In 1783 a volcanic fissure in Iceland erupted with enormous force,
pouring out cubic kilometers of lava. Layers of poisonous ash snowed
down upon the island. The grass died, and three-quarters of the livestock
starved to death, followed by a quarter of the people. A peculiar
haze shadowed western Europe for months. Benjamin Franklin, visiting
France, noticed the unusual cold that summer and speculated that it
might have been caused by the volcanic "fog" that visibly dimmed the
sunlight.(1) |
- LINKS - |
Better evidence came from the titanic 1883 explosion of
Krakatau (Krakatoa) in the East Indies, which sent up a veil of
volcanic dust that measurably reduced sunlight around the world
for months. The planet had so few weather stations that scientists
were unable to learn for sure whether the eruption affected the
average global temperature. But from then on, scientific reviews
of climate change commonly listed volcanoes as a natural force that
might affect large regions, perhaps the entire planet. Looking at
temperatures after major volcanic eruptions between 1880 and 1910,
a few scientists believed they could see a distinct temporary cooling.
(The most impressive confirmation came much later, when examination
of older records showed that the1815 eruption of Tambora, scarcely
noted at the time outside Indonesia, had affected the world’s
climate much more than Krakatau. Crops were frozen as far away as
New England.)(2*)
Perhaps the smoky skies
in an era of massive volcanic eruptions were responsible for the
ice ages, or had even killed off the dinosaurs by cooling the Earth?(3) This image of climate change became familiar in popular as
well as scientific thinking. In the 1950s, experts noted that the
Northern Hemisphere had been getting warmer over the last several
decades, a time when volcanoes had been relatively quiet, whereas
the preceding century had experienced a number of huge eruptions
and had been severely colder.(4)
|
=>Solar variation
=>Climate cycles
=>Public opinion
Full discussion in
<=Modern temp's
|
The climate scientist J. Murray Mitchell,
Jr. took up the question, with the help of improved data on how minuscule
particles (aerosols) moved through the upper atmosphere. Studies of
fallout from nuclear bomb tests had shown that fine dust injected
into the stratosphere would linger for a few years, but would not
cross from one hemisphere to the other. With that in mind, Mitchell
pored over global temperature statistics and put them alongside the
record of volcanic eruptions. In 1961, he announced that large eruptions
caused a significant part of the irregular variations in average annual
temperature in a given hemisphere. On the other hand, average temperatures
had fallen since 1940, a period in which the world had seen few major
eruptions. Mitchell concluded that the recent cooling was an "enigma."
He thought it might signal a new phase of a decades-long "rhythm,"
the sort of cycle that generations of climatologists had tried to
winkle out of their data.(5) |
<=External input
![climatologist J. Murray Mitchell](images/mitchell-sm.jpg)
Murray
Mitchell |
Maybe aerosol science itself could solve the enigma. If it was
plausible that volcanic emissions could alter the climate, what about
particles from other sources? Meteorologists recognized that dust
and other tiny airborne particles could have important influences.
Simple physics theory suggested that such aerosols should scatter
radiation from the Sun back into space, cooling the Earth. Through
the first half of the 20th century, measurements and theory were inadequate
to say anything about that, beyond vague speculation.(6*) Such speculation gradually came to focus on something that
people were beginning to recognize as a major source of atmospheric
particles: human activity. |
|
Aerosols as Global Pollution (1920s
through 1960s) TOP
OF PAGE |
|
Hints that human emissions made a difference in the atmosphere
went back to measurements by the pioneering oceanographic vessel Carnegie
and other ships on voyages between 1913 and 1929. Analysis of the
sea air showed a long-term decrease in its conductivity, a decrease
which seemed to be caused by smoke and gases from ships and perhaps
from industry on land. "Thus we see," the researcher concluded, "that
like a living thing, the conductivity of the lower atmosphere finds
survival increasingly difficult in our modern industrial age."(7) Still, a 1953 review concluded that scientists
simply did not know whether pollution had significantly affected the
transmission of solar radiation.(8)
|
|
There was little prospect of getting an answer. Nobody had foreseen
the need for a series of uniform measurements over the decades to
show what was happening on a global scale. There existed only a few
indirect indications, like the Carnegie's measurements of
air far out at sea. It happened that some astronomical observatories
had kept regular records of the clarity of the air at their sites.
But nobody took on the challenge of hunting down such data and trying
to correct the numbers for local changes such as the growth of a nearby
city. As late as 1977, one expert lamented that "the time and energy
put into discussion perhaps outweigh the time and energy which have
been put into measurements."(9) Worse, since air pollution seemed to be a problem only near
the cities and factories where it was emitted, nobody had studied
the relationship between pollution on the one hand and the chemistry
of the global atmosphere on the other. The two fields engaged two
different groups of investigators. |
|
Aerosols not only intercepted sunlight, but
might also affect climate by helping to create clouds. Research early
in the century had shown that clouds can only form where there are
enough "cloud condensation nuclei," tiny particles that give a surface
for the water droplets to condense around. In the 1950s, scientists
began to consider whether people might be able to deliberately change
their local weather by injecting materials into the atmosphere to
help clouds form. "Seeding" clouds with silver iodide smoke, in hopes
of making rain, became a widespread commercial enterprise. Less visible
to the public were government studies of the use of aerosols as a
weapon of climatological warfare, to inflict droughts or blizzards
on an enemy. For good or ill, it was becoming plausible that aerosols
emitted on an industrial scale could alter the climate of an entire
region. |
<=>Rain-making
|
Perhaps we were already doing something like that inadvertently.
