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. 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.