Ralph Cicerone's views contrast starkly with those of the Bush administration, which emphasises uncertainty about how much carbon dioxide and other industrial gases are incessantly warming the atmosphere like a greenhouse.
"Carbon dioxide in the atmosphere is now at its highest level in 400,000 years, and it continues to rise," said Cicerone on Wednesday, an atmospheric scientist who left as chancellor of University of California-Irvine to become the academy's president this month.
"Nearly all climate scientists today believe that much of Earth's current warming has been caused by increases in the amount of greenhouse gases in the atmosphere, mostly from the burning of fuels."
Cicerone, testifying before a Senate Commerce subcommittee on global climate change, also bolstered a 2004 Defence Department report that two private consultants prepared on potential global impacts of an abrupt and severe change in the world's climate.
When the report was issued, it was met with scepticism and disbelief, even by the Pentagon official who commissioned the study.
"Nearly all climate scientists today believe that much of Earth's current warming has been caused by increases in the amount of greenhouse gases in the atmosphere"
Ralph Cicerone, US National Academy of Sciences chief
Among dire consequences sketched in that report were seas surging to breakdown levels in the Netherlands in 2007, making The Hague uninhabitable; and the climate of Europe becoming "more like Siberia's" by 2020.
They saw possible "megadroughts" in southern China and northern Europe. "It was well done," Cicerone said of the report. "I didn't think it was fictional."
Cicerone cited data from weather stations and ships that indicate the surface of the Earth generally is hotter by about seven-tenths of one Fahrenheit than what it was as recently as the early 1970s.
Administration officials testifying before the committee stressed that the government is spending $5 billion annually on climate programmes, mostly on research.
David Conover, a principal deputy assistant energy secretary, said President George Bush would lead on the issue, although "the scientific and technology challenges are considerable".
James Mahoney, assistant commerce secretary for oceans and atmosphere, said: "We know that the surface of the Earth is warmer, and that an increase in greenhouse gases caused by humans is contributing to the problem." He went no further than that.
"We see economic growth, addressing the climate change problem and energy security as integrally related," said Daniel Reifsnyder, director of the State Department's Office of Global Change.
Just three senators - Republicans David Vitter of Louisiana and Ted Stevens of Alaska and Democrat Frank Lautenberg of New Jersey - were at the hearing. All three spoke of fears about coastlines disappearing.
Last month, the National Academy of Sciences - an independent organisation chartered by US Congress to advise the government on scientific matters - joined with similar groups from 10 other nations in calling for prompt action to reduce greenhouse gas emissions.
The other nations were Brazil, Britain, Canada, China, France, Germany, India, Italy, Japan and Russia. Bush said this month he recognised that human activity contributes to a warmer Earth.
President Bush has steadfastly
rejected the Kyoto treaty
But he continues to reject the Kyoto Protocol on global warming that all other G-8 industrialised nations signed, citing developing nations being exempted from its preventive measures as his reason.
His administration has argued strongly against mandatory climate-related emissions caps, contending that its voluntary programme is countering the growth of the emissions but not actually reducing the tonnes being released annually into the atmosphere.