In June 1988, a government scientist named James E. Hansen told a Senate committee that the greenhouse effect was changing the climate. “We have already reached the point where the greenhouse effect is important,” he said.

It is now generally agreed that Hansen’s presentation to the committee alerted other scientists, policymakers, and the general public to the reality of changes in global climate and the role humans played in it.

In June 2008, Hansen returned to Capitol Hill to testify to a House committee and also appeared at the National Press Club, two of several occasions commemorating the 20th anniversary of his first warning. Hansen was then and continues to serve as the director of the NASA Goddard Institute for Space Studies and adjunct professor of earth and environmental sciences at Columbia University.

Time and the need for action were again emphasized, but the sense of urgency was greater. Hansen told lawmakers and journalists: “The difference is that now we have used up all slack in the schedule for actions needed to defuse the global warming time bomb. The next President and Congress must define a course next year in which the United States exerts leadership commensurate with our responsibility for the present dangerous situation.”

Using the analogy of a “perfect storm,“ Hansen listed warming events now pushing us toward the tipping point: “Climate can reach points such that amplifying feedbacks spur large rapid changes. Arctic sea ice is a current example. Global warming initiated sea ice melt, exposing darker ocean that absorbs more sunlight, melting more ice. As a result, without any additional greenhouse gases, the Arctic soon will be ice-free in the summer…West Antarctic and Greenland ice sheets are vulnerable to even small additional warming. These two-mile-thick behemoths respond slowly at first, but if disintegration gets well underway it will become unstoppable.”

He predicted hat the disintegration of the ice sheets will raise sea levels six feet in this century, creating hundreds of millions of refuges as well as unstable shore lines far into the future.

Noting the many other dire consequences of unchecked greenhouse gas emissions—extinction of species, expansion of subtropical climate zones, shortage of fresh water—Hansen said we have already gone too far, adding “We must draw down atmospheric carbon dioxide to preserve the planet we know.”

He referred to a paper he has written with climate experts that puts the safe level of atmospheric carbon dioxide at no more than 350 parts per million (ppm). Already the amount is 385 ppm and rising by 2 ppm per year.

With no time to waste, Hansen urges us to phase out of coal use except where the carbon is captured and stored below ground. Because we are running out of oil used in vehicles and eventually must find other sources, Hansen tells us to move promptly to carbon-free energy.

Putting little hope in politicians or CEOs of fossil-energy companies to lead, Hansen says citizens must demand a moratorium on new coal-fired power plants. “We must block fossil fuel interests who aim to squeeze every last drop of oil from public lands, off-shore, and wilderness areas. Those last drops are no solution. They yield continued exorbitant profits for a short-sighted self-serving industry, but no alleviation of our addiction or long-term energy source.”

Using greenhouse gas emissions from China and India as excuses for doing nothing to cut emissions in the United States is dismissed as demagoguery by Hansen. He points out that we have produced most of the excess carbon in the air today. Other countries will follow because they too have much to lose from climate change.

Since the scientist’s first warning about greenhouse gases, little has been done to cut emissions in the United States although climate change has become a “global cause,” according to an article in the Washington Post on the day of Hansen’s testimony, June 23, 2008. The paper reported: “In the two decades since Hansen’s testimony, Congress has not passed any law mandating major cuts in greenhouse-gas emissions. In that interval, 21 new coal-fired generating units have been built at power plants around the United States. The country’s total emissions of carbon dioxide have climbed by about 18 percent, according to the latest statistics.”

 The NASA Goddard Institute for Space Studies (GISS), which Hansen heads, is a laboratory of the Earth Sciences Division of NASA’s Goddard Space Flight Center and a unit of the Columbia University Earth Institute. Research at GISS emphasizes a broad study of global climate change.