If you want the best information and research about global warming you need to go to the foremost expert on Global Warming Research. Here is what he has to say about Global Warming in the present and future.

As a government scientist, James Hansen is taking a risk. He says there are things the White House doesn't want you to hear but he's going to say them anyway.

Hansen is arguably the world's leading researcher on global warming. He's the head of NASA's top institute studying the climate. But as correspondent Scott Pelley first reported last spring, this imminent scientist says that the Bush administration is restricting who he can talk to and editing what he can say. Politicians, he says, are rewriting the science.

But he didn't hold back speaking to Pelley, telling 60 Minutes what he knows.

Asked if he believes the administration is censoring what he can say to the public, Hansen says: "Or they're censoring whether or not I can say it. I mean, I say what I believe if I'm allowed to say it."

What James Hansen believes is that global warming is accelerating. He points to the melting arctic and to Antarctica, where new data show massive losses of ice to the sea.

Is it fair to say at this point that humans control the climate? Is that possible?

"There's no doubt about that, says Hansen. "The natural changes, the speed of the natural changes is now dwarfed by the changes that humans are making to the atmosphere and to the surface."

Those human changes, he says, are driven by burning fossil fuels that pump out greenhouse gases like CO2, carbon dioxide. Hansen has a theory that man has just 10 years to reduce greenhouse gases before global warming reaches what he calls a tipping point and becomes unstoppable. He says the White House is blocking that message.

"In my more than three decades in the government I've never witnessed such restrictions on the ability of scientists to communicate with the public," says Hansen.

Restrictions like an e-mail Hansen's institute received from NASA in 2004. "… there is a new review process … ," the e-mail read. "The White House (is) now reviewing all climate related press releases," it continued.

Why the scrutiny of Hansen's work? Well, his Goddard Institute for Space Studies is the source of respected but sobering research on warming. It recently announced 2005 was the warmest year on record. Hansen started at NASA more than 30 years ago, spending nearly all that time studying the earth. How important is his work? 60 Minutes asked someone at the top, Ralph Cicerone, president of the nation’s leading institute of science, the National Academy of Sciences.

"I can't think of anybody who I would say is better than Hansen. You might argue that there's two or three others as good, but nobody better," says Cicerone.

And Cicerone, who’s an atmospheric chemist, said the same thing every leading scientist told 60 Minutes.

"Climate change is really happening," says Cicerone.

Asked what is causing the changes, Cicernone says it's greenhouse gases: "Carbon dioxide and methane, and chlorofluorocarbons and a couple of others, which are all — the increases in their concentrations in the air are due to human activities. It's that simple."

But if it is that simple, why do some climate science reports look like they have been heavily edited at the White House? With science labeled "not sufficiently reliable." It’s a tone of scientific uncertainty the president set in his first months in office after he pulled out of a global treaty to reduce greenhouse gas emissions.