Pacific leaders have sought to remind the world what is at stake for the most vulnerable, amid warnings not enough is being done to stave off the most damaging impacts of climate change.

Key points: World leaders are meeting in New York for a UN Climate Action Summit

World leaders are meeting in New York for a UN Climate Action Summit Fiji's Frank Bainimarama said climate change was a "living nightmare" for island nations

Fiji's Frank Bainimarama said climate change was a "living nightmare" for island nations Teen activist Greta Thunberg said world leaders had "stolen" her childhood

World leaders have gathered at the United Nations in New York for a Climate Action Summit, where countries have been urged to account for the sluggish progress being made on reducing global greenhouse gas emissions.

A new report from the World Meteorological Organisation released ahead of the conference warned the world was falling far behind in the race to avert a climate disaster.

The last five years have been the hottest on record, the report said, with ice sheets melting and sea levels rising at an unprecedented rate.

Opening remarks from UN secretary-general Antonio Guterres set the tone early with another urgent warning.

"Nature is angry. And we fool ourselves if we think we can fool nature, because nature always strikes back and around the world, nature is striking back with fury," he said.

It was a theme later picked up by Fiji's Prime Minister Frank Bainimarama, whose country has borne the brunt of that fury in recent years.

Forty-four people were killed when Cyclone Winston struck Fiji in February 2016. ( ABC News: Brant Cumming )

"The brutality of our changing climate has already driven vulnerable communities into a nightmare scenario, one in which the hellscape of storms like Cyclone Winston and Hurricane Dorian have become the new normal," he told the summit.

"Acceptance of this living nightmare is morally unthinkable, and denial is unconscionable."

Mr Bainimarama warned that even if temperature rises were restricted to the 1.5 degrees Celsius limit outlined in the Paris Agreement, many more innocent people would die without urgent adaptation measures.

'How dare you'

Before world leaders started to deliver their speeches, 16-year-old climate activist Greta Thunberg gave an emotional appeal in which she chided the leaders with the repeated phrase, "How dare you."

Ms Thunberg's lone protest outside the Swedish Parliament more than a year ago sparked a global movement, culminating in Friday's global climate strikes

Marshall Islands President Hilda Heine told the summit falling short of that 1.5 degrees target "would mean the greatest failure of humanity that we have ever seen."

Marshall Islands' President Hilda Heine called for better climate change adaptation for vulnerable countries. ( AP: Jason DeCrow )

The average elevation in the Marshall Islands is barely 2 metres above sea level — the Pacific nation is even looking into raising the elevation of its more than 1,000 islands, in response to the threat posed by rising seas.

"Not enough is being done to adapt our world to the impacts that we have already locked in, let alone the worst that's yet to come," Dr Heine said.

"This summit must be the moment we choose survival over selfishness, communities over coal and planet over profits."

Trump's surprise appearance

Donald Trump did not speak at the summit, but briefly listened in to one of the speeches. ( AP Photo: Evan Vucci )

Leaders were only permitted to speak at the event if they could offer up new climate action plans — even major emitters like the United States, Japan and Saudi Arabia did not take the podium.

However, this didn't stop US President Donald Trump from making a brief appearance, listening in to a speech by German Chancellor Angela Merkel, before making a silent exit.

Fiji, Marshall Islands, Palau and Tonga were all given speaking slots due to their commitment to global efforts.

Mr Bainimarama said Fiji was in the process of relocating vulnerable communities and had submitted its roadmap for reaching net zero emissions by 2050, among other measures.

"While Fiji did not cause the climate crisis, we are fully awake to its reality. Someone must act with clear purpose and resolve, someone must clear a path for others to follow," he said.

With climate impacts such as extreme weather, thawing permafrost and sea-level rise unfolding much faster than expected, scientists say the urgency of the crisis has intensified since the Paris agreement was struck.

However pledges made so far under the agreement are nowhere near enough to avert catastrophic warming, scientists say, and last year carbon emissions hit a record high.

The agreement will enter a crucial implementation phase next year, and Dr Heine said history would not forgive leaders who failed to step up to the challenge.

"This is the lens through which history will judge this summit and all of us," she said.

"The time has come for leaders to do just that — lead," she said.

Mr Bainimarama and Mr Morrison have previously locked horns over climate change action. ( ABC News: Tamara Penniket )

Australian Prime Minister Scott Morrison, who did not attend the conference, has previously sparred with Pacific leaders over climate change: most recently at the Pacific Islands Forum in Tuvalu, where Mr Morrison's stance on the issue sparked a fairly public spat.

Mr Bainimarama told media at the time that Mr Morrison had brought up Australia's aid commitments to the Pacific after he was urged to endorse a statement calling for a ban on new coal mines and faster cuts to greenhouse gas emissions.

Mr Morrison refused, and the statement did not make it into the forum's final communique. Mr Bainimarama described Mr Morrison's behaviour was "very insulting, very condescending".

The summit comes days after millions of people around the world took to the streets to demand emergency action on climate change — including more than 100,000 in Australia, across all capital cities and 104 other places.

ABC/Wires