KATOWICE, Poland (AP) — U.N. Secretary-General Antonio Guterres issued a dramatic appeal to world leaders Monday to take the threat of global warming seriously and to act boldly to avert a catastrophic rise in temperatures before the end of the century.

Guterres, who spoke at the opening of the U.N. climate conference in Poland, called climate change "the most important issue we face."

"Even as we witness devastating climate impacts causing havoc across the world, we are still not doing enough, nor moving fast enough, to prevent irreversible and catastrophic climate disruption," Guterres told delegates from almost 200 countries who gathered in the city of Katowice.

Famed British naturalist Sir David Attenborough echoed his warnings, telling the gathering that the "collapse of our civilizations and the extinction of much of the natural world is on the horizons" if no urgent action is taking against global warming.

The 92-year-old TV presenter blamed humans for the "disaster of global scale, our greatest threat in thousands of years."

The U.N. chief chided countries, particularly those most responsible for greenhouse gas emissions, for failing to do enough to back the 2015 Paris climate accord, which set a goal of keeping global warming well below 2 degrees Celsius (3.6 degrees Fahrenheit) — ideally 1.5 degrees C (2.7 degrees F) — by the end of the century.

Citing a recent scientific report on the dire consequences of letting average global temperatures rise beyond 1.5 degrees, Guterres urged countries to cut their emissions 45 percent from 2010 levels by 2030 and aim for net zero emissions by 2050.

Net zero emissions mean that any greenhouse gases emitted need to be soaked up by forest or new technologies that can remove carbon from the atmosphere.

Such cuts, which experts say are the only way to achieve the 1.5-degree goal, would require a radical overhaul of the global economy and a move away from using fossil fuels.

"In short, we need a complete transformation of our global energy economy, as well as how we manage land and forest resources," Guterres said.

He said governments should embrace the opportunities rather than cling to fossil fuels such as coal, which are blamed for a significant share of human-caused greenhouse gas emissions.

The U.N. chief's remark was directed at conference host Poland, which relies on coal for 80 percent of its energy. Polish President Andrzej Duda said during a news conference later Monday that the coal-rich country would work to reduce its reliance on coal but never entirely give up its "strategic fossil fuel."

Guterres also urged negotiators not to forget that the challenges they face pale in comparison to the difficulties climate change already is causing millions of people around the world whose homes and livelihoods are threatened by rising sea levels, drought and more powerful storms.

A 15-year-old Swedish activist who takes time out of school to highlight the danger of global warming echoed his appeal. sentiment. old Greta Thunberg said world leaders who skip the climate summit are "very irresponsible."

Thunberg, who protests outside Sweden's parliament each week and has inspired students in other countries, said absent leaders such as U.S. President Donald Trump and German Chancellor Angela Merkel "don't realize how much power they have."

"I think that in the future we will look back and we will either laugh at them or we will hate them," she told The Associated Press. "It's very sad, but if they don't do anything right now that is the truth."

A goal of the two-week conference in Poland's southern coal mining region of Silesia is finalizing how governments report on their efforts to reduce greenhouse gas emissions and keep global warming within the limits of the Paris climate deal.

Guterres called for a "huge increase in ambitions" during the negotiations in Poland, adding "we cannot afford to fail in Katowice."

"This is the challenge on which this generation's leaders will be judged," he said.

He told reporters that the reality of global climate change has been "worse than expected, but the political will is relatively faded after Paris" and not matching the current challenges.

Arnold Schwarzenegger, the action film star and former governor of California, insisted the United States is "still in" the Paris accord to curb global warming despite Trump's decision to walk away from it.

"America is more than just Washington or one leader," he said, adding that he wished he could travel back in time — like the cyborg he portrayed in "The Terminator" — to stop fossil fuels from being used.

Calling Trump "meshugge" — Yiddish for "crazy" — for deciding to withdraw from the Paris accord, Schwarzenegger insisted that the climate deal has widespread support at local and state levels in the U.S.

Duda, the Polish leader, said participants at the conference have backed his country's proposal that governments should ensure a "just transition" for workers in fossil fuel industries who stand to their jobs as the world shifts to renewable energy.

But Fiji's prime minister, Frank Bainimarama said any "just transition" should consider the fate of all the people around the world whose lives are affected by climate change.

Residents of the world's smaller islands, many of whom face catastrophic flooding from higher sea levels in a warming world, have been among the world's most vocal backers of measures to combat climate change.

___

Read more stories on climate issues by The Associated Press at https://www.apnews.com/Climate