Royal Swedish Academy of Sciences

Direct Climate Forcing by Anthropogenic Sulfate Aerosols: The Arrhenius Paradigm aCentury LaterAuthor(s): Robert J. CharlsonSource: Ambio, Vol. 26, No. 1, Arrhenius and the Greenhouse Gases (Feb., 1997), pp. 25-31Published by: Allen Press on behalf of Royal Swedish Academy of SciencesStable URL: http://www.jstor.org/stable/4314546Accessed: 02/10/2009 17:55

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Robert J. Charlson

Direct Climate Forcing by Anthropogenic Sulfate Aerosols: The Arrhenius Paradigm a Century Later

The so-called "greenhouse gases", or "hothouse" gases as Arrhenius called them, are not the only radiatively active substances being added to the atmosphere by humans. A different class of matter, aerosols, consisting of small particles of condensed matter suspended in the air, has several climatic effects besides a small amount of absorp- tion of infrared radiation. The particles that constitute the smoke and haze of industrial regions arise both as smoke from combustion and as the products of atmospheric chemical reactions, e.g. of sulfur dioxide from burning of coal. The direct effect of aerosols on the Earth's heat balance consists of their reflection of solar radiation back to space. Their indirect effect derives from their roles as the nuclei around which cloud droplets form. Adding more of these cloud condensation nuclei increases the reflectivity of clouds, a very important determinant of climate as discussed by Arrhenius. Aerosols may also influence the longevity of clouds and thus the fraction of earth covered by them-the "nebulosity" mentioned by Arrhenius. An additional direct effect of aerosols is light absorption by black soot particles, which can influence both clear and cloudy skies. Although quantitative assessments of these effects of aerosols have been developed only in recent years, there have been extensive studies and observations of their roles in climate, particularly the ancient observation that large volcanic eruptions caused large-scale cooling to occur. Interestingly, several of the early key studies were also conducted in Sweden, notably by Angstrom and Bergeron. The present understanding is that man-made aerosols have a cooling effect, but that it is somewhat smaller in the global mean than the warming effect of the man-made greenhouse gases and is spatially and tempor- ally very different. Thus, no simple cancellation of the two effects can occur. This paper reviews the historical devel- opment of the understanding and quantification of direct aerosol effects on climate. Particular attention will be paid to comparing the parallel approaches to quantifying the effects of greenhouse gases and anthropogenic sulfate aerosols, respectively, which were both developed in Swe- den, albeit almost a century apart.

INTRODUCTION Radiatively active components of the atmosphere-gases, aero- sol particles and clouds-have received rapidly increasing rec- ognition as factors that can influence the heat balance of the Earth. Of particular interest are those components that cause cli- mate forcing, that is, externally imposed changes in heat balance at the top of the atmosphere as measured or calculated in watts per square meter (W m-2). Externally imposed changes include such factors as emissions from large volcanoes and from human activities, but not those factors that are internal to the climate system itself. Particular emphasis is placed on the anthropogenic, i.e. generated by humans, emission of climate-active substances. Climate-active substances are the gases that cause changes in absorption of heat by the atmosphere and aerosol particles that either reflect sunlight or change the reflectivity or area of cov-

erage of clouds. The influences of all of these were discussed qualitatively in the latter part of the 19th century, and the first to submit to quantification was carbon dioxide as presented by Arrhenius (1). Quantitative estimates of forcing due to other anthropogenically influenced greenhouse gases (methane, nitrous oxide, chlorofluorocarbons and ozone) occurred much later (2, 3); however, climate forcing by anthropogenic aerosols has only been partly analyzed a whole century after CO2. While this par- tial analysis has demonstrated that the climatic forcing by hu- man-caused aerosols is quantitatively significant, it has also re- vealed a number of uncertainties and complexities, particularly regarding the effects of aerosol particles on clouds.

While these multiple influences of aerosol particles on clouds can only be qualitatively assessed, the direct forcing, e.g. the re- flection of sunlight by the particles themselves, is much simpler to address. Also, while these indirect effects as well as the ef- fect of greenhouse gases are not visibly perceptible, the interac- tion of sunlight with aerosol particles causes visible haze, par- ticularly in geographical regions where aerosols are produced. Figures 1 and 2 are photographs taken from the U.S. Space Shut- tle in actual colors showing the reflection of sunlight toward space-a visible negative forcing-over eastern parts of North America and southeast of China. That such masses of haze could be detected photographically from space has been known for decades (4, 5); the new realization is that the upwelling light con- stitutes a significant climate forcing.

