Video report by ITV News Science Correspondent Alok Jha.
Some 171 countries have signed the Paris climate agreement in New York as part of a landmark deal hailed by UN Secretary-General Ban Ki-moon as "a new covenant for the future".
Around 15 nations - mainly small island states - had already ratified the agreement, which aims to keep temperatures below 1.5-2C.
The agreement will come into legal force after it is ratified by at least 55 countries - accounting for at least 55% of global greenhouse gas emissions.
Speaking at the opening ceremony, Ban said the planet was experiencing record temperatures.
"We are in a race against time," he said. "I urge all countries to join the agreement at the national level.
"Paris will shape the lives of all future generations a profound way - it is their future that is at stake."
The gathering of world leaders - the largest ever of its kind - heard from people who will be affected most by climate change in the future, such as 16-year-old UN youth representative Gertrude Clement from Tanzania.
Actor Leonardo DiCaprio also addressed delegates in a rousing speech in which he called for urgent action against climate change.
"You are the last best hope of Earth," he told world leaders. "We ask you to protect it or we, and all living things we cherish, are history."
US President Barack Obama is keen for the new agreement to take effect before he leaves office next January.
A clause in the treaty means it would take four years if a new leader, not committed to tackling climate change, wanted to take the US out of the agreement.
UN climate chief Christiana Figueres said: "Most countries, though not all, need to take the signed document and go back home and go to ratification procedures that in most countries requires parliamentary discussion and decision."