Former secretary of state John Kerry signed the 2015 Paris Agreement on behalf of the United States, a decision subsequently reversed by President Donald Trump.
Now, he is set to be President-elect Joe Biden's climate envoy, in a clear sign of the upcoming administration's renewed commitment to fighting climate change.
"I'm returning to government to get America back on track to address the biggest challenge of this generation and those that will follow," Kerry tweeted shortly after his appointment.
"The climate crisis demands nothing less than all hands on deck."
The 76-year-old, a longtime Senate colleague, friend, and political ally of Biden who stood by the president-elect when his candidacy was in crisis, brings to the table the clout and connections associated with being ex-president Barack Obama's top diplomat.
The chief architect of the Iran nuclear deal will need all his skills as a statesman as the US looks to rebuild its strained credibility when it returns to the Paris accord, which Biden has vowed to do on the first day he takes office.
Despite his advancing age, observers say the Democratic Party grandee, who unsuccessfully ran for president in 2004, has lost none of his zest for international affairs.
Last year he pivoted towards making climate his signature issue, launching a cross-party coalition called "World War Zero" that included top military officials, Republican Arnold Schwarzenegger and celebrities like Leonardo DiCaprio and Emma Watson.
"Things are getting worse, not better. So we have our unlikely allies coming together here... to treat this like a war," he said.
The years of climate inaction under Trump have made the war harder to win.
Emissions from the world's second-biggest polluter have been falling in recent years thanks to the increased contribution of natural gas and renewables -- and this year by the Covid-19 pandemic.
But the rate isn't close to what is needed to achieve the goal Biden has set for the United States, of net carbon neutrality by 2050.
The Paris agreement aims to limit end-of-century warming below 1.5 degrees Celsius (2.7 degrees Fahrenheit), to avoid triggering a series of catastrophic climate tipping points that could confine most of humanity to the planet's northern and southern bands.
Scientists have calculated that carbon neutrality -- which would involve reducing emissions and increasing the amount of carbon captured from the atmosphere -- must be achieved by the middle of the century to reach that goal.
Back in 2015, setting a national carbon neutrality target of 2050 seemed radical. But since then, many have done just that, including the European Union.
China recently announced its intentions to get there by 2060, all the while scolding Washington for "obstructing" the global fight.
Kerry's first task, therefore, will be to regain the trust of international partners whose faith was shaken by Trump's unilateral withdrawal from the agreement and disdain for climate science, which eased the way for countries like Australia and Brazil to weaken their ambitions.
It's a tall task, but Kerry has an established track record.
Beyond Paris and Iran, his diplomatic accomplishments include reaching a deal with his Russian counterpart Sergei Lavrov to destroy Syria's chemical weapons, and pacifying Pakistan in the wake of the Osama Bin Laden raid, after the wayward ally was kept in the dark.
That's in addition to being a highly-decorated veteran from the Vietnam war, which he signed up for while still a student at Yale University, only to grow disillusioned with the futility of the conflict.
He trained as a lawyer, but was drawn into the world of politics, becoming elected senator for Massachusetts.
The tall, patrician Bostonian, who sports a thick mane of gray hair and is known to speak good French, will need to hit the ground running. Biden will be expected to unveil America's new climate plan to the world ahead of a UN conference in Glasgow next November.