Worldwatch Institute is partnering with Grist to bring you this three-part series commemorating the 20-year anniversary of NASA scientist James Hansen’s groundbreaking testimony on global climate change next week. It is written by Worldwatch staff writer Ben Block. Here follows part one. Part two is here; part three is here.

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The speakers at a Washington, D.C., climate rally this past Earth Day, April 22, showcased the range of the modern environmental movement. They included an activist who engaged in a hunger strike, an outspoken preacher from the Hip Hop Caucus, and a folk duo that performed, “Unsustainable,” a parody of Frank Sinatra’s “Unforgettable.”

Yet it was a comparatively dry, 20-minute scientific presentation that brought the crowd to its feet. The speaker, introduced as a “climate hero,” was James Hansen, a long-time scientist with the U.S. National Aeronautics and Space Administration.

Hansen is not a revolutionary by character. He is a mild-natured man who speaks with a soft, Midwestern tone. Raised in southwest Iowa, the fifth child of tenant farmers, Hansen would later commit his life to studying computerized climate models. With human-induced climate change now widely regarded as the greatest challenge of this generation, Hansen is considered a visionary pioneer.

Theories of climate change first surfaced more than a century ago. But it was Hansen who forever altered the debate on climate change 20 years ago this month.

On June 23, 1988, in the sweltering heat, Hansen told a U.S. Senate committee he was 99 percent certain that the year’s record temperatures were not the result of natural variation. It was the first time a lead scientist drew a connection between human activities, the growing concentration of atmospheric pollutants, and a warming climate.

“It’s time to stop waffling so much and say that the evidence is pretty strong that the greenhouse effect is here,” Hansen told reporters.

Scientists first expressed concern about possible climate change more than a decade before Hansen’s testimony. The most-publicized report came from the National Academy of Sciences in 1977. It warned that average temperatures may rise 6 degrees Celsius by 2050 due to the burning of coal.

Around the same time, Hansen, a space scientist at NASA’s Goddard Institute for Space Studies in New York, began studying the effect of greenhouse gases on climate. His first paper on the subject, published in the journal Science (PDF) in 1981, predicted that burning fossil fuels would increase global temperatures by 4.5 degrees Fahrenheit (2.5 degrees Celsius) by the end of the 21st century.

The incoming Reagan administration responded to Hansen’s predictions by cutting funding for GISS. But Hansen, encouraged by former Friends of the Earth president Rafe Pomerance, continued to raise the issue. “It was truly important for him to be heard. The issue had no traction at that point,” said Pomerance, now president of Clean Air Cool Planet.

Al Gore, then a young member of Congress, began organizing some of the first Congressional hearings on climate change in the early 1980s, which featured Hansen’s input. As more studies suggested a link between burning fossil fuels and climate change, the media gave the issue greater coverage throughout the decade. But in 1986, two separate polls found that most Americans (55 percent) still had not heard about the greenhouse effect.

Sen. Tim Wirth (D-Colo.) was aware of the growing evidence of climate change, in part from his constituents at the National Center for Atmospheric Research and the National Oceanic Atmospheric Administration’s Aeronomy Laboratory, both based in Colorado. He wanted to make a difference.

Meanwhile, the United States in 1988 was suffering from a terrible drought. Wirth knew that if he arranged a hearing that drew a link between the present weather conditions and a trend of global warming, it would generate considerable media attention.

Wirth’s legislative assistant, David Harwood, called Hansen for his input. Hansen responded that the observed temperatures were warmer beyond the range of natural variability. The year 1988 was on pace to be the warmest on record. “I didn’t know much, but I knew that was significant,” Harwood recalls.

Plans for a groundbreaking hearing were under way. Hansen would soon become the leader of climate science in a warming world.