John F. Kerry was on a Boeing 757 government jet in November 2016, en route to Antarctica to observe scientists studying the impact of climate change, when he learned that Donald Trump would be the next president.

There, high above the ocean, the then-secretary of state realized that an achievement central to the Obama administration, and to his own legacy, would almost certainly be dismantled. Kerry would relive that moment repeatedly over the next four years as — out of office for the first time in decades — he watched the United States turn away from international climate efforts and become the only country to withdraw from the landmark Paris accord he had helped negotiate.

These days, Kerry, at 77 and serving as President Biden’s climate envoy, is flying commercial, catching red-eyes to Europe and meeting with leaders from France to India, trying once again to cement the United States as a world leader on climate action — and, not incidentally, salvage the fight he has long considered central to his own stamp on history.

This week’s global climate summit offers a first major test for Kerry, providing early clues as to whether his style of energetic international diplomacy is paying off. It’s an unusual final act for someone who rose to the near-pinnacle of American politics; the former presidential nominee and senator from Massachusetts has taken a job that is lower-profile than some of his former roles, and he occupies a State Department office down the hall from the one he occupied as secretary.

But the assignment itself — helping to steer the nation and world toward a less calamitous future than scientists have warned awaits as climate change intensifies — is one he views as more critical than ever.

Kerry already has visited key allies in Europe and made diplomatic stops in India, Bangladesh, the United Arab Emirates, China and South Korea, often as the first prominent Biden official whom the leaders of those nations have met in person.

The man who was a military hero at 25, who was almost president, who served as a senator for 28 years, who thought his last public job was as secretary of state under President Barack Obama, will begin finding out this week what it might take to persuade other countries to emulate Biden’s ambitious climate goals.

Kerry’s supporters say he’s well-positioned for the job in part because so much of it involves convincing allies that they can rely on the United States after the Trump years.

“I just literally cheered when I got the news about his appointment,” said Al Gore, who after losing the 2000 presidential election poured his energy into a Nobel Prize-winning climate change campaign and has been in frequent touch with Kerry. “He’s not in it for the ego or for recognition or any kind of steppingstone or some personal development goal. He’s in it to try and help. Because the help is needed. It is the challenge of our time.”

That challenge has become more daunting since Kerry helped forge the Paris climate accord six years ago. The evidence of the perils of a warming climate is even starker. And because most major countries have not fully lived up to the promises they made in Paris, the math on achieving those goals has become harder, the timeline for achieving them shorter.

The United States’ credibility, meanwhile, waned as it largely abandoned the climate battle in the Trump years. The only way to win it back, Kerry argues, is for the nation to follow through on ambitious goals. “You can’t run around the world talking to people if you’re not doing what you’re talking about,” he said in an interview during a recent trip to India.

Kerry insists that he does not view the effort through a personal lens.

“No. Profoundly no. I’ve never thought about that,” he said Wednesday during an interview with Washington Post Live. “This has nothing to do with me and legacy, which I know is the way sometimes the media has to organize itself.”

Rather, he said, he views the fight to counter climate change through the perspective of a grandfather who hopes to leave behind a more sustainable world, and of a longtime public official who has traveled to the ends of the Earth and witnessed the profound changes already taking place.

“You see these things, and nobody could not be moved to act,” he said, adding: “That’s why I’m here. There are a lot of other things that would be fun to be doing. But this is the big one.”

Kerry has always had an abundance of zeal, attempting to fight global problems to the point where critics sometimes liken him to Don Quixote, while admirers say he’s willing to remain optimistic and unafraid, against all odds, to try.

For a figure of Kerry’s stature and ambition to work as part of a White House team is, at a minimum, unusual, and it’s not fully clear how he will fit in. A former chairman of the Senate Foreign Relations Committee, Kerry was once derided as “Live Shot” for his tendency to seek television coverage.

Days before the Iowa caucuses in 2020, an NBC News analyst overheard Kerry — who was in Iowa to campaign for Biden, then doing poorly in the race — talking on the phone in a hotel lobby about the steps he would need to take to get into a presidential race himself. (Kerry later denied that he was contemplating a run, calling the notion “categorically false” and claiming that he was telling a friend the reasons doing so would be complicated.)

Now, he is working out of the State Department he used to run. Antony Blinken, who once served under Kerry, is now secretary of state, and Kerry has tried to remain deferential, even if his early enthusiasm has caused some administrative quandaries.

Shortly after Biden took office, Kerry sought to begin a direct dialogue with Xie Zhenhua, his Chinese counterpart and a longtime diplomat whom the Chinese brought out of semi-retirement to work with Kerry ahead of a critical U.N. climate summit this fall in Scotland. But their dialogue was postponed once State Department officials made clear that as a matter of protocol, Blinken needed to speak with his counterpart first.

