This afternoon, I had a short meeting with President Obama that left me more convinced than ever that he's serious about tackling the climate crisis. Sure enough, later under a sweltering sun at Georgetown University, I watched him calmly and forcefully restate the case for taking action on the climate crisis in one of the most important speeches of his presidency. He also outlined a Climate Action Plan that will help curb carbon pollution, develop clean energy sources, promote energy efficiency, and assert American global leadership on climate issues. Taken together, the new policies directly address what the president rightly calls "the global threat of our time."

Coming on the heels of an unprecedented string of extreme weather disasters, the plan recognizes that we must work on both the causes and the consequences of climate disruption.

But the two most significant commitments the president made were bona fide game-changers: First, he said that he will use the full authority of the Clean Air Act to limit air pollution from both new and existing power plants. Second, he declared that he will not approve the Keystone XL pipeline if it harms the climate, because to do so would not be in the national interest.

The science on Keystone's potentially catastrophic effect on climate could not be more clear. The rejection of this carbon pollution pipeline will be a major climate disaster averted.

Coal-fired power plants, however, are a disaster that has persisted for far too long and, as I listened to the president's speech, I shared the exuberance of the Sierra Club's Beyond Coal activists and so many others in the movement who have fought to end this injustice. Coal-fired power plants are currently responsible for nearly one-third of U.S. carbon pollution; although only a decade ago, that share was greater than one-half. The recent and welcome decline in U.S. carbon emissions to 1986 levels is the result of a decade-long trend away from using coal to generate electricity. Extending clean-air standards to older coal plants, many of which have been polluting for decades, will speed that trend. Not only will this significantly reduce our carbon pollution, but it will also save tens of thousands of lives, since the plants emit many other toxic air pollutants, from sulfur dioxide to mercury.

To meet the challenge of the climate crisis, however, we must do much more than simply celebrate the end of the Coal Age -- we need to hasten a new era of smart, clean energy, energy efficiency, and the jobs that support them. Here, too, the president's plan lays out a practical vision for the future. The president is justifiably proud that generation of renewable energy from wind and solar doubled during his first term; now he has committed to seeing it double again. One of the ways his administration will make that happen is by responsibly siting more renewable-energy projects on public lands. The goal is to install enough such projects to power 6 million homes by 2020.

Other major initiatives will promote energy efficiency in both the public and private sectors, begin the critical work of developing a "smart grid" energy infrastructure, raise the bar on fuel economy standards for heavy-duty vehicles, and tackle the problem of climate-polluting hydrofluorocarbons and methane. Leakage and flaring of methane, which currently accounts for 9 percent of U.S. carbon pollution (and has a global warming potential that is more than 20 times greater than carbon dioxide), is one of the reasons why natural gas doesn't deserve its reputation as a "cleaner" fossil fuel.

Is everything in the Climate Action Plan workable -- or even a good idea to begin with? Of course not. Some ideas, like pursuing "clean coal" technology, investing in nuclear power, fracking, and building overseas markets for U.S. natural gas are either wrong-headed or dead ends. On balance, though, the plan offers a way for our nation to move forward strongly. Even if not every path offered is a good one, it's never been clearer what our destination must be -- and that this president wants to get us there.

Beyond the president's specific commitments, however, the most important takeaway from his speech is that he is determined to "personally own" this issue. That means taking responsibility in the face of what he has called a "moral obligation." He is far from alone in recognizing such an obligation. A national poll earlier this year found that 93 percent of Americans agree that we have "a moral obligation to future generations to leave them a planet that is not polluted or damaged."

Although the president's desire to save the planet certainly resonates with environmentalists like myself and the Sierra Club's 2.1 million members and supporters, that alone can't account for the overwhelming support of more than 90 percent of the American population. Our "moral obligation to future generations," though, is a different matter. If I ever need to get re-energized about fighting the climate crisis, all I need to do is look into the eyes of my kids. I know the same is true for President Obama. His exact words today: "As a president, as a father, and as an American, I’m here to say we need to act."

The president's plan may one day be seen as a critical turning point, but let's not forget that this struggle is far from over. The president himself emphasized that this will be a long and rocky road. In the near term, at least, powerful special interests will continue to throw up roadblocks and obstacles at every turn. Congress, for its part, has resolutely and shamefully shirked its own moral obligation. What matters today, though, is that President Barack Obama has reasserted his leadership on climate with both words and deeds. For that, he deserves both our deepest gratitude and our whole-hearted support (and here's where you can send it to him).

Hope is back in the game. Let's win it.