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(CNN) World leaders opened pivotal climate talks Monday in Paris saying the stakes are too high to end the conference without achieving a binding agreement to help slow the pace of global climate change.

"A political moment like this may not come again," U.N. Secretary-General Ban Ki-moon told leaders gathered for the conference. "We have never faced such a test. But neither have we encountered such great opportunity."

The talks began with a moment of silence for victims of the November 13 terror attacks in Paris, and the tragedy served as a touchstone for world leaders urging unity and action.

"What greater rejection of those who would tear down our world than marshaling our best efforts to save it," President Barack Obama said in his speech.

French President Francois Hollande noted that "never have the stakes been so high because this is about the future of the planet, the future of life."

"And yet two weeks ago, here in Paris itself, a group of fanatics was sowing the seeds of death in the streets," he said.

Legally binding agreement

Leaders of 150 nations, along with 40,000 delegates from 195 countries, are attending the conference, called COP21 . COP stands for Conference of Parties, an annual forum to try to tackle climate change on a global political level.

The leaders have one mission: Agree on legally binding reductions in greenhouse gas emissions meant to hold global average temperatures short of a 2 degrees Celsius increase over preindustrial global temperatures.

Speaking aboard the papal plane on his way back to Rome, Pope Francis said that the time to do something was now or never.

"We are on the brink. We are on the brink of a suicide, to use a strong word, and I am sure that most of those at the COP have this conscience, and want to do something," he said.

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On Monday, the leaders of the main players necessary to achieve the ambitious goal -- China and the United States -- sat down together at the COP21 . They are the largest producers of greenhouse gases

Obama told the conference that the United States recognizes its role in creating climate change and its role in solving the issue.

But he said the agreement should be global in nature, assertive and flexible.

"Here in Paris, let's secure an agreement that builds in ambition, where progress paves the way for regularly updated targets," he said.

He also addressed economic issues associated with climate change, saying recent economic growth in the United States has come despite a lack of growth in carbon emissions, proving that climate advancements need not come at the expense of the economy or individual livelihoods.

"That's what we seek in these next two weeks -- not simply an agreement to roll back the pollution we put into the skies, but an agreement that helps us lift people from poverty without condemning the next generation to a planet that is beyond its capacity to repair," he said.

He also said developed countries must help island nations and others that have contributed little to climate change but are the first to be feeling its effects.

Chinese President Xi Jinping said that the conference "is not a finish line, but a new starting point" and that any agreement must take into account the differences among nations.

"Countries should be allowed to seek their own solutions, according to their national interest," he said.

Russian President Vladimir Putin called climate change "one of the greatest threats humanity is facing."

"Russia not only prevented the increase of greenhouse emissions, it has reduced them," he said, promising a 70% reduction in emissions from 1990 levels by 2030.

Indian Prime Minister Narendra Modi called for a "comprehensive, equitable and durable agreement that leads us to restore balance between humanity and nature."

And German Chancellor Angela Merkel reminded the leaders of the "billions of people pinning their hopes on what we do in Paris."

"Let us do everything we can not to dash those hopes," she said.

Terror, security, clashes

The conference is being held amid even more intense security than usual for such a gathering of global leaders following the terror attacks that left 130 people dead this month.

Activists laid out shoes where demonstrators might have stood.

Authorities have clamped down on anti-global warming demonstrations in the city out of security concerns.

Nevertheless, disappointed demonstrators turned out Sunday, and brief clashes erupted with police at the Place de la Republique, where peaceful protesters had placed rows of shoes and name tags to represent the crowds not allowed to show up.

But violent protesters pelted officers with shoes and bottles. Police said that protesters even threw candles taken from memorials to those killed in the terror attacks , and that they arrested more than 200 people.

Paris police Chief Michel Cadot said taking the candles and using them against police showed "an extreme lack of respect to those events."

Riot police responded with tear gas.

Hollande called the clashes "scandalous." The French President said authorities knew "troubling elements" would arrive in Paris for the talks, and that is why "these sorts of assemblies were banned and some were ordered to stay home."

In many countries, people gathered to protest against human-made climate change Sunday. There is a broad consensus among scientists that global warming is driven by human activity, foremost the burning of fossil fuels.

Previous failures

A look at previous global climate change negotiations illustrates the challenge in achieving this year's goals, especially when it comes to the biggest greenhouse gas emitters.

Photos: Climate change is real Photos: Climate change is real A single summer stole more than 200 feet (60 meters) of ice from the snout of Sólheimajökull, a glacier in Iceland, seen in April 2006. Hide Caption 1 of 11 Photos: Climate change is real What a difference a few months made for Sólheimajökull, seen from the same vantage in October 2006. The glacier has withdrawn a third of a mile (0.5 kilometer) over the past decade, in step with rising temperatures. Hide Caption 2 of 11 Photos: Climate change is real Years ago, says a veteran skier, "conditions were fantastic" at the world's highest ski area, on 17,250-foot (5,260 meters) Chacaltaya Glacier near La Paz, Bolivia. Hide Caption 3 of 11 Photos: Climate change is real Today, few attempt the ski run down Bolivia's Chacaltaya Glacier, even after a snowfall. The glacier has shriveled in the past decade, turning much of the slope into a boulder field. Hide Caption 4 of 11 Photos: Climate change is real As icons such as Montana's once well-formed Grinnell Glacier vanish, "people feel a sense of loss," says Dan Fagre of the USGS. Hide Caption 5 of 11 Photos: Climate change is real Montana's Grinnell Glacier, a robust body of ice decades ago, is melting. Hide Caption 6 of 11 Photos: Climate change is real Mount Everest's East Rongbuk Glacier lost some 350 vertical feet of ice between August 1921 (above) and October 2008 (next). Hide Caption 7 of 11 Photos: Climate change is real Mount Everest's East Rongbuk Glacier lost some 350 vertical feet of ice between August 1921 (previous) and October 2008 (above). Hide Caption 8 of 11 Photos: Climate change is real Columbia Glacier in Alaska has retreated 11 miles since 1980. Since then, it has diminished vertically an amount equal to the height of New York's Empire State Building. Hide Caption 9 of 11 Photos: Climate change is real Columbia Glacier in 2012. Hide Caption 10 of 11 Photos: Climate change is real The November 2015 issue of National Geographic magazine. Hide Caption 11 of 11

Probably the best-known milestone to come out of a previous conference was the adoption of the Kyoto Protocol, a nonbinding agreement by 192 parties to lower greenhouse gas emissions to 5% below 1990 levels between 2008 and 2012.

The United States did not ratify the Kyoto Protocol and dropped out of it completely in 2001. Canada dumped it, too, and China, India and other developing countries were exempt from it.