The gathering of world leaders in Paris yesterday is seen as perhaps the last chance to clinch a global deal to fight climate change after more than two decades of fraught negotiations.
With 2015 a near certainty of becoming the hottest year on record, the leaders are expected to press negotiators to seal an ambitious pact that commits all nations to cutting emissions blamed for heating up the planet.
The leaders and heads of state and government from more than 150 nations sounded the alarm on the need for a deal that steers the planet away from rapidly rising temperatures that could derail economies and trigger mass migration that could dwarf the current migrant crisis in Europe.
"History is calling," United Nations Secretary-General Ban Ki Moon told leaders and delegates. "This is a pivotal moment for the future of your countries, your people and our common home, our planet. You can no longer delay," he said, urging negotiators to reach agreement by the end of the 12-day talks that are meant to end on Dec 11 but are likely to run into overtime.
French President Francois Hollande, as host, said: "Never have the stakes of an international meeting been so high because it concerns the future of the planet, the future of life. The hope of all of humanity rests on all of your shoulders."
US President Barack Obama and Chinese President Xi Jinping both stressed that terrorism would not stop efforts to fight climate change. Both pledged yesterday to work towards
getting a deal in Paris.
"We have come to Paris to show our resolve," Mr Obama said as he paid tribute to those killed in the Nov 13 terror attacks in the city. "Nothing will deter us from building the future we want for our children."
In an impassioned speech, he said the Paris talks should lead to a world that is worthy of the next generation. "The growing threat from climate change could define the contours of this century more dramatically than any other. And what should give us hope that this is a turning point, that this is the moment we finally determine we would save our planet."
Mr Xi called on negotiators to seal a binding agreement to fight climate change in Paris and expressed confidence that the conference would yield "satisfying results".
He urged a collaborative approach to the greening of economies and more support for vulnerable poorer nations. He also said the principle of recognising developing nations had common but differentiative responsibilities from rich nations should be adhered to, a reference to a principle that has proved very divisive in UN climate talks.
Earlier in the day, leaders swept into the vast purpose-built conference centre on the north-east edge of Paris that is being guarded by nearly 3,000 security personnel and protected by a ring of steel fencing. Some motorways were locked down for the myriad motorcades.
They gathered for a group photo and then from noon until deep into the night, each leader made short speeches in two large plenary rooms. In between, leaders also held bilateral meetings, though Russian President Vladimir Putin rejected a meeting with his Turkish counterpart Recep Tayyip Erdogan, as a dispute rages over Ankara's downing of a Russian warplane, Agence France-Presse reported.
Once the leaders leave, most by today, negotiators and ministers get down to the real work of thrashing out deep differences, such as climate finance for poorer nations and a system to transparently monitor and verify all national climate action plans.
While the outlines of a deal in Paris are clear, a lot of hard-bargaining lies ahead, negotiators say.