Climate Forcing by Anthropogenic Aerosols
24 Jan 1992-Science (American Association for the Advancement of Science)-Vol. 255, Iss: 5043, pp 423-430
TL;DR: The aerosol forcing has likely offset global greenhouse warming to a substantial degree, however, differences in geographical and seasonal distributions of these forcings preclude any simple compensation.
Abstract: Although long considered to be of marginal importance to global climate change, tropospheric aerosol contributes substantially to radiative forcing, and anthropogenic sulfate aerosol in particular has imposed a major perturbation to this forcing. Both the direct scattering of shortwavelength solar radiation and the modification of the shortwave reflective properties of clouds by sulfate aerosol particles increase planetary albedo, thereby exerting a cooling influence on the planet. Current climate forcing due to anthropogenic sulfate is estimated to be –1 to –2 watts per square meter, globally averaged. This perturbation is comparable in magnitude to current anthropogenic greenhouse gas forcing but opposite in sign. Thus, the aerosol forcing has likely offset global greenhouse warming to a substantial degree. However, differences in geographical and seasonal distributions of these forcings preclude any simple compensation. Aerosol effects must be taken into account in evaluating anthropogenic influences on past, current, and projected future climate and in formulating policy regarding controls on emission of greenhouse gases and sulfur dioxide. Resolution of such policy issues requires integrated research on the magnitude and geographical distribution of aerosol climate forcing and on the controlling chemical and physical processes.
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Stockholm Environment Institute1, Stockholm University2, Australian National University3, University of Alaska Fairbanks4, Université catholique de Louvain5, University of East Anglia6, Wageningen University and Research Centre7, Royal Swedish Academy of Sciences8, Potsdam Institute for Climate Impact Research9, University of Oxford10, James Cook University11, Arizona State University12, Royal Institute of Technology13, University of Minnesota14, University of Vermont15, Stockholm International Water Institute16, California State University San Marcos17, Goddard Institute for Space Studies18, Commonwealth Scientific and Industrial Research Organisation19, University of Arizona20, University of Copenhagen21, Max Planck Society22
TL;DR: In this article, the authors proposed a new approach to global sustainability in which they define planetary boundaries within which they expect that humanity can operate safely. But the proposed concept of "planetary boundaries" lays the groundwork for shifting our approach to governance and management, away from the essentially sectoral analyses of limits to growth aimed at minimizing negative externalities, toward the estimation of the safe space for human development.
Abstract: Anthropogenic pressures on the Earth System have reached a scale where abrupt global environmental change can no longer be excluded. We propose a new approach to global sustainability in which we define planetary boundaries within which we expect that humanity can operate safely. Transgressing one or more planetary boundaries may be deleterious or even catastrophic due to the risk of crossing thresholds that will trigger non-linear, abrupt environmental change within continental- to planetary-scale systems. We have identified nine planetary boundaries and, drawing upon current scientific understanding, we propose quantifications for seven of them. These seven are climate change (CO2 concentration in the atmosphere <350 ppm and/or a maximum change of +1 W m-2 in radiative forcing); ocean acidification (mean surface seawater saturation state with respect to aragonite ≥ 80% of pre-industrial levels); stratospheric ozone (<5% reduction in O3 concentration from pre-industrial level of 290 Dobson Units); biogeochemical nitrogen (N) cycle (limit industrial and agricultural fixation of N2 to 35 Tg N yr-1) and phosphorus (P) cycle (annual P inflow to oceans not to exceed 10 times the natural background weathering of P); global freshwater use (<4000 km3 yr-1 of consumptive use of runoff resources); land system change (<15% of the ice-free land surface under cropland); and the rate at which biological diversity is lost (annual rate of <10 extinctions per million species). The two additional planetary boundaries for which we have not yet been able to determine a boundary level are chemical pollution and atmospheric aerosol loading. We estimate that humanity has already transgressed three planetary boundaries: for climate change, rate of biodiversity loss, and changes to the global nitrogen cycle. Planetary boundaries are interdependent, because transgressing one may both shift the position of other boundaries or cause them to be transgressed. The social impacts of transgressing boundaries will be a function of the social-ecological resilience of the affected societies. Our proposed boundaries are rough, first estimates only, surrounded by large uncertainties and knowledge gaps. Filling these gaps will require major advancements in Earth System and resilience science. The proposed concept of "planetary boundaries" lays the groundwork for shifting our approach to governance and management, away from the essentially sectoral analyses of limits to growth aimed at minimizing negative externalities, toward the estimation of the safe space for human development. Planetary boundaries define, as it were, the boundaries of the "planetary playing field" for humanity if we want to be sure of avoiding major human-induced environmental change on a global scale.
