Kerry will travel to Scotland this weekend and put his 30 years’ of climate experience on the line as almost 200 nations gather for a nearly two-week summit. But as his political star power at home wanes, he’ll likely arrive in Glasgow without the bold domestic climate action that Biden had sought as Democrats in Congress back off from their most ambitious proposals for eliminating the country’s use of oil, natural gas and coal. Lawmakers may not even be able to deliver a shrunken climate compromise before the summit begins.

Kerry returned from retirement to lead Biden’s team in a climate envoy position specially created for him by his colleague of 20 years in the Senate, and whom he succeeded as chair of Foreign Relations Committee in 2009 when Biden became vice president. But he may find that his prestige around the world on the issue does little to persuade countries like China and India to abandon the fossil fuels they see as critical to their economies — if the United States isn’t willing to take the first step.

The U.S. has been pressing China to accelerate its plans to eliminate greenhouse gases, but Beijing has countered that the U.S. itself walked away from climate diplomacy under former President Donald Trump. Beijing also says the deterioration in relations between the two countries was undermining progress on climate cooperation.

And without major U.S. climate legislation, “I think that people will wonder whether the U.S. can get its act together,” said John Podesta, the former Obama White House climate adviser who is close to Biden’s team. “They’ll begin to wonder about the credibility of the United States to get the job done.”

Democrats in Congress saw their most ambitious plan to tackle climate change stymied by coal-state Sen. Joe Manchin, who opposed a measure to pay utilities who increase carbon-free power and penalize those that don’t. That left their package — which is still in flux — to rely heavily on tax incentives to promote clean energy, measures that experts say won’t drive a speedy enough transition to keep the world from suffering the ravages from higher temperatures.

“If we want to be seen with global credibility when it comes to climate — one of the major issues of our time — then we need to deliver on substantive policies that are reducing emissions and reducing subsidies for fossil fuels,” said Rep. Ro Khanna (D-Calif.). “It’s up to Congress to deliver.”

Meanwhile, the European Union, which has a record of regulating industries to wring carbon out of its economy, is weighing the creation of a carbon border tax that would penalize imports of some carbon-intensive products from countries that don’t have their own strong climate policies. That measure is intended to protect the bloc’s industries that fear addressing climate change will impose costs that make them less competitive, but is likely to stir up trade tensions with trading partners, like the U.S.

For Kerry, who sits at the nexus of Biden’s deep bench of climate experts, it’s never been more critical to achieve the goal of staving off the surging sea levels, devastating droughts and wildfires, and brutal storms that a changing climate is bringing.

Six years after he led the diplomatic effort that cemented the landmark Paris climate deal, he’ll try to cajole the often recalcitrant governments of China and India to step in line with the effort to phase out fossil fuels. And he’ll have to do that as many in the United States — including skeptical members of his own party — are wary of turning off the spigot that has made the country the biggest oil and gas producer in the world.

The son of a foreign service officer and social activist, Kerry has often cast his causes in moral terms, from his 1971 testimony against the Vietnam War in front of a Senate committee to his calls this year to heighten the the urgency of reducing greenhouse gases. The goal of those efforts is to prevent global temperatures from rising 1.5 degrees Celsius above pre-industrial levels — warming that scientists predict would bake in sea-level rises that would render several island nations uninhabitable in the coming decades and threaten millions of people.

As it stands now, the world is on track to see a 2.7 degrees Celsius increase by the end of this century which would raise seas, make entire regions unbearably hot and spark even more disastrous flooding, wildfires and droughts.

“How many politicians,” Kerry asked in an interview earlier this year, “how many scientists, how many people have stood up and said, ‘This is existential for us on this planet’? Existential. That means life and death. And the question is, are we behaving as if it is? And the answer is no.”

Kerry’s job officially is as Biden’s special presidential envoy for climate, and he answers only to the president — with whom he’s in frequent contact — and his former subordinate Antony Blinken, the U.S. secretary of state.

He’s also in regular touch with officials atop the Treasury Department — the crucial agency for creating financial networks to slow fossil fuel financing, guarding the economy from climate shocks, and delivering the promised billions of dollars to developing nations.

And he regularly confers with members of the Senate, where he served for 28 years representing his home state of Massachusetts.

So why, at 77, does he still do it? According to people who know Kerry, it’s primarily passion for the topic. He was at the first United Nations climate conference in Rio de Janeiro during the George H.W. Bush administration, where Kerry met his wife, Teresa Heinz.

It’s also a sense of righting the ship after Trump abandoned the global effort and withdrew from the Paris pact, even if the U.S. was out of the deal for only a few months before Biden took office and rejoined the rest of the world.

And, as Kerry frequently recounts, it’s about the granddaughter who sat on his lap as he signed the Paris climate agreement.

None of those things surprise the people who worked for him, who say he has expressed no plans to stop the work, regardless of whether the Glasgow gathering ends in success or failure.

