Aerosols: Effects of Haze and Cloud

Haze from small particles surely affected climate, but how? Old speculations about the effects of smoke from volcanoes were brought to mind in the 1960s, when urban smog became a major research topic. Some tentative evidence suggested that aerosols emitted by human industry and agriculture could change the weather. A few scientists exclaimed that smoke and dust from human activities would cause a dangerous global cooling. Or would pollution warm the atmosphere? Theory and data were far too feeble to answer the question, and few people even tried to address it. Among these few, the uncertainties fueled vigorous debates, in particular over how adding aerosols might change the planet's cloud cover. Finally, in the late 1970s, powerful computers got to work on the stupefyingly complex calculations, helped by data from volcanic eruptions. It became clear that overall, human production of aerosols was cooling the atmosphere. Pollution was significantly delaying, and concealing, the coming of greenhouse effect warming.

Sulfates, Soot and Clouds (mid 1970s-1988)

When thinking about aerosols, the public and most scientists had attended chiefly to the visible and obvious. That meant the fine carbon soot making up smoke (from factories, slash-and-burn forest clearing, and natural forest fires), mineral dust from dried-out soil (perhaps increased by human agriculture), and other solids such as salt crystals (from ocean foam). When scientists thought about climate change that volcanic eruptions might cause, they chiefly considered the minute glassy dust particles that snowed down thousands of miles downwind from an eruption. ⁽¹⁾ Well into the 1970s, meteorologists concerned with aerosols mostly continued to assume they were dealing with such coarse mineral particles. However, anyone looking at city smog--or smelling it--might guess that chemicals could be a main component of a haze. The intense studies of urban smog that began in the 1950s focused the attention of a few scientists on the production and evolution of simple chemicals.

One of the most important of these molecules was sulfur dioxide, SO₂. Emitted profusely by volcanoes as well as by industries burning fossil fuels, SO₂ rises in the atmosphere and combines with water vapor to form minuscule droplets and crystals of sulfuric acid and other sulfates. These can linger in the stratosphere for years. The particles reflect some of the radiation coming from the Sun and absorb some of the thermal radiation rising from the Earth's surface.

To the considerable surprise of atmospheric scientists, studies in the early 1960s found that sulfuric acid and other sulfate particles were the most significant stratospheric aerosols. The sulfate haze was especially thick for a few years following a huge volcanic eruption in 1963, when Mount Agung in Indonesia blasted some three million tons of sulfur into the stratosphere. That was an order of magnitude more sulfur than human industry produced in a year, and most specialists thought human emissions of sulfates must be comparatively unimportant. ⁽²⁾ Flights in the stratosphere in the early 1970s (part of a huge government effort to study whether airplanes might harm the ozone layer) demonstrated conclusively that the principal aerosol there was droplets of sulfuric acid, presumably from volcanoes. ⁽³⁾

Outside the smoggy cities, haze was commonly assumed to be a "natural background" from soil particles and the like, with occasional extra material from volcanoes. That was challenged in 1976 by two leading experts, Bert Bolin and Robert Charlson. Analyzing air purity data collected by government agencies, they showed that sulfate aerosols seriously affected wide regions. Sulfates measurably dimmed the sunlight not only in cities but across much of the eastern United States and Western Europe. This confirmed what McCormick and Ludwig had reported a decade earlier, a widespread haze somehow connected with urban smog. ⁽⁴⁾

Bolin and Charlson drove their point home with some calculations. Although they repeatedly admitted that the data were fragmentary, and the theory so oversimplified that they could be off by a factor of ten, their results strongly indicated that sulfates were a significant factor in the atmosphere. Indeed among all the aerosols arising from human activity, sulfates played the biggest role for climate. The old view of aerosols as simply a dust of mineral particles had to be abandoned. In fact the haze was a mixture of the dust with tinier chemical droplets.

