Campaigner hopeful humankind can reverse the damage and avert global warming disaster.
A so-called "rockstar" climate change campaigner says he is hopeful - but not confident - that the world can avert a catastrophic global warming scenario.
Prominent United States environmentalist and author Bill McKibben is visiting New Zealand as part of his international Do The Math speaking tour, making the point that the world cannot burn more than 565 gigatonnes of carbon if it wants to stay below the internationally agreed limit of 2C of warming.
He claims the world is on track to chew through that budget within 15 years - and that the planet's reserves of fossil fuels hold a much more alarming threat.
"Within the last couple of years, financial analysts have found that the world has about five times that supply already in its reserves - so in essence the fossil fuel industry has about 2800 gigatonnes of carbon."
An article he penned for Rolling Stone on his calculations is one of the magazine's most shared stories in its history.
Though he spent most of his time lobbying major carbon producers such as the US and China, he said the issue was no less relevant to New Zealanders.
"A bit of leadership in this case from New Zealand would be important, and would mean that its Government is not aggressively pursuing exploration for more hydrocarbons at a moment when there is already too much in the atmosphere."
Mr McKibben has been a vocal advocate of divestment in fossil fuel companies as a way to highlight the issue.
"There's no way that we can use divestment to bankrupt Exxon, but what we can do is make it clear to people that these are no longer on the list of normal countries."
The group he founded, 350.org, which seeks to reel back the amount of CO2 in the atmosphere from its current levels of nearly 400 parts per million to below 350 ppm, has also been fighting the proposed Keystone Pipeline System to transport oil sands bitumen across North America.
Asked if he was confident mankind could rein in climate change, Mr McKibben said he was hopeful, "but I wouldn't say my confidence level is high".
"I'm pretty sure that as we get further into the greenhouse era, more people will rally to the cause.
"But the trouble is, if we wait until everybody is rallying to the cause, because it's so bloody obvious what is going on, it'll be too late - and there are some that argue that it's closing in on too late at the moment."
He figured that putting simple numbers across was the best way to communicate the problem and its urgency.
"It gives you a very strong sense of our predicament, and once you know those numbers, you kind of know how the story comes out, unless you change the script very quickly and dramatically."
Doing the math
* US climate change campaigner Bill McKibben says the message he is pushing in New Zealand this week boils down to a few numbers.
* The world can emit only 565 more gigatonnes of carbon dioxide and stay below 2C of warming - anything more than that risks catastrophe for life on earth, he says.
* But the only problem is that burning the fossil fuel that corporations now have in their reserves would result in emitting 2795 gigatonnes of carbon dioxide - five times the safe amount.