Joe Biden's victory over President Donald Trump has brought hope to climate activists who now see a better chance to save the planet through massive projects to limit global warming.
Activists and scientists feared further climate change destruction from another four years of Trump in denial in the White House.
His defeat totally changes the landscape.
The veteran Democrat leader has pledged to take the United States back in to the Paris climate agreement that Trump forced the country to leave, and which happened officially only on November 4.
The Democrats have drawn up a $1.7-billion-dollar plan to turn the US carbon neutral by 2050.
"Joe Biden's historic election win is the first step towards avoiding climate catastrophe," tweeted Jennifer Morgan, executive director of Greenpeace International.
"The American people are demanding a climate champion in president-elect Biden and in Kamala Harris," his vice-president, she added.
France's former prime minister Laurent Fabius, who chaired the Paris agreement negotiations, cheered Biden's success saying it "raises new hopes in the indispensable fight against climate change".
"Now is the moment to relaunch global, concrete and coordinated climate action" ahead of next year's COP15 conference on biodiversity and COP26 on climate, Fabius urged.
For Laurence Tubiana, one of the architects of the Paris accords, "The Biden-Harris Administration has an historic opportunity to enact one of the world's largest green stimulus efforts, to accelerate the US economy toward sustained emissions reductions while rebuilding and creating a fairer society."
To stay within the Paris deal's 1.5C warming limit since pre-industrial times and cut the growing ferocity of wild weather, greenhouse gas emissions need to be slashed almost 50 percent by 2030, UN climate experts say.
That will require radical economic reform on a global scale, a challenge experts hope will be more within reach under a Biden presidency.
The Climate Action Tracker group put out a statement saying the election outcome could prove "a tipping point" that puts the Paris agreement's 1.5C limit "within striking distance".
That would however need the US to achieve carbon neutrality by 2050 and for Europe, China and Japan to also keep their climate commitments.
Said Johan Rockstrom, director of the Potsdam Institute for Climate Impact Research: "If the US adopts net-zero emissions by 2050, we would have the four largest economic regions in the world aligning with science and showing the path towards a safe, clean and modern future".
Pennsylvania State University climate scientist Michael Mann declared himself "cautiously optimistic" with Biden at the helm.
"But make no mistake. The sobering reality is that even if every country meets their commitments under the Paris agreement (and many, including the US and EU are currently falling at least a bit short), that gets us less than half way to where we need to be."
"I.e. on a path to limiting warming below 2C (let alone the more stringent 1.5C many are now calling for)," Rockstrom said.
Biden's room for manoeuvre will depend greatly on his ability to push through ambitious climate legislation.
And for that he will need the US Senate, which may yet stay under a Republican majority. With both parties equal on 48 senators, two seats will be run-off in the state of Georgia on January 5.
Without the Senate, Biden will have to count on the multitude of non-federal bodies from the states and cities to companies in the effort to meet previous Paris targets. According to Climate Action Tracker that means by 2025 a 26-28 percent reduction in emissions from a 2005 base.
With the Paris agreement's fifth anniversary next month, the green lobby is hoping at the very least for a return of American leadership to the climate front.
"By re-entering the Paris Agreement on Day One, President-elect Biden can boost confidence in the international cooperation and begin to restore US standing the world," said World Resources Institute president Andrew Steer.
"This is a new day for the climate, the environment and the American people... a better tomorrow is possible," the think-tank head added.
However, Potsdam Institute co-director Ottmar Edenhofer warned: "Generations to come can either remember the Biden-Harris Administration as one that failed great expectations -- or as one that really served the US people, and the world."