But the conference this year could be called the "COP of Youth," as it was not only delegates who spoke about the climate crisis but young activists from around the world.
Activists travelled to Madrid to address what they say is their generation's defining issue.
Swedish activist Greta Thunberg has emerged as the movement's unofficial leader.
"Finding holistic solutions is what the COP should be all about," she said. "Instead, it seems to have turned into some kind of opportunity for countries to negotiate loopholes and to avoid raising their ambition."
But the UN talks are also giving rise to new voices.
At a news conference, young people from developing nations made a plea to delegates to understand the emergency. Their countries are vulnerable to torrential rains and rising sea levels linked to climate change.
"My home is only two meters above water. With the threat of climate change, we'd lose two meters of our culture, " said Carlon Zackhras, a climate activist from the Marshall Islands.
"We have a massive decline in fisheries that's inevitable with the rise in sea level and rising temperatures," said Kisha Erah Muana, a Filipino climate activist.
A 22-year-old Ugandan student, Hilda Flavia Nakabuye, said she was in Madrid to represent the millions of young Africans affected by climate change.
"You have dreams, but we have dreams too," she said. "But these dreams have turned into nightmares of our own future, which is being negotiated right now by the world leaders without our say."
Hilda's activism took root two years ago, when heavy rain followed by continuous dry spells devastated her village. Her family had no choice but to sell their land and livestock to survive. She said the threat of extreme weather is only increasing.
Hilda took center stage at high-level COP25 meetings to deliver a stern message.
"Dear leaders, we need leadership on climate action, not talks," she said. "For how long will you keep negotiating? You've been negotiating for the last 25 years, even before I was born."
She received a standing ovation, but Hilda said it's not about generating attention, but guaranteeing action.
Sweden's Environment and Climate Minister, Isabella Lovin, backed the views of the activists.
"The young generation have every right to speak out now and to be angry with the leaders that haven't reacted to the facts," she said. "If we don't cut emissions it will be catastrophic for all future generations."
The European Union unveiled its own climate policies the day after Hilda's speech.
The bloc is aiming to strengthen its greenhouse gas emissions targets to at least a 50% cut from 1990 levels by 2030.
Hilda's battle is just beginning. She said when she returns home she'll continue advocating for a better future, keep watching, and keep speaking for the millions of people who are suffering from the climate crisis.
The conference concluded with delegates unable to find common ground on rules for implementing the Paris Agreement. Major polluters resisted calls for ramping up efforts to halt global warming, and negotiators postponed debate about rules for an international carbon market.
UN Secretary-General Antonio Guterres said he was disappointed by the result.
But some experts say it was better to delay any decision than accept rules that would compromise the Paris Agreement.
The Paris accord goes into effect next year, but the world is already experiencing weather disasters including torrential rains, wildfires and record heat waves.
Time for discussion is running out.