US Secretary-General Antonio Guterres addresses the 78th United Nations General Assembly at UN headquarters in New York City on September 19. (Timothy A. Clary/AFP/Getty Images)

(CNN) — United Nations Secretary General António Guterres issued a stark warning as he gathered world leaders for a high-level summit on the climate crisis: “Humanity has opened the gates to hell.”

“Horrendous heat is having horrendous effects. Distraught farmers watching crops carried away by floods. Sweltering temperatures spawning disease,” he said in a speech to open the Climate Ambition Summit, happening alongside the UN General Assembly in New York.

“Climate action is dwarfed by the scale of the challenge,” he added, warning that if nothing changes we are heading “towards a dangerous and unstable world.”

The one-day conference – which comes as the the world grapples with devastating floods and fires – is intended to build global momentum toward slashing planet-heating pollution ahead of the UN’s COP28 climate summit in Dubai in December.

Wednesday’s summit is unusual due to Guterres’ decision to restrict the speaker list to countries he deemed to have clear and effective climate plans, and those prepared to send a high-level leader to speak.

Of the nearly 200 countries present in New York for the General Assembly, only 34 nations and seven non-governmental bodies secured speaking slots at the UN chief’s summit.

Notably absent from the speaker list were some of the world’s biggest polluters, including China, India and the United States, though US climate envoy John Kerry is attending.

Also missing from the list was UK prime minister Rishi Sunak, who is facing a backlash at home over plans to weaken the country’s climate commitments.

The aim of the summit is to increase ambitions on climate action, Guterres said.

Selwin Hart, a special adviser to the UN Secretary-General on climate action and just transition, said there has been “massive backsliding” on commitments.

“The countries that committed to net-zero by 2050, and to the 1.5-degree goal of the Paris Agreement, they’re expanding fossil fuel licensing at a time when science tells us this is totally incompatible with this 1.5-degree goal,” he told CNN in a Tuesday interview.

In his Wednesday speech, Guterres called on developed countries to reach net-zero emissions – removing from the atmosphere at least as much planet-warming pollution as they produce – by 2040, at least ten years earlier than most current commitments.

He also asked countries to commit to timelines to phase out fossil fuel emissions, as well as to significantly increase finance to help low- and middle-income countries quickly move to clean energy and invest in climate resilience measures to better cope with increasingly severe extreme weather events.

“We are decades behind,” Guterres said. “We must make up time lost to foot-dragging, arm-twisting and the naked greed of entrenched interests raking in billions from fossil fuels.”