World leaders and lawmakers are in Paris today for the latest United Nations climate summit, where they hope to determine an international plan to halt the effects of man-made climate change.

The two-week affair, during which each of the 150 leaders will lay out their negotiating terms, is seen as a last-ditch effort to formulate a coordinated global strategy against climate change, at a time when experts say that the planet is on the cusp of being fastened onto an irreversible course of destructive and dangerous warming.

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Here are five reasons why the Paris summit, known as COP21, has been so anticipated, and why people are talking about it.

1. "Never have the stakes been so high"

That's a quote from French President François Hollande, who was speaking on the importance of the timing of COP21 Monday. Hollande and others have noted the critical timing of the summit.

Lawmakers have a chance to agree on a plan of action to avoid surpassing the two-degrees Celsius warming mark—beyond which climate scientists say some of the worst effects of climate change will be unavoidable. Two degrees refers to the rise in Celsius above preindustrial temperatures, and entered the lexicon of climate change in the 1990s.

While some experts are skeptical that keeping warming below the two degree threshold is a realistic goal—we're already well on our way to surpassing it—world leaders are vigilant that sufficient action is necessary and attainable. "Your presence here is a sign of hope," said Hollande on Monday.

2. Climate change is framed as a deadly threat, a force of evil

The deadly attacks in Paris earlier this month, in which 130 were killed and hundreds more injured, loomed large on the opening day of the COP21 negotiations. But as much as they contributed to a blanket of security concerns and fear in the city of lights, the attacks played into a narrative of overcoming fear and tragedy, and resilience in the face of uncertainty.

"We have come to Paris to show our resolve," said President Obama on Monday, "to protect our people, and to uphold the values that keep us strong and keep us free. We salute the people of Paris for insisting that this crucial conference will go on."

"[We want] a declaration that, for all the challenges we face, climate change will define the contours of this century more dramatically than any other," he continued, adding that the summit itself was a "rejection of those who would tear down our world."

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3. Legally binding agreement

The aim of the conference is "to achieve a legally binding and universal agreement on climate" that would keep warming below the two degree mark. It's unclear what the outcome of the talks will be, but the terms of the negotiations and a blueprint for how to enforce them seem pivotal to the success of any agreed-upon action against climate change.

On Monday, leaders seemed committed to building out an ambitious plan—but also one that allows for some wiggle room. "Here in Paris, let's secure an agreement that builds in ambition, where progress paves the way for regularly updated targets," Pres. Obama said.

Others from pollutant-heavy nations seemed to echo the sentiment. India's Prime Minister Narendra Modi said there is a need for a "comprehensive, equitable, and durable agreement that leads us to restore the balance between humanity and nature," while Russian President Vladimir Putin said that climate change is "one of the greatest threats humanity is facing."

"Countries should be allowed to seek their own solutions, according to their national interest," said Chinese President Xi Jinping.

4. Previous failed summits

As its name suggests, COP21 is the latest in a series of annual international conventions, which have been held to stave off climate change since the 1990s. It's also one reason some remain skeptical that meaningful change can emerge, as world leaders have tried and to some degree failed to take decisive action 20 previous times.

That doesn't mean there's no potential for meaningful strides, however. One other frequently cited summit in 1997, in Kyoto, Japan, produced something of a landmark regulatory recommendation against carbon dioxide production. Known as the Kyoto Protocol, the agreement sought to place limitations on carbon production in signatory countries in 2005. The U.S., under the Bush administration, fought back against the agreement, and dropped out in 2001. Though many other countries signed on, major polluters such as China, India, and Canada were exempted, and the treaty didn't become international law until about halfway through its 22-year reduction window.

5. The world's top two polluters are actively involved in negotiations

Unlike the Kyoto Protocol, which saw the world's top two polluters—the U.S. and China—unbeholden to its restrictions, COP21 includes the notable participation of both countries. President Xi Jinping said it was "very important for China and the United States to be firmly committed to the right direction of building a new model of major country relations[.]" The partnership, he said, was important "to help the climate conference deliver its expected targets."

"As the two largest economies in the world and the two largest carbon emitters, we have both determined that it is our responsibility to take action," said Obama, echoing the Chinese president's words. "And so our leadership on this issue has been absolutely vital, and I appreciate President Xi's consistent cooperation on this issue."

The two nations came to a historic agreement on capping emissions almost exactly one year ago, though those promises have since been complicated by unfortunate realities: earlier this month, the New York Times reported that China had been underreporting coal consumption, which data suggested had risen by up to 17 percent each year.

Check back at ATTN: for more reporting on climate change and COP21.