Net-zero targets must be brought forward by a decade to stop the climate "time bomb".
That was the grim message yesterday from the UN at the launch of a new climate-change report.
Rising emissions in recent years mean cuts in the next two decades will have to be more extreme than current plans, according to the Intergovernmental Panel on Climate Change (IPCC).
An 80% global reduction in CO2 emissions is needed to limit warming to 1.5C, the upper aim of the Paris Agreement, its new report says.
But the UN said richer countries must move faster than developing nations, by "super-charging" their net-zero goals and helping poorer countries cut their own emissions.
The UK, like most other developed nations, has set a target for net-zero emissions by 2050, and its climate-change advisers have said getting there quicker will "stretch feasibility".
Thin ice
Speaking at the launch of the report, Antonio Guterres, the UN Secretary-General, said "humanity is on thin ice - and that ice is melting fast".
"The climate time bomb is ticking," he said, and added that the 1.5C limit was "achievable", but would require a "quantum leap in climate action".
Developed countries should "commit to reaching net zero as close as possible to 2040" while emerging countries, including China and India, should aim for 2050.
Chris Jones, from the Met Office Hadley Centre and a co-author of the report, told the Telegraph that the scale of the global challenge was "massive".
"In 2020, during the Covid lockdowns, CO2 emissions dropped by about 6%," he said. "So we need to achieve that year-on-year for the rest of this decade, and obviously we can't do that by locking people down."
The new report, which brings together six other IPCC publications since 2018, was agreed at the end of week-long talks in the Swiss town of Interlaken and will be the last report from the UN until 2030, considered to be a crunch point for reducing emissions.
Cash flows
It says cash flows to help developing countries reduce their emissions must be increased six times above current levels to keep climate change to 1.5C.
And it highlights that richer countries have a greater responsibility for climate change and emissions, with the 10% of highest emitting households, generally the most wealthy, contributing about 40% of all emissions.
The report follows the push at the last Cop27 climate summit for compensation for poorer countries for the loss and damage caused by climate change. It will inform the next meeting, Cop28, to take place in the UAE later this year.
Dieter Helm, a professor of economic policy at Oxford University, and a government adviser, said the UN was ignoring the reality of reaching more-ambitious targets.
"The UN is very good at proposing targets, but what is patently obvious is the gap between the existing targets and what is actually happening on the ground - before we get to new targets," he said. "For the developed countries to get to net zero in 27 years is a big stretch. To do it in 17 years would take a massive transformation."
Michal Liebreich, the founder of Bloomberg New Energy Finance, said the 2040 goal was "not feasible" and governments should instead "double down" on trying to stay below 2C.
"We're not going to achieve 1.5C. That doesn't mean we give up" he said. "We have to stop fooling ourselves. If we're going to find it hard enough to hit 2050, then screeching about 2040 is not actually helpful."