4,771 citations
Cites background from "Climate Forcing by Anthropogenic Ae..."
...Aerosolsiii influence the Earth’s radiation balance directly by scattering incoming radiation back to space (Charlson et al. 1991, 1992) or indirectly by influencing cloud reflectivity and persistence (Twomey 1977, Albrecht 1989)....
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TL;DR: In this paper, the AERONET network of ground-based radiometers were used to remotely sense the aerosol absorption and other optical properties in several key locations, and the results showed robust differentiation in both the magnitude and spectral dependence of the absorption, a property driving aerosol climate forcing.
Abstract: Aerosol radiative forcing is a critical, though variable and uncertain, component of the global climate. Yet climate models rely on sparse information of the aerosol optical properties. In situ measurements, though important in many respects, seldom provide measurements of the undisturbed aerosol in the entire atmospheric column. Here, 8 yr of worldwide distributed data from the AERONET network of ground-based radiometers were used to remotely sense the aerosol absorption and other optical properties in several key locations. Established procedures for maintaining and calibrating the global network of radiometers, cloud screening, and inversion techniques allow for a consistent retrieval of the optical properties of aerosol in locations with varying emission sources and conditions. The multiyear, multi-instrument observations show robust differentiation in both the magnitude and spectral dependence of the absorption—a property driving aerosol climate forcing, for desert dust, biomass burning, urban‐industrial, and marine aerosols. Moreover, significant variability of the absorption for the same aerosol type appearing due to different meteorological and source characteristics as well as different emission characteristics are observed. It is expected that this aerosol characterization will help refine aerosol optical models and reduce uncertainties in satellite observations of the global aerosol and in modeling aerosol impacts on climate.
2,653 citations
Cites background from "Climate Forcing by Anthropogenic Ae..."
...The lack of detailed knowledge of the optical properties of aerosols results in aerosol being one of the largest uncertainties in climate forcing assessments (cf. Charlson et al. 1992; Houghton et al. 1996; Tegen et al. 1996; Hansen et al. 1997, 2000; Heintzenberg et al. 1997)....
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TL;DR: This paper examined the sensitivity of a climate model to a wide range of radiative forcings, including changes of solar irradiance, atmospheric CO2, O3, CFCs, clouds, aerosols, surface albedo, and a "ghost" forcing introduced at arbitrary heights, latitudes, longitudes, seasons, and times of day.
Abstract: We examine the sensitivity of a climate model to a wide range of radiative forcings, including changes of solar irradiance, atmospheric CO2, O3, CFCs, clouds, aerosols, surface albedo, and a “ghost” forcing introduced at arbitrary heights, latitudes, longitudes, seasons, and times of day. We show that, in general, the climate response, specifically the global mean temperature change, is sensitive to the altitude, latitude, and nature of the forcing; that is, the response to a given forcing can vary by 50% or more depending upon characteristics of the forcing other than its magnitude measured in watts per square meter. The consistency of the response among different forcings is higher, within 20% or better, for most of the globally distributed forcings suspected of influencing global mean temperature in the past century, but exceptions occur for certain changes of ozone or absorbing aerosols, for which the climate response is less well behaved. In all cases the physical basis for the variations of the response can be understood. The principal mechanisms involve alterations of lapse rate and decrease (increase) of large-scale cloud cover in layers that are preferentially heated (cooled). Although the magnitude of these effects must be model-dependent, the existence and sense of the mechanisms appear to be reasonable. Overall, we reaffirm the value of the radiative forcing concept for predicting climate response and for comparative studies of different forcings; indeed, the present results can help improve the accuracy of such analyses and define error estimates. Our results also emphasize the need for measurements having the specificity and precision needed to define poorly known forcings such as absorbing aerosols and ozone change. Available data on aerosol single scatter albedo imply that anthropogenic aerosols cause less cooling than has commonly been assumed. However, negative forcing due to the net ozone change since 1979 appears to have counterbalanced 30–50% of the positive forcing due to the increase of well-mixed greenhouse gases in the same period. As the net ozone change includes halogen-driven ozone depletion with negative radiative forcing and a tropospheric ozone increase with positive radiative forcing, it is possible that the halogen-driven ozone depletion has counterbalanced more than half of the radiative forcing due to well-mixed greenhouse gases since 1979.
2,044 citations
TL;DR: Increases in aerosol concentration and changes in their composition, driven by industrialization and an expanding population, may adversely affect the Earth's climate and water supply.