Still, the effect seemed minor. Bolin and Charlson figured that human sulfate emissions noticeably affected scarcely one percent of the Earth's surface. The sulfates were cooling the Northern Hemisphere by scarcely one-tenth of a degree. Most scientists thought that was negligible (even if the calculation were accurate, which seemed unlikely). They continued to assume that the problem of human aerosols was strictly local, or at worst regional. Bolin and Charlson themselves, however, noted that emissions were climbing steeply. They warned that "we are already approaching the time when the magnitude of the indirect effects of increasing use of fossil fuel may be comparable to the natural changes of the climate over decades and centuries." ⁽⁵⁾

Sulfates were a new worry for the scientists who were concerned about future climates. That included in particular the Russian expert Mikhail Budyko. In 1974, he suggested that if global warming became a problem, we could cool down the planet by burning sulfur in the stratosphere, which would create a haze "much like that which arises from volcanic eruptions." He calculated that just a few airplane flights a day would suffice. ⁽⁶⁾ That kind of freewheeling speculation was about all one could do at this point in thinking about sulfates.

The question attracted few workers, if only because the prospects were poor for solid, publishable studies. For one thing, the amount and type of aerosols (unlike CO₂) varied greatly from region to region. For another, their net effect on the radiation balance depended on the angle of sunlight (the low-angle illumination of Arctic zones doesn't interact with clouds in the same way as the plunging rays of the tropics). And so forth. The only thing likely to get anywhere would be a full-scale computer attack, and that would have to wait for faster machines.

In the mid 1970s, some groups studying greenhouse gases managed at last to develop computer models that plausibly connected climate to variations of radiation. A few groups tried to apply these models to aerosols. First they needed reasonably accurate information on the spectrum of aerosols normally in the atmosphere--the sulfuric acid droplets, salt crystals, rock dust, soot, and so forth. What were the sizes of the particles, their chemical composition, and their effects on radiation at various heights in the atmosphere? There were far fewer observations than the scientists needed, but some approximate numbers were laboriously worked out in a form usable for modeling studies. ⁽⁷⁾ The scientists also had to give up their preoccupation with the smog-ridden lower atmosphere, considering also the clear stratosphere. A few extra particles there, lingering for months, could make a big difference to the passing radiation. Despite daunting theoretical complexities and ignorance of many aerosol properties, the enterprise made progress. Different groups of modelers, using different techniques, converged on some tentative ideas.

The first big idea was that the formation of clouds was not in fact already saturated by natural aerosols. Thus adding some particles to the atmosphere should noticeably affect climate. The second big idea was that the net effect of adding aerosols, an effect which could now be reliably calculated, was to increase the planet's reflectivity and thus bring modest cooling. ⁽⁸⁾

Especially impressive was work published in 1978 by a NASA group under James Hansen, studying how climate had changed after the 1963 Mount Agung eruption. They found that the changes calculated by their simple model corresponded in all essential respects--including timing and approximate magnitude--to the observed global temperature changes. Hansen undertook the study mainly to check that his climate modeling was on the right track. But the results also showed that "contrary to some recent opinions," volcanic aerosols could significantly cool the surface. ⁽⁹⁾

Another sign that sulfates mattered came literally from another planet--Venus. The hellish greenhouse effect that astronomers observed there could not be caused by CO₂ alone, and during the 1970s, sulfuric acid was identified as a main force in the planet's atmosphere. ⁽¹⁰⁾ Another telling sign came from a 1980 study of Greenland ice cores. The level of sulfuric acid in the layers of ice pointed directly to ancient volcanic eruptions. Where clusters of giant eruptions were found, there had been episodes of cooling ("which further complicates climatic predictions," the authors remarked). ⁽¹¹⁾

