The chances of limiting global warming to 1.5°C are close to zero, the former head of a UN climate panel has said.
The target was set under the Paris Agreement and is regarded as crucial to preventing environmental catastrophe.
Britain describes the Cop26 climate summit which it is hosting in November as the last chance to ensure necessary action is taken.
But Prof Sir Robert Watson, chairman of the Intergovernmental Panel on Climate Change from 1997 to 2002, said he doubts that this would happen.
He said emissions would need to halve by 2030 for the goal to be met, when major polluters such as China and India would be at similar levels to today.
Britain’s pledge to slash emissions over the next decade is good on paper but experts doubt whether the UK will hit its climate targets, he said.
“I don’t see that there is much political chance of having emissions in 2030 being 50 per cent lower,” he said in an online discussion with scientists.
“I think the odds of hitting 1.5°C, I’ll be honest, I think they’re slim to zero. I think the odds of hitting 2°C are pretty slim, to be quite candid.”
The Paris Agreement said countries should limit warming to 2°C above pre-industrial levels but pursue efforts to keep it to 1.5°C.
An IPCC report published last week said the effects of climate change would be “dramatically more severe” if the 1.5°C threshold were breached.
Heatwaves would be far more common at 2°C warming, while droughts, fires and extreme rainfall would occur more frequently, it is feared.
The panel said global warming was likely to reach 1.5°C by the early 2030s if the pace of climate change continued at its current rate.
The report was described as a “code red for humanity” by the head of the UN and led to fresh calls for action at Cop26.
Summit president Alok Sharma said the report showed that “1.5°C is still achievable … but that it is retreating and it is retreating fast”.
He said diplomats would seek agreement on phasing out coal in wealthy countries over the next decade and around the world by 2040.
But Prof Watson said the UK government had not put the policies in place to stimulate the private sector and people to meet green targets.
He said: “We may end up with a whole bunch of pledges in Glasgow that look very good on paper but are they real?
“I would hope that [governments] don’t only tell us what their goals and targets are – what are the actions, what are the policies that will give us any hope of meeting these?”
Prof Watson said the IPCC report’s language was stronger than when he chaired the panel but the overall message had not changed.
“The messages are clear – the climate is changing, we’re seeing more extreme events,” he said.
“The cause of the changes is human-induced and the situation will continue to get worse as the emissions continue to increase.”