TONY JONES, PRESENTER: On the eve of his Australian tour, the renowned US environmentalist Bill McKibben is urging Australians to do the math. That's the catchphrase of his US roadshow and media campaign highlighting what he describes as the terrifying consequences of CO2 emissions. Mr McKibben puts a figure on how much more CO2 can go into the atmosphere to avoid catastrophic climate change. It's one-fifth of the remaining fossil fuel reserves in the world. But how can Australians do the math when recent reporting shows scientists at odds over the rate and severity of global warming? Well shortly I'll be speaking with Bill McKibben in Washington. First this report from Kerry Brewster.

KERRY BREWSTER, REPORTER: Environmentalist Bill McKibben is drawing huge crowds in the United States.

BILL MCKIBBEN, ENVIRONMENTALIST (footage from Do the Math documentary): All I ever wanted to see was a movement of people to stop climate change and now I've seen it.

KERRY BREWSTER: McKibben has declared war against the fossil fuel industry, what he calls corporate outlaws against the laws of physics, the companies who he says are intent on leaving the world uninhabitable in their pursuit of vast profits.

BILL MCKIBBEN: If they carry out their business plan, the planet tanks.

KERRY BREWSTER: McKibben says "Do the math": 1,000 gigatons of CO2 in the atmosphere being the limit to avoid dangerous warming of more than two degrees. By his calculation, there's 565 gigatons of CO2 left to go.

BILL MCKIBBEN: The problem is we pour 30 billion tonnes a year in now and it goes up three per cent a year. Do the math and it's about 15 years before we go past that threshold.

KERRY BREWSTER: But is Bill McKibben right? One of Australia's leading climatologists says yes and no.

ANDY PITMAN, UNIVERSITY OF NSW: He's simply pointing out that we have already emitted a sizable fraction of that thousand billion tonnes, which leaves us with not as much as we'd like to think in terms of avoiding warming of two or three degrees. Now I actually don't agree with that assertion that two degrees is safe. I think we've seen heatwaves in Australia that have been unprecedented in our history covering most of the continent and the globe has only warmed 0.8 degrees. So, where this "two degrees is safe" idea comes from, I'm not really clear.

KERRY BREWSTER: McKibben wants Australians to accept his maths at a time when according to the media scientists are disagreeing over the pace of climate change. First came the bad news that atmospheric CO2 had passed the concentration of 400 parts per million with dire climate consequences. Then, only days ago, what appeared to be good news: a credible British study concluding that extreme warming was unlikely.

ANDY PITMAN: I don't think they're at all contradictory. The next Geoscience paper, which is a very nice piece of work, does mean that the upper range of the warming for a doubling of CO2 is less likely, but the warming at the lower range, the two or three degrees, is still catastrophic and the 400 parts per million being reached earlier than I expected and the onwards trajectory of those CO2 in the atmosphere upwards, both of those papers lead us to the same conclusion that we have significant climate change ahead of us if we don't cut emissions.

JOHN COOK, RESEARCH FELLOW, UNI. OF QLD: If I had to give the Australian media a mark on how they've been covering climate change, I'd probably give them a fail.

KERRY BREWSTER: Research fellow John Cook has studied thousands of scientific papers to gauge the level of consensus among climate scientists.

JOHN COOK: What we've done is actually look at the last 21 years of climate research and looked at - just identified all the papers that state a position on whether - just that simple question: whether humans are causing global warming. And we identified about 4,000 papers that stated the position on this, and among those 4,000 papers, more than 97 per cent endorse human-caused global warming.

KERRY BREWSTER: John Cook says Bill McKibben's message is scientifically sound.

JOHN COOK: It's a very powerful way of communicating the realities of climate change and scientifically it's pretty sound.

KERRY BREWSTER: Kerry Brewster, Lateline.