Environmental activist William E. McKibben ’82 implored Harvard affiliates this week to recognize the imminent dangers of climate change and push for an international cap on carbon emissions.

“In this past year, this has gone from ‘This is a big problem’ to ‘This is a big freaking emergency,’” McKibben said at one of a series of campus talks. “Climate change is happening on a way faster and a much larger scale than we thought it would. It is truly scary.”

The urgency of the issue inspired McKibben, a former president of The Harvard Crimson, to put aside his 20-year career as a prolific journalist and author. Writing, he said, is “too slow” to effect change, while talks and large-scale events have greater potential to both “spark” citizens to participate and pressure politicians to act.

In the spring, McKibben spearheaded 1,400 simultaneous demonstrations across the country advocating that the new White House commit to an 80 percent reduction of greenhouse gas emissions by 2050—a promise that President-Elect Barack Obama reaffirmed on Tuesday that he will uphold.

McKibben’s newest initiative stems from research by NASA’s top climate scientist, James E. Hansen, that carbon dioxide levels should be limited to 350 parts per million (ppm) in order to ensure a climate agreeable to civilization. That calls for a sizable reduction from the current level, which is 385 parts ppm.

Titled simply “350,” the project aims to influence policy-makers at an international climate conference in Copenhagen next December to adopt that standard as the global benchmark.

“Our plan for this year is to communicate this number everywhere around the world,” McKibben said Wednesday. “We picked a number because it is simple...It travels easily across linguistic boundaries and holds politicians to one thing.”

Various groups have pledged their support already, McKibben said, including several farmers in Cameroon who have planted 350 trees on the edge of their village.

Though he calls the proposal’s acceptance a “long shot”—because of commercial interests and the “wickedly fast spread” of mass consumerism—he touted the internet as a powerful ally.

“The fact that we live in a connected world is the only wild card we have,” he said. “It’s not impossible to manage a situation where we are able to muster enough people though the web to make a difference.”

In McKibben’s view, an international agreement to cap carbon emissions is the only viable way to mitigate the effects of climate change.

The problem “is too hard to take on sector by sector politically,” he said. “The only lever big enough is to change the price of fossil fuel.”

Some of McKibben’s audiences throughout the week only conditionally accepted his assertion.

“I agree about the urgency of the crisis, but the only thing that holds a lot of promise is being open-minded to a lot of solutions—not canceling anything out of hand,” said the former co-chair of the Environmental Action Committee, Mitchell C. Hunter ’09, after McKibben’s Wednesday talk. “We don’t have enough knowledge right now to know which technologies, or ways that we change societies, will be the way out.”

But McKibben holds that while the role of individuals’ and institutions’ eco-minded actions are important, such as Harvard’s commitment to reduce greenhouse gas emissions, they should be de-emphasized in favor of organizing direct and symbolic action.

“You can’t make the math work in any practical way by changing your light bulb. Symbolic action offers the practical hope for actually changing things on a large scale,” McKibben said.

It is more effective, he added, to focus on “political action that seems to have some chance of payoff, and less towards perfecting one’s personal life and one’s campus.”

Many attendees said they responded to McKibben’s passionate call to action.

“I thought the talk was wonderful, compelling, necessary, and in no way apocalyptic,” said Kirkland House Master Tom Conley. “We have to bet that these will be the consequences if we do nothing. There is no good reason not to act.”

Listeners at McKibben’s talk at the Divinity School yesterday afternoon said the gravity of the situation inspired them to engage with the cause.

A 2005 graduate of the Divinity School, Brent G. Was, said this is the “first time since the early ’80s” that he is uneasy about the future. As a result, he said he will “definitely get more involved” in spreading the message about climate change, such as working with his church on the North Shore to ring its bell 350 times.

—Staff writer Natasha S. Whitney can be reached at nwhitney@fas.harvard.edu.