aerosols relative to preindustrial times.
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From page 17... ...
Indirect Forcing To quantify indirect climatic effects of aerosols requires relating increased mass concentrations of aerosol from anthropogenic sources to increased number concentrations of aerosol particles, to increased numbers of cloud condensation nuclei (CCN) , to increased numbers of cloud droplets, to altered cloud radiative properties or lifetime.
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From page 18... ...
Enhancing the concentration of CCN does not always lead to higher cloud drop concentrations. Similarly, enhancing cloud drop concentrations does not always lead to increased liquid water contents and cloud albedos.
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From page 19... ...
Based on two sets of experiments, the study suggests that the indirect radiative forcing by anthropogenic aerosols could be -0.6 or -1.0 W m~2 averaged in the 0°-50°N latitude band. The uncertainty in these estimates was judged to be at least 50 percent.
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From page 20... ...
9. The observed pattern of temperature change shows no increasing likeness to that predicted by climate models that do not include forcing by anthropogenic aerosols, whereas there is an increasing correlation with time between the observed pattern of temperature change and patterns predicted with climate models that do include anthropogenic sulfate aerosol forcing (Mitchell et al., 1995; Santer et al., 1995a)
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From page 21... ...
Although a comparison of estimated global means might suggest that anthropogenic aerosol to some degree has compensated for GHG forcing, the spatial/temporal differences preclude any simple cancellation. Not only are GHG and aerosol forcings fundamentally different, so that their climate effects cannot be expected a priori to compensate for each other even if global mean values of radiative forcing cancel, but also fundamentally different approaches are required for determining their respective magnitudes.
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From page 22... ...
Cooling of the Northern relative to the Southern Hemisphere occurs in numerical model simulations that incorporate tropospheric aerosol forcing (Taylor and Penner, 1994; Roeckner et al., 1995~. Model computations indicate that the transient increase in the stratospheric optical depth from the eruption of Mt.
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From page 23... ...
RADIATIVE FORCING OF CLIMATE BY STRATOSPHERIC AEROSOLS The quantification of climate forcing by stratospheric aerosols, together with climate model calculations of response, provides a unique and independent method of testing climate models. It is important also to quantify possible influences of volcanic aerosols on clouds and cloud forcing in order to be able to separate natural (volcanic)
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From page 24... ...
Radiative forcing by stratospheric aerosols is governed essentially by the column burden of the particles and their sizes. Surface-troposphere forcing by stratospheric aerosols is less sensitive to aerosol composition and location within the stratosphere, but warming within the lower stratosphere does depend on altitude (Pollack and Ackerman, 1983; WMO, 19893.
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From page 25... ...
This, in turn, would lead to adiabatic cooling and reductions in ozone concentrations (Kinne et al., 19921. Increases in stratospheric aerosols raise the potential for ozone destruction in the lower stratosphere via heterogeneous reactions on particles (Hoffman and Solomon, 19891.
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From page 26... ...
In many respects, the episodic stratospheric aerosol forcing problem is far less complicated than the tropospheric aerosol climate problem. Nonetheless, the relatively better understanding of stratospheric aerosol climate effects does provide an extremely useful reference with which to compare and contrast forcings and responses from secular increases in tropospheric aerosols, since the optical depths in the two cases are comparable, albeit operating on different time and space scales.
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From page 27... ...
~ Current DeSI eSllmaleS OI source strengths and aerosol properties, anthropogenic sulfates, organics, and soot are very likely to contribute a substantial fraction of the aerosol optical depth of the whole atmosphere. Table 1.3 summarizes estimates of climate forcing for sulfates, smoke from biomass combustion, and soot, with an additional entry for chemically undifferentiated "anthropogenic aerosols." With the exception of the early, low estimate by Bolin and Charlson (1976)
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From page 28... ...
Optical Depth Natural Primary Soil dust (mineral aerosol) 1,0003,0001,500 o.7a 0.023 Sea salt (mass mean diameter = 5 am, fig = 21,00010,0001,300 0.4b 0.003 Volcanic dust410,00033 2.0 0.001 Biological debris268050 2.0 0.002 Secondary Sulfates as (NH4)
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From page 29... ...
Table 1.4 lists key anthropogenic aerosol types, their forcing mechanisms, and brief assessments of current understanding. CONCLUSIONS It is our judgment that · climate forcing by anthropogenic aerosols is likely to be of sufficient magnitude to necessitate its representation in models of climate change over the industrial period; and · present estimates of anthropogenic aerosol forcing are sufficiently uncertain as to be inadequate to usefully represent this forcing in models of climate change over the industrial period.
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From page 34... ...
multiagency program to answer these scientific questions and, thereby, to improve climate models. Chapters 3 and 4 contain our recommendations for implementing a focused, integrated research program.
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