Two years ago, the first People's Climate March took place on a crisp, blue-sky September day in Manhattan. An estimated 400,000 people, representing the full display of American diversity, were united in the same righteous purpose: to demand that our leaders act fast to address the climate crisis.

The day was filled with promise, and in the following years our enthusiasm was reciprocated with progress. The Paris Agreement. The Clean Power Plan. The rejection of the Keystone XL pipeline. We could say that, powered by a movement of millions, the United States was truly leading on climate.

Now the political landscape is different. Donald Trump's election will upend U.S. climate policy. I doubt that many citizens voted for Trump because they were enthusiastic about his views on climate change, but that's beside the point.

The Trump-Pence administration has no mandate to roll back environmental progress. Polling before the election showed that seven in 10 Americans agreed the government should do something about global warming. Polling after the election showed that 86 percent of voters—including three out of four of those who voted for Trump—support "action to accelerate the development and use of clean energy."

Despite so many people agreeing about what we need, we're getting exactly the opposite. Our federal climate and energy policy has been handed over to science deniers and fossil fuel apologists. As Bruce Springsteen once sang, "there's a darkness on the edge of town."

Although that darkness is real, we are not powerless against it. Even the Trump appointees with extreme views will find that dismantling environmental protections such as the regulation of carbon pollution (which is required by the Clean Air Act) won't be easy. The courts don't look kindly on arbitrary reversals of rules that have been finalized and implemented, especially when they're supported by a mountain of scientific evidence. Even if federal progress on climate and clean energy slows or stalls, cities and states, major corporations, and other nations will continue to push toward a clean energy economy.

That said, we can't afford to underestimate the Trump administration. Unchecked, Donald Trump and Mike Pence are a threat to our climate and the civil rights and liberties guaranteed by our Constitution. This is a dangerous moment in U.S. history.

It's time to march again. On Saturday, April 29, we will come together once more in the streets—this time in Washington, D.C.—in the name of climate justice.

The first march was about empowerment and encouragement. The 2017 People's Climate March will inspire a movement of resistance—resistance to a hateful and retrograde agenda that would take our country backward.

The march will be powerful and visible, in a U.S. tradition going back to the women's suffrage movement a century ago and the civil rights protests 50 years after. We'll send a message that cannot be ignored.

Across our country, many people are appalled at what they see as the new administration takes shape. They are asking what they can do to oppose Trump. Here it is: Of all the individual actions you can take, nothing will be more powerful than joining with others to say that climate change demands action, and that communities of color, workers, women, Muslims, and immigrants will not stand aside while our rights are threatened.

This is the moment. If the Trump-Pence administration attempts to roll back the progress we've made in the past 50 years, we do not have to stand for it. Instead, we will stand up against it. We will march, organize, and keep marching—and we will not give up.

Illustration by Peter Kuper