In the early 1960s, Walter Orr Roberts, a prominent astrophysicist
at the University of Colorado, noticed that something was changing
in the broad and sparkling skies above Boulder. Roberts had a long-standing
interest in climate. One of the things that had driven his career
in astrophysics was a hope of connecting climate with sunspot cycles.
He had been especially impressed by the terrible drought of the 1930s,
which he had seen firsthand when he drove through the Dust Bowl on
his way to Colorado. Aerosols stayed on his mind. One morning as he
was talking with a reporter from the New York Times, Roberts
pointed out the jet airplane contrails overhead. He predicted that
by mid afternoon they would spread and thin, until you couldn't tell
the contrails from cirrus clouds. They did, and you couldn't.(10) The Times made it a front-page
story (Sept. 23, 1963). "Until recently, Dr. Roberts explained, cirrus
clouds were thought to be more of an effect than a cause of weather
conditions. But data from balloon and satellite experiments now suggest...
[clouds] may trap enough heat beneath them to affect the weather."
Since jets evidently made cirrus clouds, they "might be altering the
climate subtly along major air routes." |
|
The idea was controversial, like anything
that sounded like "cloud seeding." Many scientists believed that seeding
with particles could cause rain only under unusual conditions
or never. The "cloud chamber" studies around the start of the 20th
century, which had shown that clouds could not condense in very pure
air, did not seem significant. Most scientists believed that there
were always plenty of nuclei in the air, from sources like soil dust
stirred up by the wind and salt crystals from ocean foam. Therefore
clouds would form wherever the temperature and humidity were right.
Nobody had carefully tested this assumption. The theory of how particles
affected clouds was complex beyond reckoning, and field tests were
too costly to pursue far, especially since their results turned out
to be contradictory and confusing. Scientists avoided the intractable
study of cloud formation. As one of them later recalled, they viewed
tiny particles mainly "as air-quality indicators."(11) |
<=>Rain-making |
Nevertheless, by the early 1960s the question of human influence
on clouds was starting to attract at least some scientific attention.
Roberts's observation of contrails was joined by other hints that
various types of anthropogenic aerosols microscopic solid particles
or droplets of chemicals produced by human activity could indeed
increase the amount of cloud cover. That would increase the amount
of sunlight reflected away from the Earth. A 1966 study of satellite
photos of the oceans found linear clouds that might have been seeded
by smoke from ships. Another study, tracking rainfall downwind from
paper mills, concluded that humans were probably causing more precipitation
inadvertently than by deliberate cloud seeding.(12) |
|
Another line of speculation about the effects of dust from human
activities addressed pollution that settled on the polar icecaps.
Could that lower their reflectivity enough to change the climate?
The idea was inspired by one of those quirky speculations that both
harassed and stimulated climate scientists a suggestion that
"dusting of the ice caps" by volcanoes and soil blown off of dry lands
had caused the irregular changes in sea-level that had been recorded
in historic times.(13) This was only one of countless theories
of climate change, and did not get much credence or attention. |
|
Roberts's tentative
ideas about clouds did get a chance to catch sustained attention.
The opportunity was a growing public concern over the U.S. government's
plans to build a fleet of supersonic transport airplanes. Hundreds
of flights a year would inject water vapor and other exhaust into
the high, thin stratosphere, where natural aerosols were rare and
any new chemical might linger for years. Some scientists feared that
the flights would seriously affect the climate.(14*) A 1970 review by a panel of experts warned that aircraft
were already polluting the stratosphere with hydrocarbons and sulfur
and nitrogen compounds, all of which might interfere with radiation
directly as well as increasing cloudiness. They reported that high
cirrus clouds had in fact increased in the United States since the
1940s. The effects of aircraft on climate might be significant, they
concluded indeed particles emitted by a fleet of supersonic
transports might alter the stratosphere as much as a volcanic eruption.
But a calculation of the actual effects was still far beyond
reach.(15*) Such work was admittedly closer to plausible story-telling
than scientific rigor. Neither laboratory nor theoretical studies
had gone far in studying the kind of particles that mattered for climate.
|
<=>Other gases
=>Public
opinion
|
Aerosol science was just emerging as a field standing on its own.
Like many other fields it had gotten a strong impetus from warfare,
where smokes, poison gases, and disease-carrying aerosols could be
mortal concerns. The field first began to coalesce during the Second
World War, and its first handbook was based on studies done under
the Manhattan Project.(16) After the war, rainmaking jumped to
the top of the list of practical interests. But it was too intractable
and controversial for most aerosol specialists. Still less were they
interested in tackling the physics of clouds or the physics of radiation
high in the atmosphere, topics that were dauntingly intractable and
remote from any useful application. Concern over fallout from nuclear
weapons tests motivated some research (not always openly published)
that was potentially relevant to climate. Most aerosol specialists,
however, were far more interested in the practical problems of air
at ground level, especially public and occupational health. And these
problems, involving dust and pollution, "had no glamour to offer for
young researchers," as one pioneer admitted. The field's first journal
(named, naturally enough, the Journal of Aerosol Science)
was not founded until 1970, and the editor remarked that even then
"academic status has not been achieved."(17) |
|
The people who were
coming together to form an aerosol science community were mostly scattered
among industrial and government laboratories. In these organizations,
as the new journal's editor remarked, "it is extremely difficult for
a scientist to concentrate over the many years which are necessary
for the mastery of a subject."(18*) Many of the aerosol experts were kept
busy studying local air pollution, driven by rising public dismay
over urban smogs. Chemists were drawn in during the 1950s to analyze
the smog of Los Angeles, which turned out to be a fascinating (and
sometimes lethal) mixture of chemicals as well as particles. Meanwhile
other aerosol experts worked on industrial processes like "clean rooms"
for manufacturing electronics, and still others investigated military
problems such as the way particles scattered laser light. These researchers
had only occasional contact with their colleagues in different areas
of the proto-field of aerosol science, and still less with anyone
in other fields of science that might relate to
climate.(19) |
<=Public opinion
<=>Climatologists
|
Most of the aerosol scientists' attention
went to "pollution" particles that fell out of the atmosphere (or
were washed out by rain) within a few days. But there were also microscopic
particles that could linger longer and travel farther. Entire regions
were intermittently hazed over, raising questions about possible world-wide
effects. Already in 1958, one expert had remarked that "there can
no longer be any sharp division between polluted and unpolluted atmospheres."(20*)
It was some time before many others recognized how far pollution spread
beyond cities. Understanding came only after people studying smog
set up a network of stations that regularly monitored the atmosphere's
turbidity (haziness). In 1967, Robert McCormick and John Ludwig of
the National Center for Air Pollution Control in Cincinnati reported
a gradual increase in the general turbidity over regions spanning
a thousand kilometers. Further checks of the record of turbidity turned
up increases even in remote areas like Hawaii and the North and South Poles.(21) Could humanity's emissions be
affecting the global climate, not in some abstract future but right
now? |
<=External input
|
These studies contributed
to, and at the same time responded to, a broad change of thinking.
This had started with observations that radioactive fallout and chemical
pesticides could be found far from the places where they were emitted.
The world's oceans and air could no longer be seen as a virtually
infinite dumping-ground. Fewer and fewer people believed that the
atmosphere could safely absorb (as one aerosol expert acidly put it)
"any effluents which mankind might see fit to disgorge into it."(22)
Through the early 1960s, ideas about human influence on climate had
focused on greenhouse effect warming caused by industrial emissions
of carbon dioxide gas (CO2). At the time the
effect seemed no more than a fuzzy speculation, and it sounded all
the less attractive after weather experts reported that a world-wide
cooling trend had been underway for a decade or so. Now McCormick
and Ludwig suggested that the cooling trend itself might be due to
human activities. |
<=>Public opinion
<=Modern temp's |
Reid Bryson, a University of Wisconsin meteorologist, joined the
discussion. In 1962 he had flown across India en route to a conference.
He was struck by the fact that he could not see the ground
his view blocked not by clouds but by dust. Later he saw similar hazes
in Brazil and Africa. The murk was so pervasive that local meteorologists
took it for granted and failed to study it. Bryson realized, however,
that the haze was not some timeless natural feature of the tropics.
He was seeing smoke from fields set on fire by the growing population
of slash-and-burn farmers, and dust from overgrazed lands turning
to desert. The effects of ever more widespread farming and grazing,
together with pollution from industry, seemed large enough to alter
the climate of the entire planet. Bryson suggested that a rapid and
world-wide rise of atmospheric turbidity was counteracting the CO2
greenhouse effect. The consequence, he suspected, would be a drop
in temperatures. Calling for more intense study, Bryson and a collaborator
wrote that they "would be pleased to be proved wrong. It is too important
a problem to entrust to a half-dozen part-time investigators."(23) |
|
Concern grew when studies
showed that recent decades had seen a great increase in the amount
of aerosols in the lower atmosphere ("troposphere"). The air over
the North Atlantic was twice as dirty in the late 1960s as it had
been in the 1910s, suggesting that the natural processes that washed
aerosols out of the atmosphere could not keep up with human emissions.(24) As a back-page New York Times item (Oct. 18, 1970,
p. 92) reported, "This is disturbing news for those weather experts
who fear that air pollution, if it continues unchecked, will seriously
affect the climate and perhaps bring a new ice age." |
=>Public opinion
= Milestone
|
But how much of the
haze was really caused by humans? In 1969 Murray Mitchell pushed ahead
with his statistical studies of temperatures and volcanoes. He calculated
that about two-thirds of the cooling seen in the Northern Hemisphere
since 1940 was due to a few volcanic eruptions. He concluded that
"man has been playing a very poor second fiddle to nature as a dust
factory."(25) Other respected climatologists agreed
that volcanic dust could account for a substantial part of the temperature
variations in the last century or so. The most impressive work was
done by the British meteorologist Hubert Lamb, who burrowed through
many kinds of historical records to compile a "Dust Veil Index." His
tables revealed a telling connection between dust and cooler temperatures.
