LE BOURGET, France — The largest gathering of world leaders in history on Monday began a multinational effort toward forging what many called the planet's last, best hope to stave off the worst consequences of climate change.
"Never have the stakes of an international meeting been so high, since what is at stake is the future of the planet, the future of life," President François Hollande of France told a packed United Nations plenary session at a convention center in this suburb north of Paris.
President Obama said the United States is at least partly to blame for the damage that environmental change has wrought, and he urged world leaders to join him in fixing the problem.
During the next two weeks, 30,000 diplomats and delegates will labor to forge a new pact that would, for the first time, commit nearly every country on earth to enact new policies to reduce their planet-warming greenhouse gas emissions.
The talks were kicked off by world leaders including Obama, President Xi Jinping of China, Prime Minister Narendra Modi of India, and President Vladimir V. Putin of Russia, as well as leaders from smaller nations and those most endangered by the effects of rising temperatures.
Many remarked upon the urgency of the task — but also upon a new optimism that a successful deal could be closer at hand than ever before, despite significant obstacles in the negotiating days ahead.
The Paris talks represent the culmination of more than 20 years of efforts to enact such a deal, including two previous meetings of world leaders. Both of those gatherings — in Kyoto in 1997 and in Copenhagen in 2009 — ended with documents that were ultimately viewed as failures.
New scientific reports find that the destructive effects of climate change have already begun to sweep the planet, with the global economy firmly on track to produce a level of emissions that would lock in a future of rising sea levels, intense droughts and food shortages, more destructive storms and floods, and other catastrophic effects.
If the Paris talks collapse or end in failure, it may be many years before leaders will again try to negotiate a similar deal.
Already, a series of analyses have concluded that the best deal that could emerge from Paris would most likely cut emissions by only about half the level needed to avert the worst effects. That would leave a Paris deal as a step forward to solving climate change, but not the solution in and of itself.
In past failed efforts to strike such a deal, the United States and China, the world's two largest greenhouse gas polluters, have negotiated as adversaries. But after striking a bilateral deal last year to jointly reduce emissions, Obama and Xi urged their counterparts to join them in solving the problem — a sharp contrast from what has come before.
"I've come here personally, as the leader of the world's largest economy and the second-largest emitter," Obama said, "to say that the United States of America not only recognizes our role in creating this problem, we embrace our responsibility to do something about it."
Huge hurdles remain ahead of striking a deal, which must be agreed to unanimously by the nearly 200 countries in order to be legally binding.
The greatest threat to reaching a binding climate accord may be a loose coalition of developing nations, led by India, who argue that they should not be asked to limit their economic growth as a way of fixing a problem that was largely created by the others. Obama conceded that point.
With India the world's third-largest greenhouse gas polluter, Obama has invested heavily in his relationship with Modi in hopes of securing his cooperation for a deal here.
Modi has insisted that emissions reductions in his country must be paired with billions of dollars of investment by the developed world into helping poor countries make the transition from fossil fuel to cleaner forms of energy.
There was evidence of a shift in tone from some other major world leaders. Justin Trudeau, the new prime minister of Canada, sought to signal the about-face on climate policy that he represents from his predecessor, Stephen Harper, who had strongly opposed climate change policies.
Even Putin, who has in the past expressed skepticism about the established science of climate change, reflected the tone of the day. Russia, which relies heavily on the production of fossil fuels, had previously been viewed as a likely obstacle to a final deal.
"Climate change has become one of the gravest challenges humanity is facing," he said. "Russia has been contributing actively to addressing global warming."
Main summit issues at a glance
■ No exemptions: The previous climate treaty, the 1997 Kyoto Protocol, divided the world into developed and developing countries and required only the former to cut their greenhouse gas emissions.
The United States, the European Union, and other developed countries say this time all countries must chip in and that the rich-poor firewall is outdated anyway because it classifies countries like Qatar, the wealthiest country on Earth per capita, as developing.
Even though almost all countries taking part are presenting emissions pledges, India and many others want the Paris agreement to state clearly that the developed countries have a bigger responsibility to fight global warming.
■ Aid and technology: Even if the agreement required all countries to cut their emissions, many countries wouldn't be able to do so without help.
Developing countries need money and technology to make the switch to clean energy sources such as solar and wind power. They are also asking for money to adapt to climate change, which would continue for decades even if emissions were to stop today.
The developed countries are reluctant to make firm commitments. They also want to expand the pool of donors to include advanced developing countries like China.
■ Binding language: Many participants, including the European Union, are insisting on a legally binding agreement. The United States has a problem with that because an international treaty imposing emissions limits on the United States isn't likely to be approved by the Republican-controlled Congress.
Negotiators are trying to find a compromise in which parts of the deal are binding and others, such as the emissions targets, are not. That may allow President Obama to approve the deal without going to Congress.
■ Long-term goal: Many countries want the deal to include a long-term goal that spells out what it is they are trying to accomplish. How to spell that out has proven very difficult.
Big oil producers like Saudi Arabia don't want language that suggests fossil fuels have to be phased out. The current draft of the agreement contains multiple options, including ''net zero emissions'' by 2050 or later. That means no more emissions than the world can naturally absorb.
■ Losses and damages: Small island nations say there must be a mechanism in the agreement that deals with climate impacts that they cannot fully adapt to, such as rising seas and more devastating storms.
This issue makes the United States and other wealthy countries uncomfortable because they worry it's going to pave the way for claims of compensation and liability from countries ravaged by climate-related disasters.
SOURCE: Associated Press