After President Donald Trump said the United States was getting out of the Paris climate agreement because it put the U.S. at a "big economic disadvantage," the last two holdouts said they were getting in.
Nicaragua and Syria announced late last year that they would join the global agreement to reduce emissions of planet-warming gases.
Experts said it's one way that Trump's decision to pull back from tackling climate change has galvanized others to step up.
But whether others will fill the gap the U.S. has left remains an open question.
No other country has followed his lead, said former lead climate negotiator Todd Stern.
"The first, most important piece of good news, and it wasn't a foregone conclusion, is that other countries stayed in," he said.
Stepping up
Some countries have announced plans to step up their efforts. China, France, Britain and several other countries have said they will end sales of fossil fuel-powered vehicles, though not all have set a deadline.
More than 60 countries, states, cities and companies have promised an end to coal-powered electricity generation.
In the U.S., experts note that states, cities and businesses have been taking action to fight climate change, even when the federal government has not.
Following Trump's announcement, an alliance representing more than half of the U.S. economy pledged to meet the nation's Paris greenhouse gas-reduction commitment anyway.
Counted among the "We Are Still In" coalition's 2,770 members are New York, California and seven other states; 230 cities, including nine of the 10 most populous; and Unilever, Intel, Gap Inc. and other Fortune 500 companies.
Some states announced plans to do more to cut greenhouse gas emissions. Virginia and New Jersey moved to require power plants to pay for their carbon pollution, joining a nine-state cap-and-trade program.
"A lot of this work would have occurred naturally," noted Virginia deputy secretary of commerce and trade Angela Navarro, but Trump's decision "gave us a galvanizing point."
More than 400 companies worldwide have promised to reduce their emissions in line with global climate goals, and 26 U.S.-based companies, including McDonald's, Walmart and PepsiCo, have already set targets.
Market forces have also helped U.S. greenhouse gas emissions fall steadily since 2007. Hydraulic fracturing, or fracking, has created a boom in natural gas, replacing dirtier coal in power plants. And the cost of wind and solar energy has been plummeting.
Tipping the balance
But it's unclear whether the trend will continue. The Trump administration is working to undo regulations aimed at limiting greenhouse gases from power plants, vehicles and other sources.
"The question is, how will it all pencil out?" asked Rhodium Group climate policy analyst Kate Larsen. "Are the federal rollbacks more than enough to tip the balance?"
State, city and business action is "a really good place to start," she added, "but over time, it's not a great replacement for federal action."
The world pledged in Paris to keep global warming to less than 2 degrees Celsius above pre-industrial levels. It is currently falling far short of that goal.
All countries have to ramp up their efforts. But with the Trump administration stepping back, former U.S. climate negotiator Todd Stern said other countries may be less willing to step up.
"You see the United States — the biggest historic emitter, the second biggest emitter now — suddenly saying, 'Never mind.' What's the impact of that? Obviously not good," Stern said.
Negotiators will meet in Poland in December aiming to finalize the "rule book" for how to implement the Paris climate agreement. Experts said that will be one of the first indications of how serious countries are about increasing efforts to meet their climate goals, with or without the United States.
