PARIS — First, the good news. On Saturday afternoon, world leaders ratified a universal pact to slow global warming, ending a decades-long political stalemate and, according to the best possible science, lowering the risk of ecological collapse.

The decisive moment arrived inside a high-security airplane hangar on the outskirts of Paris, where delegates from nearly 200 nations fought over the deal line by line for two weeks. Finally, the French foreign minister called an all-hands meeting, and asked if there were any objections to the final 31-page agreement.

Seeing and hearing none, he banged a tiny green gavel, sealing the deal into international law. The room erupted in a man-on-the-moon-like moment of rapturous applause. The online world followed.

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President Barack Obama praised the deal as “the most ambitious climate change agreement in history.” “Beautiful,” added French president Francois Hollande. “I used to say we must,” commented Christiana Figueres, the United Nation’s climate chief. “Today, we can say we did!”

The agreement commits the world to hold “the increase in the global average temperature to well below 2°C above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5°C above pre-industrial levels.” That’s an ambitious goal here in 2015, which is expected to be the hottest year on record, amid the three hottest decades on record. Close video Activism in the age of terror Currently in Paris even basic public demonstrations are against the law due to the three month state of emergency after the ISIS terror attacks. MSNBC’s Tony Dokoupil reports on activists who are still trying to make a difference during the recent… Currently in Paris even basic public demonstrations are against the law due to the three month state of emergency after the ISIS terror attacks. MSNBC’s Tony Dokoupil reports on activists who are still trying to make a difference during the recent… share tweet email Embed

The agreement also commits the world to a “global peaking of greenhouse gas emissions as soon as possible,” and “rapid reductions thereafter.” That could spell the end of the fossil fuel era, since the primary source of those emissions is oil, coal, and gas.

Finally, the agreement pledges a minimum of $100 billion a year for developing countries to adapt to the ravages of our already overheated climate. That’s an acknowledgement of the vast inequities of climate change: The fact that the richest, most powerful countries have done the most to cause climate change, yet suffer the least as a result.

But there’s a whole lot of bad news in this agreement, as well: It’s vague. It’s aspirational. And critics say it’s several orders of magnitude away from being enough to avert a crisis in the decades ahead.

“The Paris Climate Agreement is not a fair, just or science-based deal,” said Erich Pica, president of the eco-watchdog group Friends of the Earth. “The result is in an agreement that could see low-lying islands and coastlines swallowed up by the sea, and many African lands ravaged by drought.”

Take the long-term temperature goal of global warming “well below” 2 degrees Celsius, which is 3.6 degrees Fahrenheit.

We’ve already warmed more than half that much, and the national emission pledges in Paris put the world on track for more warming still. If fully implemented, the Paris agreement would warm the earth between 2.5 and 3.7 degrees Celsius, according to independent analysis recognized by the United Nations.

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That’s a range that risks “irreversible” damage to the Earth’s system, according to the Intergovernmental Panel on Climate Change, which is why activists flooded central Paris even as diplomats celebrated a few miles away. In rallies near the The Arc de Triomphe and the Eiffel Tower, thousands of people unfurled long red banners to symbolize the “red lines” crossed by negotiators.

“It’s what we expected,” the author and activist Bill McKibben said of the deal. “It’s like a very fat man announced that he’s going on a diet but never changed a thing about the way he eats. Our job, the job of the climate movement, is the same: We’re the fat man’s personal trainer.”

The pledge to reach a “global peaking of greenhouse gas emissions” as soon as possible is also shaky, McKibben and others noted. It does nothing to control the burning of fossil fuels, which already exist in quantities large enough to melt all the ice in Antarctica, according to climate scientists.

Sure, the agreement calls for “rapid reductions” that are “in accordance with best available science.” But it also offers a pair of “get out of jail free” cards to heavy polluters.

The first is the chance to balance human emissions with “sinks” such as new trees. The second is the potential for technology that somehow sucks emissions out of the sky, cooling the planet even we burn through trillions of dollars in fossil fuel reserves.

Finally, the $100 billion a year in climate finance for developing countries sounds large, and it is large. But it’s not large enough, advocacy groups argue. Oxfam has estimated that the cost of adaptation and mitigation will be as much as $800 billion by 2050.

The deal is also weak on the related issue of “loss and damage.” When you strip away the United Nations jargon, the phrase means money for property destroyed by climate change. Under almost any scenario, there will be a lot of such property in the decades ahead.

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But the bulk of the agreement is merely a foundation for future action. So it’s the uncertain parts — more than the good or the bad — that will determine the fate of the atmosphere.

Every five years, the nations of the world are supposed to return to the negotiating table, raising their intention to slash greenhouse gas emissions. They’re also supposed to participate in a “global stocktake,” which evaluates the world’s progress toward zero net emissions. And they’re supposed to do all this within an as-yet-nonexistent “enhanced transparency framework for action and support.”

In other words, Paris is only the beginning.

But Paris is also largely voluntary, a quirk devised by the U.S. delegation, which calculated that it could never get a binding treaty through Congress. So at any time, this “beautiful” and “historic” agreement could come undone.

Correction: This article previously quoted President Obama from pro-Obama Twitter account Organizing For Action.