Thousands of young climate activists flooded Downtown streets today as part of the Global Climate Strike, happening in 150 countries and over 800 U.S. locations. The scope of the marches was fitting, as the fight against climate change will require dramatically reshaping society, from how we live to how we move. Yet the most impactful thing Los Angeles residents can do right now to support climate action is a lot more simple than you think—and it’s something you can do today.

The decision to back the U.S. out of the Paris climate accord sparked an overwhelming wave of support from city governments. More than 400 mayors—including the Southern California mayors of LA, Long Beach, and Santa Monica—pledged to adopt the Paris accord with or without federal commitment.

As part of that pledge, many of those cities, including LA, Long Beach, and Santa Monica, are moving eliminate emissions within a few decades. California passed a similar plan to not only have the entire state running on renewable energy, but also to achieve carbon-neutrality by 2045.

But transportation specifically is the fastest-growing contributor to the greenhouse gases that cause climate change nationwide, and it represents the biggest chunk of total emissions in California.

It’s true that California is leading the country when it comes to reducing vehicular emissions, but reducing emissions alone won’t be enough to meet the state’s climate goals—or the guidelines in the Paris climate accord.

A report by the California Air Resources Board shows that the only way for the state to achieve its climate goals is for Californians to shift a substantial number of trips from cars to other modes like walking, biking, or public transit.

How many trips are we talking about? This Los Angeles Times analysis breaks it down: Everyone in Southern California has to drive 12 percent less. Just 12 percent less time in our cars—about two car-free trips per week.

One easy way city leaders can help residents drive less is to add housing density to the transit-accessible corridors that already exist—or the many more that will be formed across LA County thanks to Measure M funding. But many local governments—as well as their residents—don’t see the connection among climate change, transportation, and density.

“You can’t be for fighting climate change and against building density in urban centers,” says Long Beach Mayor Robert Garcia. “Building a sustainable future includes creating smart growth that is centered around housing, mass transit, and walkability. That’s why Long Beach is committed to increasing density by building additional residential units and promoting growth along our transit corridors.”

Just adding taller buildings along the forthcoming Purple Line could add housing for as many as 1 million people to the urban core of Los Angeles. That could not only accommodate new arrivals to the city, it would allow more people who already live here to move closer to their jobs, without forcing them to spend money on cars or commute far distances.

Southern California has produced some good policy work in this area over the last few years. Long Beach has prioritized development along its downtown transit corridors. Los Angeles’s Green New Deal includes a roadmap for building new housing that’s transit-accessible and plan to get more cars off the road.

Santa Monica has a proposal for an expedited process to approve housing development along transit corridors and has made car-free living part of its sustainability strategy.

But the problem is that even when a city recommends such changes, some local leaders and neighborhood groups are working in direct opposition to the city's climate action: championing the expansion of freeways, pushing to repave roads for drivers, stifling the development of bike lanes, stopping tall buildings adjacent to train stations, and trying to pass ballot measures that incentivize sprawl.

If you support any of these efforts, you’re not combatting climate change.

If you really want to do something about climate change, you can change your own transportation behavior to more clearly align with the goals set by 400 U.S. cities and nearly 200 other countries that are honoring this commitment—and become a strong voice for the kind of urban-scale changes that will help more people change their behavior as well.

“The world cannot wait—and neither will we,” was the rallying cry from the 400 mayors who pledged to honor the Paris agreement

But young transportation activists had a message for their elected leaders at today’s rallies. “Friday is about putting the entire political establishment on notice that our generation is energized and mobilized, and we’re watching you,” said Varshini Prakash, co-founder and executive director of Sunrise Movement.

But there’s no reason for you to wait for your city to implement new policies—you can endorse global climate action every time you choose not to use your car.