Fortune: You co-founded a non-profit called 350.org. What does the name mean?

Bill McKibben: It’s the most important number in the world. Five years ago, NASA scientist Jim Hansen put out a paper saying we know how much carbon is too much—anything over 350 parts per million in the atmosphere. Now we’re already over 400 parts and that number is growing by 2 parts per million a year.  If we don’t reverse that trend and get back to 350 parts, we’ll see global temperature rise by 3 or 4 degrees Celsius by the end of the century.

And the consequences of that would be severe?

Already 80% of the summer Arctic ice is gone, the ocean is 30% more acidic, and the atmosphere is about 5% wetter. Look at the rains we’re getting in Colorado, which they’re calling biblical. Last year, Jim Hansen said we’re already in a planetary emergency.

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You have an initiative at 350.org that encourages universities and pension funds to divest themselves of stocks in companies that form the carbon economy. What are you hoping to accomplish?

The biggest reason we haven’t taken action on climate change is that the richest industry on earth profits handsomely from carbon. Exxon (XOM) is the most profitable company in history. The fossil fuel industry has five times as much carbon locked up in reserves than even the most conservative governments say is safe to burn. So the industry will do anything to block anything that gets in its way.

It’s not like you can put Exxon out of business.

Before we can get reasonable climate change policy, we have to lessen the power of the fossil fuel companies to influence politicians through mechanisms such as Citizens United. The point is not that we’re going to bankrupt Exxon, but we’re going to politically bankrupt them by showing people how they’re on the wrong side of this issue. One way people have been able to stand up to corporations and make them change is to divest.

That strategy helped fight apartheid and Big Tobacco but you’re dealing with, I think, an even tougher problem here.

You’re right. The last time divestiture was effective was 25 years ago with apartheid in South Africa when cities, states, and universities all divested.  Yes, it’s a tough problem but we’re making progress. Desmond Tutu said if you could see the drought and famine in Africa from climate change that his people didn’t cause, you will know why we need to divest.

In 10 months since the 350.org campaign started, seven colleges have already divested including San Francisco State, Hampshire, and Unity College. They have been joined by, among others, religious organizations like the United Church of Christ and the cities of Seattle, San Francisco, and Providence, who say they have plans to divest. The biggest action will be in the states, which control big pension funds. Bills are moving through the Massachusetts and Vermont legislatures to do just that. Internationally, the movement has spread as well. A number of big pension funds in Australia are offering fossil-free portfolios.

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Some investors who are sympathetic to your cause say it’s more effective to stay invested and wage proxy fights.

Proxy fights are usually the right recourse when things go wrong. For example, you can use a proxy fight to get Apple (AAPL) to raise wages of the Asian labor it uses and the price of an iPhone goes up $1. In this case, the problem is different. It’s not like there is a flaw in the business plan—the flaw is the business plan.

You’ve also been battling the Keystone pipeline, which would move shale oil from Canada to refineries on the Gulf Coast. Will the project get approved this fall?

I’m the wrong person to ask. I’ve never been in the White House and the last time I was outside it, I was arrested from protesting the Keystone pipeline. It is a crucial issue, however. If Obama becomes the first world leader to block a big project because of climate change, it will send a strong signal around the word. It is the iconic environmental fight of our generation.