The purpose of this paper is to review the historical develop- ment of recognizing, understanding and quantifying direct cli- matic forcing by anthropogenic aerosols. The main conclusion that will be drawn from this review is that the paradigm origi- nally proposed by Arrhenius, in 1896, for CO2 is still a useful guide for quantifying the direct effects of aerosols.

HISTORICAL DEVELOPMENT The term aerosol was originally formulated early in the 20th cen- tury to describe a gas-phase analog of hydrosols or aqueous- phase colloids, i.e., particles of liquid or solid matter suspended in a gas such as air (6). However, recognition of particular types of aerosols and their effects on climate predate even the discov- ery of the greenhouse effect by centuries. Table 1 provides a con- cise summary of the various fields of study that recognized the influence of aerosols on climate or as a factor known to influ- ence climate. The earliest observations consisted of empirical connections between large volcanic eruptions and cool summers/ poor harvests in mid-latitude regions. Figure 3 shows a 15-year history of the optical depth, 6, at 535 nm over the interior desert of the northwestern US, illustrating the large variability of at- mospheric transparency due to aerosols in general and volcanic aerosols in particular. The best-documented case of volcanic in- fluence is for Pinatubo (1991-ca. 1993) which had a global-mean optical depth of 0.15 and was observed to cool this planet by ca. 0.5?C (7, 8). Also shown are data for the optical depth over the eastern US indicating that such industrial regions have opti- cal depths much greater than those caused by a large volcano.

The brief listing in Table 1 of the origins of thinking about the climatic effects of atmospheric aerosols reveals that these

Ambio Vol. 26 No. 1, Feb. 1997 ? Royal Swedish Academy of Sciences 1997 25

Figure 1. (a) Photograph shows an extensive (1000 km +) aerosol-laden air mass, which modifies the regional radiation energy balance. It was taken by astronauts using a hand held camera on board Space Shuttle STS-31, at 1300 Z,

Is the peninsula of Florida, as well as the coastline of the southeastern US. The haze aerosol is centered on the mid- hzAtlantic states and was also detected by surface visibility observations made over the same day. (b) For orientation, the approximate field of view is given in the map along with the 1200 Z surface barometric pressure and the location of NWS rawinsonde sites. The map reveals a stagnant high pressure

2.system located over the southeastern US. -~ .t ~ Analysis of relative humidity showed that

the near-surface air was humid but not saturated. The 1200 Z temperature and dewpoint soundings show sub-saturated air up to 500 mb with only isolated clouds. Thus the visible haze is neither cloud nor ground fog. Long-term averaged chemical analyses of the haze particles indicate approximately 60% are composed of sulfate compounds from industrial sources, and 40% organic compounds from agricultural sources. (Acknowledgments: The photograph was made available by Dr. Kamlesh Lulla and his associates at the NASA Lyndon B. Johnson Space Center. Meteorological analysis by J. D. Wheeler).

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studies increasingly resorted to scientific reductionism that did not always result in a free exchange of scientific results between different areas of endeavor. It is especially relevant that atmos- pheric aerosols were treated mainly as a topic in physics, leav- ing their chemistry, i.e. sources via chemical reactions in the at- mosphere, their chemically determined properties like water solu- bility or their measurement via chemical analysis, as topics that only have been connected in the last 10 to 20 years. In spite of the insightful beginning of studies of atmospheric aerosols by Aitken and others around 1880 (9), the cloud chamber studies by C. T. R. Wilson published in 1899 (10) that led to an ability to observe ionizing radiation also led to a reductionist surge to- ward the understanding of the structure of atoms and existence of sub-atomic particles. While obviously important in its own right, this surge also depleted the ranks of those working on at- mospheric aerosols until well after World War II. Perhaps the same kind of surge toward other topics in chemistry and geo- physics resulted in very little work being done on atmospheric CO2 between the flurry of activity regarding CO2 and the green- house effect in the 1890s, a few lone papers in the 1920s and 1930s and the first definitive atmospheric measurements that appeared in the early 1970s. Even when the resurgence in at- mospheric aerosol research occurred after the 1950s, most of it was focused on problems with immediate practicality-air pol- lution, acid rain, atmospheric visibility, military application of lasers, clean rooms for the semiconductor industry, etc.-and it really wasn't until the late 1980s that an integration of the nec- essary pieces of chemistry, aerosol microphysics and optics ac- tually began to come into place.

