How can volcanic haze cause extinction

Warming or Cooling? A few scientists did have their curiosity and concern aroused to the point where they pursued a modest number of studies in the early s. They failed to find solid evidence for a global increase of turbidity. But the studies did confirm that there were regional hazes — episodes of pollution spreading a thousand kilometers or so downwind from industrial centers. While Mitchell continued to insist that humanity was "an innocent bystander" in the cooling of the past quarter-century, in he calculated that our emissions might begin to cause substantial cooling after the end of the century.

But nobody trusted anyone else's calculations, which were in fact much too crude to give reliable answers. Adding to the uncertainty, Mitchell gave plausible arguments that aerosols could produce a warming effect. It depended on how much they absorbed or reflected radiation coming down from the Sun, and how much they trapped heat radiation rising up from the Earth's surface. It also depended on the height in the atmosphere where the aerosols floated, and on whether they floated above bright regions like deserts which reflect sunlight or dark ones like the oceans which absorb sunlight.

Ichtiaque Rasool and Stephen Schneider entered the discussion with a pioneering numerical computation. This was the first atmospheric science paper by Schneider, who would become a well-known commentator on global warming.

As an engineering graduate student, he had been alerted to environmental issues when he heard a talk by the biologist Barry Commoner, warning that pollution could trigger either an ice age or global warming. Their calculation gave cooling as the most likely result. Estimating that dust in the global atmosphere might have doubled already during the century, and might double again in the next fifty years, they figured that this might cool the planet by as much as 3.

That could be disastrous, especially in view of some simplified calculations just published by others which suggested that the climate system could be very sensitive to small changes of temperature. Rasool and Schneider also believed the greenhouse effect would not counteract the cooling, since according to their model, adding even a large amount of CO 2 would bring little warming. The dip caused by aerosols, they exclaimed, "could be sufficient to trigger an ice age!

But if the paper was wrong, what did aerosols in fact do? Another stimulus to work on aerosols came from a spacecraft that reached Mars in and found the planet enveloped by a great dust storm. The dust had caused the Martian atmosphere to warm up substantially — an undeniable demonstration that aerosols could profoundly affect climate. Yet the calculations were still too uncertain to say for sure whether the net result would be to increase or decrease the reflectivity, whether dust would cool the Earth or warm it.

Beyond the direct effects of aerosols absorbing or scattering radiation, an even tougher puzzle remained: how did particles help create particular types of clouds? And beyond that loomed the enigmatic question of how a given type of cloud might affect the temperature. Depending on whether clouds were thick or thin, and where they floated in the atmosphere, they might bring some amount of cooling, by reflecting sunlight, or they might even bring warming, by trapping heat radiation in a sort of greenhouse effect.

The one sure thing was that aerosols could make a difference to climate, and perhaps a big difference. Bryson felt more certain than most about the effects of aerosols, and more worried. His studies of the distant past had convinced him that the climate had sometimes veered dramatically in the span of a single century. Could a similar cataclysm befall our civilization? Weren't the deadly droughts in Africa and South Asia a sign that we were destroying our climate with pollution?

He suggested that the pattern of pollution would change the gradient of temperature from equator to pole. A change of only a few tenths of a degree in this gradient, he calculated, could shift the entire general pattern of atmospheric circulation.

That might alter, for example, the annual monsoon that was crucial for the peoples of India and the African Sahel. The entire balance of climate could be tipped, he said, by aerosols pouring from what he called "the human volcano. The amount of the gas coming from volcanoes was negligible, barely as much in a century as what human industry emitted each year.

To the confusion of onlookers, an entirely different prediction about cooling was meanwhile emerging from an entirely different field of science. New data on past ice ages showed that they followed a remarkably regular schedule. The warmest part of a cycle typically lasted barely ten thousand years, so it seemed likely that the Earth was now past the peak of the current cycle and was scheduled to descend into another glacial epoch.

Decades later it was learned that the current cycle is atypical, likely to last a few tens of thousands of years, but nobody at the time could guess that. In the natural course of things the temperature would fall gradually over the next few thousand years.

But perhaps human emissions were getting large enough to interfere with the natural process. Would greenhouse gases prevent the projected cooling? Or would pollution accelerate it? Newspapers and television in the early s were regularly running stories on the appalling droughts in the Sahel and elsewhere, and the public was starting to worry about climate change.

Would more dust and gases of human origin afflict us with even more deadly droughts or floods? That depended critically on the effects of aerosols. The scientists who had studied this recondite topic began to feel the public eye upon them, and they debated their technical questions with heightened intensity. They increasingly saw that it was theoretically possible for a small change of conditions to bring large changes of climate. But it would be another three decades before computer models of climate became good enough to confirm that industrial pollution had indeed contributed to the Sahel drought.

