The highly anticipated COP26 climate change summit has begun in the Scottish city of Glasgow.
Delegates from around 200 countries are 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 ahead of the two-week conference, Mr Johnson urged leaders to make the most of it: "The question everyone is asking is whether we seize this moment or let it slip away."
COP26 President Alok Sharma said agreement would be "tougher than what we achieved in Paris" five years ago, when almost all the world's nations agreed a treaty to "pursue efforts" to limit the global temperature rise to 1.5C.
"This is on leaders," he told BBC One's Andrew Marr Show. "They need to come forward and we need to collectively agree how we are going to meet this goal."
He said "we expected more" of countries such as China, the world's largest carbon emitter, and called the summit a "real opportunity" for them to show leadership.
Day one of the summit 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.
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. (Source: BBC website, headline composed by Newsroom, photo from internet)
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