PARIS - Declaring that the future of the planet is at stake, more than 150 world leaders assembled outside Paris on Monday to launch an ambitious attempt to confront climate change by reducing greenhouse gas emissions.

After decades of difficult negotiations and an unsuccessful attempt to strike an agreement six years ago, President Obama said a turning point may have been reached.

"What should give us hope," he told fellow leaders, "is the fact that our nations share a sense of urgency about this challenge and a growing realization that it is within our power to do something about it."

The United Nations conference comes at a time of record high temperatures, more extreme droughts and storms, shrinking glaciers, and melting ice packs - events that have helped make climate science more widely accepted. That, coupled with major advances in cleaner energy sources, has increased the willingness of some of the world's biggest polluters to act.

The leaders of nations responsible for more than 95 percent of the world's greenhouse gas emissions arrived at the conference at Le Bourget, on the northern edge of Paris, armed with plans to reduce their outputs. But organizers warned there was still a long way to go.

Critically, the proposals on the table are not projected to limit rising global temperatures to 2 degrees Celsius above preindustrial levels, the threshold at which scientists believe most of the worst effects of climate change could be avoided. It's unclear whether the gap between those proposals and the 2-degree goal can be closed in the next two weeks.

"The future of your people, the future of the people of the world, is in your hands," U.N. Secretary-General Ban Ki-moon told leaders as the talks kicked off. "We cannot afford indecision, half measures, or merely gradual approaches. Our goal must be a transformation."

The high-level meeting began with a moment of silence for the victims of recent deadly attacks in France, Lebanon, Tunisia, and elsewhere - a wave of violence that threatened to overshadow longer-term climate concerns.

"These tragic events . . . force us to concentrate on the essentials," French President Francois Hollande said. "We must leave our children more than a world free from terror. We must leave them a planet that is preserved from catastrophes, a viable planet."

Major issues have still to be resolved, however.

Developing countries have long argued that countries that grew rich through the use of fossil fuels should assume the burden of shifting poorer economies to solar, wind, and other renewable energy sources without hurting their potential for growth.

Addressing the conference Monday, President Xi Jinping of China said the deal reached in Paris must recognize differences between developing and more established economies, and include aid for poor countries.

"Addressing climate change should not deny the legitimate needs of developing countries to reduce poverty and improve living standards," he said.

Indian Prime Minister Narendra Modi said developed nations could make the biggest difference to the environment. "National commitments must be consistent with the carbon space nations occupy," he said.

As the summit got underway, the capitals of India and China were blanketed in smog.

Modi said his country is seeking to produce 40 percent of its power from nonfossil fuels, but said wealthier nations have a responsibility to make cleaner energy affordable and accessible in the developing world. "This is in our collective interest," he said.

Many leaders who spoke Monday want the agreement to include legally binding targets. That could be a deal-breaker for the United States, which needs approval from the GOP-controlled Senate to sign a binding treaty.

In his address to the conference, Obama acknowledged that taking strong action on the environment has not always been a political winner. But he noted that the U.S. and other global economies have grown even as fossil-fuel emissions have leveled off.

"We have proved that strong economic growth and a safer environment no longer have to conflict with one another," he said. "And that should give us hope."

Speaking at a heavily guarded conference center, Obama said the determination of world leaders to act as one in pursuit of a common goal was itself a rebuke to the Islamist extremists who killed 130 people in Paris on Nov. 13.

Obama highlighted steps that his administration has taken to curb emissions, and praised the many other nations that have committed to do the same in the lead-up to the summit.

"Our task here in Paris is to turn these achievements into an enduring framework for human progress - not a stopgap solution, but a long-term strategy that gives the world confidence in a low-carbon future," Obama said. "That's what we seek in these next two weeks, not simply an agreement to roll back the pollution we put into our skies, but an agreement that helps us lift people from poverty without condemning the next generation to a planet that's beyond its capacity to repair."

By inviting Obama and other leaders to the opening days of the summit, organizers hoped to send a clear signal: Paris is not Copenhagen.

A similar effort to reach a binding agreement to address climate change had all but collapsed by the time Obama and his counterparts arrived in the Danish capital near the conclusion of the summit in 2009.

Officials hope the participation of world leaders at the start of this year's conference, with commitments for action in hand as they arrive, will provide the momentum needed to reach a strong accord, even if it falls short of the 2-degree target.

A number of initiatives were announced by governments and private investors Monday, including billions of dollars in investments to support the development of clean energy technologies and to help the most vulnerable countries cope with the effects of climate change.