Leonardo DiCaprio’s climate change documentary, Before the Flood, released for free on YouTube and iTunes, is 96 minutes of sometimes depressing but always vital-feeling discussion, full of appearances by true luminaries: President Barack Obama, Elon Musk, and even the Pope. But the most powerful appearance was by a NASA scientist you wouldn’t recognize on the street.

When DiCaprio sat down with Piers Sellers, an astronaut and NASA scientist who was diagnosed with pancreatic cancer in late 2015, the science and humanity of climate change merged as they stood silhouetted in front of a rainbow globe that showed the rising temperatures of ocean waters:

Sellers is head of the Earth Sciences division at NASA’s Goddard Space Flight Center in Maryland. There, he oversees scientists who monitor climate data gathered by satellites, which means he’s seen a lot of climate models over the years. He does this despite the diagnosis.

“We think the biggest impact from climate change is the moving of the precipitation belts from the equators to further out,” Sellers explains to DiCaprio. “You know the Dust Bowl region? We expect that to get much, much drier over the next few decades.” As Oklahoma becomes more arid, Europe will get colder as the Gulf Stream is disrupted and no longer brings warmer water from the tropics.

Sellers
Sellers

“Let’s be realistic. Let’s find a way out of it. And there are ways out of it,” Sellers tells DiCaprio. “You know, if we stopped burning fossil fuel right now, the planet would still keep warming for a little while before cooling off again.”

Sellers is an obvious recipient of the overview effect: After his three stints aboard the International Space Station, he’s seen the biggest possible picture in addition to the reams of data at Goddard.

“When you’re up there in orbit and you can see 1,200 miles in any direction, I mean, let me tell you, it’s kind of a revelation,” he tells DiCaprio. Seeing the atmosphere as a “tiny little onion skin around the Earth” made Sellers realize its fragility:

Onion skin.
Onion skin.

In late October of 2015, Sellers was diagnosed with stage IV pancreatic cancer. “I had to figure out what to do with the rest of time,” he thought, after he was told his life expectancy was a year to 18 months. “It’s really one place. It’s our home, we ought to take care of it,” Sellers told CNN earlier this year.

“The odds are I won’t be around for very long,” he tells DiCaprio. “That’s really motivated me to think about what’s important to do, and what can I contribute in the time I have left.”

Amazingly, Sellers is incredibly optimistic. “I have faith in people. I really do have faith in people,” he says. As for the Earth? “I wish it all well.”

DiCaprio’s interview with Sellers begins at about 1 hour, 15 minutes:

Photos via YouTube screenshot (1, 2)

Kelsey Kennedy is a science journalist from Oregon, now based in New York City. She's written about science, technology, and the environment for Quartz, Undark, and Scienceline.