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. Part three of three follows. Part one is here; part two is here.

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In May 1989, a few months after NASA scientist James Hansen declared that global warming had arrived, he would provide another testimony to clarify the risks of future climate change.

But before Hansen could make his presentation to Sen. Al Gore’s subcommittee, the White House’s Office of Management and Budget intercepted the testimony and rewrote its conclusion. According to the revised copy, the cause of climate change was still unknown. NASA headquarters said Hansen could accept the changes or not testify, he later recalled.

It was not the first OMB revision of a Hansen testimony. This time, he decided, would be different. Hansen notified Gore that his testimony did not reflect his actual opinion, which led Gore to frame the hearing’s questions to reveal the OMB edits. It was the lead story on all major television networks that night.

Twenty years after Hansen’s 1988 landmark testimony, the U.S. government remains largely in a state of denial about the urgency of global climate change. Yet Hansen remains a source of reason, despite government efforts to silence him and industry campaigns to obscure his research.

“Hansen has a real intense inner light,” says Rafe Pomerance, president of Clean Air Cool Planet. “What’s someone who sees the future to do? Keep his mouth shut? Hansen’s not going to do that.”

While Hansen espoused confidence about the science of climate change, few other scientists were willing to make such clear predictions. In its initial 1990 report, the Intergovernmental Panel on Climate Change, composed of leading climate scientists from around the world, merely questioned whether anthropogenic climate change was occurring. Meanwhile, the coal, oil, and automotive industries unleashed a propaganda campaign to dispute the science of climate change.

The industry-funded Global Climate Coalition spent tens of millions of dollars to raise doubts about the evidence for climate change and to minimize the potential consequences. During the 1990s, the group managed to reshape media coverage of human-induced climate change from fact into theory by recruiting a handful of skeptical scientists who were paid to speak with the press and public. “It sowed confusion and doubt into the public that is now irremediable,” says Spencer Weart, a climate change historian at the American Institute of Physics.

These efforts helped derail U.S. climate legislation in the 1990s and complicated efforts to negotiate the Kyoto Protocol, the international treaty that most industrial countries adopted to limit their greenhouse gas emissions. When George W. Bush became president in 2001, he quickly rejected the Kyoto Protocol and brought to a halt serious discussion of climate policy in the United States.

Hansen remained relatively quiet during those years and instead focused on his research. That quickly changed when he delivered a speech (PDF) to the American Geophysical Union in December 2005. In addition to announcing that the year would prove to be the hottest on record, Hansen warned that the rise in sea levels was evidence that humans were causing global climate instability. “Jim took a step beyond that usual dissonance in the scientific community. He said ‘six to eight feet increase in sea level, I call that dangerous, don’t you?'” says Rick Piltz, director of Climate Science Watch, a watchdog program of the Government Accountability Project.

The speech, which received widespread media attention, led White House-appointed NASA administrators to silence Hansen and other scientists. The White House Council on Environmental Quality, influenced by ExxonMobil lobbyists, singled out scientists who had worked in the Clinton administration, such as former IPCC chair Robert Watson, who could be “removed from their positions of influence,” according to a released memo. American Petroleum Institute attorney Philip Cooney was appointed chief-of-staff of the CEQ. He would repeatedly edit government reports on climate change in an effort to lessen the certainty of the science.

At NASA, orders authorized by administration-appointed public relations officers “reduced, marginalized, or mischaracterized climate change science,” an agency investigation stated recently. Climate scientists were not allowed to conduct media interviews without prior approval. Hansen had to remove the 2005 temperature data from NASA’s website. Even Hansen’s daily schedule suddenly required prior consent.

Hansen decided he had seen enough. He sent an email in January 2006 about the NASA constraints to New York Times reporter Andrew Revkin, who first uncovered the restrictions. During an interview on the CBS program 60 Minutes, Hansen said, “In my more than three decades in the government I’ve never witnessed such restrictions on the ability of scientists to communicate with the public.”

Federal scientists, from NASA, the National Oceanic and Atmospheric Administration, and other agencies have since acknowledged that their climate findings were also being repressed. “[Hansen] did a great deal to help unmask the Bush administration’s collusion with the global warming disinformation campaign,” said Piltz, who helped expose the White House when he publicly resigned from the U.S. Climate Change Science Program. “He’s a bit like a lone wolf. Nobody can tell him what to say or what to do. They made a mistake when they tried to mess with him.”

Today Hansen rallies openly for drastic cuts in greenhouse gas emissions. He writes personal letters to governors urging them not to approve new coal-fired power plants in their states. He decries the increased role of fossil fuel lobbyists in American politics — once testifying to Congress (PDF) that NASA’s mission had apparently become to “protect special interests’ backside.”

Above all, Hansen has continued to produce groundbreaking research. He and eight of the world’s leading climate scientists will soon publish a paper arguing that total atmospheric greenhouse gases must be reduced to 350 parts per million — not 450, which many scientists have long stated — to avoid “irreversible catastrophic effects.”

Over the past 20 years, supporters have lauded Hansen as a visionary scientist and a brave public servant. At a time when the United States has refused to act on climate change, Hansen has jolted the nation awake. Will the country — and its new leader — heed his advice? He says the world cannot wait another 20 years.