The US withdrawal from the Paris climate pact could "in a worst case scenario" add a 0.3 degree Celsius rise in global temperatures over the 21st century, the UN said Friday.
The head of the World Meteorological Organization'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.
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.
Terblanche also cautioned "it is still fairly early days" to assess the impact of Trump's "complex announcement" .
UN officials also stressed that the deal was structured to require action from multiple levels of government, including municipal, regional and federal authorities.
With some US cities and states immediately voicing support for the Paris deal following Trump's announcement, it is unlikely that the country as a whole will see zero implementation of the pact in real terms.
It will take several years before the world "will have a proper understanding of what the implications are" of the US withdrawal, Terblanche said.