Governments have adopted a global agreement that for the first time asks all countries to reduce or rein in their greenhouse gas emissions.
Loud applause erupted in the conference hall outside of Paris after French Foreign Minister Laurent Fabius gaveled the agreement Saturday. Some delegates started crying. Others embraced.
More than 190 countries had been negotiating the pact for four years after earlier attempts to reach such a deal failed.
In the "Paris agreement," countries would commit to keeping average global temperatures from rising another degree Celsius (1.8 Fahrenheit) between now and 2100, a key demand of poor countries ravaged by rising sea levels and other effects of climate change. But the pact doesn't have any mechanism to punish countries that don't or can't contribute toward that goal.
Countries would also commit to limiting the amount of greenhouse gases emitted by human activity to the same levels that trees, soil and oceans can absorb naturally, beginning at some point between 2050 and 2100.
In practical terms, achieving that goal means the world would have to stop emitting greenhouse gases altogether in the next half-century, scientists said. That's because the less we pollute, the less pollution nature absorbs.
Achieving such a reduction in emissions would involve a complete transformation of how people get energy.
Negotiators had a few hours to analyze the draft before going into a plenary meeting for possible adoption. French President Francois Hollande, who joined the meeting Saturday to add weight to the negotiations, urged them to approve it.
"The decisive agreement for the planet is here and now," Hollande said. "France calls upon you to adopt the first universal agreement on climate."
The deal, meant to take effect in 2020, would be the first to ask all countries to join the fight against global warming, representing a sea change in the U.N. talks, which previously required only wealthy nations to reduce their emissions.
"This is a good text," said Brazilian Environment Minister Izabella Teixeira. "Brazil can accept this."
The deal commits countries to keeping the rise in global temperatures by the year 2100 compared with pre-industrial times "well below" 2 degrees Celsius (3.6 degrees Fahrenheit), and says they will "endeavor to limit" them even more, to 1.5 degrees Celsius. The world has already warmed by about 1 degree Celsius since pre-industrial times.
Ben Strauss, a sea level researcher at Climate Central, said limiting warming to 1.5 degrees instead of 2 degrees could potentially cut in half the projected 280 million people whose houses will eventually be submerged by rising seas.
More than 180 countries have already presented plans to limit greenhouse gas emissions- a breakthrough in itself after years of stalemate. But those pledges are not enough to achieve the goals in the accord, meaning countries will need to cut much more to meet the goal.
"We've agreed to what we ought to be doing, but no one yet has agreed to go do it," said Dennis Clare, a negotiator for the Federated States of Micronesia. "It's a whole lot of pomp, given the circumstances."
The draft agreement sets a goal of getting global greenhouse gas emissions to start falling "as soon as possible"; they have been generally rising since the industrial revolution.
It says wealthy nations should continue to provide financial support for poor nations to cope with climate change and encourages other countries to pitch in on a voluntary basis. That reflects Western attempts to expand the donor base to include advanced developing countries such as China.
In what would be a victory for small island nations, the draft includes a section highlighting the losses they expect to incur from climate-related disasters that it's too late to adapt to. However, a footnote specifies that it "does not involve or provide any basis for any liability or compensation" - a key U.S. demand because it would let the Obama administration sign on to the deal without going through the Republican-led Senate.
In fact, three experts on international climate law told The Associated Press that nothing in the draft would require Senate approval.
Activists who say the agreement won't go far enough held protests across Paris on Saturday, calling attention to populations threatened by melting glaciers, rising seas and expanding deserts.
"This puts the fossil fuel industry on the wrong side of history," said Kumi Naidoo of Greenpeace. "This deal alone won't dig us out of the hole we're in."
Still, if the more than 190 nations gathered in Paris agree to the accord, it would be a breakthrough in climate negotiations. The U.N. has been working for more than two decades to persuade governments to work together to reduce the man-made emissions that scientists say are warming the planet.
The previous emissions treaty, the 1997 Kyoto Protocol, included only rich countries and the U.S. never signed on. The last climate summit, in Copenhagen in 2009, ended in failure when countries couldn't agree on a binding emissions pact.
The talks were initially scheduled to end Friday. U.N. climate conferences often run over time, because of the high stakes and widely differing demands and economic concerns of countries as diverse as the United States and tiny Pacific island nations.
The main dispute in Paris centered over how to anchor the climate targets in a binding international pact, with China and other major developing countries insisting on different rules for rich and poor nations. The draft strikes a middle ground, removing a strict firewall between rich and poor nations and saying that expectations on countries to take climate action should grow as their capabilities evolve. It does not require them to do so.
Some scientists who had criticized earlier drafts of the pact as unrealistic praised Saturday's version for including language that essentially means the world will have to all but stop polluting with greenhouse gases by 2070 to reach the 2-degree goal, or by 2050 to reach the 1.5-degree goal.
That's because when emissions fall, nature compensates by absorbing less carbon dioxide - and can even release old pollution once there's less of it in the air, said Princeton University's Michael Oppenheimer. Forests, oceans and soil currently absorb about half the world's man-made carbon dioxide emissions.
"It means that in the end, you have to phase out carbon dioxide," said John Schellnhuber, director of the Potsdam Institute for Climate Impact Research in Germany.
In addition to the cuts in emissions, the goal could be reached in part by increasing how much carbon dioxide is sucked out of the air by planting forests or with futuristic technology, said Princeton University's Michael Oppenheimer. But, he added, such technology would be expensive and might not come about.
Sylvie Corbet, Seth Borenstein, Andy Drake and Matthew Lee in Le Bourget contributed to this report.