Five major advances signaled the arrival of the opportunity to address the question of direct forcing of climate by anthro- pogenic sulfate aerosols: i) The growing realization that the atmospheric aerosol usu-

ally is a binary physical mixture of coarse dust and/or sea salt particles (diameters greater than ca. 1 gm) and fine (sub

26 ? Royal Swedish Academy of Sciences 1997 Ambio Vol. 26 No. 1, Feb. 1997

Figure 2. View of aerosol emanating from the SE coast of China Including the island of Taiwan (left, center); northwest wind direction. Photograph taken on 4 March 1996 at 01:29:432. Photo No. STS-75-773065. (Acknowledgments: The photograph was made available by Dr. Kamlesh Lulla and his associates at the NASA Lyndon B. Johnson Space Center).

~~~~~~ ... .. -r- . . . ....~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.. .... . ~ . ..

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,um) particles of materials mostly condensed from gas-phase precursors like SO2. This external mixture results in fime par- ticles that are not chemically mixed with the coarse parti- cles and that have chemical and physical properties that are nearly independent of the coarse particles (1 1). Prior to the 1970s, the view was that the entire aerosol consisted of mix- tures of soluble salts and insoluble dust-so-called mixed particles (mischkerne) as articulated by Junge (12). Finding

0

0

C,,

1979 1981 1983 1985 1987 1989 1991 1993 1995

Year D E A B C

Figure 3. Time series of optical depth measured at 535 nm wavelength during noncloudy periods at Rattlesnake Mountain, WA, USA; compared to the December and July 1970-1974 means (solid line) and range (as standard deviation, shaded) for 26 eastern US sites. Also included are markers for volcanic eruptions: A. Mt. St. Helens; B. Alaid; C. El Chichon; E. Pinatubo and D. Wildfires in the vicinity. (Rattlesnake Mountain database: Multispectral Optical Depth Measurements: 1979-1994; contributors: M. R. Larson, J. J. Michalsky, B. A. LeBaron; available online at http://edlac.esd.ornl.gov Eastern US data ref. 39).

an atmospheric chemical source of the very particles that controlled the scattering and extinction of sunlight allowed recognition of the need to connect such physical effects to the chemical cause. It was also significant that quantifica- tion of the relative magnitude of natural and anthropogenic aerosols became possible through chemical considerations.

ii) The development of understanding of aerosol microphysi- cal properties in the 1970s and 1980s via the development of instruments and measurement methods for characterizing atmospheric aerosols. One instrument in particular, the in- tegrating nephelometer (13, 14), found use in quantifying the amount of visible light that is scattered due to the presence of fine particles. The key aerosol property-scattering effi- ciency in m2 per gram of aerosol substance, a-was found to be sufficiently constant to allow quantitative connections to be made between the mass concentration of sulfate-com- pound aerosols and the amount of light retro-scattered into the blackness of space by the aerosol. Large amounts of data exist at a wavelength in the middle of the solar spectrum (550 nm) for this ratio of scattering coefficient to mass concen- tration showing a = 3 m2 g-1 for fine particle aerosols at low humidity (15), almost irrespective of their chemical compo- sition and variations in size distribution.

iii) The advent of three-dimensional global models of atmos- pheric chemistry. The first one, code name Moguntia (16, 17), was by present standards rather crude (e.g. 100 x 100 horizontal resolution, 10 layers, 1 month averages) but it did allow in one of its earliest applications the description of the natural and anthropogenically perturbed sulfur cycle (18). Once the results from these model computations were avail- able, it became obvious to connect the optical information acquired by nephelometry to the model-calculated sulfate aerosol mass concentration.

iv) The quantification of the natural sources of aerosols, particu- larly of sulfate compounds from the oxidation of the gase-

Ambio Vol. 26 No. 1, Feb. 1997 ? Royal Swedish Academy of Sciences 1997 27

ous precursors SO2 and dimethylsulfide (DMS). When the natural sources, largely S02 from volcanoes and DMS from phytoplankton, where shown to be relatively small (20-30 Tg yr-l of sulfur globally) compared to the anthropogenic SO2-sulfur source (ca. 70 Tg yr-'), it became obvious that the anthropogenic source had to dominate atmospheric sulfate aerosols (19).

v) A refinement in thinking within the climate research com- munity that it would be useful to separate the measurement or calculation of climateforcing (imposed change in heat bal- ance) from the response. While this was briefly mentioned by Arrhenius in 1896 (1) and was occasionally referred to in a few papers, explicit focusing on forcing as a separate issue began to occur in the mid-1980s (2).

