I was at the People's Climate March in New York City on Sunday. It was mind-bogglingly huge.

The gathering was so big that the pre-march starting line-up stretched a mile and a half, from Columbus Circle at 59th Street all the way up to 86th Street, with numerous cross-streets jammed with participants, too. For people standing at 77th Street, it took more than an hour and a half for the parade to even get moving. The event was so big that when I finally reached the end of the parade route at 40th Street and 11th Avenue — three miles and five hours later — and turned back to rest my aching feet, I could see that Times Square (many avenues away) was still jammed with marchers.

The most immediate point of comparison was to the Occupy movement. On one level, there was a very similar vibe: lots of music, chanting, and anti-capitalist slogans. But it was a much more organized affair than Occupy — a prearranged moment of silence was executed without a hitch, followed by a deafening march-wide cheer that rattled windowpanes on the other side of Central Park.

More than 400,000 people participated, according to organizers' estimates. That makes this the biggest climate rally in history — roughly comparable to the Iraq War protests of early 2003 — and thus the biggest political rally of any sort in over a decade.

But protest marches — even massive ones — do not translate neatly or reliably into public policy goals or changing public opinion. Sometimes they are forgotten nearly as soon as they end.

So what happens now?

It's hard for protests to get political traction without good coverage from the media. In this sense, the climate marchers cannot feel super optimistic. The Sunday shows totally ignored the march, bringing up memories of how the national media deliberately undercounted and muffled the gigantic 2003 anti-war protests. Glenn Beck's The Blaze tried some good old red-baiting based on the presence of numerous socialists and radicals, something which caused deep unease back in 2003, too.

Still, these are very different times. Today, we don't have anything even remotely close to 2003's suffocating miasma of hysterical jingoism. On the contrary, climate change is about as close to a mainstream issue as anything gets anymore. Basically everyone starting from Mike Bloomberg and heading leftward is committed to some degree of "climate justice," as marchers called it. Sen. Bernie Sanders (I-Vt.) and New York Mayor Bill de Blasio were there on Sunday. UN Secretary-General Ban Ki-Moon was there, too, along with Mark Ruffalo, Leonardo DiCaprio, and Kevin Bacon. And while the (increasingly irrelevant) Sunday shows pretended like the march didn't exist, online coverage was generally good. Even Politico and Fox News' website had decent coverage.

In that sense, it's fair to say that the protests achieved their most immediate political goals: to demonstrate the numerical strength and impressive organizational chops the green movement is developing, to put some political energy behind President Obama and the EPA as they go through the grinding process of implementing new regulations on coal-fired power plants, and to pressure world leaders ahead of the big climate summit at the U.N. on Tuesday.

Still, "political energy" and "pressure" can be squishy things. And with the European Union wracked by depression and political chaos, and Republican science deniers ready to block any new treaty or legislation in America, world climate policy is largely reduced to informal agreements between nations. President Obama will speak at Tuesday's summit, and will likely announce a fairly aggressive future decarbonization target, with the hope that EPA regulations and rapidly advancing technology will be enough to get there. That, in turn, will put pressure on developing nations, particularly China — now by far the largest emitter in the world, with nearly twice the emissions of the United States — to agree to and implement some aggressive climate policy. Now, that may sound like a long shot. But air pollution from coal power is reaching dystopian levels in China, providing a serious impetus to decarbonize.

The sheer size of Sunday's march makes it a magnificent achievement. It set the stage for Tuesday's summit. The question moving forward is whether the movement can move beyond huge rallies to put concrete pressure on the political system, which means above all winning some elections, something Occupy notably failed to do.

There aren't many signs, as yet, that this new green movement is planning to storm future elections, the most important way to influence policy in a democratic system. In interviews, Bill McKibben — who has become the most prominent voice in the movement — hasn't articulated a clear electoral goal, instead focusing on trying to pressure the president and other world leaders.

But I think the future prospects are bright. The movement is fairly diffuse, but there are still some leaders, important for organization and concentrating media intention. There is evidence of logistical skill: Organizing 400,000 people to be in the same place at the same time is an awesome task. They've even got the outlines of an agenda — stop the Keystone XL pipeline, speedily implement EPA regulations on carbon from coal power, and put a general price on carbon pollution. If and when they decide to start swinging some elections, they are in excellent position to wield serious influence.