But if the experts now agreed that volcanic explosions could affect
temperature, they disagreed on how strong the effect was.(26*)
|
=>Simple models
=>World winter |
One thing scientists were coming to agree on was that the problem
was significant enough to merit a sustained attack. Mitchell for one,
even while denying that human aerosols had done much so far, thought
they could become significant within a few decades. McCormick and
Ludwig told a New York Times reporter (June 9, 1970, p. 60)
that their experiments proved that fine particles could noticeably
reduce the sunlight reaching the surface of the Earth. Their main
message was a call for better monitoring of turbidity. "What we are
trying to do," Ludwig added, "is get scientists' curiosity and concern
aroused." |
|
Warming or Cooling? (Early 1970s)
TOP
OF PAGE |
|
Enough scientists had their curiosity and
concern aroused to the point where they pursued a modest number of
studies in the early 1970s. They failed to find solid evidence for
a global increase of turbidity. But the studies did turn up regional
hazes episodes of pollution spreading a thousand kilometers
or so downwind from industrial centers.(27)
Everyone now admitted that human pollution was growing headlong. While
Mitchell continued to insist that humanity was "an innocent bystander"
in the cooling of the past quarter-century, in 1971 he calculated
that our emissions might begin to cause serious cooling after the
end of the century.(28) Other scientists claimed that the increase
of aerosols was important already, perhaps even more of a concern
than CO2. 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 instead of cooling. It depended on how much they absorbed radiation
coming down from the Sun, and how much they trapped heat radiation
rising up from the Earth's surface. And these numbers depended on
the level 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).(29*) |
=>Other gases
|
S. 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.)(30) Rasool and Schneider, like Mitchell, recognized that aerosols
might not cool the atmosphere but warm it. The tricky part was to
understand how aerosols absorbed radiation. 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 3.5C. |
=>Modern temp's
=>World
winter
|
That could be serious, 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 CO2 would bring
little warming. The dip caused by aerosols, they exclaimed, "could
be sufficient to trigger an ice age!" In fact their equations and
data were rudimentary, and scientists soon noticed crippling flaws
(as did Schneider himself, see below).
But if the paper was wrong, what did aerosols
in fact do?(31*) |
<=Radiation math
<=Simple models
=>Public opinion
|
Another stimulus to work on aerosols came
from a spacecraft that reached Mars in 1971 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.(32*) New studies confirmed how aerosols could affect a planet's
reflectivity, by scattering and absorbing sunlight and by catching
infrared rays coming up from the surface. 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. |
<=Venus & Mars |
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,
through a sort of greenhouse effect. The one sure thing was that aerosols
could make a difference to climate, and perhaps a big difference.
|
<=Simple models
|
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
1973 droughts in Africa and South Asia a sign that we were destroying
our climate with pollution?(33) In 1974, Bryson noted that humans emitted aerosols mostly
in northern mid-latitudes, just where the recent cooling trend was
most evident. 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. "Our climatic pattern is fragile rather
than robust," he warned.(34)
Bryson took his concerns to the public. The entire balance of climate
could be tipped, he said, by aerosols pouring from what he called
"the human volcano." |
<=Rapid change
![climate scientist Reid Bryson](images/bryson-sm.jpg)
Reid
Bryson
=>Public
opinion |
Newspapers and television in the early 1970s
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 debated their technical
questions with heightened intensity. Although they increasingly saw
that it was theoretically possible for a small change of conditions
to bring large changes of climate, few experts were even halfway convinced
by Bryson's "human volcano" warnings. |
<=Public opinion |
The prominent meteorologist William W. Kellogg told a 1975 World Meteorological
Organization symposium not to worry about cooling. 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.
Besides, many nations were vigorously working to lower air pollution.
