Political leaders and climate diplomats are meeting in Madrid for two weeks of talks amid a growing sense of crisis.
According to UN Secretary General António Guterres, “the point of no return is no longer over the horizon”.
Meanwhile, Save the Children says that climate shocks have left millions in Africa facing hunger.
The charity says 33 million people are at emergency levels of food insecurity due to cyclones and droughts.
This conference of the parties, or COP25, was due to be held in Chile but was cancelled by the government due to weeks of civil disturbances.
Spain then stepped in to host the event, which will see 29,000 attendees over the two weeks of talks.
Speaking ahead of the meeting the UN secretary general said the climate crisis was imminent and political leaders had to respond.
Almost every country in the world has now signed and ratified the Paris climate agreement and under the terms of the pact they will all have to put new climate pledges on the table before the end of 2020.