The highly anticipated COP26 climate change summit begins later in the Scottish city of Glasgow.
Delegates from about 200 countries will be there to announce how they will cut emissions by 2030 and help the planet.
With the world warming because of fossil fuel emissions caused by humans, scientists warn that urgent action is needed to avoid a climate catastrophe.
UK Prime Minister Boris Johnson said the summit will be the “world’s moment of truth”.
Speaking before the two-week conference, he urged leaders to make the most of it: “The question everyone is asking is whether we seize this moment or let it slip away.”
Day one will see a report on the state of the climate released by the World Meteorological Organization (WMO).
The WMO’s provisional report by climate scientists will compare global temperatures this year so far with previous years.
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Extreme weather events linked to climate change – including heatwaves, floods and forest fires – are intensifying.
The past decade was the warmest on record, and governments agree urgent collective action is needed.
The United Nations gathering is one of the biggest summits the UK has ever hosted, and was delayed for a year because of the pandemic. COP stands for ‘conference of the parties’ and it is the 26th such conference.
Most of the leaders will arrive late on Sunday evening. Many are travelling from another summit, the G20, in Rome.
Sunday is largely a ceremonial opening day, with various speeches from people including Abdulla Shahid, the president of the UN General Assembly and Foreign Minister of the Maldives. The low-lying islands of the Maldives are threatened by climate change because of rising sea levels.
Countries from every region of the world will have representatives in Glasgow, ready to discuss their plans to cut emissions by 2030.
They all agreed in 2015 to make changes to keep global warming “well below” 2C above pre-industrial levels, but since then, as extreme weather events have intensified, climate scientists have urged nations to aim for 1.5C to limit the risk of environmental disaster.