THE SIMPLE MODEL OF DIRECT CLIMATE FORCING BY ANTHROPOGENIC SULFATE AEROSOLS Due to the complexity of either in situ or remote measurements and a complete lack of data in the pre-industrial epoch it is not possible to directly measure the climate forcing by any type of anthropogenic aerosols. However, measured aerosol properties can be used to calculate the forcing due to a measured or calcu- lated amount of aerosol substance in the atmosphere, with the history of its development coming from records of industrial and other human activities.

Assuming that the anthropogenic sulfate aerosol is in the form (NH4)2SO4, the average burden, Baeroso, is most conveniently es- timated for an average vertical column of the atmosphere by:

Baerosol - Qaerosol.taerosol - (4.6 x 106 g s-1)(5 x i05 s)

Aeaxth 5 x 1014 m2

- 4.6 x 10-3 g m-2 Eq. 1

where the source-strength, Qaerosol represents the mass of (NH4)2SO4 created per unit of time (second) from the atmos- pheric oxidation of about half of the 70 x 1012 g yr-' of S02- sulfur emission and the reaction of those products with ammo- nia. The lifetime of such aerosol particles, taerosol, in the air can be deduced from measurements of rainwater and air composi- tion and is ca. 6 days or 5 x 105 seconds. AE1h is the area of the Earth. This burden has to be increased because sulfate compo- nents are hygroscopic, grow in size and scatter more light at el- evated relative humidity. At the global-mean relative humidity near the Earth's surface of close to 80% the mass approximately doubles, and we find:

Baerosoli 9.2 x I0-3 g m-2 Eq. 2

In order to connect Baerosol to the amount of sunlight transmitted through the atmosphere, a term called optical depth, 6aerosol, is needed. If IL is the intensity of sunlight outside the atmosphere at some effective wavelength and I is its intensity at the Earth's surface, then:

1 - 1= e- aerosol r ~~~~~~~~~~~~~Eq. 3

Properly, this equation should be quantified over the entire spec- trum of the sun's emission (ca. 300-3000 rim wavelength). How- ever, most data on 8 are available at a wavelength of 550 nm near the peak of the solar emission spectrum and those values will be used in this illustration.

The connection to Baerosoi is made by recognizing that:

f 6 (Z)aerosol dz 8aerosol 0

Baerosol Eq. 4 f m (Z)aerosol dz 0

where 6aerosol is the global mean of optical depth, O(YZ)aerosol is the extinction coefficient at altitude z and m(z)aerOsoi is the mass con- centration of anthropogenic (NH4)2SO4 aerosol, also at altitude z. Since both the numerator and denominator are integrals over the same altitude range:

6aerosol (Taerosol (m1) 2 1

Baerosol (gm) - maerosol (g m - Eq. 5

where Oeaerosol iS called the extinction efficiency. Happily, OXaerosol

has been measured directly with the combination of an integrat- ing nephelometer and a filter for weighing maerosol, and for a wide variety of conditions as noted above:

aerosol =_3 m2 g-1

So, since B _9.2 xO-3 g m-2.

6aerosol 0.03

which means that on average, with sun overhead, a 3% loss of intensity of the direct solar beam from the Earth's surface. How- ever, this does not mean that there is a 3% loss of solar heat from the Earth-atmosphere system because much of that extinction is caused by scattering toward the Earth's surface. Only 10-20% of the extinction actually results in the reflection of sunlight into space, so the actual loss is roughly between 0.3 and 0.6%.

Finally, the Earth is about half-covered by clouds and we pre- sume that this reflection of light by aerosols is only operative when skies are clear. Since the incident flux, I 300 W m-2, the climate forcing, AFR, can be estimated as: being crudely - 0.45 to -0.9 W m-2. The daytime average is double that.

Simple though it is, this exercise illustrates four things: (i) the nature of the model and the data that are needed; (ii) that the approximate magnitude is not insignificant compared to the forc- ing by anthropogenic greenhouse gases of ca. +2 W m-2; (iii) that the sign of the anthropogenic sulfate-compound aerosol forc- ing is opposite to that of the man-made greenhouse gases; and (iv) the short lifetime of aerosols dictates a patchiness to the forc- ing unlike the more uniform greenhouse effect.

Needless to say, many models (20) have been formulated in the past six years that are much more detailed and thorough. While most of these models also have relied on in situ data for O(aerosol or have calculated it from a size distribution, many fea- tures make these newer model formulations more realistic: - 2.50 to 10? latitude and longitude horizontal resolution; - more layers in the vertical; - numerous spectral intervals throughout the visible and near

