When the Industrial Revolution started, the atmosphere of our home planet had a carbon dioxide concentration of roughly 280 parts per million. That has, of course, increased. We’re now at about 385 parts per million.

The big question is: How high can we go before things get really, really bad?  Before things are irreversible?

As the Intergovernmental Panel on Climate Change gathered its multitude of scientists and produced its voluminous reports, 450 became the figure of choice. After that, average global temperatures are likely to increase 3.6 degrees Fahrenheit by 2100, they said.

But now, more and more scientists are saying that’s too high. They say they’re seeing the signs of global warming now, and we can’t afford any more CO2. In fact, we need to reduce. According to recent wire reports, more than 80 of the world’s poorest and most climate-vulnerable nations — places with large populations living near sea level, for instance — have declared that CO2 concentrations must be scaled back to below 350 ppm.

So maybe author and eco-activist Bill McKibben, who started an international campaign to garner public support — and public pressure — for 350 wasn’t so fringe after all. In advance of climate meetings in Copenhagen in December, the group (check them out at www.350.org) is aiming for a “planetary day of action” on Oct. 24.

The mission: “We hope to have actions at hundreds of iconic places around the world - from the Taj Mahal to the Great Barrier Reef to your community - and clear message to world leaders: the solutions to climate change must be equitable, they must be grounded in science, and they must meet the scale of the crisis.”

Yesterday, the movement got a super-size boost from Rajendra Pachauri, the U.N.’s top climate scientist. In an interview with Agence France Presse reporter Marlowe Hood he said that as chairman of the IPCC he couldn’t take a position “because we do not make recommendations.”

Then he continued: “But as a human being I am fully supportive of that goal. What is happening, and what is likely to happen, convinces me that the world must be really ambitious and very determined at moving toward a 350 target.”
So, like an overweight person who is risking cardiac problems unless he or she reduces, they’re saying the planet has to not just limit its carbon emissions, but reduce them.

NASA scientist James Hansen also has been saying 350 is the limit. In a 2008 article in the journal, Open Atmospheric Science, he and others warned that “If the present overshoot of this target CO2 is not brief, there is a possibility of seeding irreversible catastrophic effects.”

If they are right, an even bigger question confronts us: How?