NEW YORK—Speaking to a United Nations climate summit that drew more than a hundred heads of state, Secretary-General Ban Ki-moon on Tuesday urged the world's leaders to "do their fair share" to protect the planet by limiting global warming to 2°C.

"The human, environmental, and financial cost of climate change is fast becoming unbearable," Ban said in an address to the UN General Assembly.

U.S. President Barack Obama told world leaders that the United States has reduced emissions more than any other country on Earth over the past eight years. "There should be no question that the United States of America is stepping up to the plate," he said.(Read "When the Snows Fail" in National Geographic magazine.)

Reporting that the U.S. is on track to cut emissions to 17 percent below 2005 levels by 2020, Obama said he had spoken with Chinese leaders on the need for the world’s two largest emitters to do much more.

"We have a special responsibility to lead," Obama added. "That's what big nations do."

The president's comments were short on specifics, which drew criticism from environmental activists.

Bill McKibben, co-founder of the climate awareness group 350.org, ran with President Obama's baseball analogy, saying that he had "dropped down a bunt single when we're behind by ten runs in the ninth inning."

Ban convened the summit to push world leaders to take bolder actions to cut greenhouse gas emissions, stop deforestation, and help poor countries withstand the ravages of climate change such as severe drought, deadly storm surges, and sea-level rise.

"No one is immune from climate change. Not even these United Nations headquarters, which were flooded during Superstorm Sandy," Ban said, referring to the 2012 hurricane that flooded low-lying areas in the northeast U.S. and cut off power to millions of home and businesses. (Read "Rising Seas" in National Geographic magazine).

Wrangling Over Strategy

Ban hoped to create a spirit of one-upmanship, with presidents and industries competing to launch more ambitious policies to fight climate change in the 15 months before world leaders gather in Paris at a meeting of the United Nations Framework Convention on Climate Change.

In the meantime, France is leading the effort to negotiate a new, more rigorous strategy for combating global warming.

French President Francois Hollande said France agreed to do the job because global warming is "the only topic which over the long run really threatens our planet."

Hollande announced that France will spend $1 billion (U.S.) in the next few years to help poor countries invest in clean power and protect themselves from deadly consequences of climate change like droughts and storm surges. He challenged his counterparts to do the same in the coming months.

Neither China nor India, the third largest emitter, sent their heads of state to the summit. Chinese Vice Premier Zhang Gaoli, who represented his country at the event, said his country is doing its part by investing in clean power, using electricity more efficiently, and expanding its forests.

In a summit short on tangible results, the biggest announcement was a pledge by dozens of countries and big food and paper producers to cut the loss of forests in half by 2020 and stop deforestation by 2030.

View Images Farmers in Brazil cleared a part of the Amazon rain forest to grow corn. Photograph by George Steinmetz, National Geographic Creative

"Consumers have sent companies a clear signal that they do not want their purchasing habits to drive deforestation, and companies are responding," said Paul Polman, chief executive officer of Unilever, a consumer products company that signed on to the agreement.

But the initiative was hamstrung from the start because Brazil, home to the vast Amazon rain forest, did not sign on.

"I think that it's impossible to think that you can have a global forest initiative without Brazil on board. It doesn't make sense," Brazilian Environment Minister Izabella Teixeira told the Associated Press.

Greenhouse Gases Rising

The international community has been negotiating strategies to limit greenhouse gas emissions for more than 22 years. Yet 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.

"During all of this time, notwithstanding the focus, we are not meeting the challenge," U.S. Secretary of State John Kerry said on Monday. As a senator, Kerry was a leader in congressional efforts to pass climate legislation, but the attempts never succeeded. (Related: "Climate Change May Put Half of North American Birds at Risk of Extinction.")

On Monday Kerry said that if global sea levels rose by a meter by the end of the century, as some scientists predict, it would put 20 percent of greater New York City under water.

"Just one meter would displace hundreds of millions of people worldwide and threaten billions in economic activity," he said.

Kerry also said he is "personally committed" to keeping the issue at the center of U.S. diplomatic efforts.

Treaty Faces Long Odds

Countries have agreed to limit global warming to 2°C (3.6°F) to avoid the dangerous consequences of climate change, such as melting ice sheets.

In April the UN's Intergovernmental Panel on Climate Change reported that the best science shows that to limit global warming to 2°C would requirereductions of greenhouse gas emissions of 40 to 70 percent by 2050.

To put the world on that track, the European Union, much of Africa, and many island countries are pressing for a binding treaty that would mandate significant reductions of global greenhouse gases.

But that would likely exclude the United States, because Republicans and some Democrats in the Senate would oppose it. Treaties need to be ratified by two-thirds of the Senate.

Current and former administration officials say the Paris agreement is likely to be a collection of voluntary commitments.

"It's a tug-of-war right now," says Ronny Jumeau, ambassador for climate change for the island nation of Seychelles and spokesman for a group of 43 small island nations.

Many island nations already struggle with impacts from climate change on 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."