More than 150 leaders have gathered for the United Nations climate talks in Paris, with rallying calls to take action to secure a new deal to curb global warming.
UK Prime Minister David Cameron, the Prince of Wales and US President Barack Obama were among the leaders to address the conference, at which negotiators from 195 countries will attempt to hammer out a deal to prevent temperatures rising by more than 2C above pre-industrial levels and avoid dangerous climate change.
Mr Cameron called for an agreement that keeps the door open to the 2C goal, telling delegates: "Let's imagine what we would have to say to our grandchildren if we failed.
He set out what was needed to get a "good deal" in Paris: one which had a binding legal mechanism, with five yearly reviews to see how the world was progressing, a deal which provided finance for the poorest and most vulnerable countries, which transferred technology from rich to poor nations and had transparency.
Environmentalists said the Prime Minister must match his words on the world stage with action at home, after the Government made a series of moves to curb support for renewables, energy efficiency and a £1bn competition to develop technology to capture and store carbon emissions from power plants.
The Prince of Wales told delegates: "Rarely in human history have so many people around the world placed their trust in so few. Your deliberations over the next two weeks will decide the fate not only of those alive today, but also of generations yet unborn."
Referring to the terror attacks in Paris, Mr Obama said: "What greater rejection of those who would tear down our world than marshalling our best efforts to save it."
As the summit opened, organisers of 2,300 events around the world at the weekend, calling for action on climate change, said more than three-quarters of a million people had taken part in 175 countries.
Meanwhile, in Belfast, MLAs were warned that Stormont has yet to act on extending climate change laws to Northern Ireland, eight years after the legislation was passed.
The attack came from Anna Lo, chair of the Assembly's environment committee. She said: "The Executive has agreed the extension of the Climate Change Act 2008 to Northern Ireland, but nearly eight years on we have still seen no sign of it."
The UK Act of 2008 set a target for greenhouse gases to be at least 80% lower than the 1990 baseline by 2050, giving Ministers powers to introduce measures necessary to achieve targets. Ms Lo said: "The need to act on climate change is urgent - for our society, our economy and those in developing countries paying the price for our actions."
Minister Mark H Durkan has set out proposals for a Northern Ireland Climate Change Bill.
He is calling on the public and interested parties for their views on the plans set out in a discussion document.