The first frictions that will mark two weeks of climate talks were already apparent just minutes after the formal launch of the COP21 summit on Monday, as leaders spelled out their visions of what a climate deal should look like.

The conference kicked off with an ambitious call for even tougher emissions reduction goals — an effort to push countries to do much more than what they’ve already pledged.

“Our current ambition must be the floor not the ceiling for future efforts,” said Ban Ki-moon, the U.N. chief.

He pointed out that the promises already made by about 180 countries were “a very good start” but that “we need to go much further, much faster” if the world is to avoid warming by more than the 2 degrees Celsius most climate scientists say marks a danger point.

He was echoed by François Hollande, the French president, who has staked his reputation on a successful outcome of the climate conference.

“The greatest danger is not that we aim too high and that we miss. The greatest danger is that we aim too low and hit it,” Hollande said.

Angela Merkel, the German chancellor, wanted a deal that is “comprehensive, fair and binding.”

But that goal is going to be tough to push past some of the world’s largest countries — and biggest emitters.

U.S. President Barack Obama gave a soaring speech that ran far longer than the three minutes allotted to each leader, in which he spelled out the urgent need to tackle climate change to avoid a world with “floods of desperate peoples seeking sanctuaries of nations not their own.” He called for a deal that ramped up ambition over time, but significantly avoided mentioning whether the agreement should be legally binding. That’s a non-starter for the Republican-dominated Congress.

For his part, Chinese President Xi Jinping emphasized that rich countries should shoulder much of the burden of steps to reduce the risk of global warming. Paris should accommodate the “national conditions” of various countries, he said.

That underscores the two thorniest issues in Paris — how much of any agreement will be legally binding, and who should do the heavy, and expensive, lifting in halting climate change.

In all, 147 leaders were slated to speak on Monday. Their presence is an indication of the weight the world ascribes to a negotiating process aimed at preventing catastrophic global warming.

While no breakthroughs are anticipated out of Monday’s set-piece speeches, the leaders are expected to provide momentum for the technical negotiations, which are set to last until December 11.

The summit’s goal is to create a new treaty on climate policies, including emissions reductions as well as financial aid, with the aim of shifting away from fossil fuels and compensating countries that have already suffered loss and damage from climate change.

Though such gatherings are always held under intense security, the terrorist attacks in Paris have heightened jitters and left a somber backdrop for the summit.

Indeed, as urgent as the climate discussions are, leaders are also sending a political message. By meeting in Paris little more than two weeks after the attacks, leaders hope to signal not just solidarity with France, but also a refusal to allow terrorism to divert the global agenda.