In 2007, Fisher Stevens was on a diving trip when Jim Clark, cofounder of Netscape, pulled him aside. He told Stevens about a documentary he was funding, directed by underwater photographer Louis Psihoyos, about ocean degradation. Stevens was interested—an avid diver, he'd witnessed dead corrals on previous expeditions. The three decided to join forces. 

The result was the 2009 blockbuster documentary The Cove, which uncovered the brutal slaughter of dolphins near the Japanese village of Taiji. While making the film, Stevens learned the extent to which the planet and its wildlife are in peril.

But that wasn't Stevens's first exposure to human impacts on the oceans. Earlier in 2007, he had discovered the reason for his lack of energy and the metallic taste in his mouth. The New York City Board of Health informed him that he had mercury poisoning, and that unless he stopped eating seafood, the health consequences could be disastrous.

That proved to be a seminal year in Stevens's evolution from longtime Hollywood fixture to environmental activist.

The Chicago native was 14 when an acting school rented his mother's loft and he started taking classes. Stevens became recognizable in the 1980s as a character actor in such films as The Flamingo Kid and Short Circuit. Over time, he demonstrated a facility for a wide range of roles in film (Reversal of Fortune, Hackers), on television (Early Edition, Lost), and on and off Broadway.

His mother, Sally Fisher, a longtime advocate on issues ranging from HIV/AIDS to violence against women, has modeled activism for her son throughout his career. "She infused in me a need to use my voice, as an actor, as someone in this business, to try to make change," he told Sierra.

Now he has reinvented himself as an eco-activist filmmaker. After producing The Cove, for which he won an Academy Award, Stevens directed the Emmy Award-winning Mission Blue (2014), about Sylvia Earle's work to protect marine sanctuaries, and produced Racing Extinction (2015) to document the plight of endangered species.

Of the eco-advocacy films produced since Al Gore invigorated the genre with An Inconvenient Truth, few sound the alarm with such ferocity as Stevens's latest directorial effort, Before the Flood. A partnership with his friend Leonardo DiCaprio, from whose point of view the story is told, the film is relentless. In place of Gore's nerd-cool charts and graphs is a deep rope burn that will leave viewers squirming and looking for a way out. 

Before the Flood tracks DiCaprio traveling to climate change front lines around the world, documenting everything from melting sea ice off Baffin Island in Canada to historic flooding in India. There is no mistaking DiCaprio's dejection as he circles the globe seeking answers from scientists, government leaders, and affected communities.

"That's Leo's role," Stevens said, "to be you and me and everybody else, just finally realizing, 'Oh wow. Climate change is real. It's really happening, and it's happening right now.'"

In one of the most moving scenes, NASA astronaut and climate scientist Piers Sellers sits with DiCaprio before a giant digital display of Earth. Sellers, who has terminal stage-four pancreatic cancer, walks him through the science of, and solutions to, climate change. In the end, it's a dying Sellers who delivers the answer DiCaprio has been seeking: There's still time. If we do the right things, we can head off catastrophe.

"That's the point of the film," Trent Reznor—who scored Before the Flood in partnership with Atticus Ross, Gustavo Santaolalla, and Mogwai—told Sierra. "We want you to leave the theater feeling concerned, but also with a sense of hope."

This article appeared in the January/February 2017 edition with the headline "The Messenger."

Read Sierra's full review of Before the Flood and go behind the scenes with Fisher Stevens: sc.org/beforetheflood.

For insight into the soundtrack, check out our conversation with film composer Trent Reznor: sc.org/trentreznor.