NOVA's History of "Global Dimming" Series

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/
	Laki Eruption Cools Europe 1783 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.
	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.
	Cloud "Seeding" Attempted 1950s 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.
	Pollution's Far Reach 1960s 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.
	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.
	Early Computer Climate Models Mid-to-late 1970s Early computer models 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.
	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.
	Shipping Lane Clouds 1987 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.
	Pinatubo Confirms Climate Models 1991 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.
	Indian Ocean Study Late 1990s 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.
	Dimming Recognized Worldwide Mid-1980s to present In the mid-1980s, when meteorologist Gerry Stanhill reported that 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."
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