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We may look back on November 4th as the day the global effort to stop climate change truly began. Today, the Paris Agreement—the international treaty that aims to limit global warming to 2°C (and ideally 1.5°C)—enters into force and becomes legally binding. All 97 countries that have ratified the treaty, including many of the world’s biggest carbon emitters, are officially responsible for their pledges to transition away from fossil fuels.

“It’s alive,” says Andrew Light, director of the Institute for Philosophy and Public Policy at the George Mason University, who helped draft the agreement when he worked for the U.S. Department of State. “It’s there. It now can’t be dismantled by any one party.”

The Paris Agreement is the culmination of decades of work by scientists, environmentalists, politicians, and many others to get the world rallied behind a collective effort to adapt to the changing climate and impede its extent. Since the 1970s, about climate change and its ability to reshape the world as we know it. A degree or two (in Celsius) may not seem like much, but it has the potential to redistribute natural resources, make populated areas unlivable, disrupt our tightly knit economy—and intensify threats to birds and other wildlife. Climate change is the number one threat to birds, , as the food sources and habitat on which they rely undergo change.

Because climate change is a collective problem, it can only be addressed through a coordinated global effort. And that effort is the Paris Agreement: the playbook by which countries develop and share their plans to reduce carbon emissions while holding each other accountable. It was drafted less than a year ago, when world leaders gathered in Paris for the 2015 United Nations Climate Change Conference.

“It was the best experience of my life, other than the days my children were born,” Light says of the negotiation process. Before the conference began, 188 countries submitted pledges that detailed how they would reduce their carbon emissions and by how much. Over the next two weeks, leaders hashed out the details and managed to develop a set of rules to guide the decades-long process of transforming the world’s economy.

“People came in with serious disagreements and resolved them, and we got a better agreement than I thought we would get," Light says. "The text got more ambitious as the negotiation went along. It was certainly in my experience the best example of global cooperation that we’ve ever seen.”

The draft’s completion in December was celebrated, and rightly so. But that was only the first step in the laborious process of making the treaty legally binding. In April, world leaders , an act that confirmed their intention to stick with it. Then they returned home to instate policies to reduce emissions and gain the support of their governments; only then could they ratify the treaty, confident in their ability to follow through on their promises.

But a climate treaty with a fraction of the world’s support doesn’t do much; it needs the support of many countries and the majority of the world’s carbon emissions to reach its goal. To that end, the Paris Agreement required the ratification of 55 countries representing 55 percent of global carbon emissions before it would take effect.

U.N. leaders expected the process to take years. But today is less than a year since the completion of the treaty’s drafting. “This is one of three U.N. agreements—other than the U.N. charter itself—to enter into force within a year,” says Light, who’s also a distinguished senior fellow at the World Resources Institute.

It seemed to all happen at once; within the past two months, a flurry of the world’s biggest and smallest nations signed on. On September 3rd, President Obama and President Xi Jinping of China ratified the treaty in a joint ceremony and brought the count from 24 countries representing 1.1 percent of emissions to 26 countries representing 39 percent of emissions—a giant leap forward in the fight against climate change.

The ratification by the world’s two biggest carbon emitters and economies triggered a landslide of support. One by one, countries ratified the treaty—and on October 5th, the ratification threshold was crossed, slating November 4th (30 days later) as the Paris Agreement’s official start date. , 97 countries representing 69 percent of carbon emissions are legally bound to their pledges. These include many of the world’s top emitters, including India, Germany, Brazil, and Canada, in addition to China and the U.S. More countries intend to ratify soon.

Light attributes the swift ratification to several factors. To his credit, President Obama made climate change a priority of his second term and rallied world leaders behind the effort. Additionally, there’s now a substantial market—including solar panels, wind turbines, batteries, energy-efficient appliances, and electric vehicles—to power the world through an economic transformation away from fossil fuels.

Most importantly, he says, climate change is now considered an urgent issue, even compared to a few years ago. “We’re experiencing impacts at an increase of 1°C over the preindustrial level that are more frequent and more dramatic than many people anticipated,” Light says. “It’s been a giant global wake-up call.”

Now that the treaty has entered into force, the hard work of fulfilling those pledges begins.  and designed the process to meet it. The U.S., for instance,  through a combination of reducing greenhouse gas emissions (including methane, hydrofluorocarbons, and carbon dioxide from power plants), increasing vehicles’ efficiency so they use less gas, and regulating the energy use of buildings and appliances.

Because the pledges are voluntary, some critics say the Paris Agreement won’t be able to get the job done; countries have no incentive to promise more than they’re certain is achievable. As it stands, the countries' commitments are not enough to limit temperature rise to 2°C. Even if every country keeps its pledge from the Paris Agreement, warming will increase by 2.9 to 3.4°C by the end of the century, by the U.N. Environment Programme. Scientists agree that we must hit the 2°C target to avoid the most severe impacts like stronger storms, droughts, and sea level rise.

“If we don’t start taking additional action now . . . we will grieve over the avoidable human tragedy,” Erik Solheim, head of the U.N. Environment Programme, . “The growing numbers of climate refugees hit by hunger, poverty, illness and conflict will be a constant reminder of our failure to deliver. The science shows that we need to move much faster.”

This need to beef up the climate pledges to meet the 2°C target is built into the Paris Agreement. Every five years, countries have to update their pledges. Crucially,  and made more ambitious. “There’s no backsliding,” Light says. The idea is that over time, as progress is made and new technologies developed, the pledges will evolve and standards will be raised.

If the past month is any indication, world leaders are serious about following through on their commitments. Besides the Paris Agreement, they’ve finalized two climate treaties that have been years in the making. On October 15th, 197 nations , powerful greenhouse gases that trap thousands of times more heat than carbon dioxide; they could warm the planet by 0.5°C by 2100 if left unchecked. And in another long-awaited accord, on October 6, the airline industry—backed by 65 countries—, or else pay for their emissions by funding environmental initiatives.

“We went from constantly trying, coming up to the goal line, and failing for years,” Light says of the two efforts. “Then we get Paris. And now there’s tremendous momentum to get more agreement. Success breeds success.”

And there’s probably more to come. All of this happened just in time for this year’s U.N. Climate Change Conference in Marrakech, which starts on Monday November 7. There, world leaders will discuss how to implement the Paris Agreement, and identify new avenues for reducing emissions collectively.

“They will continue to find every nook and cranny of the existing international system where they can find more ways of reducing emissions or enhancing our capacity for adaptation,” Light says. “That’s the new world that Paris has created. And it’s great.”