A Paris deal to slow climate change is set to be signed by more than 165 countries, including Australia, at the United Nations.
It is the most number of states to endorse an international agreement on day one and backers hope this 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 percent of man-made greenhouse gas emissions.
UN 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 percent thresholds can be reached this year. The UN said 13 countries, mostly small island developing states, are due to deposit instruments of ratification on Friday (local time).
The UN expects about 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.
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 US 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.
Australia will be represented by Environment Minister Greg Hunt. Environmental activists are calling for greater action from Canberra after extensive surveying of the Great Barrier Reef found 93 percent of the 2300km-long natural wonder has been affected by coral bleaching as a result of warmer ocean temperatures.
The signing ceremony is expected to take place at 2230 AEST on Friday.
Reuters