The feeling that scientists were getting a handle on aerosols was strengthened in 1981 when Hansen's group fed their computer model a record of modern volcanic eruptions. They combined the temporary cooling effect of volcanoes with estimates of changes due to solar variations and, especially, to the rising level of CO₂. The net result fitted pretty well with the actual 20th-century temperature curve, adding credibility to their model's prediction of future global warming. ⁽¹²⁾ (This result was robust: vastly more sophisticated computer models at the end of the 20th century continued to get a good match to modern temperature fluctuations if, and only if, they added together eruptions, solar activity, and the rise of greenhouse gases. Adding industrial aerosol pollution further improved the match.) ⁽¹³⁾

The cooling effect of sulfates was confirmed by computer studies that took advantage of a colossal explosion of the Mexican volcano El Chichón in 1982. From this event scientists learned more about the effects of volcanic aerosols, one of them declared, "than from all previous eruptions combined." Satellite observations of clouds that were affected by the eight million tons of sulfur aerosols blown into the upper air could be matched with a noticeable cooling of regions beneath the clouds. ⁽¹⁴⁾ Alongside the progress in dealing with volcanoes came increasing evidence that the natural background of aerosols always present in the atmosphere also tended to cause mild cooling. The first calculation that many experts accepted as reasonably accurate gave a year-in, year-out global cooling effect of 2-3C (roughly 4-5F). ⁽¹⁵⁾

These calculations, however, dealt only with the effects of aerosols directly on radiation. They included cloud cover (if they calculated it at all) as a simple consequence of the moisture in the atmosphere. But since the 1960s, a few scientists had pointed out that the direct effects of aerosols might be less important than their indirect effects on clouds. This was the kind of thing Walter Orr Roberts had talked about, when he had pointed to cirrus clouds evolving from jet contrails. These clouds had seemed a temporary, local phenomenon. Now some wondered whether human emissions, by adding nuclei for water droplets, might be causing more cloudiness world-wide?

These speculations had been reinvigorated in 1979, when a pair of scientists at the University of Utah had managed to insert aerosols and cloudiness in a reasonable way into a basic radiation-balance computer model. The researchers confessed that their calculation was massively uncertain. But if the worst case was correct, then increased cirrus clouds could lower the Earth's surface temperature several degrees. It was another case of scientists warning that we might "initiate a return to ice age conditions." ⁽¹⁶⁾ Other scientists, in particular Hansen's group, doubted that aerosols could be so powerful. While admitting that nobody knew how to model cloud feedbacks reliably, they concluded that aerosols from human activity and even from volcanoes could not produce enough cooling to halt the "inevitable" warming by greenhouse gases. ⁽¹⁷⁾

Progress would depend upon more accurate knowledge of the intricate chemistry of the atmosphere. In the 1980s, aerosol physicists and atmospheric chemists finally established close contacts. It was becoming clear that the most important aerosols humanity produced were not dust and smoke particles, but products of chemical reactions of the gases we emitted. As in almost any field of geophysics, recognition of an important area of ignorance drove rapid improvements in measuring instruments and also in theory (which by now was done mostly through computer models).

One important finding in the early 1980s was that human chemical emissions tended to turn into sulfate particles whose sizes fell exactly within the range most effective for scattering sunlight. Thanks to research on atmospheric quality sponsored by environmental protection agencies, scientists increasingly agreed that regional sulfate hazes were a serious issue. Since the mid 1970s, studies had proved that such hazes could significantly dim sunlight for thousands of kilometers downwind from the factories. But the effect on the rest of the planet's climate, if any, remained debatable. ⁽¹⁸⁾

The need to resolve the problem was driven home by undeniable evidence that dimming of sunlight by aerosols was increasing sharply all across the Northern Hemisphere. By one estimate, the reduction was as great as 18% per decade. ⁽¹⁹⁾ Even in the Arctic, where the immense empty landscapes promised only pristine air, scientists were startled to find a visible haze of pollutants drifting up from industrial regions. There was so much soot that it might alter the northern climate. ⁽²⁰⁾

(I have seen it myself. After backpacking in the Sierra Nevada and the Canyonlands, decades after my first treks in these sacred places, I am pained to confirm that the views of distant cliffs, and even of the stars, are never as sparkling clear as I was once used to seeing.)