Eventually the warming due to the increase in CO2
a gas which lingered in the atmosphere for generations
must necessarily dominate the climate.(35*)
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 "CO2
warming dominates the surface temperature patterns soon after 1980."(36) Other experts agreed that human production of aerosols,
although it might alter weather slightly in industrial regions,
had no noticeable global effect. Only a few people noted that if
pollution continued to increase, it might at some point cancel out
some of the greenhouse warming.(37)
|
=>Solar variation
=>Public
opinion
=>CO2 greenhouse
|
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 CO2 (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.(38) Bryson would not concede that his group's observations,
analysis of data, and theoretical understanding were too uncertain
to produce a definitive answer. 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, although
in ways that they would not work out reliably for decades more. |
|
Most of the debaters 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 nuclei. "Although the
changes are small," one scientist remarked, "the long-term effect
on climate can be profound." Conceivably the clouds would dim sunlight
so much that the whole climate system would flip into a new ice age.(39)
|
|
An important 1975 review panel concluded that the impact of particles
on global temperatures "cannot be reliably determined," for it depended
on many factors that were scarcely known. Warning that the particle
load in the atmosphere might rise another 60% by the end of the century,
they called, in the usual fashion of study groups, for further study.(40) In sum, no scientist felt confident enough of an imminent
ice age to publish such a prediction in a scientific journal. [See
discussion and references by RealClimate
and W. Connolley.]
|
|
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 CO2 greenhouse warming. Critics promptly
tried to poke holes in the study's limited data.(41) Another group analyzed global weather statistics, found
that the recent drop in temperatures was restricted to northern latitudes,
and argued that this demonstrated a cooling effect of industrial particle
emissions, which were far greater in the Northern Hemisphere. This
approach too was quickly criticized, for lack of enough data on Southern
Hemisphere temperatures.(42)
Many other studies invoked physical models, data, and the history
of volcanic eruptions and the ice ages as they debated the relation
of particle size to albedo, clouds, and temperature. Like most aspects
of climate studies, only even more so, progress on aerosol impacts
would require help from many different fields.(43)
|
<=Modern temp's
=>World
winter
|
In 1977 some light was cast into the shadows by Sean Twomey at
the University of Arizona's Institute of Atmospheric Physics. (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.(44) |
|
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."(45) Besides, to figure out the effects
on the world's climate, in principle you would need to start with
a map of the globe showing for each region the amount of every type
of smoke and dust particle and industrial pollutant in each layer
of the atmosphere. 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. As other scientists
made theoretical calculations of scattering, the results were often
difficult to reconcile with measurements. It was not clear whether
the theories or the measurements were worse. 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 1970s, using instruments that, like so much in aerosol science
and the rest of geophysics, could be traced back to a military application.
(In fact one of the inventors of the original instrument that measured
visibility through the atmosphere had died during a meteorological
reconnaissance flight during the Second World War.)(46) |
|
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 1974 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.(47) |
<=International
|
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.(48) Better still, dust from volcanoes and
other sources was found in layers of ancient ice, drilled from the
frozen plateaus of Greenland and Antarctica. 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.)(49)
|
<=Climate cycles
<=Solar variation
|
As scientists calculated the physics of aerosols more accurately,
they realized they could not figure out any way that smoke and dust
from an eruption could cause long-term effects on temperature. For
these were solid particles which mostly dropped out of the air within
a few weeks. The answer was hidden in something else thrown into the
air. |
|
Sulfates, Soot and Clouds (mid
1970s-1988) TOP
OF PAGE |
|
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.(50) 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.(51)
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.(52) |
<=Government
|
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.(53) |
|
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. |
=>International
|
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."(54)
|
|
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.(55) That kind of freewheeling speculation
was about all one could do at this point in thinking about sulfates.
|
=>Rain-making
|
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.(56) 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.(57) |
<=>Models (GCMs) |
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.(58) |
<=>Simple models
|
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.(59) 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).(60) |
<=Venus & Mars |
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.(61) (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.)(62) |
<=>Radiation math
=>Modern temp's
<=Solar variation
=>Public
opinion
= Milestone
=>Models (GCMs) |
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.(63) 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 2-3°C (roughly 4-5°F).(64)
|
|
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."(65) 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.(66) |
|
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.(67) |
=>Other gases
|
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.(68) 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.(69*) |
|
(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.(70) 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.(71)
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.(72) |
<=World winter
|
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.
|
=>Public opinion
|
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.(73*) 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.(74) |
|
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 could indeed create clouds, enough to outweigh the particles'
direct interactions with radiation. (Later studies showed there were
inconsistencies, as usual with aerosols. In some cases emissions from
ships made for more cloudiness, in some cases less.)(75) It was also becoming clear that humans
were the dominant source of the atmosphere's sulfate aerosols.(76) 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").(77)
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.(78) It was another feedback dependent on
temperature which might stabilize the climate or might not.
|
<=Biosphere
|
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.(79)
|
|
After 1988 |
=>after88
|
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."(80) Interest remained focused on greenhouse
gases, which were expected to dominate climate change sooner or later.
|
=>Models (GCMs)
|
Nevertheless some scientists were starting
to realize they might need to take into account what the rapid increase
in aerosols would do to climate. Hansen called for better monitoring
and more studies.(81) 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."(82)
Workers in the various fields that dealt with aerosols increasingly
exchanged information and ground out measurements and computations.