infrared regions; - temporal resolution, now approaching fractions of an hour.

The Intergovernmental Panel on Climate Change (21) has de- veloped a consensus statement as given in Table 2, illustrating the current best estimates of climate forcing by sulfates and other anthropogenic aerosols over the industrial period with the en- hanced greenhouse effects of gases and the June 1991 eruption of the large Philippine volcano, Pinatubo, as references. T^hree further points emerge from this analysis of the evolving models of climate forcing by aerosols: i) The total, globally-averaged forcing by anthropogenic aero-

sols is judged to be negative and is a significant fraction of

28 ? Royal Swedish Academy of Sciences 1997 Ambio Vol. 26 No. 1, Feb. 1997

the positive forcing due to anthropogenic greenhouse gases. ii) The aerosol forcing occurs mostly in the Northern Hemi-

sphere (NH), and mainly in and near areas of aerosol pro- duction which, when added to the positive forcing of green- house gases, leads to a very patchy net forcing that may be zero or negative in some locations.

iii) This NH anthropogenic forcing is roughly comparable in magnitude to the average forcing of a large volcano. The dif- ference between the latter two is that the volcanic aerosol from an eruption disappears after 1' to 2 years while the an- thropogenic forcing is more nearly permanent and still grow- ing, e.g. in Asia.

Figure 3 also compares the variable aerosol optical depth in a relatively unpolluted location to the observed mean value over the eastern US showing how large the anthropogenic perturba- tion can be over a polluted region.

A COMPARISON OF PARADIGMS: 1896 VS. 1996 The approach taken by Arrhenius in 1896 to the problem of quantifying the effect of CO2 can be directly compared to the approach used in the previous section for the influence on heat balance of a key type of aerosol. Many similarities exist, along with one major difference.

The very strong parallel in approaches is evident if the key steps are listed as they are below. The Roman numerals refer directly to sections of Arrhenius' 1896 paper; this allows iden- tification of the corresponding step of the approach for the aero- sol forcing problem. Several key features are common to calcu-

lating both the effects of greenhouse gases and aerosols: Ia Observation (actual measurements) of change in the radiative

balance of the Earth's surface due to the presence in the at- mosphere of trace substances that interact with radiation.

lb Identification of the chemical composition and quantification of the relevant optical properties of each of the key trace sub- stances.

II Calculation of the spectrally-resolved magnitude of the in- teraction of the trace substance or substances with the radia- tion field (upwelling infrared radiation for the greenhouse gases, solar radiation for the aerosols). Quantification of the total magnitude of the effect by integrating over wavelength and, in the case of aerosol, the geometry of reflection away from the Earth.

III For greenhouse gases, calculation of the effect of these in- teractions on the temperature of the Earth's surface. For aero- sols, calculation of the effect on the magnitude of change in heat balance (forcing), AFR (W m-2).

IV Calculation of the sensitivity of the Earth's surface tempera- ture (or the heat balance) to changes (increases or decreases) in the amount of the radiatively active trace substance in the air.

V Examination of the implications of the chemical changes of the atmosphere on climatic events such as ice ages and on geochemical mass balances, e.g. in the carbon or sulfur cy- cles, "Geological Consequences" in Arrhenius' terms.

It will be noted in III that a subtle but fundamental difference in approach arises between the two cases; i.e., Arrhenius treat- ment of CO2 and the modem treatment of aerosols. In his ex-

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Ambio Vol. 26 No. 1, Feb. 1997 ? Royal Swedish Academy of Sciences 1997 29

tensive calculations, Arrhenius never did extract the actual change in heat balance, instead focusing on the dependence in temperature of the two bodies, the Earth's surface and the at- mospheric gases, as a function of the CO2 and water vapor con- tent. As noted above, it is very clearly the case that most cli- mate modeling prior to about the mid-1980s involving changes in greenhouse gases or aerosols had calculated temperature change as the key output rather than the magnitude of the forced change in heat balance, i.e. the forcing. Part of the reason for this is that, prior to the mid-1980s, most modeling was for well- mixed gases and was done with one-dimensional radiative con- vective models in which temperature response was the output. The few GCMs prior to this time considered only a limited number of forcing factors (e.g. C02, CH4 or the solar constant), also with global mean temperature as the desired main output.

