The accord, adopted by more than 190 nations in December, relies on individual countries to set goals for reducing greenhouse-gas emissions and to regularly revise those goals upward every five years.

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NEW YORK — In a striking declaration that the world is finally ready to change its polluted ways, global leaders gathered at United Nations headquarters in New York City to sign the historic Paris climate accord, an agreement that is seen as the blueprint for rescuing the planet.

The question is whether the plan will work.

Year after year, decade after decade, the planet has been getting warmer. This March was the warmest in history. Scientists say the future will be filled with food shortages, drought, rising seas and extreme weather if bold action is not taken quickly.

“We are in a race against time,” said United Nations Secretary-General Ban Ki-moon.

Some 175 nations signed the agreement Friday, the first day it could be signed. That is far more than had done so on a single day for any previous global agreement — but it is also only a step toward the accord becoming international law.

For that to happen, at least 55 countries representing at least 55 percent of global emissions have to formally “join,” or approve it within their national governments, some by executive action, others through legislative action. That process could last into 2017, though U.S. Secretary of State John Kerry suggested late Friday that the goal could be within reach this year. The United States plans to join the agreement this year.

Even those who helped draft the Paris Agreement acknowledge that its terms do not meet the goal it outlines of holding the increase in global temperatures to “well below” 2 degrees Celsius, or 3.6 degrees Fahrenheit, above preindustrial times. Experts said the current agreement would hold the average temperature increase to perhaps just below 3 degrees Celsius.

But nearly 25 years after international efforts to address climate change began, world leaders say this time, finally, momentum has shifted, and nations can confidently invest not in fossil fuels that create greenhouse gases but in clean, renewable energy — and be assured that their economies will grow. Coal companies in the United States are going bankrupt. Access to lower-cost solar and wind power is increasing worldwide.

The accord, adopted by more than 190 nations in December, relies on individual countries to set goals for reducing emissions and to regularly revise those goals upward every five years “to reach global peaking of greenhouse-gas emissions as soon as possible.”

The United States, which has committed to reducing its greenhouse-gas emissions by 26 to 28 percent below its 2005 level in 2025, is among the countries that likely will have to revise its goals upward.

Complicating matters, in February the Supreme Court issued a temporary stay to the Obama administration’s plan to reduce emissions from power plants.

Kerry, who signed the agreement while holding his young granddaughter, told the U.N. on Friday that it misses the point to focus on the fact that the accord does not guarantee to hold temperature rise to below 2 degrees, or a stronger goal that many groups support of 1.5 degrees.

“The power of this agreement is the opportunity that it creates,” he said. “The power is the message that it sends to the marketplace. It is the unmistakable signal that innovation, entrepreneurial activity, the allocation of capital, the decisions that governments make, all of this is what we now know definitively is what is going to define the new energy future.”

On Friday, some countries vowed to exceed goals they outlined just a few months ago for reducing their emissions. China, the world’s largest emitter of greenhouse gases, said it intends to formally approve the agreement before it hosts the G-20 leaders summit representing the world’s largest economies in September. China also reiterated its plans to establish a so-called cap-and-trade market that would put limits on how much carbon emissions industries can burn.

“The Chinese people honor their commitments,” Zhang Gaoli, the vice premier, said.

Prime Minister Justin Trudeau of Canada, the West’s new face of progressivism, prompted applause when he said his country would give money to help developing nations fight climate change and invest in clean energy, a crucial issue in negotiations leading to the agreement.

“They shouldn’t be punished for a problem they didn’t create,” he said.

Ban warned that the work ahead will be enormously expensive. “Far more than $100 billion — indeed, trillions of dollars — is needed to realize a global, clean-energy economy,” he said.

French President François Hollande, the first to sign the agreement, said Friday he will ask Parliament to ratify it by this summer. France’s environment minister is in charge of global climate negotiations.

“There is no turning back now,” Hollande told the gathering.

The ceremony brought together a wide range of states that on other issues might sharply disagree. North Korea’s foreign minister made a rare U.N. appearance to sign Friday, and Zimbabwe’s President Robert Mugabe brought applause when he declared, “Life itself is at stake in this combat. We have the power to win it.”

Countries that have not indicated they would sign include some of the world’s largest oil producers, including Saudi Arabia, Iraq, Nigeria and Kazakhstan, the World Resources Institute said.

The ceremony was held on Earth Day, an event started in 1970.

Leonardo DiCaprio addressed the delegates, quoting Abraham Lincoln, and a young woman from Chad described a once vast but now vanishing lake on which her country depends.

“True climate justice is renewable energy for all,” she said.