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The United States recognizes its role in contributing to climate change and its responsibility to address it, U.S. President Barack Obama said at the COP21 in Paris on Monday. But it is not just a U.S. problem, he said. Obama said that a global climate change agreement should have updating targets. “Here in Paris, let’s secure an agreement that builds in ambition, where progress paves the way for regularly updated targets,” he said.

[Previous story, posted at 7:07 a.m.]

(CNN) — The crucial global summit on climate change started Monday in Paris with a moment of silence to honor the victims of the city’s terror attacks.

Shortly afterward, French President Francois Hollande acknowledged the magnitude of both events.

“France is receiving 150 heads of state,” the President said. “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.”

Hollande said that while the fight against terrorism and the fight against climate change are separate battles, “these are two great challenges we must rise to.”

Ban Ki-moon

UN Secretary-General Ban Ki-moon followed him and told gathered world leaders they faced a unique window of opportunity.

“A political moment like this may not come again,” he said. “We have never faced such a test. But neither have we encountered such great opportunity. You have the power to secure the well-being of this and succeeding generations. I urge you to instruct your negotiators to choose the path of compromise and consensus.”

Legally binding agreement

The world leaders are meeting in Paris with one mission: Agree on legally binding reductions in greenhouse gas emissions.

As global average temperatures soar, they will try to get a handle on the cause.

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.

COP stands for Conference of Parties, an annual forum to try to tackle climate on a global political level.

Terror, security, clashes

The COP21, also named the Paris Climate Change Conference, kicked off under the specter of the November 13 terror attacks in Paris that killed 130, and world leaders will observe a moment of silence in honor of the victims Monday.

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, bottles; and even candles, police said, were taken from memorials to those killed in the terror attacks, and police 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.

French President Hollande called the clashes “scandalous” and said authorities knew “troubling elements” would arrive in Paris for the talks and said that was 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 on 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 the history of the COP illustrates the challenge in achieving this year’s goal of a legally binding agreement — especially when it comes to the biggest greenhouse gas emitters.

Probably the best-known milestone to come out of a previous COP was the adoption of the Kyoto Protocol, a nonbinding agreement by 192 states 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.

It has taken 20 years of U.N. negotiations to reach this attempt at a legally binding global emissions agreement to prevent a global average temperature rise of 2 degrees Celsius higher than during preindustrial times, COP organizers have said.

More than 40,000 delegates from 195 countries are attending COP21.