The two-week long negotiations which extended till Sunday saw no agreement on major issues such as Article 6, loss and damage, and long term finance.
Under the Article 6 of the Paris agreement signed in 2015, the countries had agreed to set up a global carbon market system to help developing countries decarbonise their economies at lower cost. However, the countries have tried and failed to agree the rules governing this mechanism.
Despite holding the longest climate talks ever in 25 nearly annual editions, the sleep-deprived negotiators left one of the thorniest issues for next summit in Glasgow - how to deal with carbon emissions.
"I am disappointed with the results of COP25. The international community lost an important opportunity to show increased ambition on mitigation, adaptation and finance to tackle the climate crisis," UN Secretary-General Antonio Guterres said in a tweet.
The major climate summit brought together delegates of nearly 200 countries to finalise implementation of the 2015 Paris agreement, but competing national interests proved insurmountable despite global calls for action in the face of extreme weather phenomena and increasingly dire warnings from climate scientists.
"We must not give up, and I will not give up," Guterres said.
"I am more determined than ever to work for 2020 to be the year in which all countries commit to do what science tells us is necessary to reach carbon neutrality in 2050 and a no more than 1.5 degree temperature rise," he said.
Few countries came to this year's talks with updated plans to reach the Paris agreement, even as the EU agreed its long-term target of reaching net zero emissions by 2050. Experts say more ambitious emissions cuts are needed globally if the Paris pledge is to be met.
The Paris agreement commits the US and other countries to keep rising global temperatures "well below" 2 degrees Celsius.
The observers blamed G20 countries for the poor outcome in Madrid.
"I have the hope that we can use this material to reach agreement at COP26," said COP25 President Carola Schmidt on the draft decision on Article 6 of the Paris agreement.
"COP25 was a success for the fossil fuel industry - their interest have won, effectively blocking the process and undermining the end result," said May Boeve, Executive Director of 350.org, a global environmental organisation.
"In the end, after forcing negotiators to keep at it for three days straight, they got what they wanted - a weakened text that kicks most of the big issues down the road to COP26," Boeve said.
The governments need to completely rethink how they do this, because the outcome of COP25 is totally unacceptable. This COP exposed the role of polluters in politics and the youth's deep distrust of government, as climate blockers like Brazil and Saudi Arabia, enabled by an irresponsibly weak Chilean leadership, peddled carbon deals and steamrolled scientists and civil society, said Jennifer Organ from Greenpeace International.
"We need systemic change that people can trust, and the Paris agreement is just one part of the puzzle. Decision-makers now need to go home, regroup and think about how to move forward as we head into a critical 2020," she said.
"I've been attending these climate negotiations since they first started in 1991. But never have I seen the almost total disconnect we've seen here at COP25 in Madrid between what the science requires and what the climate negotiations are delivering in terms of meaningful action by the youth," said Alden Meyer, Director of Policy and Strategy, Union of Concerned Scientists.
Nearly 70 countries - most of them climate vulnerable, developing nations - have risen to the challenge by committing to enhance the ambition of their Paris pledges. But most of the world's biggest emitting countries are missing in action and resisting calls to raise their ambition, Meyer said.
Leading economies were faulted for their complacency and their failure to deliver more support to vulnerable nations in the face of brutal impacts and push for a tougher collective response in 2020 when new climate plans are mandated under the Paris agreement.
COP25's final decision text "re-emphasises with serious concern the urgent need to address the significant gap between the aggregate effect of parties' mitigation efforts in terms of global annual emissions of greenhouse gases by 2020," according to the final draft.
"The impacts of not acting adequately on climate change are clear and visible not just in the global south, but also in the global north," said Aarti Khosla, Director, Climate Trends.
"Yet, the recently concluded COP shows that the political will to act is absolutely missing across governments," she said.
Laurence Tubiana, CEO European Climate Foundation and Paris Agreement Architect, said: "The result of this COP25 is really a mixed bag, and a far cry from what science tells us is needed".
"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 said.
