"Shall we all just kill ourselves?"

It was an odd title for a comedy night, but British stand-up Carl Donnelly turned out to have chosen an environmental theme with impeccable timing.

With temperature records tumbling daily in last week's European heat wave, a crowd in an east London bar seemed uniquely primed to appreciate his darkly humorous riffs on the existential threat posed by climate change.

That foretaste of a radically hotter world underscored what is at stake in a decisive phase of talks to implement the 2015 Paris Agreement, a collective shot at avoiding climate breakdown.

With study after study showing climate impacts from extreme weather to polar melt and sea level rise outstripping initial forecasts, negotiators have a fast-closing window to try to turn the aspirations agreed in Paris into meaningful outcomes.

"There's so much on the line in the next 18 months or so," said Sue Reid, vice president of climate and energy at Ceres, a US nonprofit group that works to steer companies and investors onto a more sustainable path.

In October, the UN-backed Intergovernmental Panel on Climate Change (IPCC) warned emissions must start falling next year at the latest to stand a chance of achieving the deal's goal of holding the global temperature rise to 1.5 C.

With emissions currently on track to push temperatures more than three degrees higher, UN Secretary-General Antonio Guterres is working to wrest bigger commitments from governments ahead of a summit in New York in September.

Telling world leaders that failing to cut emissions would be "suicidal," the Portuguese diplomat wants to build ­momentum ahead of a fresh round of climate talks in Chile in December.

By the time Britain convenes a major follow-up summit in late 2020, plans are supposed to be underway - in theory at least - to almost halve global emissions over the next decade.

"In the next year and a half, we will witness an intensity of climate diplomacy not seen since the Paris Agreement was signed," said Tessa Khan, an international climate change lawyer and co-director of the Climate Litigation Network.

As the diplomatic offensive intensifies, the latest scientific studies have offered negotiators scant comfort.

US climatologist Michael Mann believes emissions need to fall even more drastically than the IPCC assumes since the panel may be underestimating how far temperatures have already risen since preindustrial times.

"Our work on this indicates that we might have as much as 40 percent less carbon left to burn than IPCC implies, if we are to avert the 1.5 C warming limit," said Mann.