UN secretary-general Antonio Guterres on Sunday said he was disappointed by the outcome of two weeks of climate talks that ended in Madrid with a deal described by activists as watered down and weak.
After two extra days and nights of negotiations, delegates from almost 200 nations eventually agreed on a battle plan to tackle global warming.
The plan calls for improved carbon cutting targets by 2020 when the Paris accord kicks in.
The summit in Madrid was meant to have finalised the rulebook for that 2015 deal, which enjoins nations to limit global temperatures to below two degrees Celsius.
That did not happen. It leaves the UK, as a co-host of next year's talks, with a diplomatic mountain to climb to iron out all the disputes that marred COP25.
"I am disappointed with the results of COP25," said Guterres. "The international community lost an important opportunity to show increased ambition on mitigation, adaptation and finance to tackle the climate crisis.
What derailed the talks?
Splits emerged over how to create a global carbon trading market, an issue that scuppered the last climate talks in Poland, and which has again been delayed until the next meeting in Glasgow.
The Madrid summit--moved at the last minute from Chile due to unrest--at times teetered on the brink of collapse as rich polluters, emerging powerhouses and climate-vulnerable nations struggled to find common ground.
Brazil, China, Australia, Saudi Arabia and the United States had led resistance to bolder action, delegates said.
Meanwhile, Laurence Tubiana from the European Climate Foundation, and an architect of the Paris agreement, said: "Major players who needed to deliver in Madrid did not live up to expectations.
"But thanks to a progressive alliance of small island states, European, African and Latin American countries, we obtained the best possible outcome, against the will of big polluters," Tubiana added.
The US, which is leaving the Paris deal next year, was accused of playing the villain at the climate summit, notably dragging its feet on so-called "loss and damage" funding to disaster-hit countries.
"They continue to block the world's efforts to help people whose lives have been turned upside down by climate change," Harjeet Singh, a climate expert with the charity ActionAid told AFP.
Sunday's agreement fell well short of what science says is needed to tackle the climate emergency.
Despite a year of deadly extreme weather and weekly strikes by millions of young people demanding action, campaigners criticised a deal they described as watered down and weak.
Pulse of Paris still alive
Many wanted to see more explicit language spelling out the importance of countries submitting bolder pledges on emissions.
Yet, in the world of climate diplomacy, the difference between "shall" and "should" can be debated for days, and determine whether a treaty has teeth or is toothless.
"Based on the adopted text, there is a glimmer of hope that the heart of the Paris Agreement is still beating," said Mohamed Adow, firector of Power Shift, referring to the treaty inked in the French capital. "But its pulse is very weak."