The Sunday morning rush hour is not usually known for packing people into subway cars like sardines. But September 21, 2014 was not your average Sunday commute as hundreds of thousands of New Yorkers, Americans from across the country and foreign contingents converged on Columbus Circle and Central Park West for the People's Climate March.

People lined up three deep at certain subway stops, just to cram onto passing trains with big signs and cheerful attitudes. By the end of the day, what I could tell just by riding the subway had been confirmed: at least 310,000 people marched to demand action to combat climate change—and possibly tens of thousands more.

I can tell you this: as far as I walked, and as far as I could see, struggling against the tide of cheerful humanity—I found no end of people. And the people's climate march turned out to be in good part a children's crusade.

There were your typical environmentalists, contingents protesting everything from fracking to free natural gas to GMOs (genetically modified organisms) and geoengineering. But there were also union members, including SEIU, the Teamsters and even the United Auto Workers, whose main product helps to exacerbate climate change. A contingent of nurses' leader perhaps put it best when she said: "the same people who exploit our labor, exploit the planet."

And then there were my fellow New Yorkers, particularly from those communities (including, my own, Gowanus) hit hardest by Hurricane Sandy. The call went out: "What do we want?" And the answer came back: "Climate justice." Or as one speaker had it: "We are the people. We are survivors. And we are not leaving."

There were, of course, scientists, but also doctors worried about the human health impacts of climate change.

"Clean coal? You're just blowing smoke" went the slogan on a handmade sign carried by a contingent from communities in the Appalachian mountains, where coal has been mined for generations with a legacy of health effects.

The main parade of protesters kicked off around 11:20 AM, with a whistle, a cheer and chanting.

There were plenty of ways to observe the march, from live video streams on the Internet to overhead shots from helicopters beating the air. There was even a steady slew of pics from drones operated by protesters.

The streets of midtown, including 6th Avenue and 42nd Street, through the heart of Times Square had been cleared, the sidewalks blocked off with interlocking metal barriers. All waited quietly for the music of the marchers to arrive, like many a shot from a post-apocalyptic film.

Just before noon, a moment of silence halted the parade—only to be broken a few minutes later by a wave of noise that swept through the ranks from north to south as protesters raised voices, instruments and whatever else to call for action on climate change. And there was plenty of music on hand, from a New Orleans-style brass band brought by the unions to Native American rappers.

The protesters had a message for President Obama and other world leaders about to gather at the United Nations: Yes, We Can, when it comes to combating climate change. The singsong call went out: "Tell me what democracy looks like." And the answer came shouted back: "This is what democracy looks like!"

Already, the European Union has led the way in shifting to cleaner energy, though Germany and potentially France's shift away from nuclear power could tarnish that legacy. But the protesters main message appeared to be: listen to the science. It really is simple—more CO2 in the air means more heat trapped, which in turn means a changed climate and extreme weather, among other impacts.

Unfortunately, the world is headed in the opposite direction. The Global Carbon Project estimates that this year's pollution will top 40 billion metric tons for the first time in history, just as atmospheric concentrations of CO2 topped 400 parts-per-million for the first time in human history last year. That doesn't leave a lot of room for continuing climate change pollution, just as the climate marchers had to stop and wait for traffic at certain intersections.

The protest wound its way through midtown before turning south on 11th Avenue, just short of the Chinese embassy—a country that has the fate of the climate in its more than 1 billion people's hands. And it will take a lot to clean up the world's skies.

At the end of the line sat a line up of bored motorcycle cops to make sure the march progressed no further than the southern end of the Javits Center. The last few blocks boasted booths touting environmental colleges or environmental groups as well as food carts to feed weary marchers who walked for miles in the heat and humidity of a muggy late September day in New York City. And the street fair did its best to fulfill the slogan: Cook organic, not the planet. After all, as the large contingent of vegans toting signs, could have told you, cuisine is part of combating climate change.

In the end, the message of the march may be best summed up by the WWF t-shirt "Save Humans" seen on many a protester. Or perhaps it's put most succinctly by this sign: