Twenty years ago Monday, James E. Hansen, a climate scientist at NASA, shook Washington and the world by telling a sweating crowd at a Senate hearing during a stifling heat wave that he was “99 percent” certain that humans were already warming the climate.

“The greenhouse effect has been detected, and it is changing our climate now,” Dr. Hansen said then, referring to a recent string of warm years and the accumulating blanket of heat-trapping carbon dioxide and other gases emitted mainly by burning fossil fuels and forests.

To many observers of environmental history, that was the first time global warming moved from being a looming issue to breaking news. Dr. Hansen’s statement helped propel the first pushes for legislation and an international treaty to cut emissions of greenhouse gases. A treaty was enacted and an addendum, the Kyoto Protocol, was added.

Even as the scientific picture of a human-heated world has solidified, emissions of the gases continue to rise.

On Monday, Dr. Hansen, 67, plans to testify at a House committee hearing that it is almost, but not quite, too late to start defusing what he calls the “global warming time bomb.” He will offer a plan for cuts in emissions and also a warning about the risks of further inaction.

“If we don’t begin to reduce greenhouse gas emissions in the next several years, and really on a very different course, then we are in trouble,” Dr. Hansen said Friday at NASA’s Goddard Institute for Space Studies in New York, which he has directed since 1981. “Then the ice sheets are in trouble. Many species on the planet are in trouble.”

In his testimony, Dr. Hansen said, he will say that the next president faces a unique opportunity to galvanize the country around the need for a transformed, nonpolluting energy system. The hearing is before the House Select Committee on Energy Independence and Global Warming.

Dr. Hansen said the natural skepticism and debates embedded in the scientific process had distracted the public from the confidence experts have in a future with centuries of changing climate patterns and higher sea levels under rising carbon dioxide concentrations. The confusion has been amplified by industries that extract or rely on fossil fuels, he said, and this has given cover to politicians who rely on contributions from such industries.

Dr. Hansen said the United States must begin a sustained effort to exploit new energy sources and phase out unfettered burning of finite fossil fuels, starting with a moratorium on the construction of coal-burning power plants if they lack systems for capturing and burying carbon dioxide. Such systems exist but have not been tested at anywhere near the scale required to blunt emissions. Ultimately he is seeking a worldwide end to emissions from coal burning by 2030.

Another vital component, Dr. Hansen said, is a nationwide grid for distributing and storing electricity in ways that could accommodate large-scale use of renewable, but intermittent, energy sources like wind turbines and solar-powered generators.

The transformation would require new technology as well as new policies, particularly legislation promoting investments and practices that steadily reduce emissions.

Such an enterprise would be on the scale of past ambitious national initiatives, Dr. Hansen said, like the construction of the federal highway system and the Apollo space program.

Dr. Hansen disagrees with supporters of “cap and trade” bills to cut greenhouse emissions, like the one that foundered in the Senate this month. He supports a “tax and dividend” approach that would raise the cost of fuels contributing to greenhouse emissions but return the revenue directly to consumers to shield them from higher energy prices.

As was the case in 1988, Dr. Hansen’s peers in climatology, while concerned about the risks posed by unabated emissions, have mixed views on the probity of a scientist’s advocating a menu of policy choices outside his field.

Some also do not see such high risks of imminent climatic calamity, particularly disagreeing with Dr. Hansen’s projection that sea levels could rise a couple of yards or more in this century if emissions continue unabated.

Dr. Hansen is a favorite target of conservative commentators; on FoxNews.com, one called him “alarmist in chief.” But many climate experts say Dr. Hansen, despite some faults, has been an essential prodder of the public and scientific conscience.

Jerry Mahlman, who recently retired from a long career in climatology, said he disagreed with some of Dr. Hansen’s characterizations of the climate problem and his ideas about solutions. “On the whole, though, he’s been helpful,” Dr. Mahlman said. “He pushes the edge, but most of the time it’s pedagogically sound.”

Dr. Hansen said he was making a new public push now because the coming year presented a unique opportunity, with a new administration and the world waiting for the United States to re-engage in treaty talks scheduled to culminate with a new climate pact at the end of 2009.

He said a recent focus on China, which has surpassed the United States in annual carbon dioxide emissions, obscured the fact that the United States, Britain and Germany are most responsible for the accumulation of greenhouse gases.

Dr. Hansen said he had no regrets about stepping into the realm of policy, despite much criticism.

“I only regret that we haven’t gotten the story across as well as it needs to be,” he said. “And I think we’re running out of time.”