The prominent meteorologist William W. Kellogg, for one, told a World Meteorological Organization symposium not to worry. He noted that industrial aerosols, and also the soot from burning debris where forests were cleared, absorbed sunlight strongly — after all, smog and smoke are visibly dark. They would thus retain heat. He calculated that the chief effect of human aerosols would be regional warming although he admitted that the calculation relied on properties that were poorly known.

Anyway, as Kellogg also pointed out, rains washed aerosols out of the lower atmosphere in a matter of weeks. Eventually the warming due to the increase in CO 2 — a gas that lingered in the atmosphere for centuries — must necessarily dominate the climate.

Similarly, Stephen Schneider and a collaborator improved his rudimentary model, correcting his earlier overestimate of cooling see above by checking against the effects of dust from volcanoes. They got a decent match to temperatures over the past thousand years, after they added an estimate for changes of solar intensity.

The model now predicted that "CO 2 warming dominates the surface temperature patterns soon after Bryson and his co-workers continued to insist that smoke from burning fossil fuels and forest clearing had a powerful cooling effect.

After all, the haze visibly dimmed the solar radiation that reaches the surface. They expected pollution would more than balance the effects of increased CO 2 , since the more fuel humanity burned, the more aerosols were emitted along with the gas. Taking everything into account, they calculated that "an expected slight decrease in surface temperature" was already underway.

The real value of this work was not in the purported findings, but in the way it forced scientists to pay attention to a topic that was indeed highly important. Most of the studies were not even addressing all the key problems. Ideas about human emissions focused on an image of dark smog and smoke obscuring the sky.

Some scientists pointed out, however, that such direct effects of particles interfering with radiation could be outweighed by indirect effects. They emphasized new observations that nuclei for the condensation of water droplets into rain or snow were sparse under natural conditions.

Thus "the most sensitive" leverage point for pollution particles might be their role as cloud condensation nuclei. So it continued, as some scientists concluded that aerosols would cause warming, others expected cooling, and still others expected no significant global effect.

One widely noted example was a survey of dusty days in Arizona by Sherwood Idso and Anthony Brazel, who concluded that additional aerosols from human activity would warm the Earth. They urged people to abandon any thought that industrial pollution would serve as a brake on CO 2 greenhouse warming.

Critics promptly tried to poke holes in the study's limited data. This approach too was quickly criticized, for lack of enough data on Southern Hemisphere temperatures. Like most aspects of climate studies, only even more so, progress on aerosol impacts would require help from many different fields.

The name of the institute hints how scientists were regrouping to attack complex questions involving the environment. Twomey showed that reflection of sunlight from clouds depends on the number of nuclei in a curiously intricate way. Adding particles would normally create more water droplets, and thus thicker light-reflecting clouds. Past some point, however, the drops might fall as rain and the clouds would disappear altogether.

On the other hand, if there were a great many nuclei the water could end up not as raindrops but as myriads of tiny droplets — a long-lasting mist. And as Twomey also showed, the amount of reflection and absorption depended strongly on the average size of the droplets with smaller mist droplets there is more surface area for a given amount of water.

In short, adding more aerosol particles might either raise or lower cloud reflectivity, depending on quite a variety of factors. Overall, for thin clouds Twomey calculated that added pollution would increase the reflectivity and thus cool the climate , whereas for thick clouds absorption would dominate hence warming.

He concluded that since thin clouds are most common, the net effect of human pollution should be to cool the Earth. This did not close the debates. As another pioneer recalled, "Twomey's insights were largely ignored by the climate modeling community — perhaps because it seemed unlikely that such a simple analysis could capture the behavior of such a complex object as a cloud. Next you would have to calculate the direct interaction of each type of particle or chemical molecule with sunlight, and also calculate the effects of each type in forming various types of clouds, and finally calculate how each kind of cloud interacted with visible and infrared sunlight.

Little was known about any of this. The debates made one thing clear: climate change could not be properly understood without a better grasp of aerosol effects. When scientists made theoretical calculations of scattering, the results were often at odds with field and laboratory measurements.

It was not clear whether the theories or the measurements were wrong — if not both. Much more work would have to be done to get even the most basic data, such as how the various kinds of particles of various sizes scattered or absorbed light of various wavelengths. Several groups undertook these measurements in the s, using instruments that, like so much in aerosol science and the rest of geophysics, could be traced back to a military application.

