In the early 21st century, it's become clear that air pollution can
significantly reduce the amount of sunlight reaching Earth, lower temperatures,
and mask the warming effects of greenhouse gases. Climate researcher James
Hansen estimates that "global dimming" is cooling our planet by more than a
degree Celsius (1.8°F) and fears that as we cut back on pollution, global
warming may escalate to a point of no return. Regrettably, in terms of possibly
taking corrective action, our current understanding of global dimming has been
a long time in the coming, considering the first hints of the phenomenon date back
to 18th-century observations of volcanic eruptions. Below, follow a series of
historic events and scientific milestones that built the case for global
dimming.—Susan K. Lewis
Thanks to Spencer Weart of the American Institute of Physics, whose book
The Discovery of Global Warming made this feature possible. A hypertext
version of the book can be found at www.aip.org/history/climate/
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Laki Eruption Cools Europe
1783
For millennia humans no doubt have noticed that smoke and ash from volcanic
eruptions can block sunlight for many days. But Benjamin Franklin went a step
further in 1783, proposing that a massive volcanic eruption of the Laki fissure
in Iceland caused months of unusually cold weather in Europe. By the early
1900s, scientists had begun trying to quantify how volcanic eruptions affect
climate, but measurements and climate models were too crude to conclusively
link the two. It wasn't until the late 20th century that scientists understood
precisely how Laki's eruption and the subsequent strange blue haze that wafted
over Europe cooled temperatures, and how this related to the "human volcano" of
air pollution.
Left: Part of the 16-mile-long Laki fissure today. When this volcanic ridge
opened in 1783, altered weather patterns led to the greatest famine in
Iceland's history.
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WWII Spurs Aerosol Science
1940s
Before scientists could even be in a position to suspect such a subtle
phenomenon as global dimming, they had to gain a better understanding of
aerosols, particles suspended in a gas. World War II provided them with that
opportunity, as researchers investigated various types of smoke, poison gas,
and nuclear fallout. Studies done for the Manhattan Project, in fact, led to
the first handbook of aerosol science. Nevertheless, for decades after the war,
few aerosol experts tackled issues directly related to climate change.
Left: How far would radioactive particles from nuclear explosions travel? The
need to understand such issues sparked aerosol science, but wartime research
was not always openly published.
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Cloud "Seeding" Attempted
1950s
Scientists knew that clouds form only when there are tiny particles in the air
such as sea salt or pollen around which water droplets can condense. In the
1950s, various government and commercial groups began exploring whether it
would be possible to "seed" clouds with silver iodide smoke and other
substances in order to make rain and control local weather. The widespread
cloud seeding efforts ultimately had very limited success. But they prompted
suspicion, borne out decades later, that human pollution was inadvertently
creating clouds that intercepted sunlight and impacted the climate of a broad
region.
Left: Cloud-seeding studies took place in skies around the world. Here, a 1957
effort near Sydney, Australia
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Pollution's Far Reach
1960s
By midcentury, rising levels of smog in cities had triggered public outcry and
captured the attention of aerosol scientists. Most of the focus initially was
on large particles that fell from the sky in a matter of days. In the 1960s,
however, experts began studying how microscopic particles could linger longer
and travel farther. Experts set up networks of monitoring stations to regularly
measure atmospheric turbidity, commonly known as haze. In 1967, Robert
McCormick and John Ludwig of the National Center for Air Pollution Control
reported a rise in turbidity over regions spanning as much as 600 miles. Even
remote areas of the Arctic appeared affected. It seemed likely that both
industrial pollution and haze caused by slash-and-burn agriculture in the
developing world were having a global reach.
Left: The infamous smog of Los Angeles, despite countless clean air reforms
since the 1940s, still plagues the city today.
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Ice Cores Reveal Past Trends
Late 1970s-early 1980s
Looking at ice cores from Greenland spanning hundreds of centuries, scientists
repeatedly saw telltale signs that giant volcanic eruptions of dust and
sulfuric acid were followed by cooler temperatures in subsequent years. On a
different but related front, other experts realized that sulfuric acid and
other small particle sulfates could stay in the stratosphere for years,
persisting much longer than larger particles. Sulfate aerosols, whether
generated by volcanoes or by humans burning coal, oil, and other fossil fuels,
now seemed likely culprits in climate cooling.
Left: Layers of ancient ice drilled from the immense Greenland ice sheet
offered clues that volcanic eruptions had triggered climate cooling.
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Early Computer Climate Models
Mid-to-late 1970s
Early computer models of the global climate attempted to factor in aerosols. It
was a daunting task: A wide spectrum of aerosols exists in the
atmosphere—small sulfate particles, salt crystals from the oceans, soot,
and many others. How these particles, at various heights, cause absorption or
reflection of the sun's radiation was poorly understood. Yet different groups
of modelers came to the same tentative conclusion. Human-made aerosols, they
found, were contributing to cloud formation, increasing the planet's
reflectivity, and causing a modest cooling. Some scientists even suggested that
air pollution, if unrestrained, might trigger a new ice age. Yet great
uncertainty remained over how the complex mix of pollution affected the
climate.
Left: More recent and sophisticated computer models confirm what earlier models
glimpsed. Today, using a version of NASA's global climate computer model, even
high school students can run climate change scenarios.
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Dinosaur Extinction Theory
Early 1980s
In 1980, 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 1983, 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.
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Shipping Lane Clouds
1987
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 1987, 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 2003 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.
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Pinatubo Confirms Climate Models
1991
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 (0.9°F), with the greatest changes in the
higher northern latitudes. The prediction proved remarkably on target. By the
mid-1990s, 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 1783 after Laki erupted in
Iceland.
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Indian Ocean Study
Late 1990s
In a $25 million multinational study spanning four years, climate scientists
led by Veerabhadran Ramanathan documented how pollution was severely dimming
areas of the Indian Ocean. 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 2001 satellite image captures the toxic aerosol haze blanketing
northern India and Bangladesh, south of the Himalayan Mountains.
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Dimming Recognized Worldwide
Mid-1980s to present
In the mid-1980s, when meteorologist Gerry Stanhill reported that a dramatic 22
percent reduction of sunlight had occurred in Israel between the 1950s and the
1980s, 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.S., 16 percent in parts
of Great Britain, and almost 30 percent in one region of Russia. Alarmed by the
trend, Stanhill coined the term "global dimming." By the turn of the 21st
century, with mounting evidence of just how human pollution had caused it,
global dimming finally gained the attention it warranted.
Left: NASA's Aqua satellite, launched in 2002, is filling in details in the big
picture of global dimming. Data from Aqua indicates that aerosol pollution is
cooling the climate by more than a degree Celsius (1.8°F).
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