A parade of more than 120 heads of state—including President Obama—will pledge their commitment to tackling rising greenhouse emissions at Tuesday's United Nations climate-change summit.
But the impact of the giant meeting will be tough to measure—and it might take awhile.
U.N. Secretary General Ban Ki-moon designed the summit largely as a very public launchpad for the final stages of formal diplomatic efforts to forge a meaningful new global climate pact in Paris in December 2015.
Peter Ogden, a former climate-policy aide in Obama's White House and the State Department, said the summit can provide momentum for the intricate talks unfolding over the next 15 months.
"I think what success looks like at this meeting is if you have not only political leaders but leaders from business, the faith community, and civil society all interacting together and demonstrating that there is a real alignment of interest and effort in combating climate change," said Ogden, now the director of international energy and climate policy with the liberal Center for American Progress.
The summit is part of a constellation of climate events in New York this week.
Already, a march that organizers said drew more than 300,000 people wound through Manhattan on Sunday to call for aggressive steps to confront global warming, with participants that included the U.N. chief, Leonardo DiCaprio, Al Gore, and several lawmakers.
Summit side-events have featured new climate pledges from corporations, investors, and nations, with more to come.
Ogden points to the suite of climate announcements emerging ahead of the summit, such as the recent White House effort with several big companies to cut emissions from strong greenhouse gases called hydrofluorocarbons used in refrigeration and air conditioning.
"At the end of the day, without being about the negotiations per se for a new climate agreement, all of this is very much wind at the back for countries and negotiators who have to ... begin the final sprint over the next year," Ogden said.
Between the march and various other events, the full spectrum of the climate movement is out in force. Activities range from the confrontational—such as fresh pledges by philanthropies to dump holdings in fossil fuel companies and a Wall Street sit-in—to corporate initiatives on green energy that include major companies like Ikea and insurance giant Swiss Re.
On Monday, the World Bank touted the growth in the number of businesses and national and regional governments supporting some form of carbon pricing, which is typically accomplished through an emissions-trading program or carbon tax.
According to the World Bank, there's support for some kind of pricing from 73 national and 22 state, regional, and local governments that jointly comprise 54 percent of global greenhouse gas emissions. There is also support from more than 1,000 companies, such as Lego, Dupont, Nestle, and Shell Oil.
On a call with reporters last week, Obama administration officials said there will be a new partnership with six oil companies to cut emissions of methane, a major greenhouse gas.
Paul Bledsoe, a senior fellow on climate at the German Marshall Fund of the United States, said this week's events in New York show that the United Nations has learned that it needs to rally public support to make the diplomatic process work.
"Clearly a major goal of the summit is to rally public attention to the need for near-term climate action leading to a global agreement," he said. Galvanizing a popular movement is something that Bledsoe says the U.N. has done a lousy job of in the past, but he calls the U.N. leader's decision to take part in environmentalists' big march on Sunday a noteworthy change.
"Ban Ki-moon's participation in the march is a clear recognition of the need for popular support," said Bledsoe, who was a climate-policy aide in Bill Clinton's White House.
Tuesday's summit will unfold against fresh evidence of the failure of global leaders thus far to stem soaring greenhouse gas-emissions. The research group called the Global Carbon Project released findings Sunday showing that worldwide carbon emissions from burning fossil fuels grew 2.3 percent to reach a new record in 2013, with 2.5 percent growth projected this year.
Bill McKibben, whose 350.org group was a major organizer of Sunday's march, said ahead of the event that it could push global political leaders to move beyond the speechifying that will mark Tuesday's U.N. gathering.
He told The New Yorker that he's not expecting "immediate results" in New York. "This is all a warm-up for a big negotiating session in Paris in 2015. To the extent that we can build a large movement, we can help push these countries some," he said.
Obama administration officials say the president will use his speech to tout U.S. domestic actions and announce plans to help boost nations' resilience to the dangers of climate change that are unavoidable.
But a bigger announcement will come in the months after the summit, when U.S. officials offer the U.N. climate pact negotiations the country's post-2020 emissions-reduction pledge. The existing U.S. commitment is to cut its emissions by 17 percent from 2005 levels by 2020.
John Podesta, a top White House adviser on climate change, told reporters that Obama will use this week's summit to push other countries ahead of the final rounds of U.N. negotiations.
"You can expect the U.S. to make public by the first quarter of 2015 a strong national target for reducing greenhouse-gas emissions in the post-2020 time frame. The president will use his speech at the climate summit to call on other leaders to keep their ambition high and to work toward a strong global framework to cut emissions," he told reporters on a conference call late last week.