Many of the earlier papers on direct aerosol effects in the 1970s and 1980s also were aimed at estimating the magnitude of the global-mean temperature decreases expected from in- creases in anthropogenic aerosols. For example, Rasool and Sch- neider (22) concluded that a fourfold increase in "global back- ground opacity" would decrease global temperature by as much as 3.5 K which might "be sufficient to trigger an ice age". Coakley and others (23) considered the direct effect of the "back- ground aerosol", concluding that it produced a "2-3?C surface cooling". Robock (24) concluded that anthropogenic aerosols would likely produce cooling but that "even the sign" is "open to much question". None of these recognized the chemical na- ture of the aerosol or the atmospheric chemical processes that produce the aerosol. As a result, none could quantitatively par- tition the effect between natural and anthropogenic aerosols. Bolin and Charlson (25) did manage to identify a likely domi- nant role of anthropogenic sulfates but still used the change in global-mean temperature as the single index of the direct effect of aerosols. Their calculated cooling was 0.03< AT(K) <0.06, which is very small compared to either of the above papers and to modem model calculations, largely due to the incorrect as- sumption that the area of the Earth influenced by anthropogenic sulfates is only ca. 1% of the planetary surface. An assumption about area was necessary in 1976 because of a lack of geographi- cally distributed data on optical depth, data on chemical com- position and global 3-D models of sulfate aerosol.

This focus on temperature, especially in its global mean, posed a serious limitation to progress toward quantifying the effects of aerosols. Specifically, all in situ aerosol data necessarily had to be made at one point in space and time. Column integrals like optical depth also were local; as a result, it was virtually impos- sible to extrapolate the necessarily limited set of available meas- urements made at the Earth's surface or from aircraft to the glo- bal scale. Thus, there was a clear need for either (or both) a way to extrapolate to the global scale or a way to relate local aerosol

properties to local effects. One way to do this for local effects was proposed by Ball and Robinson (26) who attributed a 7.5% annual-average depletion of solar irradiance due to sulfate-con- taining haze in the industrialized eastern US. However, they too did not extrapolate this result to the global scale.

CONCLUSION Having thus explained the need for relating local aerosol meas- urements to local forcing and the need for a means to extrapo- late from local to the global scale, it is appropriate to return to the Arrhenius CO2 paradigm to seek the key factor(s) that slowed earlier progress toward quantification of aerosol forcing and, conversely, the ways in which the five key advances mentioned above aided and accelerated the quantification of the direct aero- sol effects.

Progress toward understanding aerosol as part of the totality of climate forcing factors was definitely slowed by the lack of attention to the study of atmospheric aerosols in general from ca. 1900 until the 1960s. In addition to this late start, the preoc- cupation with global-mean temperature as the sole index of cli- mate had two effects. First, it buried the relatively simple cal- culation of climate forcing underneath the immense complexity of calculating the climatic response to forcing. Second, it encour- aged the climate analysis community to seek empirical relation- ships of observed temperature change to aerosol changes.

It seemed logical enough to seek observations of temperature response to aerosols from the "human volcano" to parallel the known cooling due to volcanic dust. It is not at all surprising, then, that in the years preceding the acceptance of the empirical demonstration of increasing atmospheric CO2 content from fos- sil fuel combustion that suggestions were made that increasing aerosols might hasten the arrival of another ice age (22).

Then, in the early to mid-1980s, the capabilities and clues pro- vided by advances, mentioned above, led to the opportunity to quantify the effects of anthropogenic aerosols on radiative bal- ance. Prior to that time, a consensus existed that coarse-particle aerosols like soil dust and sea salt far outweighed anthropogenic aerosol sources, like sulfates, and that the anthropogenic pertur- bation to the "background aerosol" was relatively small. Also prior to that time, most estimates of the fluxes in the atmospheric sulfur cycle indicated dominance by natural emission, e.g. of H2S which we now know was incorrect. The understanding that sulfate compounds like (NH4)2SO4 are created from anthropo- genic gaseous precursors exactly in the size range that is most effective in scattering sunlight shifted the consensus toward rec- ognition of regional-scale "blobs" of sulfate haze that caused degradation as visibility and dominated optical depth (26, 44). While quantification of scattering efficiency was begun in the late 1960s (27), substantial amounts of data, particularly those

