The result? A deadlock and a dodge.
The 12-day talks extended two days into overtime but still punted the carbon market conundrum to next year's COP26 in Glasgow.
A non-binding pledge, meanwhile, to revisit deeply inadequate national plans for slashing greenhouse gas emissions was apparently too big an ask.
The European Union was the only major emitter to step up with an ambitious mid-century target ("net zero"), and even then it was over the objection of Poland and without a crucial midway marker.
UN Secretary-General Antonio Guterres labelled COP25 "disappointing". Others were more blunt.
"The can-do spirit that birthed the Paris Agreement feels like a distant memory," said Helen Mountford of Washington-based think tank World Resources Institute (WRI).
"The world is screaming out for climate action but this summit has responded with a whisper," noted Chema Vera, executive director of Oxfam International.
So what went wrong?
At least five factors contributed to the Madrid meltdown. To an unsettling degree, the outcome of a UN climate summit - where 196 nations must sign off on every decision - depends on the savvy and skill of the host country, which acts as a facilitator.
The stars were not aligned for the chaotic Copenhagen summit of 2009 and the Danish prime minister's less-than-deft manoeuvering did not help. By contrast, the 2015 climate treaty was in no small measure made possible by France's diplomatic tour-de-force.
This year, Chile's environment minister Carolina Schmidt wielded the hammer after the conference was moved at the last minute to Madrid due to massive protests on the streets of Santiago. From Day One, when Schmidt's mishandling of a request from the African negotiating bloc mushroomed into a diplomatic incident, veteran observers worried that she was not up to the job.
For Greenpeace International executive director Jennifer Morgan, "an irresponsibly weak Chilean leadership" enabled Brazil and Saudi Arabia to push agendas destined to derail the talks.
"Chile played a bad hand poorly," noted another insider.
A marginal factor, perhaps, but not a negligible one.
Among the nearly 30,000 diplomats, experts, activists and journalists accredited to attend the summit were hundreds of high-octane fossil fuel lobbyists.
They are collectively the elephant in the room: everyone knows what causes climate change but it is considered impolitic within the UN climate bubble to point fingers.
Even the Paris Agreement turns a blind eye: nowhere in its articles does one find the words oil, natural gas, coal, fossil fuels or even CO2.
"We need to engage with them," UN Climate executive secretary Patricia Espinosa told AFP when asked whether it was time to exclude such lobbyists from the room.
"There is no way we will achieve this transformation without the energy industry, including oil and gas."
But the incongruity of their participation in a life-and-death struggle to wean the world from their products has become harder to ignore.
"Is there no space free from greenwashing," asked Mohamed Adow, director of climate think tank Power Shift Africa.
"The UN climate negotiations should be the one place that is free from such fossil fuel interference."
On November 4, 2020 - the day after US voters will renew Donald Trump's mandate or turn him out of office - the United States is set to formally withdraw from the Paris Agreement.
It will be the second time that a Republican White House has plunged a dagger in the heart of a climate treaty nurtured by the Democratic administration that preceded it - the Kyoto Protocol was the previous one.
From the moment Trump was elected - on Day Two of COP22 in Marrakesh - advocates of climate action have played down the negative impact of the world's largest economy and second biggest carbon polluter pulling out of the Paris deal. But the corrosive "Trump effect" was palpable in Madrid, as was the anger at Washington for twisting arms even as it walked out the door. "There are one or two parties that seem hell-bent on ensuring any calls for ambition, action and environmental integrity are rolled back," said Simon Stiell, Grenada's environment minister.