The head of NASA, Michael D. Griffin, said Thursday that he was not sure whether global warming was "a long-term concern or not."
Griffin, whose agency is facing criticism in Congress for cutting programs to track climate change, said in an interview on National Public Radio that he did not doubt that the burning of fossil fuels was warming the planet.
But, he said, "I am not sure that it is fair to say that it is a problem we must wrestle with."
Climate experts, including those at NASA, said they were baffled by Griffin's comments.
"It is mind-boggling that he is not better informed on this," said James Hansen, head of the NASA Goddard Institute for Space Studies.
"If I had the opportunity, I would welcome the chance to sit down and talk to him," said Richard Alley, a geoscientist at Pennsylvania State University who studies glaciers.
Griffin's remarks contradict the recent findings of the United Nations' Intergovernmental Panel on Climate Change.
In a series of reports this year, the thousands of scientists who make up the panel warned of an apocalyptic future -- widespread drought, rising sea levels, more frequent hurricanes and mass extinctions by the end of the century -- if the world did not act soon to curb its emissions of carbon dioxide and other greenhouse gases.
To avoid the worst effects of global warming, the panel reported in May, emissions must peak within a decade and fall to 50% to 85% of 2000 levels by midcentury. That would limit the temperature increase to 3 degrees Fahrenheit.
The Bush administration has opposed mandatory emissions cuts, instead placing its hopes on voluntary reductions and future technologies that would be cleaner and cheaper.
Griffin, an aerospace engineer who became the NASA administrator in 2005, said it was "arrogant" for anyone to suggest that the current climate was optimal for humans.
"I don't think it's within the power of human beings to assure that the climate does not change," he said.
Geological and ice-core records suggest that the Earth is entering its warmest period in the history of humans.
Rep. Bart Gordon (D-Tenn.), chairman of the House Science and Technology Committee, countered Griffin's comments with a statement criticizing NASA's decision to reduce funding for satellites that monitored the atmosphere and oceans.
"I remain concerned that NASA is not doing as much as needs to be done on climate-change data collection and research," he said.
Later in the day, Griffin released a statement saying "it is not NASA's mission to make policy regarding possible climate-change mitigation strategies.