Thinking about cooling from aerosols took a spectacular turn in 1983, when a group of scientists, mostly people who had already been studying the question, went public with warnings of an even worse danger. If the blasts of a nuclear war injected smoke and dust into the atmosphere, a lethal "nuclear winter" might envelop the planet. The Russian meteorologist Kirill Kondratyev went on to point a finger at the nitrates (NOx) that had already been put into the atmosphere by weapons tests. These had produced aerosols which, he surmised, might have been responsible for the decreased transparency of the atmosphere, and thus the cooling, observed during the 1960s. Like others, Kondratyev warned that aerosols from human activity had temporarily masked the tendency toward global warming due to increased CO₂. ⁽²¹⁾ Only think how much cooling might follow a thousand nuclear explosions!

An even more horrendous effect of aerosols had been proposed back in 1980 by Walter and Luis Alvarez: the doom of the dinosaurs when a giant meteor struck the Earth 65 million years ago. Calculations showed that dust from an asteroid impact could have fatally cooled the planet. ⁽²²⁾ All this was sharply contested by other scientists. The leading alternative that they developed to explain the doom of the dinosaurs was a series of gargantuan volcanic eruptions. That just showed another way that aerosols could change climate on an apocalyptic scale. ⁽²³⁾

The "nuclear winter" and dinosaur extinction controversies contributed almost nothing to scientific study of ordinary climate change. But they encouraged a planetary-scale viewpoint, and sharpened awareness of the mortal fragility of the Earth's climate. Especially aroused was the aerosol community, or rather the scattering of researchers in diverse specialties who were gradually coalescing into a community. The furious controversies encouraged them to communicate with one another, and with meteorologists and other climate scientists.

Turning back to the way routine pollution might affect climate, scientists were slowly hacking a way through the jungle of complexities. A few meteorologists gradually worked out the implications of Twomey's studies, noticing how aerosols could create lingering misty clouds that might reflect enough sunlight to cancel the greenhouse warming. ⁽²⁴⁾ It was hard to know whether nature really acted according to these difficult calculations, and most experts paid little heed. After all, even massive direct cloud seeding had never been proven capable of doing much, despite decades of experiments. As Twomey admitted in 1980, "clear field verification has not been obtained" for various key predictions. ⁽²⁵⁾

Finally in 1987 a dramatic visible demonstration convinced many scientists that the theory deserved respect. Satellite pictures of the oceans displayed persistent clouds reflecting sunlight above shipping lanes--a manifest response to ship-stack exhaust. Apparently aerosols did create clouds, enough to outweigh the particles' direct interactions with radiation. ⁽²⁶⁾ It was also becoming clear that humans were the dominant source of the atmosphere's sulfate aerosols. ⁽²⁷⁾ Nevertheless, many scientists continued to think of aerosols as "local" pollution and worried little about global implications.

The closer scientists got to definite answers, the more they noticed additional factors that they ought to figure in. Especially troublesome was the fact that any climate change would alter the natural background emission of aerosols. For example, if deserts expanded (whether from direct human activity or climate change) there would be more airborne dust. Meanwhile pollution studies showed that altering the amount of one type of aerosol in the air would alter the distribution of sizes and other key characteristics of other aerosols (and these subtle calculations themselves, the author warned, "do not do justice to the complexities of the real atmosphere"). ⁽²⁸⁾ On top of this, there could be biological feedbacks. The most intriguing suggestion was that the nuclei for condensation of clouds in the pure air over the oceans might come primarily from dimethylsulfide (DMS) molecules, whose chief source is living plankton. ⁽²⁹⁾ It was another feedback dependent on temperature which might stabilize the climate--or might not.