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30 ? Royal Swedish Academy of Sciences 1997 Ambio Vol. 26 No. 1, Feb. 1997

including a nearly-complete chemical analysis didn't appear until the late 1980s. Of particular importance was the partitioning of light scattering to individual chemical species by White (15, 28). Improved measurements of aerosol particles, e.g. of size distri- bution and scattering coefficient, provided increasing confidence in the quality of the data that were available for calculating radiative effects.

With the separation of forcing and response (3) by Dickinson and others freshly in mind, the advent of a 3-D global model of the atmospheric portion of the sulfur cycle (18) provided the es- sential means to extrapolate the local effects (26) to the global scale (29, 30).

In retrospect what can be drawn from this experience is that it probably would have been useful in the 1970s and 1980s to deliberately study both the linkages and gaps between the vari- ous areas of aerosol research. It also would have been useful to approach the problem of climate forcing systematically by writ- ing out a full equation for the Earth's heat balance in order to provide earlier warning that a missing term or terms in the equa- tion might be significant (43). By thinking along these lines, we can be assured that major terms in the equation have not been inadvertently omitted. We can also be assured that the whole matter of the indirect forcing via influences of anthropogenic

aerosol particles on clouds remains as a major term in the equa- tion with very large uncertainty.

EPILOGUE The simple model of climate forcing by anthropogenic aerosols that is summarized above was first written down while the au- thor, H. Rodhe, J. Langner and J. Ogren were simultaneously in attendance at a meeting in Germany hosted by P. Crutzen. The key to making the connection between optical depth and column burden of sulfate had been Langner's demonsration of a model that estimated sulfate content as a function of altitude, latitude and longitude. It became obvious to use that model to estimate the geographical distribution of burden, thereby provid- ing a means to estimate the geographical distribution of forc- ing. An agreement was reached that Langner, upon returning to Stockholm, would do such a calculation. The author visited the Department of Meteorology at Stockholm University the next week, viewed the results and began to prepare a paper that was first presented orally in Sweden at the symposium celebrating the retirement of Prof. Bert Bolin and subsequently was pub- lished in Tellus .3O).

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phytoplankton, atmospheric sulfur, cloud albedo and climate. Nature 326, 655-661. 42. Penner, J.E., Dickinson, R. and O'Neil, C. 1992. Effects of aerosol from biomass bum-

ing on the global radiation budget. Science 256, 1432-1434. 43. Twomey, S. 1991. Aerosols, clouds, and radiation. Atmos. Environ. 25A, 2435-2442. 44. Husar, R.B. and Patterson, D.E. 1980. Regional scale air pollution: Sources and ef-

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46. Without the frequent and substantial visits to Stockholm to work with scientists there, the opportunity for the author to make this contribution to research on climate forcing likely would not have existed. The author specifically thanks his many colleagues at the Department of Meteorology, Stockholm University and the International Meteoro- logical Institute in Stockholm for their support of those visits.

Robert Charlson is professor of chemistry and of atmospheric sciences at the University of Washington in Seattle. He received a PhD from the University of Washington in 1964 and honorary PhD from Stockholm University in 1993. He was a Fulbright Scholar studying Cloud Physics in the UK in 1964. He has published more than 170 scientific papers, has authored or edited seven books, and has six patents on instruments for atmospheric measurements. His research foci include aerosol optical and cloud nucleating properties and their relationships to the chemical properties of the particles. His address: University of Washington, Department of Atmospheric Sciences, Box 351640 Seattle, WA 98195-1640, USA

Ambio Vol. 26 No. 1, Feb. 1997 ? Royal Swedish Academy of Sciences 1997 31