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The Parasol Effect

A hazy umbrella of sulfur particles is reflecting enough sunlight and heat back into space to offset global warming. You might think that's good news. Think again.

By David Berreby
Jul 1, 1993 5:00 AMNov 12, 2019 5:30 AM


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Robert Charlson glances at a stand of dark pines a few hundred yards away, across the flat gray waters of Lake Washington. This air looks pretty clean,’’ he says.

It sure does. A cold scent of fresh water is blowing off the lake behind the parking lot of the National Oceanic and Atmospheric Administration’s Pacific Marine Environmental Laboratory in Seattle. Sparrows are cheeping all around as they flit among the red and gold leaves of trees in full autumn display. There’s a constant scritch-scritch sound coming from the lawn, where a flock of Canada geese, each approximately the size of a well-fed third grader, is munching grass. The sensible compacts in the parking lot aren’t belching exhaust, and even the smoke coming from one of NOAA’s boxy white buildings looks like harmless water vapor. It’s hard to imagine how the atmosphere could be any cleaner and still have any modern, car-driving, industry-dependent people in it.

Well, let me tell you, it’s not clean,’’ Charlson says. See the trees on the other side of the lake?’’ He points east. If it were really clear, you’d be able to see every branch over there. Instead, some of the details are lost because some of the light reflected from the trees isn’t reaching us. On its trip across the lake, the light is slogging through a thin haze of solid specks and liquid globules, most of which are sulfur compounds. Some of these particles are as small as viruses; some are no bigger than a handful of molecules. Belched forth from smokestacks and car exhausts, these airborne particles, or aerosols, don’t absorb much light, so they don’t appear dark. But light that strikes an aerosol doesn’t pass through it, either--it just bounces off at a new angle. The more haze, Charlson says, the more this optical scattering degrades the view.

Charlson, a professor of both atmospheric sciences and chemistry at the University of Washington in Seattle, has been studying aerosols since the 1960s, when standard textbooks said optical scattering would never be measured accurately (among the first of Charlson’s half-dozen patents is for a device that does just that). Like a nineteenth-century explorer painstakingly drawing hills and streams on the blank spots that were once labeled here there be tigers, he has spent 30 years creating an almanac of details about what he calls this peculiar state of material floating around in the atmosphere.

As a result of his work, one feature of haze is now very clear: there’s much more at stake than the view. Our whole climate is in jeopardy. Just as aerosols scatter light traveling from one side of a lake to another, they also interfere with light coming in to Earth from the sun. Some of it’s being reflected back, Charlson says. It goes right out into the blackness of space.

And sunlight, on our planet, means heat. Last year Charlson, together with six of his fellow atmospheric researchers, published the first reliable calculations of just how much heat is getting bounced away from Earth. Some regions, they found, are so blanketed by haze that they are undergoing an aerosol cooling, a cooling great enough that what might be called the parasol effect is neutralizing the better-known greenhouse effect. In other words, the explorer is back with news. Here be tigers, indeed.

When Charlson and his colleagues made this announcement, they noted that their finding might explain why even the best models of global warming have predicted hotter temperatures than those that have actually been measured. They also pointed out that their assessment of aerosol effects may in fact be too conservative. Charlson says it includes only the direct effect of aerosols; there’s even more cooling going on indirectly. Those colorless combinations of oxygen and sulfur--collectively known as sulfates--have a chemical affinity for water. They pull free-floating moisture out of the air and condense it into droplets of liquid water and acid; in fact, sulfates are the acid in acid rain. Put a bunch of these droplets together and you get a cloud. So wherever there are excess aerosols, clouds are more numerous, further shading the planet.

Moreover, the more aerosols that are in the air, the smaller will be the water droplets making up the clouds, because the available water vapor will be condensing around a larger number of particles. That also has a cooling effect. Try putting equal amounts of table salt and rock salt on a black tablecloth and you’ll see it,’’ Charlson says. You can see the table through the rock salt because there are fewer particles blocking your view. Everything else held constant, the cloud with more droplets will be brighter than the one with fewer droplets.’’ And a bright cloud reflects more heat than a dull one. The physics and chemistry of cloud formation are not yet understood well enough for Charlson or any other expert to make a good estimate of the scope of this indirect cooling effect, but few in the field doubt that it’s large.

