Behind all the fanfare around this week’s UN Climate Summit, which will bring 120 heads of state to New York on Tuesday, looms one big question: Will the nations of the world agree on a path to avoid the most dangerous consequences of climate change, such as dramatic sea-level rise and extreme droughts and storms?

The answer will not come during the official summit. This week’s event is not a negotiating session for the next international agreement; that will happen in December 2015, when countries that are signatories to the United Nations Framework Convention on Climate Change meet in Paris.

But it’s looking increasingly likely that the next big international agreement on climate change will not be a legally binding treaty like the 1997 Kyoto Protocol, which required developed countries to reduce greenhouse gases by specific amounts (and which was rejected by the United States and, more recently, Canada).

Nor will the next global climate deal likely require the deep reductions in greenhouse gas emissions that scientists say would be necessary to prevent catastrophic impacts from global warming, according to current and former Obama administration officials and other observers of ongoing international climate negotiations.

There are lots of reasons why a treaty is unlikely, beginning with the near certainty that the U.S. Senate would not ratify one. The United States is the world’s second biggest emitter of greenhouse gas pollution caused by the burning fossil of fuels. China and India—the world’s first and third largest greenhouse gas emitters—also would likely balk at a binding agreement. (Read "When Snows Fail" in National Geographic magazine.)

View Images Anshan Steel works in China, said to be the largest such plant in the country. Photograph by George Steinmetz, National Geographic Creative

Instead, the next international climate deal is likely to be a collection of voluntary commitments to control greenhouse gas emissions by individual countries and groups of nations. That’s a problem, some environmentalists say, because the lack of legally binding requirements would make such a deal toothless. But others point to U.S. progress in reducing greenhouse pollution even without participating in Kyoto (emissions in 2012 were 10 percent below 2005 levels) as evidence that voluntary agreements can work.

The negotiations in Paris next year are expected to be intense because many nations, including the European Union, much of Africa, and island countries, are pushing for a binding treaty and deep emissions cuts. (Related: "Ahead of UN Climate Summit, Environmental Report Sees Economic Opportunities.")

This week’s summit will give country leaders a chance to signal how aggressive—or not—they will be in cutting emissions and in helping poor countries blunt the harm caused by droughts, sea-level rise, and other climate change effects. Other types of announcements to look for include pledges to protect forests, which trap a large amount of carbon dioxide. That greenhouse gas is released into the atmosphere when trees are cut down.

Bold proposals by some presidents and prime ministers this week could encourage bigger commitments by their counterparts in the months ahead; weaker ones could give other countries cover to pull their punches. How far many nations are willing to go will become clear by March of next year, when the world's big emitters are required to submit their plans to the UN as part of the process leading up to Paris.

Reading Tea Leaves

The big question about any international pact, experts say, is whether it will push countries to adopt cleaner energy sources or make other big changes that they wouldn’t otherwise undertake.

“The odds are high that there will be an agreement,” said David Victor, professor of foreign relations at UC San Diego. “What’s open is whether the agreement will have any impact on behavior. What you’re trying to do ultimately is to get countries to do more than they would do unilaterally.”

There is no draft agreement yet, and some experts say there is not enough time before Paris to hammer out a complicated treaty that would require deep cuts in greenhouse gases.

“I think it’s too late to have an agreement with big reductions in emissions,” Victor said.

Current and former Obama administration officials share his predictions.

“It’s doubtful that it will be a treaty,” said one senior Obama administration official, who was not authorized to speak on the record.

“Developing countries will be in tears over it,” added the official

That’s because, although the framework probably will include financing to help poor countries hurt by climate impacts, the pledges will not come close to meeting their needs, said the official. “It’s not going to do the job and save the planet.”

Some observers say it would be unrealistic to expect an international agreement to force major changes. Deep reductions in greenhouse gas emissions would require massive shifts away from coal, petroleum, and natural gas. There is no political consensus for such changes in the United States or other countries that are big emitters.

“We’ve got to be clear-eyed about what we can get,” said Nathaniel Keohane, a former adviser to President Barack Obama and a vice president of Environmental Defense Fund (EDF). “We shouldn’t be thinking that this issue will be solved in one fell swoop with a legally binding international treaty.”

The most obvious reason a treaty mandating specific cuts isn’t doable: not enough support in the U.S. Senate. Treaties must be ratified by two-thirds of the Senate. Most Republicans and some Democrats would be expected to vote no. (Related: "Climate Change May Put Half of North American Birds at Risk of Extinction.")

There are a host of other reasons that getting the whole world behind a single strategy is near impossible. Most of the future growth in emissions is projected to come from China, India, and other developing nations with fast-growing economies. Coal is the cheapest way to provide the electricity needed to modernize their societies and grow their economies.

Canada likely would resist a new treaty for the same reasons it withdrew from the Kyoto Protocol three years ago. It didn’t want to face pollution limits when the world’s biggest emitters—China and the United States—didn’t. In addition, Canada’s leaders didn’t want to limit their country’s booming oil sands industry, which is a large and growing source of jobs and revenue as well as a source of greenhouse gas emissions.

However, Keohane says a looser agreement could “build credibility and trust” and over time prod countries to make increasingly aggressive commitments both to limit emissions and help poor countries adapt to climate change.

“You have to come back and revisit it and tighten it and tighten it again,” said Peter Ogden, a former State Department and White House official in the Obama administration who now directs climate and energy programs at the Center for American Progress, a liberal Washington-based think tank.

Most Vulnerable Countries

Many nations, especially European countries and the island nations that are most threatened by rising seas, will fight for a more ambitious agreement.

They want both a legally binding treaty and the major reductions in greenhouse gases necessary to put the world on track to stay below a 2°C (3.6°F) increase in temperature from industrial levels. That’s the goal nations agreed to in the 2009 Copenhagen Accord to prevent “dangerous” interference with the climate.

“It’s a tug-of-war right now,” said Ronny Jumeau, ambassador for climate change for the island nation of Seychelles and spokesman for a group of 43 small island nations. “We refuse to accept that someone says it cannot be legally binding and everybody has to live with it because they’re so powerful.”

Many island nations already struggle with impacts from climate change to their freshwater supplies, fisheries, and agriculture; over the long term, sea-level rise threatens to put many of them underwater.

“The voluntary stuff will never be enough,” Jumeau says. “We are still headed to destruction.”

Connie Hedegaard, European Union's climate action commissioner, says voluntary commitments can be put aside in tough economic times or when new leaders take power. Europe, which understands the limitations President Obama is under because of the Senate, is pushing for a “hybrid” agreement that would include both mandatory and voluntary elements, she says.

“It matters when things are not just nice intentions but when it’s something you can count on will be done,” she says.

Other recent scientific reports also make the case for bold action. Global emissions keep rising, and the concentration of greenhouse gases in the atmosphere hit a new record high in 2013, according to a report this month from the World Meteorological Organization (WMO). The annual increase in concentration was the greatest since 1984, perhaps because the world’s oceans are no longer able to absorb as much carbon dioxide as they used to from the burning of fossil fuels. (Read: "Greenhouse Gases Hit Record High Amid Fears of CO2 Saturation Point.")