Leonardo DiCaprio went to the White House earlier this month to talk to President Obama about climate change. As part of the keynote address of the South by South Lawn event, the actor sat down with the president and climate scientist Katharine Hayhoe to discuss what he has called the "defining crisis of our time." He premiered his new documentary, Before the Flood, where DiCaprio travels the world, witnessing the devastating effects of climate change: dramatic sea-level rise, deadlier storms, and longer and more devastating droughts.

"This is the most important issue of our time," he says in the film, as he stands before the United Nations, calling on leaders to take "bold, unprecedented action."

The documentary, which will air on the National Geographic channel October 30, explores the connection between political will and personal decisions. On the website, tips for reducing your own carbon footprint appear alongside a story about how a global average temperature rise of only 1.5 degrees Celsius (opposed to the far riskier 2 degrees) could make all the difference when it comes to protecting the Earth.

A few weeks earlier, DiCaprio’s name had made headlines for another reason. He was listed as one of several "concerned Angelenos" endorsing the Neighborhood Integrity Initiative, a ballot measure that would cripple LA’s efforts to grow denser and more vertical. The initiative grew out of opposition to taller buildings and transit-oriented development in Hollywood.

DiCaprio appears to be oblivious to how the anti-density movement is making the "defining crisis of our time"—climate change—worse.

He’s not alone. The connection between density and climate change isn’t immediately obvious. But it’s crucially important.

When you think of climate change, you probably think of a coal-fired power plant, the black smoke from its government-regulated incineration of fossil fuels spewing into a blue sky.

But here in the U.S., the fastest growing contributor to the types of emissions that are causing the planet to warm is no longer power plants. It’s transportation.

Americans are buying more cars and driving more than ever. Electric vehicles (while growing in popularity) still only make up one percent of all cars on the road. And even then, virtually all those cars are still powered by fossil fuels.

Less than half of the U.S.’s transportation emissions, however, are from the transportation of humans. The shipment of goods has become the most dangerous enabler of climate change. The environmental cost of moving not just our bodies but our stuff around is quickly becoming the single biggest threat to our planet’s survival.

So, quite literally, the single best thing that a city can do for the planet is locate destinations—houses, jobs, grocery stores, schools—closer together so its residents expend less time, less money, and fewer fossil fuels traveling among them.

That’s how LA needs to think about density—as a long-term solution for climate change that will also deliver short-term social and economic benefits.

That’s how LA needs to think about density—as a long-term solution for climate change.

The problem with anti-density campaigns is that their boosters aren’t thinking about our city in a way that looks beyond what they see on their own block today.

Santa Monica’s anti-density measure, LV, is the most troubling, as it would require a citywide vote to approve any new structure over 32 feet. This would make it politically (and economically) difficult to erect buildings more than two stories tall in a prohibitively expensive city that already has limited room to grow, pushing workers farther and farther away from their jobs.

The DiCaprio-supported Neighborhood Integrity Initiative, which will be on LA city’s ballot in March, is calling for a two-year building moratorium to suppress "luxury megaprojects" citywide—specifically targeting high-rise towers and development located near transit.

It’s not just ballot measures promoting this mindset: The nonprofit Fix the City routinely fights for shorter buildings, bigger parking lots, and more car-centric infrastructure. And one lawyer, with a handful of homeowners, has halted a variety of projects throughout the city specifically because of their "densifying" attributes.

Restricting building height and planning for cars goes against everything that environmental leaders and sustainability experts have been saying for decades: If you’re erecting a multi-use structure in a dense, transit-accessible neighborhood with centralized freight delivery systems, the environmental impact of that structure is lessened significantly over time.

Building a two-story building surrounded by a city-mandated parking lot on an extra wide street is not the worst thing you could do for the planet. The worst thing you could do for the planet is codify this kind of development into the land use and planning policies of your city to make building anything else impossible.

That’s why many cities and states are incentivizing dense, transit-accessible development as part of a larger climate-friendly mandate to not only decrease emissions, but also improve public health, clean the air, and slash energy costs.

This week California’s Affordable Housing and Sustainable Communities (AHSC) awarded $140 million to 25 multifamily developments statewide that encourage walking and transit. "Smart design helps us to cut down on greenhouse gas emissions by linking communities to more transportation options," said LA’s Mayor Eric Garcetti in a statement.

Even DiCaprio’s buddy President Obama called for high-rise developments, transit access, and eradicating parking minimums in his housing toolkit, released just a few days before their climate chat: "More residents with access to walking, biking and public transit options also means less congestion on the roads and overall reductions in traffic congestion, greenhouse gas emissions, and commute times."

One of the key takeaways from DiCaprio’s film is that no action is too small, and individuals can make a difference. Los Angeles, as one of the largest cities on Earth area-wise, with a statistically heavy reliance on single-passenger vehicles, could absolutely have an outsize impact on keeping the world’s temperatures below that 2 degree point of no return—and inspire countries like China and India that are adding thousands of cars to their streets every day—if we do things differently.