This might, of course, seem like good news. At first blush, it looks like we’ve created a type of good pollution that is eliminating the effects of bad greenhouse-gas pollution. Perhaps we should even be congratulating ourselves for polluting our way out of a global disaster.

Indeed, says Charlson, just this type of reasoning has been used by politicians to justify going slowly on problems associated with global warming. Since the days of the Nixon administration, he says, there have been people suggesting that aerosol pollution might counteract global warming. Some people have actually suggested that if we learn how to pollute just right, everything will be fine.

But as Charlson points out, there are a number of subtleties to the parasol effect suggesting that aerosols, far from preventing a greenhouse world, are more likely to send global warming veering in a new, unexpected, but no less dangerous direction. To understand why, he says, you have to take a closer look at the haze.

A certain amount of aerosol haze occurs naturally. Twenty-two million tons of sulfur are emitted every year by minuscule, single-celled marine algae, giving the sea its faintly musty smell. The occasional volcano contributes its share. But this natural background isn’t the cause of modern haze. For that, industry is squarely to blame. Over the past 150 years humanity has been busy adding sulfur to the natural background, gouging the element out of the earth in the form of coal, metal ores, and oil. After being cooked from those substances by industrial processes, sulfur links up with oxygen and emerges from smokestacks as sulfur dioxide gas. Charlson estimates that, worldwide, industry puts out some 90 million tons of sulfur every year--almost 500 million pounds every single day. It’s like having lots of volcanoes erupting 24 hours a day, 365 days a year, he says. In a multiple-step chemical reaction that has not been fully elucidated, many of the atoms of this gas recombine to form trillions of tiny sulfate particles.

These particles stay up for no more than a few days before they fall back to Earth. Only sulfates from the most powerful of volcanic eruptions ever reach the stratosphere, where powerful wind currents keep them suspended for a year or two and distribute them all over the globe. Those produced by human beings stay in the lower atmosphere--below 36,000 feet at the middle latitudes, 50,000 feet at the equator. The gentler winds of this part of the atmosphere can push aerosols only about 600 miles at most before they come back to Earth, often as acid rain.

So Seattle air, which blows in after a 6,000-mile journey over the industry-free Pacific, is far less aerosol-laden than the stuff people are breathing in, say, Steubenville, Ohio, the epicenter of North American haze, according to Charlson. In fact, he says, so great is the aerosol concentration everywhere east of the Mississippi that people who grew up in that part of the country don’t even know what the sky is supposed to look like. The sky they know is murky--visibility is perhaps 20 miles, as opposed to the 100 miles or more that your average Antarctic penguin enjoys--and often it’s not even the right color.

When you have lots of photons bouncing around in a scatter, the sky goes from blue to a whitish color, Charlson says. From the ground anywhere in the eastern third of North America, you look up on an otherwise sunny day, and the sky directly overhead may be blue or bluish, but off at angles it’ll be whitish. That white sky you see in the East is due to aerosol. That doesn’t happen very often in Montana.’’

Hence, for many years aerosols were considered a local problem for industrial areas and their neighbors a few hundred miles downwind. In fact, for most of the time that Charlson pursued his research, the government agencies that paid his bills were concerned about the view rather than far-flung effects on the climate. Among the customers for his instruments was the U.S. Defense Department, which wanted to understand haze so weapons guidance systems could pierce its veil.

Indeed, Charlson himself, with his longtime collaborator Bert Bolin of Stockholm University, wrote a paper in the mid-1970s that said aerosols could not have much impact on global climate. We had made a mistake, Charlson says now. We didn’t have the global chemical model. We were guessing as to numbers. We didn’t get the geographical extent of sulfates right.

Then, in the 1980s, sulfate haze began to register as more than a technical problem for tourists and bomber pilots. Sulfate aerosols were recognized as the key culprit in the acid rain that is killing lake fish, stunting forests, and corroding buildings and equipment in Europe and North America. The acid rain problem led to more support for research into sulfates.

Out of this focus on the problem came better techniques for measuring emissions, as well as new and more accurate computer models of wind patterns and chemical mixing in the lower atmosphere and of the dispersal of particles on those winds. In early 1990 this led to a big break. Charlson was attending a meeting on sulfates in a huge nineteenth- century faux-medieval castle in Bavaria. Many other climate experts were there also, of course, including two other collaborators and old friends of Charlson’s from Stockholm University, Henning Rodhe and Joakim Langner, who were showing off one of these improved computer models. The new Swedish model was the first devised to process data about industrial activity and weather, and it yielded a crucial variable in acid rain--the distribution of sulfur in the air after it leaves the pollution centers that create it.

