PARIS—The landmark agreement struck by more than 190 nations in Paris introduces a new international approach to limiting climate change, with all countries rather than just developed industrial nations joining in to pledge to curb their greenhouse-gas emissions.
But whether the deal reaches its goal—all but eliminating fossil fuels and carbon-emitting economic activity in the second half of the century—still depends almost entirely on domestic will within each individual country to overhaul how energy is produced and consumed and transform the way its economy works.
“We’re on the cusp of getting the ambition we need,” said Edward Cameron, a policy adviser for We Mean Business, a business coalition advocating for action on climate change. “This means no backsliding on what’s been agreed.”
That is because the core of the deal is a collection of voluntary plans by each country to address climate change within the context of its own domestic economic and political situations. Those plans aren’t legally enforceable—a condition some countries including the U.S. insisted upon—although the deal did legally bind countries to develop a common set of reporting standards and a review process to strengthen efforts to reduce greenhouse-gas emissions over time.
For developed countries, success will require not only actually reducing greenhouse-gas emissions starting very soon, but also channeling more than $ 100 billion annually to poorer, developing nations to help finance their efforts. Those poorer countries in turn will need to find ways to grow their economies and lift hundreds of millions of people into the modern middle class, while at the same time limiting and then ultimately slashing carbon emissions associated with fossil fuels that have driven such growth elsewhere for well over a century.
To work, the deal will also have to unleash trillions of dollars from private investors, companies and multilateral lenders. Those investments would go into energy sources such as solar, wind and nuclear that don’t create carbon emissions, new technologies such as better batteries for electric cars, ways to capture and store the emissions of carbon, increases in the energy efficiency of buildings and growing more trees and vegetation to absorb carbon from the atmosphere.
Some believe the voluntary nature of the agreement could actually spur private investments more effectively than an enforceable one. To them, that arrangement signals governments aren’t being pressed into what are some of their most ambitious plans to date—and therefore are more likely to make good on them and encourage private investors and businesses to help.
“An agreement like this might really be a game changer,” said Philippe Desfossés, chief executive of ERAFP, a pension fund for French public-service workers that has been outspoken on climate-change issues. He cited the number of countries committed to what he called an “ambitious” deal. “It’s where we’ll switch from moving millions to moving trillions of dollars.”
The focus on voluntary efforts was at the heart of the negotiations from the beginning of the two weeks during which hundreds of diplomats gathered in a conference center in a suburb of Paris. They were surrounded by tens of thousands of observers, from labor activists and environmental groups to representatives of business and media organizations.
The last major gathering to forge a climate agreement, held in Copenhagen, Denmark in 2009, focused on getting developed countries to legally bind themselves to reducing emissions. The meeting collapsed at the final hour into recriminations and finger-pointing.
It had little lasting impact, and countries backtracked on pledges and ignored the accord. The Kyoto Protocol of 1997 helped spur carbon emissions among signatories, particularly in Europe. But the U.S. and some of the fastest-growing developing economies never signed on, limiting its effectiveness and, eventually, its relevance.
Much of the time spent since Copenhagen has been devoted to building the new approach endorsed in Paris.
Meanwhile, several countries—most prominently the U.S.—firmly ruled out from the beginning of the Paris meeting any plan that made emissions cutting legally binding under international law. That reflected a deep division in Washington between the Obama administration’s support for strong climate-change policies and Republicans and some Democrats in Congress, who have staunchly opposed them as economically damaging intrusions by the government into free markets.
All of the Republican candidates running for president say they oppose international requirements to cut greenhouse gasses, while several have expressed deep skepticism about whether climate change is occurring or being caused by human activity.
“You have to take American politics into account because of their awkward political situation,” said Nozipho Mxakato-Diseko, the South African diplomat who leads the Group of 77, which represents more than 100 developing nations in the climate talks.
But fears that the U.S. won’t live up to its end of the deal, particularly if a Republican wins the White House, trouble officials in Ms. Mxakato-Diseko’s negotiating group. “You cannot bank on the Americans to follow through,” she said.
Even supporters of the deal say each country’s voluntary plan will need to be strengthened over time in order to reach the accord’s goal. That is why they cheered the periodic reviews, reporting and transparency criteria and obligations to try to ramp up the emissions cuts that the deal made legally binding.
“This process gave us most of what it could,” said Lou Leonard, a vice president with the World Wildlife Fund in Washington. “We got a blue print; we got a process.”
—Matthew Dalton contributed to this article.
Write to Bill Spindle at bill.spindle@wsj.com
