NEW YORK • The representatives of 171 nations that signed the Paris climate accord yesterday set in motion an unprecedented global effort to reduce pollution and slow rising temperatures linked to floods, heatwaves and droughts.
While the show of support was historic, implementing the agreement will require an energy revolution.
The effort will require titanic shifts in the ways societies generate electricity, fuel vehicles and run factories, in large part by forsaking coal, oil and natural gas for renewable energy. Societies must also be much more efficient in their energy use.
The Paris pact is broader than any previous climate agreement, applying to all nations, rich and poor alike.
This energy revolution will cost an estimated US$12.1 trillion (S$16.3 trillion) over the next 25 years for the 195 countries that have said they will sign the pact, according to Bloomberg New Energy Finance.
Even if the pact is fully implemented, promised greenhouse gas cuts are insufficient to limit warming to an agreed maximum, the UN says. Deeper cuts are needed, and quickly, the UN and climate scientists say.
But it's a crucial start.
"Signing the Paris Agreement is but a step on a long journey to make our world a better place. A large step, yes, but many more remain to be taken in our efforts to address the risks of climate change, reduce pollution and increase access to clean energy," said Mr Achim Steiner, executive director of the United Nations Environment Programme.
The pact will only enter into force when ratified by at least 55 nations representing 55 per cent of man- made greenhouse gas emissions.
The sooner that happens, the better, the UN says. Some experts predict the 55-per cent thresholds can be reached this year.
Yesterday's mass signing was a record. The previous first-day record for signatures was set in 1982 when 119 states signed the Convention on the Law of the Sea.
China and the United States, the world's top emitters accounting together for 38 per cent of emissions, were due to sign, along with Russia and India, who round out the top four.
China would ratify the Paris pact by September, its climate change envoy told the signing ceremony.
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.
Since the Paris pact was sealed last December, the weather extremes have intensified.
The first three months of the year have broken temperature records and 2015 was the warmest year since records began in the 19th century, with heatwaves, droughts and rising sea levels.
Exceptionally warm ocean temperatures 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 Mr Petteri Taalas, secretary-general of the World Meteorological Organisation.
US 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.