NASA climate scientist James Hansen was among the first to alert the public to the dangers of climate change. He did so at a congressional hearing conducted by then-Senator Tim Wirth.
"In 1988, he stepped to the front of the line of the scientific community to proclaim a human fingerprint on the earth's rising temperature," Wirth said. "It was a brave, a lonely leadership role he played then and he hasn't stopped for one day since."
At a Washington event recalling that hearing, Hansen said the world has long passed dangerous levels for greenhouse gasses in the atmosphere.
"We really have reached a point of a planetary emergency," Hanson said. "And it is because there are tipping points in the climate system which we are very close to and which, if we pass, the dynamics of the system can take over. So the momentum of the system will carry you to very large changes which are out of your control."
Some advocates downplay the warnings of global warming and dispute the claim the climate system is reaching a tipping point.
"Most people who work on climate change issues are a lot more optimistic than that and believe there are a suite of technological applications that we will be able to apply that can reverse, or at least adapt, to the consequences of global warming," said Steve Segal, who is the lawyer representing energy interests.
Hansen joined other climate scientists in warning of increasingly violent storms, plant and animal extinctions, melting ice sheets and rising sea levels -- if mankind fails to act.
"We have already reached one tipping point and we are going to lose all of the sea ice in the Arctic summer season," Hanson said "...and we know we are going to lose that sea ice because the planet is out of energy balance."
Hansen says Arctic ice will melt completely during each summer in five to 10 years but that it can be reversed if mankind stops burning fossil fuels and phases out coal, the chief cause of manmade greenhouse gasses.