Abstract: Anthropogenic aerosols are intricately linked to the climate system and to the hydrologic cycle. The net effect of aerosols is to cool the climate system by reflecting sunlight. Depending on their composition, aerosols can also absorb sunlight in the atmosphere, further cooling the surface but warming the atmosphere in the process. These effects of aerosols on the temperature profile, along with the role of aerosols as cloud condensation nuclei, impact the hydrologic cycle, through changes in cloud cover, cloud properties and precipitation. Unravelling these feedbacks is particularly difficult because aerosols take a multitude of shapes and forms, ranging from desert dust to urban pollution, and because aerosol concentrations vary strongly over time and space. To accurately study aerosol distribution and composition therefore requires continuous observations from satellites, networks of ground-based instruments and dedicated field experiments. Increases in aerosol concentration and changes in their composition, driven by industrialization and an expanding population, may adversely affect the Earth's climate and water supply.
2,021 citations
TL;DR: A review of the many developments in estimates of the direct and indirect global annual mean radiative forcing due to present-day concentra- tions of anthropogenic tropospheric aerosols since the Inter- governmental Panel on Climate Change (1996) is presented in this paper.
Abstract: This paper reviews the many developments in estimates of the direct and indirect global annual mean radiative forcing due to present-day concentra- tions of anthropogenic tropospheric aerosols since Inter- governmental Panel on Climate Change (1996). The range of estimates of the global mean direct radiative forcing due to six distinct aerosol types is presented. Addition- ally, the indirect effect is split into two components corresponding to the radiative forcing due to modifica- tion of the radiative properties of clouds (cloud albedo effect) and the effects of anthropogenic aerosols upon the lifetime of clouds (cloud lifetime effect). The radia- tive forcing for anthropogenic sulphate aerosol ranges from 20.26 to 20.82 W m 22 . For fossil fuel black carbon the radiative forcing ranges from 10.16 W m 22 for an external mixture to 10.42 W m 22 for where the black carbon is modeled as internally mixed with sulphate aerosol. For fossil fuel organic carbon the two estimates of the likely weakest limit of the direct radiative forcing are 20.02 and 20.04 W m 22 . For biomass-burning sources of black carbon and organic carbon the com-
1,868 citations
References
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01 Jan 1969
6,280 citations
TL;DR: The major source of cloud-condensation nuclei (CCN) over the oceans appears to be dimethylsulphide, which is produced by planktonic algae in sea water and oxidizes in the atmosphere to form a sulphate aerosol as mentioned in this paper.
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TL;DR: Increases in aerosol concentrations over the oceans may increase the amount of low-level cloudiness through a reduction in drizzle—a process that regulates the liquid-water content and the energetics of shallow marine clouds—to contribute to a cooling of the earth's surface.
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1,278 citations
TL;DR: In this paper, a three-dimensional global model for estimating the SO 4 = aerosol mass concentration, along with previously-acquired information on the scattering and back-scattering coefficients per unit mass concentration are presented.
Abstract: Anthropogenic sulfate (SO 4 = ) aerosol particles play two potential roles in the radiative climate of the earth. In cloud-free air, SO 4 = particles scatter sunlight, some of which is lost to space, thereby reducing solar irradiance at the ground. The same particles can act as cloud condensation nuclei (CCN), the number concentration of which is an important determinant of cloud albedo. This albedo effect, in turn, also influences incoming short-wave solar radiation. Development of a three-dimensional global model for estimating the SO 4 = aerosol mass concentration, along with previously-acquired information on the scattering and back-scattering coefficients per unit mass concentration allow calculation of the effects of anthropogenic SO 4 = aerosol on clear-sky optical depth. Subsequently, this can be used to estimate the change in hemispheric and global average reflected solar radiation. The conclusion is that the change of reflected solar flux due to anthropogenic SO 4 = averaged over the Northern Hemisphere is ca. − 1.1 Wm -2 , which is comparable but opposite in sign to the present-day radiative forcing by anthropogenic CO 2 , + 1.5 Wm -2 . Because of the spatial variability of the anthropogenic SO 4 = distribution, its meteorological effects must be studied regionally. That is, global models with regional resolution and regional data are required. Unlike the direct effect on solar irradiance, the relationship of CCN number concentration to mass concentration is not known. Thus it is not yet possible to make quantitatively reliable statements about anthropogenic forcing of cloud albedo, although there is qualitative evidence that the CCN effect may also be substantial. DOI: 10.1034/j.1600-0870.1991.00013.x
760 citations