Author and climate activist Bill McKibben ’82 ascended into the pulpit of the Memorial Church at Harvard University on Oct. 18, a stage so grand that legend puts it “10 feet above contradiction.”

McKibben’s message is that the Earth is warming rapidly, that climate change is too late to stop, but that it is not too late to act. Acting means getting humankind, with its proliferating autos and industries, back to 350 parts per million of atmospheric carbon dioxide, the principal gas associated with global warming. (Levels are 390 ppm now, and rising at about 2 ppm each year.)

The 350 ppm level is what scientists last year said is the maximum at which civilization can prosper. Above that, they believe, the Earth over the next century will continue its rapid baking: melting glaciers and polar sea ice, spurring insect-born diseases, and displacing populations threatened with drought, flooding, and rising seas. (By one estimate, there will be 700 million “climate refugees” clamoring for shelter by the end of this century.)

In short, said McKibben, what is ahead is a climate challenge so vast that it is now “on a civilization scale.”

McKibben’s first book, “The End of Nature” (1989), was a groundbreaking distillation of climate change warnings.

Now McKibben has founded 350.org, a worldwide action group that this weekend (Oct. 23-25) will promote demonstrations on every continent, including a Boston Under Water Festival on Saturday (Oct. 24). He was at Harvard for a 90-minute “climate convocation” sponsored by 17 groups.

It was a meeting of — among other unlikely partners — the Massachusetts Council of Churches and Massachusetts Power Shift, a student group that is calling on the state to lead another revolution by shifting to 100 percent renewable energy in 10 years. The sponsors were religious and secular, including Religious Witness for the Earth and Massachusetts Interfaith Power & Light.

Taking the lectern for a moment, along with other speakers, was Heather Henriksen, director of Harvard’s Office For Sustainability. She called the gathering “an important event on an imperative topic.”

In the coming years, Harvard’s greenhouse gas reduction goal will mean “incredible changes in the way the University operates,” she said.

Last year Harvard President Drew Faust pledged to reduce the University’s greenhouse gases 30 percent over 2006 levels by 2016. This fall Harvard Schools are completing plans for reducing energy use by that figure, “including growth,” said Henriksen.

She quoted Faust regarding the enormity of the challenge and the need to change behaviors on an individual level too: “What is at stake is nothing less than a change in culture in how we work and live.”

Before McKibben spoke, a rabbi, a Muslim, and four ministers spoke, and a minister sang, embracing climate action as an act that respects all of creation, follows the Golden Rule, and underscores human expectations. “The God of many names,” said the Rev. Jim Antal of the Massachusetts Conference, United Church of Christ, “is a God of hope.”

In the face of challenge, there is hope, McKibben said. “There’s no guarantee we’ve started in time,” he said of citizen action on the 350-goal. “But at least we’ve started.”

McKibben, a Methodist Sunday school teacher, began with a confession that organizing for 350.org has meant a lot of air travel — five continents in nine days recently, for instance. That gives him the onus of having generated more greenhouse gases in the last year, he said, than some villages will generate in a lifetime.

When he wrote “The End of Nature” two decades ago, said McKibben, “we knew everything but when” about climate change, and hoped it was far in the future, so that it would become someone else’s problem.

But by 2007 it became clear that things were dramatically out of control, he said. Sea ice melted so fast that summer that there was soon 25 percent less than the year before.

Today, “name a physical feature, and it’s in violent and dramatic flux,” he said, from weather systems and polar ice to mountain glaciers. McKibben recounted a recent visit to the Three Rivers Headwaters region of Tibet, where the Yangtze, Yellow, and Mekong rivers originate. About 600 million people live downstream. If climate change dries up the region’s snowy mountains and glaciers, he said, “there is no Plan B.”

With the concision, detail, and humor that befits the New Yorker writer he once was, McKibben delivered what was likely the first sermon in the Memorial Church on hydrology, the science of planetary water movement.

Climate change has altered hydrology, he said, causing droughts as well as deluges, recently burning suburbs of Melbourne, in which 200 died, and drenching the Philippines and other Pacific nations with record typhoons.

At the same time, climate change — by heat, flooding, and disease — is killing about 300,000 people a year, said McKibben, who himself was infected on his travels with a disease that has spread on a warmer planet: dengue fever.

Climate change is doing this, he said, but it is also revealing a pattern of injustice that should appeal to religious sentiments. Poor, low-lying parts of the world are paying the price of climate change that they did not cause.

There is good news of a sort, said McKibben. The climate-change story is a vindication of the scientific method, which predicted the rise in temperatures, and of engineering, whose practitioners have myriad ideas on how to adapt to the planet’s new realities.

“But as well as the scientific method has worked, that’s how badly the political method has failed,” he said. “We’ve done virtually nothing as a nation or as a planet to … take measures of the kind we need to take.”

The deep, serious work has to happen in the political arena, said McKibben, and citizen action such as that of 350.org can help.

The author, part of that tribe of writers, he said, who are self-selected to sit in a room and type, decided, “I wanted to do more than write or speak about this.” In 2006, he and some other Vermonters lobbied for an 80 percent reduction in carbon emissions by 2050 — a figure and a plan that eventually brought presidential candidates Hillary Clinton and Barack Obama on board, but not before a comical beginning to a “movement” started by a group of writers, whose demonstrations once roused 1,000 people to march near Burlington. “The cows were bolting in terror,” said McKibben.

Before long, the only thing the movement lacked “was the movement part,” he said. So in January 2007 McKibben and others opened the “Step It Up 2007” Web site. On April 14, there were 1,400 simultaneous demonstrations in all 50 states — a precursor to what will happen this year. “They realized,” he said of the demonstrators, “we could no longer change the world one light bulb at a time.”

A year ago, McKibben and six Middlebury seniors took climate action global. It was a “quixotic” idea, he acknowledged, in part because “people all across [the world] insist on speaking different languages.” (On Oct. 24, organizers expect 3,500 demonstrations for 350.org in 170 countries.)

McKibben admits it is odd for a movement “to take a scientific data point” — 350 ppm — “as its rallying cry.”

But 350 ppm is where the world needs to return, he said. The only negotiators on this issue are humans themselves, since it is now impossible to argue with the science of climate change, or with nature itself.

“Physics and chemistry are poor negotiators,” said McKibben, citing the Earth’s merciless turn to warming. “They don’t meet you halfway. … They do what they do.”

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