A top government scientist is speaking out about what he says is an effort to keep him quiet about global warming.
NASA's chief climate scientist James Hansen says the space agency's backlash is part of a Bush administration effort aimed at those trying to sound the alarm on climate change.
He says the administration tried to silence him after he gave a speech last month with this warning: "We're getting very close to a tipping point in the climate system. If we don't get off our 'business as usual' scenario and begin to reduce our greenhouse gas emissions, we're going to get big climate changes."
Hansen says threats from NASA officials came only by phone, with nothing in writing.
"One threat was relayed to me that there would be 'dire consequences -- not specified,'" he told ABC News.
In December, ABC News' "Good Morning America" reported NASA's announcement, linking the record high temperatures of 2005 to greenhouse gas emissions.
Said Hansen, "When 'Good Morning America' released our data showing that 2005 was probably the warmest year on record, I got calls that they were very unhappy."
NASA today issued a statement, saying its policies are similar to those of "any other federal agency, corporation or news organization" in requiring any NASA employee to "coordinate (any statements) with the Office of Public Affairs. No exceptions."
Over the past year, a growing number of American scientists who study global warming have been complaining about the federal government's efforts to silence or alter their reports or to discourage them.
Many climate scientists say the Bush administration's policy, which asks businesses to cut greenhouse gases voluntarily, simply won't work.
Hansen says NASA's stated mission includes protecting the planet, and he will continue to speak out.
ABC News' Bill Blakemore filed this report for "World News Tonight."