Nations of the world gathering in Paris on Saturday approved what may be the most historic global treaty since Versailles, pledging themselves to contain the warming of the planet by backing rapidly away from the uncontrolled combustion of fossil fuels.
But even as the agreement roused cheering and backslapping, its supporters said the road ahead would be a difficult one. After 25 years of mostly fruitless talks, many of those who have traveled together to this point said their work must continue with renewed vigor.
The turning point in Paris came at the end of what has been the warmest year in human history. And scientists predict more of the same as long as carbon dioxide continues to accumulate in the atmosphere.
Since the treaty can't bring the use of fossil fuels to a screeching halt, it will very likely be even hotter than it is now in the decade after 2020. That is when the treaty calls for the next tap on the brakes, with another tap every five years or so as nations come back to the table with tighter and tighter promises.
The Paris Agreement, a legally binding pact under international law, is meant to end that inexorable global warming, which has become increasingly understood by scientists over the past 40 years.
And the new treaty's goals are rooted in science.
It would limit emissions from burning fossil fuels and clearing forests so that warming is contained to no more than 2 degrees Celsius compared to pre-industrial times and—in even higher ambition—to try to keep the rise below 1.5 degrees Celsius.
That means seeking what the treaty calls a "balance" between sources of carbon like the burning of coal, oil and natural gas, and its absorption from the atmosphere by forest growth, or, possibly, techniques like capturing emissions of CO2 and burying them in the ground.
That would mean that sometime in the second half of this century—the sooner the better, if the 1.5 Celsius target is to be met—there should be no net additions of greenhouse gases to the atmosphere.
Environmental advocates declared that this means the end of the fossil fuel era.
While all nations are called on to decarbonize their energy systems, the treaty allows developing nations additional leeway.
On this point, the negotiations seemed at times to be foundering.
Even still, from the first days of this two-week conference, when 150 heads of state and leaders of government spoke of the urgency of action, the mood at the talks has been one of determination to settle differences.
To the last moment, questions relating to what the United Nations calls "common but differentiated responsibilities" dragged on. The French presiding officer, Laurent Fabius, had hoped to wrap up the talks by Friday. But in this kind of Rubik-cube negotiation, a delay of one day meant little.
The final, hour-long delay, which left delegates and dignitaries milling around in the plenary hall, gave time to change the auxiliary verb "shall" in one key section to the word "should"—avoiding, it appeared, the need for Senate ratification before the United States could join the pact.
The passage in question, as drafted in arduous sessions over the past few days, had said that developed nations "shall" continue to make absolute, economy-wide reductions in their emissions, ton for ton. Developing nations, it added, "should" move in that direction. Many developing countries have proposed other yardsticks, such as peaking at some distant date, or controlling emissions as a percentage of GDP.
That, officials said, was a drafting error, not an attempt at differentiation between the rich and the poor. Perhaps so. But if the error had been retained, the U.S. might have been bound by the manacles of international law into something important enough to be considered a treaty under the U.S. Constitution.
So the verb was changed, and the day was saved.
"Not everyone in this room will think this is perfect," said Secretary of State John Kerry after the decision, "that is exactly the way it should be."
"What we do next," he said, "will determine whether we are actually able to address one of the most complex challenges humankind has ever faced."
For the past two years, the United States has insisted that key parts of the Paris Agreement, such as the precise numbers in the emission pledges filed voluntarily with the UN by each country, be kept out of the binding parts of the treaty.
Instead, what all nations are now bound to do is to resubmit their plans every five years, making them more ambitious each time. And the rich countries are supposed to escalate their assistance to the poor.
Another notable victory by the United States was to include language making clear that no system of assisting the poorest states with the unavoidable impacts from global warming—storm, flood, drought and pestilence—would constitute a legal liability claim for compensation from the rich countries.
But that provision, which the U.S. had called a "red line," allowed the Paris Agreement to keep the door open to some form of financial help for climate loss and damage. It will be worked out in the years ahead.