All this was only the most simple, basic-physics aspect of aerosols. Studies increasingly confirmed that there were more complex ways that particles would surely affect the climate.

A surprising example showed up in the international GATE experiment, in which scores of research ships and aircraft crisscrossed the tropical Atlantic. They found that when winds blew dust from the Sahara desert over the ocean, significant changes in weather and the radiation balance could be seen all the way to the American coast. The best clues of all came from observing how volcanic eruptions acted on climate.

Historical research covering the past two centuries was confirming a distinct, if weak, pattern of global cooling in the few years following each major eruption. The dust in the ice cores correlated with Lamb's volcanic "Dust Veil Index" and extended much farther back.

Temperatures too could be read from the layers of ice, and analysis showed that through the past hundred millennia, dustier air had correlated with cooler polar regions. To be sure, that might only mean that cooler periods were windier, bringing dust from afar.

But it seemed likely that volcanoes did have a direct impact on climate. Later, more comprehensive studies tended to confirm that. For example, a dearth of major eruptions over several centuries may have helped cause a "Medieval Warm Period" that affected large parts of the planet — notably the North Atlantic region, when the Vikings benefitted from a benign climate to establish a colony in Greenland — although changes in solar activity were probably at least as important.

None of this supported the claims that we risked hurling ourselves into a new ice age—claims more common in excited news articles than in the scientific literature.

Few scientific papers were published in the early s on any topic related to climate change on a human time-scale, that is, faster than the thousands of years that most scientists thought glacial ages took to evolve. Only a small fraction of these few papers projected cooling within a century or two.

During the second half of the s the pace picked up as scientists published several dozen papers about century-scale global climate change. Some of these papers discussed cooling and warming factors without coming to a conclusion, but more than half projected that greenhouse warming would dominate. A study of the peer-reviewed articles of the period found that "global cooling was never more than a minor aspect of the scientific climate change literature of the era.

By the late s hardly any scientist was arguing that cooling was likely to become severe. The major industrial nations had put "clean air" laws in place.

Given that particles were washed out of the lower atmosphere in weeks, pollution was not going to double and redouble as some had feared. Moreover, improved computer models of climate had convinced many that CO 2 added to the atmosphere must bring a global warming.

The effect would be increasing rapidly along with the relentless rise of CO 2 , which humanity was emitting far more rapidly than anything could remove it from the atmosphere. As scientists calculated the physics of aerosols more accurately, they realized they could not figure out any way that smoke and dust particles from a volcanic eruption could cause long-term effects on temperature; they should drift to the ground or be rained down in a few weeks.

Then how did volcanoes affect climate for a year or even two? The answer was hidden in something else thrown into the air. 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.

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 s 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, SO 2. Emitted profusely by volcanoes as well as by industries burning fossil fuels, SO 2 rises in the atmosphere and combines with water vapor to form minuscule droplets and crystals of sulfuric acid and other sulfates.

The particles reflect some of the radiation coming from the Sun and absorb some of the heat radiation rising from the Earth's surface. To the considerable surprise of atmospheric scientists, studies in the early s suggested that sulfuric acid and other sulfate particles were the most significant stratospheric aerosols.

This was something that could linger high in the air for years, like the fine fallout particles injected by nuclear weapon tests. The sulfate haze was in fact especially thick for a few years following a huge volcanic eruption in , 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.

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 by two leading experts, Bert Bolin and Robert Charlson. Analyzing air purity data collected by government agencies, they showed that sulfate aerosols from industrial centers seriously affected wide regions downwind. Sulfates 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.

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.

The sulfates were cooling the Northern Hemisphere by scarcely one-tenth of a degree. Most scientists thought that was negligible even if the calculations 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 sulfate 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. Sulfates were a new worry for the scientists who were concerned about future climates. That included in particular the Russian expert Mikhail Budyko. In , 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.

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 CO 2 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. In the mid s, when some groups managed at last to develop computer models that plausibly connected climate to the level of greenhouse gases, a few groups tried to apply these models to study the effects of 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.

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 confirmation that the formation of clouds was not already saturated by natural aerosols. Thus adding some particles to the atmosphere should noticeably affect climate.

Especially impressive was work published in by a NASA group under James Hansen, studying how climate had changed after the 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. Another sign that sulfates mattered came literally from another planet — Venus.

The hellish greenhouse effect that astronomers observed there could not be caused by CO 2 alone, and during the s, sulfuric acid was identified as a main force in the planet's atmosphere. 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.

The feeling that scientists were getting a handle on aerosols was strengthened in 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 CO 2. The net result fitted pretty well with the actual 20th-century temperature curve, adding credibility to their model's prediction of future global warming.

