Nearly 100 world leaders met at a summit at the United Nations Tuesday to put their political weight behind achieving a global climate treaty later this year when they meet in the Danish capital.

U.N. Secretary-General Ban Ki-moon launched the unprecedented summit with a warning to world leaders that their decisions would have "momentous consequences." "The fate of future generations, and the hopes and livelihoods of billions today, rest, literally, with you," he said.

He urged action this December when the international community meets in Copenhagen. Mr. Ban said a successful deal would be one that includes ambitious reduction targets from industrialized countries and commitments on emissions from developing ones. He said it should also include financial and technological support for emerging countries.

In his first speech before the United Nations, President Barack Obama said the United States understands how serious the threat of climate change is and is ready to respond, but he did not offer any new proposals.

Instead, he pointed to U.S. efforts to invest in alternate forms of clean and renewable energy, such as wind and solar power, and to reduce carbon pollution. He also pointed to efforts to pass important climate legislation in the U.S. Congress.

"Taken together, these steps represent an historic recognition on behalf of the American people and their government. We understand the gravity of the climate threat. We are determined to act. And we will meet our responsibility to future generations," he said.

The United States and China are each responsible for about one-fifth of carbon emissions worldwide, so it came as good news when China's President Hu Jintao announced plans to cut emissions significantly by 2020, and to vigorously develop renewable and nuclear energy.

Japan's new Prime Minister Yukio Hatoyama reaffirmed his campaign pledge for Japan to reduce its greenhouse gas emissions 25 percent from 1990 levels by 2020.

African nations have contributed the least to global warming but are at greater risk from its effects and are without adequate resources to respond to its challenges. Rajendra Pachauri, chair of the Inter-governmental Panel on Climate Change warned that without significant progress to lessen the impact of climate change, Africa would be disproportionately affected.

"In Africa, by 2020, between 75 and 250 million are projected to be exposed to water stress due to climate change, and in some countries yields from rain-fed agriculture could be reduced by up to 50 percent. The impacts of climate change would be disproportionately severe on some of the poorest regions and communities in the world," Pachauri said.

But perhaps one of the most distressing calls for action came from President Mohamed Nasheed of the Maldives, whose island nation is at risk from rising sea levels.

"If things go business-as-usual, we will not live, we will die. Our country will not exist. We cannot come out from Copenhagen as failures. We cannot make Copenhagen a pact for suicide. We have to succeed and we have to make a deal in Copenhagen," he said.

The call for action at the summit did not just come from world leaders - it came from the generation with the most to lose if climate talks fail - the world's youth. Yugratna Srivastava, a 13-year-old Indian girl, spoke forcefully at the podium, sending a wake-up call to those assembled.

"We received a clean and healthy planet from our ancestors and we are gifting a damaged one to our successors. What sort of justice is this," she said.

She said the three billion young people in the world need them to take action now to protect the planet for future generations.