The world is on the verge of a climate abyss, the UN has warned, in response to a Guardian survey that found that hundreds of the world’s foremost climate experts expect global heating to soar past the international target of 1.5C.

A series of leading climate figures have reacted to the findings, saying the deep despair voiced by the scientists must be a renewed wake-up call for urgent and radical action to stop burning fossil fuels and save millions of lives and livelihoods. Some said the 1.5C target was hanging by a thread, but it was not yet inevitable that it would be passed, if an extraordinary change in the pace of climate action could be achieved.

The Guardian got the views of almost 400 senior authors of reports by the authoritative Intergovernmental Panel on Climate Change. Almost 80% expected a rise of at least 2.5C above preindustrial levels, a catastrophic level of heating, while only 6% thought it would stay within the 1.5C limit. Many expressed their personal anguish at the lack of climate action.

“The goal of limiting global warming to 1.5C is hanging by a thread,” said the official spokesperson for António Guterres, the UN secretary general. “The battle to keep 1.5C alive will be won or lost in the 2020s – under the watch of political and industry leaders today. They need to realise we are on the verge of the abyss. The science is clear and so are the world’s scientists: the stakes for all humanity could not be higher.”

Alok Sharma, the president of the Cop26 climate summit in 2021, said: “The results of the Guardian’s survey should be another wake-up call for governments to stop prevaricating and inject much more urgency into delivering on the climate commitments they have already made.” He said world leaders needed to get on and deliver on the pledge they made to transition away from fossil fuels at Cop28 in December.

Christiana Figueres, the UN climate chief who oversaw the landmark 2015 Paris climate deal where the 1.5C goal was adopted, said: “These climate scientists are doing their job. They are telling us where we are, but now it’s up to the rest of us to decide what this moment requires of us and [to] turn the seemingly impossible into the new normal.”

She said the world was on the edge of positive societal tipping points away from fossil fuels. “It doesn’t mean a utopian future – we know too much climate change is already baked into the system – but enormous positive change is coming. A world in which we pass 1.5C is not set in stone.”

The 1.5C target was initially proposed by the Alliance of Small Island States (Aosis). Fatumanava Pa’olelei Luteru, the chair of Aosis, said: “Our islands are quite literally sinking as the temperatures rise. The lack of ambition on climate change from bigger countries is consigning our states to a reality of devastating loss. The [Guardian] report must be a wake-up call to the world…”