Adding industrial aerosol pollution would further improve the match. From this event scientists learned more about the effects of volcanic aerosols, one of them declared, "than from all previous eruptions combined. 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 s, 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 , 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. 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.

Progress would depend upon more accurate knowledge of the intricate chemistry of the atmosphere. In the s, 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, an almost unknown topic. As usual, 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 s 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 s, 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. For example, ash can be very abrasive to wings. How quickly do plants begin to grow back? The answer is that it depends on how much rain falls in the particular area. This means that when you are looking at old lava flows and trying to determine how old they are based on the amount of vegetation, you have to take the climate into effect as well.

Photograph by Steve Mattox, November 14, I think that actually the long-term effects of an eruption on wildlife are usually quite small. Certainly at Mt. Helens scientists saw that both plants and animals returned to the utterly devastated areas within only a year or so of the eruption. It is usually the short-term effects that are really bad. For example, there was a very big eruption of Santa Maria volcano Guatemala in The eruption itself killed a few hundred to perhaps people as well as thousands of birds.

Pretty soon there were so many insects including disease-carrying mosquitoes that eventually people died from malaria. There are various variations on the main theory. It sounds kind of funny that either can happen but it is true.

Anyway, if you have enough large explosive eruptions, then the theory says that there will be enough ash in the stratosphere to have one of these effects. You need an eruption or series of eruptions that is much bigger than anything we have ever witnessed. Today, using a version of NASA's global climate computer model, even high school students can run climate change scenarios.

Dinosaur Extinction Theory Early s In , Walter and Luis Alvarez proposed that a giant asteroid striking Earth 65 million years ago had sent enough debris into the atmosphere to cool the planet and kill off the dinosaurs. The dinosaur extinction theory aroused public awareness of how rapidly Earth's climate might change. It also encouraged aerosol and climate scientists to look more closely at issues related to global dimming. A few years later, in , a different theory had a similar effect, when aerosol scientists warned that nuclear war could lead to an apocalyptic "nuclear winter.

Left: Whether dinosaurs died out because a giant asteroid hit Earth or because volcanic eruptions clouded the skies another leading theory , the basic mechanism of global cooling was the same. Shipping Lane Clouds Scientists had long theorized that air pollution might be "seeding" the formation of clouds.

But decades of cloud-seeding experiments had failed to provide proof, and evidence for pollution-related clouds was tenuous. More conclusive evidence came in , when satellite photos revealed persistent clouds over areas of the oceans used as shipping lanes. Smokestack exhaust from ships, dense with sulfate aerosols, was creating clouds that likely reflected sunlight and decreased the solar energy warming the ocean surface. Left: A satellite image of "ship tracks" off the Pacific Northwest coast.

The tracks appear as bright white squiggles within a thinner veil of cloud cover. Shipping lane clouds form only in extremely humid air. Pinatubo Confirms Climate Models When Mount Pinatubo in the Philippines erupted, climate scientists seized the opportunity to test their models.

The eruption released some 20 million tons of sulfur dioxide into the atmosphere, giving rise to a lingering haze of sulfate aerosols. NASA researchers led by James Hansen calculated that Pinatubo's eruption would lower average global temperatures over the next few years by roughly half a degree Celsius 0. The prediction proved remarkably on target. By the mids, most scientists agreed that human-made aerosols were acting like an ongoing volcanic eruption, and that air pollution had likely been masking the impact of global warming for decades.

Left: For months after Pinatubo's eruption, a haze of sulfate aerosols hovered in the stratosphere, just as had happened in after Laki erupted in Iceland. The study, called Project INDOEX, found that over northern regions of the ocean, where pollution streams in from India, a pollutant layer nearly two miles thick cut down the sunlight reaching the ocean by more than 10 percent—a far bigger effect than most scientists had thought possible.

Ramanathan's own models had led him to expect a dimming of only one half to one percent. Project INDOEX showed in detail how the toxic mix of soot, sulfates, and other pollutants both directly blocked sunlight and, even more critically, helped spawn clouds that reflected the sun's energy back to space. Left: This satellite image captures the toxic aerosol haze blanketing northern India and Bangladesh, south of the Himalayan Mountains. Dimming Recognized Worldwide Mids to present In the mids, when meteorologist Gerry Stanhill reported that a dramatic 22 percent reduction of sunlight had occurred in Israel between the s and the s, the news hardly made a splash in the scientific community or popular press.

But Stanhill was not alone in measuring such a drop. When he combed the scientific literature, he found that other scientists had measured declines of 9 percent in Antarctica, 10 percent in areas of the U.