PARIS—Given the daunting eleventh-hour task facing climate treaty negotiators gathered here for the long-awaited United Nations conference, they began meeting informally a day or two earlier than Monday's scheduled start. And since brinksmanship is the soul of diplomacy, the talks are more likely than not to run into overtime anyway.
But don't expect the judgment of Paris to come in a mere 15 days, give or take.
Rather, tout le monde is talking about what needs to happen in the next 15 years or so.
Only in about 2030 will it be possible to look back and determine whether Paris 2015 was the turning point that world leaders are so avidly seeking here. Will all the world's nations live up to the pledges they brought? Will they do even more? And will emissions, at long last, be heading down?
The outcome in Paris must not be "a stopgap solution, but a long-term strategy that gives the world confidence in a low-carbon future," President Barack Obama said on Monday at an extraordinary gathering of world leaders as the talks officially began. He warned that "accepting this challenge will not reward us with moments of victory that are clear or quick."
President Xi Jinping of China chimed in a few minutes later: "The Paris conference is not the finishing line but a new starting point."
Statements like these are meant to put a gloss on the widely acknowledged reality that the formal emission pledges received so far are inadequate. Those pledges—by more than 180 countries accounting for at least 95 percent of global emissions—don't come close to putting the world on a path toward holding global warming below 2 degrees Celsius, or 3.6 degrees Fahrenheit.
That's the widely used benchmark—some say 1.5 degrees C would be safer—beyond which the risks of climate change become increasingly intolerable.
Despite the shortfall, boosters of the Paris pact say it's wrong to declare it a failure at its nativity.
The most likely outcome for these talks is a pact that legally binds all the world's nations to a prolonged process of submitting, reviewing and periodically tightening their voluntary promises to cut emissions.
Rich countries, meanwhile, would commit to assisting poor countries in shifting away from fossil fuels and in building up their resilience to the impacts of climate change. The treaty would seek a path toward zero emissions in the next 50 years or so. It could encourage a global price on carbon as a powerful incentive, and favor international trading of carbon credits, although that would take time to mature.
The pact would also discourage delay, which only increases the ultimate costs.
If that all seems complex, it explains why the first week will be spent fine-tuning the latest treaty draft for the umpteenth time, putting it into a form that can be hashed over again in the second week.
"Paris is an event, but it is also a process," Christiana Figueres, the executive secretary of the United Nations climate apparatus, said at a news conference. "How do we get from where we are now to where we have to be?"
The fundamental problem is that the world has already used up much of its carbon budget—the amount of emissions beyond which heating of more than 2 degrees Celsius becomes unavoidable.
Unless there are steep cuts in future emissions, the remaining budget will be gone in just a few decades.
But the pledges submitted to the UN over the past year only go part of the way toward the needed reductions, according to the annual "gap report" issued this month by the United Nations Environment Program (UNEP).
Even so, according to another UN assessment of the climate pledges made so far, it would be possible to hit the 2 degrees C target by slamming on the brakes after 2030. That would require a much steeper glide path to zero emissions, and it would cost much more.
For those who leave Paris determined to succeed, every one of the 5,510 days from now until the end of 2030 counts—just as every ton of greenhouse gases added to the atmosphere in the meantime will add to the rising risks of climate change.
Samantha Smith, a seasoned observer of climate talks from the World Wildlife Federation, said at a news conference on Monday that the real fight against climate change "starts the minute everybody walks out" of the sprawling complex where tens of thousands of delegates, advocates, interest groups and journalists are now milling about.
Even though the new treaty won't enter into force until 2020, the five years between now and then are crucial.
For example, there's still a long way to go toward nailing down the $100 billion in annual financial assistance the developed nations promised to help developing nations transition away from fossil fuels.
That, of course, is a drop in the bucket of trillions of dollars in investments needed in the decades ahead to fulfill the ambitions of Paris—let alone to achieve even more than what will be promised this month.
In 2025, it will be time to judge the U.S. on whether it meets its pledge to cut emissions by 26 to 28 percent below its 2005 level. In 2030, it will be evident whether China has managed to reach peak emissions—something it has promised to do even earlier, if possible.
But it is the Paris accord that will determine whether the process is ambitious, fair, transparent, binding and durable – key words used in speech after speech by leaders who are seeking the most elusive attribute of all: unanimity.
"We are going to decide in a few days the future for several decades," Francois Hollande, the French president, told his peers on Monday.
"The danger is not that we aim to high and miss the target," he said. "The danger is that we aim too low and hit it."