KATOWICE, Poland — U.N. Secretary-General Antonio Guterres opened the climate summit in Poland by issuing 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 called climate change as “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 Katowice, Poland.

“We are still not doing enough, nor moving fast enough, to prevent irreversible and catastrophic climate disruption.”

The U.N. chief chided countries, particularly those most responsible for greenhouse gas emissions, for failing to do enough to meet the goals of 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.

Such a move, which experts say is 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 man-made greenhouse gas emissions.

The remark was also directed at host Poland, which relies on coal for 80 percent of its energy. But Poland’s President Andrzej Duda told a later news conference that the coal-rich country will never entirely give up its “strategic fossil fuel.”

In order to steer businesses and consumers away from heavily polluting forms of energy, the U.N. chief urged countries to embrace carbon pricing, something few countries have yet to do.

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

The two-week conference, in Poland’s southern coal mining region of Silesia, is expected to work out how governments can report on their efforts to reduce greenhouse gas emissions and keep global warming within the Paris accord limit.

“This is the challenge on which this generation’s leaders will be judged,” Guterres said.

He later told reporters that realities about global climate are “worse than expected, but the political will is relatively faded after Paris” and is not matching the challenges.

Guterres called for a “huge increase in ambitions” during the two weeks of negotiations in Poland, adding “we cannot afford to fail in Katowice.”

Famed British naturalist Sir David Attenborough warned 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.”

Duda, the Polish leader, said participants at the conference have backed its proposal of a “just transition” away from coal mining, which calls for helping those people, like coal miners, who are slated to lose their jobs as the world changes its energy mix.

Fiji’s prime minister, Frank Bainimarama, who presided over last year’s U.N. climate summit, said the “just transition” proposal shouldn’t just consider the fate of fossil fuel workers but all 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.