As an unusually early first snow fell on Harvard grounds yesterday afternoon, activist William E. McKibben ’82 spoke to an audience of over 150 people in Memorial Church, requesting their help in drawing attention to the drastic effects of climate change.

“Climate change is no longer a future threat,” McKibben said. “It is a very present crisis breaking over our heads this moment, by far the greatest crisis our species has seen yet.”

At the event, which was co-sponsored by Harvard and several statewide religious and environmental organizations, McKibben called upon audience members to participate in International Climate Day on Oct. 24, when communities across the world will publicly demonstrate their support for the “350” movement. The 350 movement aims to influence policy-makers to adopt a global standard of 350 parts per million carbon at the United Nations Climate Change Conference to be held this December in Copenhagen. This is the level necessary to avoid catastrophic warming according to NASA climate experts, McKibben said.

“The political method has failed badly,” said McKibben. “Virtually nothing has been done to grapple with the scale of this problem or to take measures of the kind we need to take.”

On International Climate Day, the advertising screens in Times Square will broadcast real-time images from demonstrations from around the world. On that day, people will be gathering to form a massive number 3 in Israel, a 5 in the West Bank, and a 0 in Jordan at the point where their three borders meet.

McKibben encouraged audience members to partake in one of Boston’s International Climate Day demonstrations, including the 350 Under Water Festival held at the Aquarium T-Stop. It will resemble a similar event in which the president of the Maldives held an underwater cabinet meeting to sign a climate change resolution.

At yesterday’s convocation, Jewish, Christian, and Muslim religious leaders said their holy texts called on believers to be “stewards of the earth.”

“It was encouraging to see so many different faiths represented here and delivering the same message,” said David Lanskov, who runs the global warming web page at the First Parish Unitarian Universalist Church in Arlington.

Director of the Harvard Office for Sustainability Heather A. Henriksen’s told the audience, “We need everyone in every field making dramatic changes so we can achieve 350.”

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