About 160 countries are expected to sign the Paris Agreement on climate change in a symbolic triumph for a landmark deal that once seemed unlikely, but is now poised to take effect years ahead of schedule.
United Nations officials say Friday's ceremony will set a record for international diplomacy - never before have so many countries put their names to an agreement on the first day of the signing period.
That could help pave the way for the pact to become effective long before the original 2020 deadline - possibly this year - though countries must first formally approve it through their domestic procedures.
"We are within striking distance of having the agreement start years earlier than anyone anticipated," Brian Deese, an adviser to US president Barack Obama, said in a speech last week at Reed College in Portland, Oregon.
The US and China, which together account for nearly 40% of global emissions, have said they intend to formally join the agreement this year. It will enter into force once 55 countries representing at least 55% of global emissions have done so.
"There's incredible momentum," former New Zealand prime minister Helen Clark, who heads the UN Development Programme, said. "We're moving as quickly as possible to action."
She said her agency was working with more than 140 countries on climate change-related issues and that financing to make the Paris Agreement a reality was "critical, and let's hope everyone lives up to commitments made".
The agreement, the world's response to hotter temperatures, rising seas and other impacts of climate change, was hammered out in December outside Paris. The pact was a major breakthrough in UN climate negotiations, which for years were bogged down with disputes between rich and poor countries over who should do what to fight global warming.
The mood was so pessimistic after a failed 2009 climate summit in Copenhagen, Denmark, that UN climate chief Christiana Figueres said she thought a global deal would not happen in her lifetime. Now she expects the Paris Agreement to take effect by 2018.
Under the agreement, countries set their own targets for reducing emissions of carbon dioxide and other greenhouse gases. The targets are not legally binding but countries must update them every five years.
That is because scientific analyses show the initial set of targets that countries pledged before Paris do not match the long-term goal of the agreement to keep global warming below two degrees Celsius (3.6 degrees Fahrenheit), compared with pre-industrial times. Global average temperatures have already climbed by almost one degree C. Last year was the hottest on record.
"Even if the Paris pledges are implemented in full, they are not enough to get us even close to a two-degree pathway," said John Sterman, of the Massachusetts Institute of Technology. "I don't think people understand how urgent it is."
The latest analysis by Mr Sterman and colleagues at the Climate Interactive research group shows the Paris pledges put the world on track for 3.5 degrees C of warming. A separate analysis by Climate Action Tracker, a European group, projected warming of 2.7 degrees C.
Either way, scientists say the consequences could be catastrophic in some places, wiping out crops, flooding coastal areas and melting glaciers and Artic sea ice.
Small island nations and other vulnerable countries managed in Paris to get others to agree to an aspirational goal of keeping the temperature rise below 1.5 degrees C, which many analysts say will not be possible without removing vast amounts of greenhouse gases from the atmosphere.
"In Paris they reached what was possible diplomatically and maybe went a little bit beyond it," Mr Sterman said. "I think we should celebrate it. But the physics of the climate are relentless."
There is some good news. Global energy emissions, the biggest source of man-made greenhouse gases, were flat last year even though the global economy grew, according to the International Energy Agency. Some say that shows countries are finally driving their economies forward without burning massive amounts of oil, coal and gas.
Still, those fossil fuels are used much more widely than renewable sources like wind and solar power.
After signing the agreement, countries need to formally ratify it. Procedures for doing that vary among nations. The UN says about 10 countries, most of them small island developing states, will deposit their instruments of ratification on Friday and that the world body will have a better idea by the end of the day which other countries intend to ratify the agreement this year.