Even if modelers set aside such issues, and even if they could resolve all the problems of cloud formation, they would still be far from knowing precisely how aerosols might affect climate. Few studies had even taken into account the fact that human activity emitted far more aerosols in some places than in others, so that the commonly used global averages could hardly represent the real situation. In some regions there would be too many particles to make normal clouds, in other regions too few. The properties of the aerosols themselves would be different in humid and dry regions. Yet climate scientists mostly continued to treat aerosols as a globally uniform background, mainly of natural origin. Atmospheric chemistry, observations of regional haze, and climate models were still such different fields that it was hard for any one person to assemble a coherent story. ⁽³⁰⁾

After 1988

By 1990, scientists understood that human activity produced somewhere between a quarter and a half of all the aerosol particles in the lower atmosphere, including industrial soot and sulfates, smoke from debris burned when forests were cleared, and dust from semi-arid lands turned to agriculture or overgrazed. The consequences, if any, were entirely uncertain--"at this stage neither the sign nor magnitude of the proposed climatic feedback can be quantitatively estimated." ⁽³¹⁾ Interest remained focused on greenhouse gases, which were expected to dominate climate change sooner or later.

Nevertheless scientists had to take into account the rapid increase in aerosols, and some called for better monitoring and more studies. ⁽³²⁾ Experts increasingly admitted that global climate change was not a matter of CO₂ alone. It came from a variety of effects ("forcings") on incoming and outgoing radiation due to a variety of gases and aerosols. A leader in the work remarked that it was this shift of viewpoint--looking at changes in the energy balance rather than attempting to calculate surface temperature changes--that made meaningful global calculations possible. He added that the calculations "would not have been possible without an enormous amount of work measuring the actual properties of atmospheric aerosols." ⁽³³⁾ Workers in the various fields that dealt with aerosols increasingly exchanged information and ground out measurements and computations.

In the early 1990s, aerosol experts began to agree that sulfates could cause significant cooling simply by scattering back incoming solar radiation. The effects of sulfate particles through stimulating cloudiness were harder to estimate, but probably added still more cooling. A pioneering 1991 calculation concluded that the scattering of radiation by sulfate particles produced by human activities was roughly counterbalancing the CO₂ greenhouse warming in the Northern Hemisphere at the present time. The calculation, however, was admittedly full of uncertainties. ⁽³⁴⁾

In 1991 Mount Pinatubo in the Philippines exploded. A mushroom cloud the size of Iowa burst into the stratosphere, where it deposited some 20 million tons of SO_2, more than any other 20th-century eruption. Hansen's group saw an opportunity in this "natural experiment." It could provide a strict test of computer models. From their calculations they boldly predicted roughly half a degree of average global cooling, concentrated in the higher northern latitudes and lasting a couple of years. ⁽³⁵⁾ Exactly such a temporary cooling was in fact observed.

Human pollution of the atmosphere should do the same, for although black soot particles absorbed radiation and would bring some warming, the cooling from cloud formation and sulfates seemed likely to outweigh that. Most scientists now agreed that aerosols emitted by the "human volcano" had indeed acted like an ongoing Pinatubo eruption, offsetting some of the greenhouse warming. Papers published in 1992 concluded that the smoke from slash-and-burn farming of tropical forests might have been enough all by itself to cancel a large share of the expected warming. ⁽³⁶⁾ As one expert remarked, "the fact that aerosols have been ignored means that projections may well be grossly in error." ⁽³⁷⁾ Thus efforts to restrict sulfate emissions, however important that might be for reducing acid rain and other unhealthy pollution, might hasten global warming. (On the other hand, reducing the emission from smokestacks of another important aerosol, soot, would help the climate as well as human health, since the black particles made for warming.)

