Sept. 25, 2006  -- In about 45 years, temperatures on Earth will be hotter than at anytime during the past one million years, says the U.S. government's top climatologist in a new report released today.

According to the report, published in the Proceedings of the National Academy of Sciences, the planet is just two degrees shy of an average temperature of 59 degrees Fahrenheit, which is what they believe the temperature was about a million years ago.

NASA's James Hansen, along with colleagues from the University of California and Columbia University, are for the first time, marking a calendar signaling the approach of temperatures that humans have never experienced.

"Humans are now in control of the Earth's climate, for better or worse," Hansen tells ABC News.

Based on a "business as usual" scenario in which greenhouse gasses continue to rise unabated, Hansen says we'll break the million-year-old record in about 45 years. But he stresses we can't wait that long to cut greenhouse gas pollution, because of the decades it takes for the climate system to respond to changes.

"We need to get started now," he says. "We can't wait another decade or two to take this seriously."

Those 2 degrees the scientists are talking about may not sound like much, but what that change means is that by mid-century, the world will experience even more record heat waves, wildfires, more intense storms and flooding.

In other parts of the world, the increase may worsen drought conditions as more mountain glaciers and snow packs vanish, no longer sending water to the valleys below.

And in a highly unusual move for a scientific paper, the authors devote eight paragraphs to systematically deconstructing the assertions of a prominent science fiction novelist. In the non-fiction sections of his 2004 book "State of Fear," best-selling author Michael Crichton wrote that Hansen's climate change calculations were "wrong by 300 percent."

Hansen says Crichton misrepresented his scientific work and, adds the scientist, has done so in testimony before Congress and in a meeting with President Bush -- even though he is not a climate expert.

"He is propagating false information to the public," Hansen says.

Crichton, through a publicist, declined ABC News' request for an interview.