GOSHEN -- A bestselling author addressed a crowd of more than 450 Elkhart County residents Wednesday night, asking them to take action on what he called "an emergency the likes of which we've never seen."

Bill McKibben, scholar in residence at Middlebury (Vt.) College and environmental activist, spoke at Goshen College Wednesday night.

He delivered a speech called, "The Most Important Number in the World: Building a Worldwide Movement to Fight Climate Change."

To McKibben, the "most important number" is 350, which refers to the amount of carbon dioxide in the atmosphere. McKibben said that once carbon dioxide levels exceed 350 parts per million, the world travels toward irreparable harm.

Currently, these levels stand at 387 parts per million. McKibben said this means that immediate change is needed.

"Unless we solve this problem soon, we're not going to solve it," he said.

That's because as global temperatures rise, they set off what McKibben called "feedback loops" or "booby traps" that further accelerate global warming.

For example, rising temperatures are melting Arctic ice. Ice reflects the sun's light where water absorbs it. So, as ice melts, there is more water, which absorbs even more heat, which, in turn, melts more ice.

He said the consequences of not interrupting cycles such as this one are extreme. Climate scientists now estimate that sea levels will rise six to seven feet this century. "It turns every coastal city into New Orleans and New Orleans into Atlantis," he said. "It's not a number we can deal with."

A Methodist Sunday school teacher, McKibben referenced not only science, but also his Christian faith.

"This is more than a scientific problem. At some level, it has a powerful moral and even theological dimension," he said.

McKibben ended his speech by asking people to participate in global demonstrations he is planning for Oct. 24.