You might want to save this date: September 21. Here's why.

Activists working to address the climate crisis have been cautiously cheering President Obama this year -- for telegraphing that he's likely to reject the Keystone XL tar sands pipeline and for the Clean Power Plan, an important set of standards that his Environmental Protection Agency has proposed for cutting carbon pollution from existing coal-fired power plants.

Yet as scientists know, as polar bears know, and as people who've experienced extreme weather know, the nation and the world are still moving too slowly to avert climate disaster.

The drumbeat for urgency is growing, however, and it's not just coming from the tree-hugging contingent. Last week, former New York mayor Michael Bloomberg, retired hedge fund manager Tom Steyer, former Treasury Secretary Hank Paulson, and other leaders known for keeping an eye on bottom lines released a report called "Risky Business." It makes a sobering case for why the nation cannot afford the economic costs of climate change.

The amount of carbon dioxide in our atmosphere has already reached levels not seen since long before we even evolved. The human suffering that results may be incalculable, but the economic consequences are not. The International Energy Agency has estimated that for every year the world delays taking significant action to curb climate change, we will have to spend an additional $500 billion down the road.

At a Senate hearing that many Republicans hoped would undercut the EPA's proposed Clean Power Act, four former heads of the EPA, under Republican presidents Richard Nixon, Ronald Reagan, George H.W. Bush, and George W. Bush testified instead that action is imperative. Noting that businesses and states are already taking the crisis seriously, William Ruckelshaus, who headed the first EPA under Nixon, said: "There is a lot happening on climate. It's just not happening in Washington."

Indeed, our government may be gridlocked by the Republicans who control the House while hamstrung by ties to a fossil fuel lobby that demands utter fealty, but local leaders and the American people are moving forward fast.

Just last year, Al Gore and I stood and watched as then-Mayor Antonio Villaraigosa announced the Los Angeles Department of Water and Power's history-making plan to stop using coal-fired power by 2025 and replace it with cleaner energy sources.

That was a proud moment for Los Angeles, and Al Gore ended an impassioned speech that day on a hopeful note about the ability of our society to quickly evolve: "If somebody had told you four years ago," he said, "that on this beautiful March day, 60 percent of the American people would say, 'we are in favor of gay marriage,' you would have said, 'no we can't change that much that fast.' But we can, and we did." The same will be true of attitudes about cutting carbon, he predicted.

My guess is that even Al Gore is surprised by how soon his prophecy has become reality. When this month the U.S. Environmental Protection Agency announced its Clean Power Plan for cutting carbon emissions from power plants, polls found about 70 percent of Americans in favor.

This really is a potentially defining American moment. We cannot let it slip from our grasp, for while momentum is on our side, time is not. And so on September 21, tens of thousands of people will converge on New York City to urge the president to show the hundreds of world leaders gathering in that city for the United Nations Climate Summit, that America is ready to lead a global response to this global crisis.

We're going to make 2014 the tipping point year in the international effort to solve the climate crisis, and contrary to what those who remain corrupted by the influence of the coal, oil, and gas industries would like you to think, the world we're already tipping toward is not one of diminished lifestyle, but one of rare historic opportunity.

Already, people across America are finding well-paying, meaningful jobs building the wind turbines and installing the solar panels that will let us walk away from the dirty 19th-century fossil fuels that are making us sick and wreaking havoc on our planet's climate. Already, investors are profiting from the technological innovation that is creating an era of clean energy prosperity, while communities that have long borne the brunt of fossil-fuel refining and burning are demanding an energy future that does not perpetuate sacrifice zones in places like Wilmington, Detroit, and Houston.

Every day, more people recognize the obvious course we need to take. And on September 21, the cross section of people rallying at the People's Climate March will state the obvious more loudly and assertively than ever before in New York City and around the country.