Computer modelers returned to their simulations of global temperature, and found they could get curves that matched the observations since the 1860s quite closely provided they included increases in sulfate aerosols as well as CO₂. ⁽³⁸⁾ Because aerosol pollution was greater in some regions than others, whereas CO₂ levels were about the same everywhere, modelers could even try to disentangle the two influences. ⁽³⁹⁾ To be sure, there was a risk that with aerosol effects poorly understood, the modelers might merely be adjusting their numbers until they reproduced the climate data, overlooking other possible factors. But the new results incorporating aerosols did give, for the first time ever, a plausible and consistent accounting of many of the main features of 20th-century climate. These convincing results led directly to the announcement by a 1995 international report that human influence on climate had probably become discernible. Global warming might have become evident decades earlier, but for the overlooked cooling effect of aerosols.

The reprieve from warming was only temporary. Rains washed aerosols out of the sky within a week or two, so that the level in the atmosphere matched the level of emissions, whereas a large fraction of greenhouse gas emissions would linger in the atmosphere for centuries, steadily accumulating. The intergovernmental panel's next report, issued in 2001, also pointed out that industrialized nations were taking steps to reduce pollution, which would reduce the cooling influence. Considering various possibilities, the panel reported a high upper limit for where global temperatures might go during the 21st century. Computer models had not changed much since the previous report. But the panel offered an additional scenario, in which use of fossil fuels continued to expand at a breakneck pace while pollution controls strongly restricted aerosols. In that case greenhouse warming might shoot up nearly 6C. ⁽⁴⁰⁾

A minority of experts dissented from the panel's confidence that the improved computer models gave solid information. The critics warned that "given the present uncertainties in aerosol forcing, such improvement may only be fortuitous." ⁽⁴¹⁾ To clear up the uncertainties, scientists needed better information not only on how aerosols interacted with weather, but also on just what kinds of aerosols human activity stirred up and just where the winds blew them. ⁽⁴²⁾ None of that was measured well enough.

The old discussion of whether pollution brought warming or cooling was still yielding surprises. In particular, evidence turned up that much more soot ("black carbon") was puffing into the air than had been suspected. For example, a team under Ramanathan deploying ships, aircraft and balloons in the Indian Ocean in 1999 detected a huge drifting "brown cloud"--a miasma caused by human activity, expanded from the haze that Bryson had noticed while flying over India a third of a century earlier. While the dark smokes shaded the surface and thus made for cooling there, higher in the atmosphere the soot absorbed radiation so thoroughly_, according to new calculations, that overall it added seriously to global warming--perhaps more than methane and second only to CO₂. ⁽⁴³⁾ Cutting this sort of pollution could probably prevent not only damage to local and global climates, but also hundreds of thousands of premature deaths from respiratory illnesses. Some scientists argued that before going all out to restrain greenhouse gases, the world should attack the rightly despised smokes, the most ancient form of technological pollution. ⁽⁴⁴⁾ Scientists would have to understand much more about such hazes, however, before they could prove beyond doubt that pollution controls would retard warming.

[Later, beginning around 2002, climatologists were surprised by evidence that hazes were having an even bigger effect than they had supposed. As far back as 1989, Atsumu Ohmura in Switzerland had published evidence that sunlight had been growing dimmer throughout the 20th century. Ohmura’s work had attracted scarcely any attention, but now evidence turned up by other scientists convinced many experts that the Northern Hemisphere, at least, had seen a dimming of 10 percent or more — much more than the experts had thought, indeed probably great enough to affect agriculture. Aerosol pollution was the only plausible cause. "There could be a big gorilla sitting on the dining table, and we didn't know about it," Ramanathan admitted in 2004.

