Secretary of State John F. Kerry, with his granddaughter on his lap, signs the Paris Agreement to curtail climate change on Friday at the United Nations General Assembly Hall in New York. (Timothy A. Clary/AFP/Getty Images)

Secretary of State John F. Kerry joined the leaders of 170 nations Friday morning in signing an agreement to lower greenhouse gas emissions as part of a global effort to ward off potentially catastrophic impacts of climate change.

The event, at the United Nations headquarters in New York, coincided with Earth Day and marked “the largest number of countries ever to sign an international agreement in a single day,” said U.N. Secretary General Ban Ki-moon.

“We are breaking records in this chambers, and this is good news. But there are also records being broken outside,” Ban said, referencing the hotter-than-ever recorded temperatures of the first three months of 2016. Other events tied to climate change also have triggered sharp concern globally: Greenland’s massive ice sheet has experienced more melting this spring than researchers have ever seen. Coral reefs known for their eye-catching colors are turning white in warming seas, with the Great Barrier Reef experiencing unprecedented bleaching.

“We are in a race against time,” Ban said.

The signing at 10:50 a.m., an hour behind schedule due to dignitaries’ lengthy speeches, was for a commitment to abide by the accord reached by an overwhelming majority of U.N. member states at climate talks in Paris late last year. Negotiators there agreed to take steps to prevent global temperatures from rising by no more than 2 degrees Celsius — 3.6 degrees Fahrenheit — by the end of the century. Island nations that took part in the talks argued that an even tougher mark of 1.5 degrees Celsius was needed to avoid devastating sea-level rise.

“This is a day to commit ourselves to actually winning this war,” Kerry stressed in his remarks near the end of the event’s opening ceremony. As 2015 closed as the warmest year since the start of the industrial age, the nations in Paris heeded the mounting evidence: “Nature is changing due to our choices,” Kerry said.

The power of the climate accord, he noted, is the message it sends to the private marketplace to make sustainable goods and technology to battle global warming. “What it’s going to do is unleash the private sector,” he said.

The Obama administration faces a difficult fight with members of Congress to implement the U.S. greenhouse gas reduction goals. Many Republican lawmakers remain skeptical about whether human activity is causing climate change — despite overwhelming consensus by the world’s leading scientists — or whether the planet is even warming long term.

Longtime skeptic Sen. James M. Inhofe (R-Okla.) has said the declaration by the U.N. International Panel on Climate Change “proves that the U.N. is more interested in advancing a political agenda than scientific integrity.”

In February, the Supreme Court issued a stay on the Environmental Protection Agency’s Clean Power Program, considered to be the Obama administration’s most ambitious effort to control greenhouse gas emissions under the Clean Air Act. The stay means the EPA cannot take industry cleanup actions at least until challenges are resolved in lower courts. Oral arguments in one case are expected to start in the U.S. Court of Appeals for the D.C. Circuit on June 2.

“While the Clean Power plan is an important element of the president’s Climate Action Plan, it is only one component of a broad set of domestic actions this administration has put in place or is in the process of putting in place across the economy to reduce carbon emissions,” EPA spokeswoman Melissa J. Harrison said.

With their actions in New York on Friday, Kerry and leaders from China, Brazil, France, Congo, Italy, Morocco and other nations affirmed that climate change is indeed real and vowed to address it. The agreement comes with no mechanism compelling governments to fulfill their pledges, however. Like President Obama, other heads of state face determined opposition against expensive regulations.

Prime Minister Justin Trudeau of Canada drew a large round of applause when he declared that developed nations such as his, the United States and those in the European Union are responsible for much of the emissions that drives climate change and should therefore help poor nations that are mostly suffering as a result.

“Canada’s ambition cannot end at home” with its own actions to curb carbon dioxide levels and other emissions, Trudeau said. “We have a role to play with poor nations, too. They should not be punished for something they did not do.”

The largest round of applause followed the appearance of award-winning actor Leonardo DiCaprio, a vocal and longtime environmental advocate who was among the last speakers. DiCaprio compared climate change to the overriding crisis of Abraham Lincoln’s time, slavery.

“Yes, we have achieved Paris,” DiCaprio said, addressing the politicians and officials before him. DiCaprio urged leaders to fight for the accord in the absence of a mandate to enforce it. “It will mean nothing if you return to your countries and fail to push beyond the promises of this agreement,” he said.

After 21 years of meetings, talks and debates, DiCaprio said, the time for talk is over. “Our planet will not be saved unless we leave fossil fuels in the ground where they belong.”

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