The head of the World Meteorological Organisation's atmospheric research and environment department, Deon Terblanche, underscored however that the likely impact of US President Donald Trump's widely-condemned decision remains far from clear.
"We haven't run new models overnight but the indications are that it could be in the worst case scenario in the order of 0.3 degrees Celsius," Terblanche told reporters in Geneva, referring to a possible rise over pre-industrial temperatures.
"That is the worst case scenario and this probably not what will happen," he added.
Under the Paris deal agreed in 2015, world nations vowed steps to keep the worldwide rise in temperatures "well below" two degrees Celsius (3.6 degrees Fahrenheit) from pre-industrial times.
What is the Paris Climate Agreement?
The fight against climate change is "unstoppable" and nations should stick to the landmark Paris accord, UN Secretary General Antonio Guterres said Friday after the US chose to exit the deal.
"Climate change is undeniable and it is one of the biggest threats to our present world, to the future of our planet," Guterres told journalists on the sidelines of a conference in Saint Petersburg.
"On the other hand, climate action is unstoppable and I urge governments around the world to stay the course, to remain committed to the implementation of the Paris agreement to the benefit of all of us," he said.
The UN chief said he was "deeply convinced" that US "states, cities, the business community, the civil society will also remain engaged, will bet in the green economy".
Guterres says climate action 'unstoppable'
World leaders have reacted with anger and defiance after President Donald Trump announced that the United States, the world's second biggest carbon emitter, was quitting the 2015 Paris climate agreement.
Led by Germany's Angela Merkel and France's Emmanuel Macron, they have branded Trump's decision as misguided and vowed to defend an accord they consider crucial to the future of the planet.
The reaction comes as the EU and China moved Friday to fill the leadership void on the Paris climate pact after President Donald Trump pulled the US out of the deal, triggering a furious global backlash.
Trump announced Thursday at the White House his administration would immediately stop implementing the "bad" 195-nation accord brokered by his predecessor Barack Obama in 2015 in tandem with Chinese leaders.
In what could turn into a global diplomatic realignment, the European Union and China held summit talks that EU chief Jean-Claude Juncker said could be a signal to the world.
"There is no reverse gear to the energy transition, there is no backsliding on the Paris agreement," Juncker said as he opened trade and climate talks with Chinese Premier Li Keqiang in Brussels.
In his speech, Li warned the world would become "a jungle" without multilateral rules, though he did not mention the Paris pact specifically.
Scientists warn of impacts of US decision
Expressions of shock and regret came from around the world, including from Pacific islands at risk of being swallowed by rising seas and who accused Washington of "abandoning" them.
In Beijing, Chinese foreign ministry spokeswoman Hua Chunying said "China as a responsible major country would uphold the pact." China is the world's biggest greenhouse gas emitter, followed by the US.
German Chancellor Angela Merkel, who met with Li on Thursday in Berlin, pledged "more decisive action than ever" to protect the climate after Trump's "highly regrettable" decision.
Germany is the driving force behind the Europe Union's economy, one of the world's biggest with more than 500 million people.
In a nationalistic "America First" Rose Garden announcement, Trump said he was withdrawing from a deal that imposes "draconian financial and economic burdens" on the United States, while going to easy on economic rivals China, India and Europe.
"We don't want other leaders and other countries laughing at us anymore. And they won't be."
'Seriously wrong'
Trump offered no details about how, or when, a formal withdrawal would happen, and at one point suggested a renegotiation could take place.
That idea was unceremoniously slapped down by furious allies in Europe, who said the deal would stand.
Juncker denounced as "seriously wrong" Trump's decision to pull out of the deal while climate action and energy commissioner Miguel Arias Canete pledged "continued global leadership" on fighting global warming.
"The agreement cannot be renegotiated," France, Germany and Italy said in a joint statement.
World leaders defiant
The United States is the world's second largest greenhouse gas emitter after China, so Trump's decision could seriously hamper efforts to cut emissions and limit global temperature increases.
Trump faced domestic shock and anger, including from Obama, who said the United States was "joining a handful of nations that reject the future."
Nicaragua and Syria are the only countries not party to the Paris accord, the former seeing it as not ambitious enough and the latter being racked by a brutal civil war.
Hillary Clinton, Trump's opponent in last year's White House race, called the decision an "historic mistake" where US workers and families would be left out of a global drive to switch to a low-carbon economy.
The Democratic governors of New York, California and Washington states formed a quick alliance, vowing to respect the standards agreed on under the Paris deal.
In New York, some major buildings, like the World Trade Center and City Hall, were lit green in solidarity with the climate agreement, echoing a move in Paris.
With much of the implementation of the accord taking place at the local level, the Paris accord's supporters hope the deal will be in hibernation rather than killed off entirely.
Trump's decision is likely to play well with the Republican base, with the more immediate damage on the diplomatic front.
Vice President Mike Pence, said that Trump "has demonstrated his commitment not just to keep his word, but to put American workers, American consumers, American energy, and the American people first."
Full Trump speech on exiting accord
Reality TV-style
Ever the showman, the 70-year-old Trump gave his decision a reality TV-style tease, refusing to indicate his preference either way until his announcement.
Withdrawal opponents -- said to include Trump's daughter Ivanka -- had warned that the US global leadership role was at stake, along with the environment.
A dozen large companies including oil major BP, agrochemical giant DuPont, Google, Intel and Microsoft had urged Trump to remain in the deal.
Ultimately, the lobbying by Trump's environmental protection chief Scott Pruitt and chief strategist Steve Bannon urging the president to leave won out.
Following the announcement Tesla and SpaceX boss Elon Musk, and Disney chief Robert Iger, announced they would no longer participate in presidential business councils.
"Climate change is real. Leaving Paris is not good for America or the world," Musk said.
White House officials acknowledged that under the deal, formal withdrawal may not take place until after the 2020 election.
'Morally criminal'
Indian Prime Minister Narendra Modi -- who is due to visit the White House shortly -- has said failing to act on climate change would be "morally criminal."
Trump's announcement comes less than 18 months after the climate pact was adopted in the French capital, the fruit of a hard-fought agreement between Beijing and Washington under Obama's leadership.
The Paris Agreement commits signatories to efforts to reduce greenhouse gas emissions that cause global warming, which is blamed for melting ice caps and glaciers, rising sea levels and more violent weather events.
They vowed steps to keep the worldwide rise in temperatures "well below" two degrees Celsius (3.6 degrees Fahrenheit) from pre-industrial times and to "pursue efforts" to hold the increase under 1.5 degrees Celsius.
