There's one figure that author and environmentalist Bill McKibben really wants you to know: 350 parts per million.

That's the number that NASA climate guru James Hansen has established as the tipping point of global climate disaster. It's how much carbon dioxide can be in the atmosphere without causing permanent climate change.

And, sadly, right now, we're at 387 parts per million.

During an appearance last night at Rochester Institute of Technology's Ingle Auditorium, McKibben gave an empowering presentation on the sobering topic of climate change. Much of what McKibben said was his standard fare: Personal action will be needed to stop global climate change, but it won't be enough. As citizens, we need to push politicians to enact top-down policies that will promote large-scale reductions in carbon emissions, he said. That includes capping those emissions.

We have a president-elect who has vowed serious action on global warming and now's the time to apply the McKibben said. The best way to do that, he said, will be a grassroots political movement.

"We may have, at least in terms of climate change, the last real opportunity to set things right," McKibben said.

If we don't take serious action by the end of President-Elect Obama's first term, the damage may be irreversible, he said.

We have a chance to enact a meaningful, ambitious climate treaty to succeed the Kyoto Protocol at Copenhagen in 2009.

Working against McKibben, and the idea that we need to take drastic action now, is the current economic climate. Politicians are already using it as cover to skip out on environmentally sound or ambitious policies. It will cost too much or stifle economic growth, they say.

The public should not accept those excuses. Looking past the environmental benefits, any serious attempt to address global warming will involve much-needed jobs, white collar and blue - in scientific research, manufacturing clean energy technology, insulating homes, and so on.

And if it seems like an insurmountable task, since when is America above a good challenge? We invented airplanes. We invented the particle accelerator. We put a man on the moon. We have a glorious history of innovation and ambition. We can certainly find ways to drastically cut our greenhouse gas emissions.

If you want to take a look at McKibben's latest project, go here.