In his new documentary, Before the Flood, Leonardo DiCaprio takes viewers on an eye-opening tour of how climate change is affecting the planet's oceans right now, from rising seas threatening Miami to the perilously melting Arctic.

One of the experts DiCaprio meets in the film is Enric Sala, a marine biologist and National Geographic explorer-in-residence. Sala is working to study, document, and protect some of the most biologically important areas of the ocean through the Pristine Seas project.

We spoke with Sala about his work and role in the new film.

Watch Before the Flood on National Geographic channels on October 30 at 9/10c, or online for free.

In Before the Flood, you take DiCaprio to the tip of Canada’s Baffin Island in the far Arctic. There, you warn that by 2040 the Arctic will have no sea ice left. Can you tell us what that will mean for the environment and for us?

The melting of the sea ice has consequences on all levels, from local to global. Locally, all the animals that live up there have evolved to live with the sea ice, so its disappearance will have major effects through the entire food web.

For example, there are certain algae that need to live under the ice. Those organisms are an important food source for little shrimplike creatures called amphipods. Those organisms, in turn, are eaten by Arctic cod, which are eaten by seals, which are eaten by polar bears. There are projections that the polar bear population is going to crash as the ice keeps melting. Already in the Beaufort Sea, we have seen a 30 percent decline in the polar bear population.

Melting sea ice also sets up a feedback loop. Ice reflects a lot of the sun’s energy back up toward space, while open water absorbs more of that heat. So the less ice and the more water, the more the planet warms. (Read about the astronaut who is using his final days to fight for climate change awareness.)

View Images Enric Sala during an expedition to the Russian Arctic in 2013 Photograph by Cory Richards, Nat Geo Image Collection

You also said the Arctic is like the air-conditioning of the Northern Hemisphere, affecting weather patterns, currents, and disasters like droughts and floods. Can you explain how this works?

A warming Arctic is going to affect our planet’s systems of ocean currents and wind patterns, which help drive a lot of our weather. Now, there is a giant conveyor belt in which cold water that forms near the edge of sea ice sinks, only to be replaced by warmer water from the south. This process makes the British Isles comfortable to live in, for example, instead of frigid. But this whole conveyor system is in danger of disruption as the poles warm.

As the Arctic warms, it may also impact air currents, such as the jet stream, which drives a lot of weather in North America by blocking or shuttling cold air.

In the film you call the warming of the Arctic “the most dramatic transformation of a large environment ever.” Can you explain why this part of the world is so susceptible to climate change?

The Arctic is warming two to three times faster than the average for the rest of the planet. It’s ironic, because the carbon emissions that are warming the planet are not produced in the Arctic, yet the Arctic is suffering the most. At the same time, what happens there is going to affect the rest of the planet.

Part of the problem is that at lower latitudes you often have more mixing of air and water, but the Arctic is more isolated at the top of the world. Also, warming of one or two degrees there has a bigger impact than other places because that’s enough to melt a huge amount of ice. That leads to dramatic changes in the landscape and sets up that feedback loop that leads to even more warming.

As the planet warms, species in temperate zones have been migrating northward. But Arctic species have nowhere to go. That’s a problem of living at the top of the world.

In Before the Flood we also meet local guide Jake Awa of Pond Inlet, who says the ice around his home used to be thick and solid, but now it is often the consistency of ice cream. Awa relies on hunting polar bears and other Arctic animals for much of his livelihood. How are indigenous people affected by climate change?

A warming Arctic is going to impact the people who live up there. The animals they depend on, such as seals and polar bears, are going to decline, so they are going to have a hard time preserving their hunting traditions.

Watch Before the Flood for Free, Everywhere

Beyond the Arctic, the rest of the world’s ocean is also under threat from a changing climate, right? How serious is the problem?

For the past 40 years or so, the Earth has absorbed more heat from the sun than it has reflected away, and about 90 percent of that has been stored in the ocean. That means if the ocean wasn’t there, the lower parts of the atmosphere—where we live—would be 36 degrees Celsius (97 degrees Fahrenheit) warmer! Earth would be like Venus: uninhabitable. Bye-bye economy, bye-bye human race. The ocean also absorbs a quarter of the carbon dioxide we put in the atmosphere every year.

Unfortunately, all that heat is raising the temperature of the ocean, which is killing corals. That leads to the decline of the fish and other organisms that depend on corals. And all that carbon dioxide dissolving in the ocean is making the water more acidic. That makes it hard for corals, oysters, and other animals to make their shells. So that puts the entire food web at risk.

How does your work leading the Pristine Seas project—which is helping save the ocean’s natural treasures—also help us address global warming?

By protecting huge chunks of the ocean from all other human impacts, such as fishing, mining, and oil exploration, we make these places stronger, healthier, and more able to adapt to warming events. As an analogy, if you get the Zika virus, you are more likely to survive if you were otherwise healthy than if you already had other health problems.

So we are trying to help make the marine environment healthier and more resilient. We’re buying time while countries figure out how to keep warming at less than 2 degrees Celsius, which is what we need to do to avoid the worst impacts of climate change.

What else needs to happen to better safeguard our future from a changing climate?

There is really only one solution to climate change: to dramatically reduce our carbon emissions. Everything else, such as reforesting, capturing carbon dioxide, et cetera, are really mechanisms to achieve that goal.

At the same time, we need to develop clean, renewable energy technologies. Several countries have shown this is feasible and great for the economy. Denmark has slashed its use of fossil fuels and will soon use none, yet their gross domestic product has gone up. And they were named the happiest country on the planet by the United Nations. Other countries are on their way, such as Germany and Norway.

Those who want to polarize us between either economic growth with pollution and economic stagnation with sustainability are wrong. (Learn about how carbon taxes may play a role.)

What do you think are the most important things ordinary people can do to address these big problems?

There are so many things people can do. But if it were one thing, I’d say eat more vegetables and less meat. One of the biggest ecological footprints is our meat consumption. To raise cattle, forests are being cut down, especially in the developing world, and that creates a huge amount of emissions. Cattle also release methane, which is a powerful greenhouse gas.

I rarely eat meat, and I have never felt better.

When you introduce DiCaprio to the narwhals off Baffin Island in the film, and he hears their unique, purrlike sounds, viewers can really see the pure joy on his face. How important do you think it is that everyone experience such joy from nature?

We only protect what we love. There is nobody who goes out into nature and experiences something extraordinary like that and doesn’t feel a sense of awe and wonder. Being in nature brings back the kid in us, that sense of wonder and curiosity. It’s also good for our health and minds. People should spend more time outside.

For those who can’t go to pristine places, we try to share that experience through our National Geographic films and media. Still, nothing beats firsthand experience.

What main message do you hope people take away from this documentary?

This film comes at a very important moment. The world is not debating anymore whether climate change exists, except for a few fringe outliers. World leaders came together in Paris last winter and agreed to reduce emissions so that global temperature does not increase more than 2 degrees Celsius compared to preindustrial levels, in order to avoid catastrophic climate change. In my opinion, one of the biggest values of the film is the strong sense of urgency. We have to act now.

And for those of us in the U.S., the film comes right before an important election. People should ask themselves, Who do we want for a leader on this issue?

What was the experience of working on the film Before the Flood like?