LE BOURGET, France — With the sudden bang of a gavel Saturday night, representatives of 195 countries reached a landmark climate accord that will, for the first time, commit nearly every country to lowering planet-warming greenhouse gas emissions to help stave off the most drastic effects of climate change.
Delegates who have been negotiating intensely in this Paris suburb for two weeks gathered for the final plenary session, where Foreign Minister Laurent Fabius of France asked for opposition to the deal and, hearing none, declared it approved.
With that, the delegates achieved what had been unreachable for two decades: a consensus on the need to shift from carbon-based fuels and a road map for the 195 nations to do so.
Though the deal did not achieve all that environmentalists, scientists and some countries had hoped for, it set the table for more efforts to slow the slide toward an unlivable planet.
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It was an extraordinary effort at global diplomacy. Supporters argued that no less than the future of the planet was at stake, and in the days before the final session, they tried relentlessly to persuade skeptical nations.
As they headed into the cavernous hall late Saturday, representatives of individual countries and blocs expressed support for a deal hammered out in a final overnight session on Friday. After a day of stops and starts, Mr. Fabius, the president of the climate conference, declared a consensus and struck the gavel at 7:26 p.m., abruptly closing formal proceedings that had threatened to go into the night.
The hall erupted in cheers as leaders like Secretary of State John Kerry and former Vice President Al Gore stood to applaud President François Hollande of France; his ecology minister, Ségolène Royal; his special envoy, Laurence Tubiana; and the executive secretary of the United Nations climate convention, Christiana Figueres.
South Africa’s environment minister, Bomo Edna Molewa, called the accord the “first step in a long journey that the global community needs to undertake together.”
At its heart is a breakthrough on an issue that foiled decades of international efforts to address climate change. Previous pacts required developed economies like the United States to reduce greenhouse gas emissions but exempted developing countries such as China and India.
The new accord changes that dynamic, requiring action in some form from every country. But the echoes of the divide persisted during the negotiations.
Delegates received the final draft of the document Saturday afternoon, after a morning when the text was promised but repeatedly delayed. They immediately began parsing it for language that had been the subject of energetic debate, in preparation for a voice vote on whether the deal should become law.
All evening, tense excitement was palpable. The delegates rose to their feet to thank the French team, which drew on the finest elements of the country’s traditions of diplomacy to broker a deal acceptable to all sides.
France’s European partners recalled the coordinated Nov. 13 terrorist attacks in Paris, which killed 130 people and threatened to cast a shadow over the negotiations. But, bound by a collective good will toward France, countries redoubled their efforts.
“This demonstrates the strength of the French nation and makes us Europeans all proud of the French nation,” said Miguel Arias Cañete, the European Union’s commissioner for energy and climate action.
Yet amid the spirit of success that dominated the final hours of the talks, Mr. Arias Cañete reminded delegates that the accord was the start of the real work. “Today, we celebrate,” he said. “Tomorrow, we have to act. This is what the world expects of us.”
The new deal will not, on its own, solve global warming. At best, scientists who have analyzed it say, it will cut emissions by about half of what is needed to prevent an increase in atmospheric temperatures of 2 degrees Celsius, or 3.6 degrees Fahrenheit. That is the point, scientific studies have concluded, at which the world will be locked into devastating consequences, including rising sea levels, severe droughts and flooding, widespread food and water shortages, and more destructive storms.
But the agreement could be an inflection point in human history: the moment when, because of a huge shift in global economic policy, the inexorable rise in carbon emissions that started during the Industrial Revolution began to level out and eventually decline.
Unlike at the climate summit meeting in Copenhagen in 2009, Mr. Fabius said, the stars for this assembly were aligned.
As negotiators from countries representing a self-described “high-ambition coalition” walked into the plenary session shortly before noon, they were swarmed by cheering bystanders. The coalition, formed to push for ambitious environmental provisions in the deal, includes rich countries such as the United States and members of the European Union; island nations like Tuvalu and Kiribati, which are vulnerable to rising sea levels; and countries with the strongest economies in Latin America, such as Brazil.
Representatives of the group wore lapel pins made of dried coconut fronds, a symbol of the Marshall Islands, whose climate envoy, Tony de Brum, helped form the coalition. Developing countries with the highest emissions, such as China and India, are not members.
Scientists and world leaders had said the talks here were the world’s last, best hope of striking a deal that would begin to avert the most devastating effects of a warming planet.
The final language did not fully satisfy everyone. Representatives of some developing nations expressed consternation. Poorer countries had pushed for a legally binding provision requiring that rich countries appropriate at least $100 billion a year to help them mitigate and adapt to the ravages of climate change. In the deal, that figure appears only in a preamble, not in the legally binding portion.
“We’ve always said that it was important that the $100 billion was anchored in the agreement,” said Tosi Mpanu-Mpanu, a negotiator for the Democratic Republic of Congo and the incoming leader of the Least Developed Countries coalition. In the end, though, they let it go.
It was not immediately clear what horse trading and arm twisting had brought the negotiators into accord. But in accord they were, after two years of international talks in dozens of world capitals, two weeks of focused negotiations in a temporary tent city here, and two all-night, line-by-line negotiations.
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While top energy, environment and foreign policy officials from nearly every country offered positions on the text, ultimately it fell to France, the host, to assemble the final document and see through its approval.
Some countries objected to the speed with which Mr. Fabius banged down the gavel. Nicaragua’s representative, Paul Oquist, said his nation favored a global cap on emissions, a political nonstarter. He said the deal unfairly exempted rich nations from liability for “loss and damage” suffered by those on the front lines of climate change.
The national pledges will not contain warming to 2 degrees Celsius. And more recent scientific reports have concluded that even preventing that amount of warming will not be enough.
Vulnerable low-lying island states had pushed for the more stringent target over the objections of major oil producers like Saudi Arabia. But that target is largely considered aspirational and is not legally binding.
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The agreement sets a vague goal of having global emissions peak “as soon as possible,” and a schedule for countries to return to the negotiating table every five years with plans for tougher polices. The first such meeting will take place in 2020.
The accord also requires “stocktaking” meetings every five years, at which countries will report how they are reducing their emissions compared with their targets. And it includes language requiring countries to monitor, verify and publicly report their emission levels.
Monitoring and verification had been among the most contentious issues, with negotiators wrangling into Saturday morning. The United States had insisted on an aggressive, uniform system for countries to publicly report their emissions, and on the creation of an outside body to verify reductions. Developing nations like China and India had demanded that they be subject to a less stringent form of monitoring and verification.
The final draft requires all countries to use the same reporting system, but it lets developing nations report fewer details until they are able to better count their emissions.
Some elements of the accord are voluntary, while others are legally binding. That hybrid structure was specifically intended to ensure the support of the United States: An accord with binding targets would be legally interpreted as a new treaty and would have to go before the Senate for ratification. Such a plan would be dead on arrival in the Republican-controlled Senate, where many question the established science of climate change and hope to thwart President Obama’s climate change agenda.
As a result, all language on the reduction of carbon emissions is essentially voluntary. The deal assigns no concrete reduction targets to any country. Instead, each government has crafted a plan to lower emissions at home based on the country’s domestic politics and economy.
The accord uses the language of an existing treaty, the 1992 United Nations Framework Convention on Climate Change, to require countries to verify their emissions and to periodically put forth tougher domestic plans.
“This agreement is highly unlikely to trigger any legitimate grounds for compelling Senate ratification,” said Paul Bledsoe, a climate change official in the Bill Clinton administration. “The language itself is sufficiently vague regarding emissions pledges, and presidents in any event have frequently used their broad authority to enter into these sorts of executive agreements.”