Fortunately, Charlson recalls, one of the talks after theirs was very boring. His mind wandered back to the Swedes’ model, which--not surprisingly--predicted strikingly high concentrations of sulfates throughout the heavily industrialized Northern Hemisphere and related that finding to acid rain. But they hadn’t related such levels of sulfates to one of Charlson’s areas of expertise--optical scattering.

Charlson won his first patent for measuring such scattering nearly 30 years ago, with an invention dubbed the nephelometer (nephelos is the Greek word for cloud). The prototype still sits on a bookshelf in his office. It’s gunmetal gray, roughly the size and shape of a bazooka. Through an inlet on the bottom, a tiny pump sucks aerosol-laden air into a chamber. On one side of the cylindrical chamber, about halfway down its length, is a halogen movie-projector lamp. At one end of the chamber is an electric light detector--the technologically more sophisticated great- grandson, Charlson says, of those electric eyes that open doors and set off alarms. By determining how much light makes it through an air sample to the light detector, Charlson can accurately measure how much light is being deflected by aerosols in the sample. It gives you the ‘scattering efficiency,’ Charlson says. You might think of it as the amount of a light beam that a particle blocks out per gram of material.

To get a complete measure of optical scattering, Charlson explains, you make a measurement with a nephelometer; simultaneously you filter the air, get the particles out of it, and do a chemical analysis of the material. That gives you an amount of sulfate per cubic meter of air. Then you take the ratio of the scattering to the concentration of material. That’s what allows you to say that given X amount of sulfate in the air, there will be Y amount of scattering.

As he sat in the Bavarian castle, listening to the high figures for sulfates that the Swedish model yielded, Charlson realized that he knew how to make the optical calculations, to get the amount of scattering in meters squared per gram of material in the air. He took out a pencil and did some rough math on a scrap of paper.

It was much bigger than I thought, he recalls. So after the boring talk was over, at the coffee break, I grabbed Langner and Rodhe and said, ‘Look at this!’ That was the light bulb, right there. That was a Thursday. I was due to see them in Stockholm the next week. When I got there on Monday, a new model, with my light-scattering calculations incorporated, was sitting on a desk waiting for me.

The computer model confirmed his rough calculation. The aerosol umbrellas over the Northern Hemisphere, he saw, are keeping, on average, about a watt of solar energy per square meter from reaching Earth’s surface. That may not sound like much--very roughly, Charlson says, it’s perhaps a fifth of the amount of heat put out by a Christmas-tree light bulb, spread out over an average desktop. But that’s enough to cool Earth substantially. It’s also, on average, equal to the amount of heat added to the planet by man-made greenhouse gases, according to some estimates.

And that, says James Hansen, director of NASA’s Goddard Institute for Space Studies in New York, could explain why models of global warming have predicted that Earth should be warmer than it actually is. Hansen gained some unwanted notoriety in 1989 when he charged that officials in the Bush administration made him lower his own estimates of the power of the greenhouse effect. His latest simulation of climate change over the past 150 years now takes aerosols into account as a global cooling force and incorporates Charlson’s model of aerosol distribution over the Northern Hemisphere. The result, Hansen says, is quite consistent with the amount of warming that has been observed in the real world. For the best estimates we can make, the aerosols are second in importance only to the greenhouse gases.

But opposite in effect. Is the aerosol umbrella, then, a mandate to do nothing about global warming? Or to do nothing about reducing sulfur emissions? In a word, Charlson says, no. To him, the notion that humanity could fine-tune a system as big and complex as the climate is laughable. There’s always this temptation to tell ourselves we can handle it, that we’re bigger than it is, he says. Personally, I find that attitude very arrogant. It assumes that we understand climate well enough to engineer it, and we don’t.

Some of Charlson’s findings about the parasol effect suggest that it won’t help at all with some serious aspects of the global warming problem, such as rising sea levels. Sulfate aerosols may even make some warming effects worse, Charlson says. The reasons lie in the fundamental difference between greenhouse gases--which rise to the stratosphere and cover the globe--and sulfates, which travel only a few hundred miles.