Kerry yielded, and he met with Xie during his recent visit to Shanghai, where the two pledged to work in tandem to fight climate change “with the seriousness and urgency that it demands.”

Adding to the complexity, Biden has tapped Gina McCarthy, a former head of the Environmental Protection Agency, to oversee the administration’s domestic climate efforts while Kerry handles the international arena.

In the early days of the Biden administration, after Kerry staff members raised concerns that he was not scheduled to be in the White House briefing room when McCarthy was to discuss a climate-related executive order, last-minute accommodations were made so Kerry could join.

“Gina McCarthy, John Kerry — neither of them could be described as shy, I suppose,” said Fred Krupp, head of the Environmental Defense Fund. “But they work very well together. I know from conversations with Secretary Kerry that he has great admiration and respect for Gina, and in fact urged her appointment to the role when Biden was president-elect.”

Krupp, who has known Kerry since he was a freshman senator in the mid-1980s, recalled working with him on climate legislation in 2009, at one point huddling for five hours on a Sunday afternoon to fine-tune an effort that ultimately failed.

“I’m sure, absolutely sure, he’s thrilled to have the chance to work on one of the most important issues, perhaps the most important threat facing humanity of our times,” Krupp said.

Kerry also knows that his last political campaign is behind him, and that the reward for doing his current job well would not be another powerful position but potentially a more secure place in history.

Paul Bledsoe, a climate adviser during the Clinton administration, said Biden’s choice of Kerry for the role sends a message that the issue matters to the administration.

“The fact that Kerry is the first administration official to go to Beijing speaks volumes about how central the White House views climate within global security,” said Bledsoe, who is now a strategic adviser at the Progressive Policy Institute. “It just shows the issue has catapulted to the head of the line.”

Climate change has been an enduring feature of Kerry’s career, often entwined with his personal life and political ambitions.

Kerry was a member of a Senate delegation that traveled to Rio de Janeiro in 1992, the first major international meeting seeking to address climate change, and he has been to numerous global gatherings on the topic since. He met his future wife, Teresa, at an Earth Day rally, and she served as the interpreter for the delegation in Brazil.

“The sparks of romance may have initially begun on that trip,” Gore said (noting that it was also during that trip that he himself received a call from Bill Clinton asking if he would be willing to be vetted to be his running mate).

In 2009, Kerry crafted the Senate’s most ambitious climate legislation to date, which would have established a cap-and-trade system for greenhouse gas emissions. The legislation ultimately failed, and Kerry considered that one of the biggest disappointments of his Senate career.

As secretary of state, he directed that all high-level meetings between American diplomats and foreign officials had to include a discussion about climate change.

Now, he has one more major shot to make an impact on the issue.

“Having known John through all the phases of his career, this assignment is in many ways a capstone,” said Tim Wirth, a former senator from Colorado who was with Kerry and Gore in Rio and at other climate conferences. “He has nothing to lose. He’s not running for office; he’s not doing anything except serving the issue.”

The challenge for Kerry — and Biden — is navigating conflicting pressures at home and diplomatic complexities abroad. Republicans are objecting to Biden’s infrastructure plan, which is critical to reaching his climate goals, while some key foreign powers have not shown the same fervor as the Biden administration for committing to new emissions-cutting targets.

The White House this week is expected to announce an ambitious pledge to cut U.S. greenhouse gas emissions by at least 50 percent, if not more, by the end of this decade, compared with 2005 levels. That pledge will allow Biden and Kerry to press other major economies to follow the U.S. lead, but how many will do so remains uncertain.

This week’s meeting of world leaders is set to begin on Earth Day, five years to the day after Kerry traveled to the United Nations to formally commit the United States to the Paris climate accord. His 2-year-old granddaughter Isabelle sat on his lap that day as he signed the historic agreement, and he quoted the late South African leader Nelson Mandela: “It always seems impossible until it is done.”

Five years later, that moment of optimism has given way to uncertainty, and a looming sense that time for action is running out.

A growing list of countries have now promised to reach net-zero emissions by 2050, or in China’s case, by 2060. But as wildfires, hurricanes and other effects of climate change intensify, the urgency is growing for nations to outline even more aggressive action, and to do so quickly.

“There is a lot of pressure on moving all the big players certainly to go faster than they were planning,” said Todd Stern, who served as Obama’s special envoy for climate. “That’s what [Biden] is up against. That’s not easy to do.”

John Hudson and Tyler Pager in Washington and Joanna Slater in India contributed to this report.