LE BOURGET, France — With the sudden bang of a gavel Saturday night, representatives of 195 nations reached a landmark 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.
The deal, which was met with an eruption of cheers and ovations from thousands of delegates gathered from around the world, represents a historic breakthrough on an issue that has foiled decades of international efforts to address climate change.
Traditionally, such pacts have required developed economies like the United States to take action to lower greenhouse gas emissions, but they have exempted developing countries like China and India from such obligations.
The accord, which United Nations diplomats have been working toward for nine years, changes that dynamic by requiring action in some form from every country, rich or poor.
“This is truly a historic moment,” the United Nations secretary general, Ban Ki-moon, said in an interview. “For the first time, we have a truly universal agreement on climate change, one of the most crucial problems on earth.”
President Obama, who regards tackling climate change as a central element of his legacy, spoke of the deal in a televised address from the White House. “This agreement sends a powerful signal that the world is fully committed to a low-carbon future,” he said. “We’ve shown that the world has both the will and the ability to take on this challenge.”
Scientists and leaders said the talks here represented the world’s last, best hope of striking a deal that would begin to avert the most devastating effects of a warming planet.
Mr. Ban said there was “no Plan B” if the deal fell apart. The Eiffel Tower was illuminated with that phrase Friday night.
The new deal will not, on its own, solve global warming. At best, scientists who have analyzed it say, it will cut global greenhouse gas emissions by about half enough as is necessary to stave off an increase in atmospheric temperatures of 2 degrees Celsius or 3.6 degrees Fahrenheit. That is the point at which, scientific studies have concluded, the world will be locked into a future of devastating consequences, including rising sea levels, severe droughts and flooding, widespread food and water shortages and more destructive storms.
But the Paris deal could represent the moment at which, because of a shift in global economic policy, the inexorable rise in planet-warming carbon emissions that started during the Industrial Revolution began to level out and eventually decline.
At the same time, the deal could be viewed as a signal to global financial and energy markets, triggering a fundamental shift away from investment in coal, oil and gas as primary energy sources toward zero-carbon energy sources like wind, solar and nuclear power.
“The world finally has a framework for cooperating on climate change that’s suited to the task,” said Michael Levi, an expert on energy and climate change policy at the Council on Foreign Relations. “Whether or not this becomes a true turning point for the world, though, depends critically on how seriously countries follow through.”
Just five years ago, such a deal seemed politically impossible. A similar 2009 climate change summit meeting in Copenhagen collapsed in acrimonious failure after countries could not unite around a deal.
Unlike in Copenhagen, Foreign Minister Laurent Fabius of France said on Saturday, the stars for this assembly were aligned.
The changes that led to the Paris accord came about through a mix of factors, particularly major shifts in the domestic politics and bilateral relationships of China and the United States, the world’s two largest greenhouse gas polluters.
Since the Copenhagen deal collapsed, scientific studies have confirmed that the earliest impacts of climate change have started to sweep across the planet. While scientists once warned that climate change was a problem for future generations, recent scientific reports have concluded that it has started to wreak havoc now, from flooding in Miami to droughts and water shortages in China.
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