[Many aerosol specialists now suspected that they had seriously underestimated how strongly greenhouse warming had been held back by the cooling effect of aerosols. If so, then temperatures would now rise more sharply. For the “global dimming” trend had reversed around 1990 in many regions, perhaps because nations imposed pollution controls and otherwise cut aerosol emissions. The temporary reduction of solar energy input at the surface had given the world “a false sense of security,” about global warming, Crutzen warned in 2003. Whatever was happening, it was more obvious than ever that the world urgently needed better measurements of aerosols ⁽⁴⁵⁾]

Large uncertainties also remained in figuring how aerosols interacted with gases, and above all with water vapor. Questions were raised once again by detailed observations that confirmed the speculation that had first started scientists worrying back in the 1960s--cirrus clouds did grow from jet contrails, visibly influencing the climate in regions beneath heavily traveled air routes. ⁽⁴⁶⁾ Experts published widely divergent models for the formation of such clouds and their absorption of radiation. Controversial measurements published in 1995 claimed that clouds absorbed much more radiation than the conventional estimates said, raising a specter of "missing physics." As one researcher complained, "The complexity of this problem seems to grow with each new study." It was reasonable to expect that improvements in theoretical models and measuring techniques would eventually lead to a reconciliation [indeed by 2003 the theory and observations had largely converged], but Ramanathan admitted that "If I wake up with a nightmare, it is the indirect aerosol effect." And this effect was only one of several more areas where studies kept showing that, as Ramanathan and a colleague remarked, people were still "in the early stages of understanding the effects" of aerosols. ⁽⁴⁷⁾

This persistent ignorance about aerosols--their direct and indirect effects, and even their concentrations--was the largest single obstacle to attempts to predict future climate, especially for a given region. Funding agencies accordingly pushed vigorous and costly efforts to measure aerosol effects, promising major improvements within the next decade. Meanwhile, most experts felt that they could at least fix a rough range for the gross global consequences. They were reasonably certain that the sum of human aerosol emissions had a net cooling influence, at least in particular parts of the world. Estimates of the magnitude of the cooling ranged from minor to quite strong. Pollution was thus delaying the appearance of greenhouse warming in some industrialized regions and perhaps everywhere. As greenhouse gas emissions continued to accumulate, few doubted that the warming would soon leap past any possible aerosol cooling effects.

Related:

General Circulation Models of the Atmosphere

Rapid Climate Change

1. Humphreys (1940), p. 595; Junge (1952).

2. Wilson and Matthews (1971), pp. 279-80, 283-84.

3. Barrett and Landsberg (1975), pp. 44-45.

4. Bolin and Charlson (1976); for other studies of regional haze, see Husar and Patterson (1980).

5. Bolin and Charlson (1976), p. 50.

6. I have not seen the original Russian language publications, including Budyko (1974a); Budyko (1974b); see Budyko and Korol (1975); Budyko (1977), pp. 239-41; quote from Geophysical Abstracts B (1977), p. 63, an English summary of Budyko and Drozdov (1976).

7. E.g., Toon and Pollack (1976).

8. Harshvardhan and Cess (1976); Harshvardhan (1979); Charlock and Sellers (1980); for an overview, see Hansen et al. (1980).

9. A one-dimensional model. Hansen et al. (1978); see also Charlock and Sellers (1980); recent opinions: e.g., B.J. Mason, see Gribbin (1976).

10. The Venus greenhouse was invoked regarding the importance of sulfuric acid in Hansen et al. (1978).

11. Hammer et al. (1980).

12. Hansen et al. (1981); see also Bryson and Goodman (1980) (eyeball comparison going back to the 1880s); Gilliland (1982b).

13. Stott et al. (2000).

14. Hofmann (1988), quote p. 196. The paper includes a historical review of 1980s work.

15. Coakley et al. (1983).

16. Freeman and Liou (1979), p. 283.

17. Hansen et al. (1981), p. 960, "inevitable" p. 966.

18. E.g., Husar and Patterson (1980) (listing 1970s studies); Ball and Robinson (1982); for useful reviews, see Charlson and Wigley (1994); Charlson (1998).

19. Peterson et al. (1981).

20. "Arctic Haze, an aerosol showing a strong anthropogenic chemical fingerprint," Shaw (1982); scientists "startled": Kerr (1981). Already in the 1950s, J. Murray Mitchell had guessed the haze was caused by distant industries.