|
<=Other gases
|
In the early 1990s, Charlson and others worked to persuade aerosol
experts 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. In a pioneering 1991 calculation, Charlson and
his allies concluded that the scattering of radiation by humanity’s
sulfate emissions was roughly counterbalancing the CO2 greenhouse warming in the Northern Hemisphere. The calculation,
however, was admittedly full of uncertainties.(83) |
|
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 SO2, 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.(84) Exactly such
a temporary cooling was in fact observed. |
<=>Modern temp's
= Milestone
<=>Models (GCMs)
<=Arakawa's math |
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. Other scientists
reported that the direct effect of sulfates blocking sunlight "completely
offsets the greenhouse effect" in the most industrialized regions.
Yet another team estimated that the indirect action of sulfates,
making clouds darker, could have a still stronger cooling effect.(85*)
As one expert remarked, "the fact that aerosols have been ignored
means that projections may well be grossly in error."(86)
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.(87) 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.(88)
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. |
<=>Models (GCMs)
|
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 6°C.(89)
|
=>International
<=>Models (GCMs)
|
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."(90) 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.(91) 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.(92*) 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.(93)
|
= Milestone
<=>Other gases
|
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, even though
some computer modellers had begun to worry that their models did not
seem to include enough aerosol absorption. 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 was not really global but regional, and it had reversed around
1990 in many regions — perhaps because some nations were cutting
aerosol emissions. Pollution controls had certainly been reducing
sulfates. The way these sulfates and other aerosols had previously
kept some sunlight from reaching 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.(93a) |
= Milestone
|
Large uncertainties also remained in figuring how aerosols interacted
with gases, and above all with water vapor (the main "indirect
effect”"or "Twomey effect"). 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.(94*)
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 within the next decade
theory and observations would largely converge), but meanwhile, Ramanathan
admitted, "If I wake up with a nightmare, it is the indirect aerosol
effect." And this effect was only one of several areas where new studies
kept showing that, as Ramanathan and a colleague remarked, people
were still "in the early stages of understanding the effects" of aerosols.(95*)
|
|
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 most parts of the
world. Estimates of the magnitude of the cooling (both directly, and
indirectly through clouds) ranged from fairly small to quite strong.
Pollution was 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. |
=>Models (GCMs)
|
|
RELATED:
Home
General Circulation Models of the Atmosphere
Rapid Climate Change
1. Franklin (1784). First
to suggest the connection was a French naturalist, Mourgue de Montredon,
in a 1783 communication to the Académie royale de Montpelier. BACK
2. Krakatau's effects were seen only on subtracting
supposed effects of the sunspot cycle. Abbot and
Fowle (1913). The classic Krakatau study (then called Krakatoa) was
Symons (1888). Tambora: Stothers
(1984). BACK
3. A principal exponent of the view that volcanoes
dominated climate change was W.J. Humphreys, see Humphreys
(1913); Humphreys (1920), repeated in the
3rd (1940) edition, pp. 587-618. BACK
4. Wexler (1952), p. 78.
BACK
5. Mitchell (1961). On fallout
studies he cites a 1960 Defense Atomic Support Agency report by A.K. Stebbins;
rhythm: Mitchell (1963), p. 180. BACK
6. It "would be necessary to bring [dust] into the
scheme" of a complete calculation, but "that will not be attempted," in
the most comprehensive effort at calculation, Richardson
(1922), p. 45, see p. 59; discussed in Nebeker (1989), pp. 93-94; Ångström (1929) ; another speculation (first
suggested by H. Shapley in 1921) was that long-term climate changes might
come when the Earth passed through clouds of interstellar dust. Hoyle
and Lyttleton (1939); Himpel (1947); Krook (1953). BACK
7. Wait (1946), p. 343.
BACK
8. Wexler (1953), pp. 94-95.
BACK
9. Twomey (1977), p. 290.
BACK
10. On Roberts: Levenson (1989),
p. 98. Reporter: John A. Osmundsen, personal communication; contrail studies
are reviewed by Barrett and Landsberg (1975);
one of the first observations was the brief report of Georgi
(1960). BACK
11. "Plenty of nuclei," "air-quality": Twomey (1980), p. 1459. BACK
12. Conover (1966); mills:
Hobbs et al. (1970), see p. 89. BACK
13. Bloch (1965); human
impact was emphasized by Landsberg (1970). BACK
14. New York Times, May 1, 1965, p. 1. Aircraft
were estimated to increase cirrus over as much as 5% of the worlds's skies,
which "is not negligible," according to Bryson and
Wendland (1970), p. 137; repeated in Bryson
and Wendland (1975), p. 146. There were also concerns about exhaust
from space shuttle flights. BACK
15. Wilson and Matthews (1971),
p. 9, see Machta and Carpenter (1971); another
major group effort found that while supersonic transports appeared to
be harmless, the effect was close enough to the threshold of harm to merit
concern. Pollack et al. (1976); in 1999 a scientific
panel concluded that aircraft would contribute roughly 5% of the human
influence on climate by the year 2050, IPCC (1999).
