The head of the United Nations says a "leadership gap" is undermining the world's efforts to curb global warming, days before presidents and prime ministers from around the globe gather for a climate summit in Glasgow, Scotland.

U.N. Secretary-General Antonio Guterres told reporters Tuesday that time is running out to cut greenhouse gas emissions and meet the goals of the 2015 Paris accord to avert global warming that he said could become "an existential threat to humanity."

"The clock is ticking," he said in New York at the presentation of a U.N. report highlighting the difference between what scientists say is needed and what countries are doing to reduce the amount of greenhouse gas being pumped into the atmosphere. "This is a moment of truth."

"The emissions gap is the result of a leadership gap," Guterres said. "But leaders can still make this a turning point to a greener future instead of a tipping point to climate catastrophe."

The new report by the U.N. Environment Programme found fresh pledges by governments to cut emissions are raising hopes but aren't strict enough to keep global warming from exceeding 1.5 degrees Celsius (2.7 degrees Fahrenheit) by the end of the century.

It concluded that recent announcements by dozens of countries to aim for "net-zero" emissions by 2050 could, if fully implemented, limit a global temperature rise to 2.2 degrees Celsius (4 F). That's closer but still above the less stringent target agreed upon in the Paris climate accord of capping global warming at 2 degrees Celsius (3.6 F) compared to pre-industrial times.

"Every ton of carbon dioxide emissions adds to global warming," French climate scientist Valerie Masson-Delmotte, who co-chaired an August U.N. climate science report, told the United Nations on Tuesday. "The climate we experience in the future depends on our decisions now."

The European Union, the United States and dozens of other countries have set net-zero emissions targets. However, the UNEP report said the net-zero goals that many governments announced ahead of a U.N. climate summit in Glasgow next week remain vague, with much of the heavy lifting on emissions cuts pushed beyond 2030.

Guterres said scientists were clear on the facts of climate change, adding that "now, leaders need to be just as clear in their actions."

"They need to come to Glasgow with bold, time-bound, front-loaded plans to reach net zero," he said.

Guterres made a direct plea to China, the top carbon polluter, to make carbon-cutting efforts go faster than previously proposed because "that would have an influence on several other countries." China hasn't updated its required emissions cut pledge.

The report came out as the U.N. General Assembly focused on climate change in a marathon session of speeches Tuesday. The presidents of vulnerable island nations Palau and the Maldives used the opportunity to plea for the world to do more because their countries are at risk of being wiped out.

"Our homes, our blue economy, our heath and our overall well-being have been ravaged by the climate crisis," Palau President Surangel Whipps Jr. told the General Assembly. "We must take radical action now."

"The fate of small islands today is the fate of the world tomorrow," Maldives President Ibrahim Solih said.

Australia became the latest country to announce a net-zero target on Tuesday, but experts swiftly pointed out that it doesn't stack up.

The U.N. Environment Programme is one of several agencies to examine the gap between government pledges and the Paris goals. Its executive director echoed the need for speed on curbing emissions.

"To stand a chance of limiting global warming to 1.5 C, we have eight years to almost halve greenhouse gas emissions," said Inger Andersen. "Eight years to make the plans, put in place the policies, implement them and ultimately deliver the cuts."

Leaders, diplomats, scientists and environmental activists will meet in Glasgow from Oct. 31-Nov. 12 to discuss how countries and businesses can adjust their targets to avert the more extreme climate change scenarios that would result in a significant sea-level rise, more frequent wild weather and more droughts.

Guterres said he would use a trip to the Group of 20 meeting in Italy to press all countries, including major emerging economies such as China, to do more on climate change.

"If there is no meaningful reduction of emissions in the next decade, we will have lost forever the possibility of reaching 1.5 degrees," he said.

Guterres said past climate summits had acknowledged that while all countries have to curb emissions, some are more able to do so than others, with leadership coming from the richest and most developed.

"But the level of emissions of the emerging economies is such that we also need the emerging economies to go an extra mile," he said. "Only if everybody does the maximum, it will be possible to get there."

The UNEP report emphasized several measures that can help boost efforts to curb global warming, including clamping down on emissions of the potent but short-lived greenhouse gas methane. It also emphasized the need to ensure pandemic recovery funds are spent on environmentally friendly measures.

The report found that most countries have missed the opportunity to use COVID-19 recovery spending to stimulate the economy while backing climate action.

"Despite these alarm bells ringing at fever pitch, we see new evidence today in the (UNEP) Emissions Gap Report that governments' actions so far simply do not add up to what is so desperately needed," Guterres told diplomats later Tuesday.

From the Associated Press