21. Kondratyev (1988), pp. 179-95.

22. Alvarez et al. (1984); Wolbach et al. (1985).

23. McLean (1985).

24. More pollution divided the water among more and hence smaller droplets, which not only made clouds linger but would also raise the reflectivity of the clouds and lower their absorption of solar radiation, keeping them cool and further lengthening their lifetime. Twomey (1980); the effect of aerosols in increasing cloud lifetimes and reflection, especially over the oceans where nuclei are rare, was worked out particularly by Albrecht (1989); "...the climatic effect is quite comparable to that of increased carbon dioxide, and acts in the opposite direction." Twomey et al. (1984).

25. Twomey (1980), p. 1461; he went on to report a verification at a single site, Twomey et al. (1984).

26. Coakley et al. (1987); the cloudiness is probably due to nitrates, see Lawrence and Crutzen (1999).

27. Schwartz (1988).

28. White (1986), quote p. 1671.

29. Charlson et al. (1987).

30. Joseph (1984); Charlson et al. (1992), p. 425.

31. Quote from chapter on "Greenhouse gases and aerosols" by R.T. Watson et al., IPCC (1990), p. 32.

32. Hansen and Lacis (1990).

33. R. Charlson, personal communication, 2002.

34. "Comparable to but opposite in sign to the current greenhouse forcing by increased CO₂ to date," Charlson et al. (1991); the first, primitive version was Charlson et al. (1990).

35. Hansen et al. (1992).

36. Penner et al. (1992); similarly, "It is becoming apparent that anthropogenic aerosols exert a radiative influence on climate that is globally comparable to that of greenhouse gases but opposite in sign," Charlson et al. (1992), p. 423; see Kerr (1992).

37. Wigley (1994).

38. Mitchell et al. (1995); IPCC (1996), chap. 8.

39. Taylor and Penner (1994).

40. IPCC (2001).

41. Ledley et al. (1999), p. 458; Singer (1999) also notes uncertainty about aerosol effects.

42. E.g., on increased dust, see Andreae (1996).

43. Satheesh and Ramanathan (2000), discussed in Wall Street Journal, May 6, 2003, p. 1; Hansen et al. (2000b); "The magnitude of the direct radiative forcing from black carbon itself exceeds that due to CH₄, suggesting that black carbon may be the second most important component of global warming after CO₂ in terms of direct forcing," Jacobson (2001). Subsequently Hansen and Nazarenko (2004) argued that decreased reflection of sunlight from snow and ice dirtied by soot gives another significant contribution to global warming.

44. Hansen et al. (2000b); Andreae (2001).

45. Ohmura and Wild (2002) and Roderick and Farquhar (2002) drew attention to the summary of evidence in Stanhill and Cohen (2001); Ohmura and Lang (1989). For "gorilla" and more see Kenneth Chang, "Globe Grows Darker as Sunshine Diminishes 10% to 37%," New York Times, May 13, 2004. Reversal: Wild et al. (2005); Pinker et al. (2005). Underestimates: Anderson et al. (2003); Crutzen quoted Pearce (2003). Work by others including Farquhar, Liepert, Ramanathan and Roderick was described on the BBC "Horizon" broadcast of April 3, 2005, transcript at http://www.bbc.co.uk/sn/tvradio/programmes/horizon/dimming_trans.shtml

46. Boucher (1999).

47. Cess et al. (1995); Pilewskie and Valero (1995); Ramanathan et al. (1995); Li et al. (1995); see Kerr (1995b); "complexity:" Kiehl (1999), p. 1273; "nightmare:" Ramanathan quoted in Schrope (2000), p. 10; "early stages," Satheesh and Ramanathan (2000), p. 62; for an argument that there was nothing serious missing, see Hansen et al. (2000a), pp. 147-54. "Because nearly all recent studies show good agreement between observations and models, the dust of the CAA [cloud absorption anomaly] debate appears to be settling down," Li et al. (2003).

Meehl et al. (2005) Bellouin et al. (2005)