“I have heard nothing whatsoever to suggest that he is going anywhere,” said Todd Stern, who led climate negotiations for the Obama administration and worked under Kerry when he was secretary of State. “He is a deeply committed guy.”

“My expectation is that he continues,” Stern added. “I have no reason to think that is not the case.”

Despite complaints from Republicans that the Paris agreement would put the U.S. energy industry at the mercy of UN bureaucrats and environmentalists, the pact has no enforcement mechanism, and in fact imposes no requirements, other than that countries submit their own self-determined targets for reducing greenhouse gases. The idea behind it was that countries would feel pressure to match the efforts of other nations, and pledge ever-more aggressive actions to avoid upsetting the global strategy.

That’s the type of pressure Kerry has been applying on trips to Beijing and New Delhi, as well as numerous other capitals around the world. Still, persuading countries to put a long-term planetary benefit over a shorter-term economic gain is no easy task.

Some experts, though, say the success of Kerry’s last big foray into climate diplomacy poses a problem: The fact that the Paris agreement that Kerry struck in 2015 lacked teeth has locked the world into a process that won’t produce the action needed to prevent disastrous climate changes.

“To me, it underlines the inability of that process to get the most responsible and accountable nations to take the kind of binding, trade-enforceable commitments that are the hallmark of every other successful treaty,” said James Connaughton, a former George W. Bush administration environmental adviser.

Still, few people have Kerry’s level of experience on the issue, an asset that will help him on the ground in Glasgow.

“He’s deeply knowledgeable on the details of climate change policy and diplomacy,” David Sandalow, a former Clinton and Obama administration climate and energy official who is now at Columbia University’s School of International and Public Affairs, said in an email. “When I was a U.S. climate change negotiator in the 1990s, no member of Congress knew more or engaged more deeply on climate change diplomacy than Secretary Kerry.”

As his former Senate and Obama administration colleague Chuck Hagel saw it, Kerry was “the environmental senator” once Al Gore had left the upper chamber to become vice president.

Yet the Nebraska Republican famously authored a congressional resolution that helped brand the U.S. as a climate spoiler by foreclosing the ratification of the Kyoto Protocol, the 1997 treaty that would have mandated carbon cuts from rich nations. The Senate resolution called for blocking that treaty unless it required emissions reductions from every country.

The measure was a shot across the bow at China and India, which sought more lenient treatment because they were considered developing nations. The resolution passed the Senate 95-0 — with even Kerry voting for it.

Hagel said he and Kerry never differed on the importance of climate change, just on whether the United States’ own actions were sufficient to compel others. He credited Kerry’s work in the Senate and later as secretary of State for getting more recalcitrant nations to step up on climate.

“What’s changed is that we’ve made progress on other nations finally stepping up to recognize that this is a global problem,” Hagel told POLITICO. “Kerry has had immense responsibility for all of that.”

That emphasis on climate became a hallmark of how Kerry approached his role as secretary. He elevated the issue to an importance akin to terrorism, poverty and weapons of mass destruction, as he told an audience in Jakarta in 2014.

Conversations with top foreign officials often veered into climate change even if it wasn’t on the agenda, said Hagel, who as defense secretary during the Obama administration sometimes joined Kerry for negotiations with counterparts from other nations. Kerry routinely confronted nations reluctant to talk about climate change, like Russia, on the topic, Hagel recalled.

“He knew that in any dealings with the Russians it was always a problem. It was a hard sell. But that never stopped John,” Hagel said. “I’ve seen him go into encyclopedic depth on the environment. No matter how far you are in the depth on this issue, John will go there with you. And he is so passionate about it that people will listen.

Aides and colleagues describe him as meticulous on the details and focused on delivering results. His steady and persistent pressure was credited with persuading China to announce an end to public financing for coal projects outside its borders in September — a big accomplishment, even if Beijing had already begun pulling back from funding those power plants.

At Glasgow, he’ll be relying on those long-established relationships to strike deals on goals like delivering more climate finance to developing nations, ratcheting down coal-fired power plant construction and buoying commitments to preserve carbon dioxide-absorbing forests.

Even China tacitly acknowledged Kerry’s role as the U.S. climate heavyweight. Shortly after Kerry was named to the climate envoy position, Beijing tapped its retired diplomat Xie Zhenhua to head its climate negotiating team.

Xie and Kerry go way back; Podesta, who ran climate policy in the Obama White House, recalled hosting Xie at a meeting in the Senate Foreign Relations Committee briefing room when Kerry was chairman of the committee.

“It’s a pretty long-standing relationship,” Podesta told POLITICO, saying it dated back to at least 2009.

Kerry would lean on his ties to Xie as the two hammered out a momentous bilateral climate deal between the U.S. and China in 2014. Climate experts credited it with setting up the Paris agreement the following year, the first to ever put all the nations on Earth into one climate compact.

“Xie clearly knew Kerry. Xie wanted to get on the phone early with Kerry and they had a good long talk,” Stern, the Obama-era climate negotiator, said of the 2014 deal.