Political leaders, diplomats and negotiators are meeting for two weeks of talks amid a growing sense of urgency.
It began with the bang of a gavel in a cavernous hall on the outskirts of Madrid.
The latest round of UN climate talks. More than 25,000 people, more than 50 heads of state.
And an urgent call to action from the UN Secretary General, Antonio Guterres.
"The decisions we make here will ultimately define whether we choose a path of hope or a path of surrender and remember we made a commitment to the people of the world through the Paris agreement and it was a solemn promise," Antonio Guterres said.
"Let us open our ears to the multitudes who are demanding change. Let us open our eyes to the imminent threat facing us all. There is no time and no reason to delay," he said.
This year's talks come amid a growing sense of crisis. Bushfires, cyclones, fast retreating glaciers. A wave of student-led protests around the world.
In the past two weeks, there've been more dire warnings about the state of the climate, with greenhouse gases hitting another record high last year.
The president of the Marshall islands, Hilda Heine, said her string of atolls in the north Pacific are inundated, hundreds are in evacuation centres.
"As we speak hundreds of people have evacuated their homes as the large waves caused the ocean to inundate parts of our capital in Majuro last week," Hilda Heine said.
"As a nation we refuse to flee. But we also refuse to die."
The talks hope to finally thrash out the rules around how to implement the Paris agreement, which was signed four years ago.
The two main hurdles will be how to build and regulate a mechanism for trading carbon, and how to compensate countries for loss and damage.
It's also the final meeting before the year 2020, when countries are supposed to come back with more ambitious targets.
Tagaloa Cooper is director of climate resilience at the Pacific intergovernmental agency SPREP. She says this meeting's critical.
"In terms of where we live, how we are affected, how we are impacted and how we stand to lose a whole lot more than people understand if we do not raise ambitions. So a lot is on the line for us," Tagaloa Cooper said.
However, the next two weeks won't be smooth.
The elephant not in the room is the United States, which has signalled it will pull out of the Paris Agreement by this time next year.
Then there's the fact that most countries - especially the big ones - are far from meeting their targets from four years ago and Antonio Guterres is far from impressed.
"We stand at a critical juncture in our collective efforts to limit dangerous global heating. Do we really want to be remembered as the generation that buried its head in the sand, that fiddled while the planet burned?," Mr Guterres said.
The Marshall Island's Hilda Heine put it another way - countries who refuse to do more are inflicting a death sentence on her country.