BACK
16. United States (1950);
see Benarie (2000). BACK
17. "glamour": Gerhard Kaspar in Preining and Davis (2000), p. 392; "academic": Davies (1970). BACK
18. Davies (1970); Othmar
Preining (personal communication) writes that an aerosol scientific community
began forming in the mid 1960s, following the publication of Fuchs
(1964); see Preining and Davis (2000),
pp. 9, 148-49, 393. The American Association of Aerosol Science was formed
only in 1981. A journal Atmospheric Environment, founded 1967,
dealt only with pollution. BACK
19. For a review of aerosol history, see Charlson (1998). BACK
20. Junge (1958), p. 95.
He asserted that "unpolluted areas... no longer exist" in Western Europe
and the northeastern United States (p. 101), but was not thinking of pollution
great enough to alter climate. BACK
21. McCormick and Ludwig (1967);
Bryson (1967); for establishment of network
in 1960-61, see Flowers et al. (1969). BACK
22. Twomey (1977), p.
1. BACK
23. Bryson and Wendland (1970),
quote p. 137; see also Bryson (1968); India:
Bryson (1967), p. 53. BACK
24. Cobb and Wells (1970);
see also Hodge (1971). BACK
25. Mitchell at meeting of AAAS, Boston, Dec. 1969,
as cited by Landsberg (1970); quote: Mitchell (1970), p. 153, from a 1968 symposium. BACK
26. "It seems probable that present changes of the
Earth's temperature are determined mainly by... the level of volcanic
activity," concluded Budyko (1969), p. 613; on the other hand, Lamb concluded
that "volcanic dust is not the only, and probably not the main, influence,"
Lamb (1970); skepticism held up publication of this paper
for five years, see Lamb (1997), p. 189. BACK
27. Barrett and Landsberg
(1975), pp. 42-43, 48. BACK
28. Mitchell (1971), quote
p. 713. BACK
29. If exponential growth continued, Mitchell foresaw
a 1°C greenhouse effect temperature rise by 2000, followed by accelerating
cooling as aerosols accumulated faster than CO2.
Mitchell (1970); "In my opinion, man-made aerosols... constitute
a more acute problem than CO2," Landsberg (1970). BACK
30. Kaiser (2000). BACK
31. This globally averaged model didn't allow for
changes in convection or clouds, and got only 2°C of warming for
an eightfold rise of CO2, an error soon corrected by other calculations. Rasool and Schneider (1971), quote p. 138, with references
to work of Budyko and Sellers; they calculated the doubling of dust from
data reported by Hodge (1971); see confirmation and priority claim by Barrett (1971); criticism: Charlson
et al. (1972); Weare et al. (1974); Chylek and Coakley (1974); possible warming was calculated
by Wang and Domoto (1974). BACK
32. Also, it was suggested that such dust storms
might initiate a radical warming by darkening the polar ice caps. Sagan
et al. (1973). BACK
33. Bryson (1973). BACK
34. Bryson (1974), quote
p. 756; for earlier mention, see Bryson (1973),
p. 9. BACK
35. Kellogg made a distinction between effects of
aerosols over land (cooling) or sea (not necessarily cooling), but held
that the pollutants were mostly over land. Kellogg
et al. (1975); Bryson's theory of cooling was "almost the opposite
of the true situation," Kellogg said at a 1980 international workshop,
Kellogg (1980), p. 282. BACK
36. Schneider and Mass (1975).
BACK
37. E.g., Barrett and Landsberg
(1975), pp. 53, p. 77. BACK
38. Bryson and Dittberner
(1976); see also Bryson and Murray (1977).
BACK
39. "Perhaps the most sensitive," Hobbs et al. (1974); "profound," referring to Budyko's
1969 paper, Twomey (1974).
BACK
40. GARP (1975), quote
p. 44; they cite Mitchell (1973); aerosol effects
were "lost in the noise": Barrett and Landsberg
(1975), p. 72. BACK
41. Idso and Brazel (1977);
Herman et al. (1978). BACK
42. Damon and Kunen (1976);
Damon and Kunen (1978). BACK
43. For example: Baldwin et
al. (1976); Pollack et al. (1976); Shaw (1976); Ninkovich and Donn (1976);
Herman et al. (1978); aerosols "can hardly have
a significant effect" except regionally: Kellogg
(1980). BACK
44. Twomey (1977); Twomey (1977); Twomey (1977);
see also Twomey (1974) (which showed that while
very few nuclei would inhibit precipitation, so would very many, multiplying
droplets too small to fall as rain); for brief review and further references
on aerosols and precipitation, see Rosenfeld (2000), p. 1793. BACK
45. Toon (2000), p. 1763.
BACK
46. Heintzenberg and Charlson
(1996), p. 987. BACK
47. Kondratyev (1981)
; Ginsburg and Feigelyson (1980) . BACK
48. E.g., "A significant dip in temperature can
be found within a few years after the major eruption dates..." according
to Taylor et al. (1980), p. 175. BACK
49. Hamilton and Seliga (1972).
Recent studies: Bauer et al. (2004.
