The world's efforts to stop climate change have been "utterly inadequate" so far and there is a danger global warming could pass the "point of no return", UN Secretary-General Antonio Guterres said.
Addressing delegates at the start of a two-week international climate conference in Spain, the UN chief said the impact of rising temperatures - including more extreme weather - is already being felt around the world with dramatic consequences for humans and other species.
The COP25 talks in Madrid will focus on finalising rules for global carbon markets and setting up a fund to help countries already reeling from climate change: enhanced heatwaves, droughts, floods and storms made worse by rising seas.
Guterres noted the world has the scientific knowledge and the technical means to limit global warming, but "what is lacking is political will".
"The point of no return is no longer over the horizon," Guterres told reporters in the Spanish capital.
"It is in sight and hurtling toward us."
Hope, not despair
Guterres cited mounting scientific evidence for the impact that man-made emissions of greenhouse gases are already having on the planet, including record temperatures and melting polar ice.
But he insisted his message was "one of hope, not of despair".
"Our war against nature must stop and we know that is possible," he said.
Guterres noted some 70 countries - many among the most vulnerable to climate change - have pledged to stop emitting more greenhouse gases by 2050.
"But we also see clearly that the world's largest emitters are not pulling their weight. And without them, our goal is unreachable," he said. The UN chief said he hoped the meeting in Madrid would see governments make more ambitious pledges ahead of a deadline to do so next year.
"Some countries like China and Japan are signalling their unwillingness to increase ambition," said Laurence Tubiana, CEO of the European Climate Foundation - and a main architect of the Paris Agreement.
United States President Donald Trump pulled the US out of the Paris deal entirely, formally notifying the UN in November this year of his government's intentions.
Organisers expect about 29,000 visitors, including some 50 heads of state and government for Monday’s opening, as well as scientists, seasoned negotiators, and activists during the two-week meeting.
More than 5,000 police officers are charged with keeping the summit safe, Spain’s Interior Ministry said on Sunday.
Although authorities have stepped-up border controls and cyber security measures, authorities have kept the country’s security alert one level under the highest, where it has been ever since attacks in Tunisia and France in mid-2015.