Because sulfates have such a limited range, almost all man-made aerosols are floating above the Northern Hemisphere, where 90 percent of industrial activity is still concentrated. By contrast, the Southern Hemisphere gets almost no such protection from man-made sulfates. Even in the relatively clean air of Seattle, Charlson says, the amount of light scattered by haze is probably 10 to 100 times higher than it is in the Southern Hemisphere. With one hemisphere bearing the full brunt of global warming while the other is protected by an umbrella of pollution, he says, seas would still rise uniformly all over the globe, as the warmer southern waters expand. In other words, sulfates can’t save the Maldives, the low- lying island nation in the Indian Ocean.

But a rise in sea levels, Charlson says, might not be the biggest effect to worry about. Much more important, he points out, could be the increased difference in temperature between the two hemispheres. That’s likely to affect the large-scale weather systems on which people depend.

More frequent occurrence of drought is a possibility, Charlson says. Or of violent storms. Or the opposite--less frequent storms. I’d give either chance equal billing. The thing people need to understand is that a slight regional shift in any direction is a big concern. Last year in the mountains around Seattle we had more precipitation as rain and less as snow than normal. And the snowpack is our reserve of water that fills the reservoirs in late spring. So just because the balance of snow to rain changed, we had a drought here.

Charlson is a neatly trimmed man who comes to work in a tie knotted tightly at the neck. The fuzzy carelessness of most public talk about world climate seems to offend him personally. To his mind, the aerosol results are a perfect illustration of the extent to which we don’t know what we’re doing. The biggest problem the public has is that it perceives that we should do research in order to solve problems--but after those problems occur. It’s wrong. It can’t work that way. You have to have the fundamental knowledge ahead of time so you can apply it when the problem shows up.

Charlson recalls the time in the 1960s when some researchers, extrapolating from measurements that showed some cooling in the globe’s average temperatures, predicted that another ice age was already starting. They were wrong, Charlson says. That’s the problem we’ve always had in this field--this kind of lurching off and making grandstand statements without a good scientific foundation. We need a decades-long intensive scientific inquiry, because in reality these things are not going to submit to quick answers.

With that in mind, Charlson is very quick to insist on what his discovery is not. He says that so much remains to be understood about aerosols--especially with regard to their indirect influence as the seeds of clouds--that any estimates about their effects could be off by an order of magnitude. There are substantial uncertainties, he says. Perhaps as much as a factor of 2 up or down, which would mean, statistically, that a calculation of, say, .6 watts per square meter could represent a reality of maybe .3 or maybe 1.2. We can’t say yet where it would fall in that range. But the key point is that even using the lowest estimates doesn’t make this effect go away. It’s definitely there.

So Charlson is continuing to chip away at the aerosol mysteries with a network of colleagues, students, and former students scattered throughout the world. One graduate student, for instance, has been dispatched to Antarctica to examine sulfate deposits trapped in ancient ice. Because the same ice that collects sulfate particles also traps carbon dioxide in bubbles, it’s possible to track the relationship between levels of sulfate and levels of the gas, which is more abundant when the climate is warmer. Not surprisingly, says Charlson, higher amounts of sulfate do seem to correlate well with lower levels of carbon dioxide. The main purpose of the work is to build a record of preindustrial sulfate levels and temperatures. A historical standard of comparison will give researchers a much better handle on the extent to which sulfates can drive the climate.

Charlson is also working with several colleagues at the National Oceanic and Atmospheric Administration lab who are assembling a shipborne expedition to get a more complete picture of the boundary between the sulfate-laden Northern Hemisphere and the more pristine southern half of the planet, to learn more about any possible aerosol carryover. As the research vessel goes steaming up north of Tahiti, Charlson says, they will see the westerly winds flowing out of Asia carrying a load of sulfate pollution from China, Japan, and Korea, so they’ll be getting measurements of the transition from clean Southern Hemisphere air to more polluted Northern Hemisphere air and quantifying the amounts of it and defining the optical properties of it. Meanwhile, airplanes will be taking measurements of aerosol and cloud properties, and an NOAA satellite will measure the amounts and wavelengths of light bouncing off the atmosphere and out into space over the ship.

The effort is very much needed. If it took this long for atmospheric scientists to get the drop on an effect as important as that of sulfate aerosols, Charlson says, who knows what other consequences of our monkeying with the climate are drifting through the air, waiting to be noticed? Most of what we do know about aerosols comes from observing our own haphazard release of the particles into our air. In a kind of sinister way we’re doing a giant worldwide meteorological experiment, Charlson says. And we don’t know what’s going to happen.

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