"Our fragile planet is hanging by a thread", he warned, adding "we are still knocking on the door of climate catastrophe."
In a follow-up tweet, the UN chief sent a message to "young people, indigenous communities, women leaders, all those leading on #ClimateAction."
"I know you might be disappointed. But we're in the fight of our lives & this fight must be won." Thunberg, arguably the world's best known environmental campaigner, was blunter in her assessment.
"The #COP26 is over," she tweeted. "Here's a brief summary: Blah, blah, blah."
"But the real work continues outside these halls. And we will never give up, ever." During the conference, Thunberg and other activists had denounced the way it was playing out, arguing that world leaders had failed to match their words with real action.
'Hard work ahead'
British prime minister Boris Johnson remained relatively upbeat. "There is still a huge amount more to do in the coming years," Johnson said.
"But today's agreement is a big step forward and, critically, we have the first ever international agreement to phase down coal and a roadmap to limit global warming to 1.5 degrees."
A European Commission statement said the deal kept the targets of the 2015 Paris climate agreement alive, "giving us a chance of limiting global warming to 1.5 degrees Celsius".
Commission president Ursula von der Leyen said delegates to the conference made progress on commitments to cut back on dangerous emissions, and on raising US dollar 100 billion a year to help developing and vulnerable countries.
"But there will be no time to relax: there is still hard work ahead," she added.
During the final negotiations, China and India insisted that language on fossil fuels be weakened in the final summit decision text. In recent days, the Australian government has vowed to sell coal for decades to come.
But Kevin Rudd, Australia's former prime minister, now the president of the Asia Society, remained hopeful.
"While the official text might have stopped short of agreeing to phase out coal, the statements made by world leaders in Glasgow leave no doubt that coal is on its way to being consigned to history."
For Britain's COP26 president Alok Sharma, the long, drawn-out negotiations had taken a toll.
"I apologise for the way this process has unfolded, said Sharma, as the final deal was clinched."I am deeply sorry," he added, before banging down his gavel.