High-level climate talks have begun in Paris, aimed at signing a long-term deal to reduce global carbon emissions.
More than 150 world leaders have converged to launch the two-week talks, known as COP21.
The last major meeting in 2009 ended in failure. But French Foreign Minister Laurent Fabius, who is chairing the meeting, said a deal was within reach.
Most of the discussions are expected to centre on an agreement to limit global warming to 2C (3.6F).
Assessments of the more than 180 national plans that have been submitted by countries suggest that if they were implemented the world would see a rise of nearer to 3C.
Peruvian Environment Minister Manuel Pulgar Vidal declared this year’s meeting open on Monday.
Strong action on carbon emissions is essential for multiple reasons, said Mr Vidal, who hosted last year’s UN climate conference in Lima.
Mr Vidal said a deal would show the world that countries can work together to fight global warming as well as terrorism.
COP21 live: The latest updates from Paris.
Christiana Figueres, the head of the UN’s climate change negotiations, addressed delegates at the start of the summit.
“Never before has a responsibility so great been in the hands of so few,” she said. “The world is looking to you. The world is counting on you.”
UN climate conference 30 Nov – 11 Dec 2015
COP 21 – the 21st session of the Conference of the Parties – will see more than 190 nations gather in Paris to discuss a possible new global agreement on climate change, aimed at reducing greenhouse gas emissions to avoid the threat of dangerous warming due to human activities.
COP21 live: The latest updates from Paris
Explained: What is climate change?
In video: Why does the Paris conference matter?
Analysis: From BBC environment correspondent Matt McGrath
More: BBC News special report
The talks are taking place amid tight security, two weeks after attacks in Paris claimed by the so-called Islamic State (IS) group.
Negotiators from 195 countries will try to reach a deal at the meeting.
This year, world leaders are attending the start of the two-week meeting to give impetus to the talks.
Some 150 heads of state, including US President Barack Obama and his Chinese counterpart Xi Jinping, have arrived at the summit.
Major points of contention include:
Limits: The UN has endorsed a goal of limiting global warming to no more than 2C over pre-industrial levels by the end of the century. But more than 100 poorer countries and low-lying, small-island states are calling for a tougher goal of 1.5C.
Fairness: Developing nations say industrialised countries should do more to cut emissions, having polluted for much longer. But rich countries insist that the burden must be shared to reach the 2C target.
Money: One of the few firm decisions from the 2009 UN climate conference in Copenhagen was a pledge from rich economies to provide $100 billion (93 billion euros) a year in financial support for poor countries from 2020 to develop technology and build infrastructure to cut emissions. Where that money will come from and how it will be distributed has yet to be agreed.
Among those attending the talks is the broadcaster and naturalist Sir David Attenborough. He said he was not confident that the Paris talks would produce a deal to tackle the “hideous problem” of climate change.
“We know the consequences of a rise of temperature, what it will do for the oceans, for example,” he told the BBC.
“Increasing temperature of the oceans will cause havoc amongst the fish stocks and similarly increasing the temperature of the Earth is causing the spread of deserts.
“The problems of a rise in temperatures are huge; it has to be avoided at all costs.”
The Prince of Wales said that humanity faces no greater threat than climate change, as he issued a call for immediate action to tackle rising temperatures.
Charles told the summit: “Rarely in human history have so many people around the world placed their trust in so few.
“Your deliberations over the next two weeks will decide the fate not only of those alive today, but also of generations yet unborn.”