BACK
50. Humphreys (1940),
p. 595; Junge (1952) . BACK
51. Wilson and Matthews (1971),
pp. 279-80, 283-84. BACK
52. Barrett and Landsberg
(1975), pp. 44-45. BACK
53. Bolin and Charlson (1976);
for other studies of regional haze, see Husar and
Patterson (1980). BACK
54. Bolin and Charlson (1976),
p. 50. BACK
55. I have not seen the original Russian language
publications, including Budyko (1974); Budyko
(1974); 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). BACK
56. E.g., Toon and Pollack
(1976). BACK
57. Harshvardhan and Cess
(1976); Harshvardhan (1979); Charlock and Sellers (1980); for an overview, see Hansen et al. (1980). BACK
58. A one-dimensional model. Hansen et al. (1978); see also the approximate calculation
by Pollack et al. (1976); Charlock
and Sellers (1980); recent opinions: e.g., B.J. Mason, see Gribbin (1976). BACK
59. The Venus greenhouse was invoked regarding the
importance of sulfuric acid in Hansen et al. (1978).
BACK
60. Hammer et al. (1980).
BACK
61. Hansen et al. (1981);
see also Bryson and Goodman (1980) (eyeball
comparison going back to the 1880s); Gilliland (1982).
BACK
62. Stott et al. (2000).
BACK
63. Hofmann (1988), quote
p. 196. The paper includes a historical review of 1980s work. BACK
64. Coakley et al. (1983).
BACK
65. Freeman and Liou (1979),
p. 283. BACK
66. Hansen et al. (1981),
p. 960, "inevitable" p. 966. BACK
67. E.g., Husar and Patterson
(1980) (listing 1970s studies); Ball and Robinson
(1982); for useful reviews, see Charlson and
Wigley (1994); Charlson (1998). BACK
68. Peterson et al. (1981).
BACK
69. "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. BACK
70. Kondratyev (1988),
pp. 179-95. BACK
71. Alvarez et al. (1984);
Wolbach et al. (1985). BACK
72. McLean (1985). BACK
73. More pollution divided the water among more
and hence smaller droplets, which not only made clouds linger (by inhibiting
precipitation) 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).
BACK
74. Twomey (1980), p.
1461; he went on to report a verification at a single site, Twomey
et al. (1984). BACK
75. Coakley et al. (1987);
Radke et al. (1989); the cloudiness is probably
due to nitrates, see Lawrence and Crutzen (1999). Inconsistencies:
e.g., Ackerman et al. (2000). BACK
76. Schwartz (1988).
BACK
77. White (1986), quote
p. 1671. BACK
78. Charlson et al. (1987).
BACK
79. Joseph (1984); Charlson et al. (1992), p. 425. BACK
80. Quote from chapter on "Greenhouse gases and
aerosols" by R.T. Watson et al., IPCC (1990),
p. 32. BACK
81. Hansen and Lacis (1990).
BACK
82. R. Charlson, personal communication, 2002. BACK
83. "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). BACK
84. Hansen et al. (1992).
BACK
85. Smoke: 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). Direct effect calculation: Kiehl and
Briegleb (1993), quote from abstract; indirect effect calculation:
Jones et al. (1994). BACK
86. Wigley (1994). BACK
87. Mitchell et al. (1995);
IPCC (1996), chap. 8. BACK
88. Taylor and Penner (1994).
BACK
89. IPCC (2001). BACK
90. Ledley et al. (1999),
p. 458; Singer (1999) also notes uncertainty
about aerosol effects. BACK
91. E.g., on increased dust, see Andreae (1996). BACK
92. Satheesh and Ramanathan
(2000), discussed in Wall Street Journal, May 6, 2003, p.
1; Hansen et al. (2000). "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. BACK
93. Hansen et al. (2000);
Andreae (2001). BACK
93a. 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). A 2005 analysis of satellite measurements indicated a disturbingly
strong aerosol effect, Bellouin et al. (2005).
Work by others including Farquhar, Liepert, Ramanathan and Roderick was
described on the BBC "Horizon" broadcast of April 3, 2005, transcript
I used is no longer online, see
http://www.bbc.co.uk/feedback/bbc_products.shtml?http://www.bbc.co.uk/search/help.shtml
BACK
94. Boucher (1999). This
and later studies showed the effect was significant but not as large as
the greenhouse effect and other sources of climate change. BACK
95. Cess et al. (1995);
Pilewskie and Valero (1995); Ramanathan et al. (1995); Li et
al.(1995); see Kerr (1995); "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. (2000), 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). BACK
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© 2003-2006 Spencer Weart & American Institute of Physics
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