For his National Geographic Channel documentary Before the Flood, Leonardo DiCaprio stopped in to chat with Elon Musk at the Tesla Gigafactory. The two celebrities talked about how a carbon tax could help countries switch from fossil fuels to clean energy, and Musk specified just how many Gigafactories it would take to enable the entire world to run on renewable energy.

On site at the Tesla Gigafactory in Nevada, DiCpario and Musk talked about the dangers the world faces with climate change precipitated by burning fossil fuels like oil and gas. According to Musk, the fossil fuel industry is the world’s largest industry with “more money and more influence than any other sector.” He spoke of the hope for a “popular uprising” against the industry but also said “we are unavoidably headed towards some level of harm.” Musk showed how his company, Tesla, is working to combat that industry with their Gigafactory.

Related: Elon Musk wants to build Tesla Gigafactories all around the world

So how many Gigafactories do we need to enable the world to run on renewable energy? According to Musk, it would take just 100 Gigafactories. But Tesla can’t build them all; Musk called for large companies in China, Europe, and the United States to “do the same thing” and build factories to make batteries for storing renewable energy.

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Governments can help out too, according to Musk, by setting up regulations or laws to ease the transition away from fossil fuels. In the video Musk said, “Unless there’s a price put on carbon…” and DiCaprio completed his sentence with, “We’re never going to be able to make the transition that we need to in time.” Musk said, “the only way to do that is with a carbon tax.”

The National Geographic Channel released DiCaprio’s documentary in 45 languages and 171 countries on October 30. The hour and a half documentary follows DiCaprio’s three-year journey to learn more about climate change, and the National Geographic Channel is offering the documentary for free so more people can view it. You can watch the entire documentary here.

+ Before the Flood

+ Tesla Gigafactory

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