United Nations: A Paris deal to slow climate change is set to be signed by more than 165 countries at the United Nations on Friday, the most states to endorse an international agreement on day one, a record backers hope will inspire swift implementation.

Many states still need a parliamentary vote to formally approve the agreement. It will only enter into force when ratified by at least 55 nations representing 55 per cent of man-made greenhouse gas emissions.

U.N. Secretary-General Ban Ki-moon ‘wants to use the event to generate momentum around implementation and early entry into force of the Paris agreement,’ said Selwin Hart, director of Ban’s climate change support team.

Some experts predict the 55 per cent thresholds can be reached this year. The United Nations said 13 countries, mostly small island developing states, are due to deposit instruments of ratification on Friday.

The United Nations expects some 60 heads of state and government at the signing ceremony. French President Francois Hollande and Hollywood actor and environmental activist Leonardo di Caprio are expected to attend.

The previous first-day record for signatures was set in 1982 when 119 states signed the Convention on the Law of the Sea.

‘It’s happening much faster than anyone anticipated or expected,’ Hart said. ‘Independent analysis suggests that at least one of the top four emitters must ratify the agreement’ for it to surpass the 55 percent of emissions threshold.

China and the United States, the world’s top emitters accounting together for 38 percent of emissions, are due to sign, along with Russia and India, who round out the top four.

Many developing nations are pushing to ensure the climate deal comes into force this year, partly to lock in the United States if a Republican opponent of the pact is elected president in November.

Even if the pact is fully implemented, promised greenhouse gas cuts are insufficient to limit warming to an agreed maximum, the United Nations says.

The first three months of 2016 have broken temperature records and 2015 was the warmest year since records began in the 19th century, with heat waves, droughts and rising sea levels.

Warm waters have done widespread damage to corals in Australia’s Great Barrier Reef, and sea ice in the Arctic hit a record winter low last month.

‘The magnitude of the changes has been a surprise even for veteran climate scientists,’ said Petteri Taalas, secretary-general of the World Meteorological Organisation.

Jake Schmidt of the Natural Resources Defence Council said the high turnout on Friday ‘increases the chances that it (the agreement) will enter into force this year.’

President Barack Obama says he does not need Senate approval to ratify the agreement. Once the accord enters into force, a little-noted Article 28 says any nation wanting to withdraw must wait four years, the length of a US presidential term.

‘There is a clear cry globally for climate action,’ a senior US State Department official said.