Returning to the French capital Tuesday for a follow-up summit, he cut a bitter and disappointed figure.
The United States' commitment to fighting climate change -- something both Kerry and former president Barack Obama hoped would be a key part of their joint legacy -- was reversed in June when Donald Trump announced he would withdraw America from the Paris pact.
The lack of American leadership in tackling what Obama had termed "an existential threat", was plain to see at the One Planet Summit in the French capital Tuesday where more than 50 world leaders and dozens of ministers met for talks.
Only Trump and Syrian president Bashar al-Assad were not invited. The American government was represented by the second-ranked diplomat in the Paris embassy who appeared nowhere on the official programme of events.
"It's very disappointing, it's worse than disappointing, it's actually a disgrace when you consider the facts, the science, the common sense, all the work that's been done," Kerry told AFP when asked for his thoughts on the US absence.
When signing the Paris accord on behalf of the United States at a ceremony at the UN in April last year, Kerry brought his two-year-old granddaughter Isabelle on stage to underline his belief that the agreement was for future generations.
The 74-year-old stressed Tuesday that Trump's rejection of the Paris Agreement amounted to more than just a waste of the Obama administration's intensive diplomatic efforts -- seen by environmentalists as compensating for Washington's decision in 2001 to withdraw from the global Kyoto climate accord.
"It's 26 years of work that's being dishonoured by people who don't even understand the science," Kerry complained.
Is Trump for turning?
French President Emmanuel Macron called on Tuesday's summit to send a signal of global support for the climate pact following Trump's abandonment of it.
Along with other world leaders, the 39-year-old has repeatedly lobbied the New York real estate mogul to change his mind, including at a one-on-one meeting in Paris in August, and an aide confirmed last week that talks were still ongoing.
"I'm pretty sure that my friend President Trump will change his mind in the coming months or years, I do hope," Macron told the CBS channel in an interview aired on Tuesday.
Pending a radical about-turn, a high-profile delegation of American entrepreneurs, city leaders, and campaigners underlined in Paris that they were the only hope for the US meeting its target of cutting greenhouse gas emissions by at least 26 percent from 2005 levels by 2025.
"We can't wait for the White House to wake up. We in America are operating from the grassroots," California governor Jerry Brown, who has become a leading advocate for emission-cutting efforts, told a panel discussion.
Like other Americans, from Microsoft billionaire Bill Gates to former New York mayor Michael Bloomberg and ex-California governor Arnold Schwarzenegger, Brown lamented the US president's unilateral decision that has isolated Washington and angered allies.
"We have a climate denier in the White House who says climate change is a hoax," Brown said.
Bloomberg, who supports a coalition of state governments, cities and companies called America's Pledge, was upbeat about America's chances of succeeding despite the lack of federal support.
He said the 1,700-member group represented more than half the US economy and would have been the world's third-biggest economy had it been a country on its own.
"President Trump has helped rally people who understand the problem to join forces and to actually do something rather than wait for the federal government... In that sense we owe President Trump a measure of gratitude for helping us meet our goals," he quipped.
Kerry holds out some hope that his hard-fought climate diplomacy might still bear fruit.
jpegMpeg4-1280x720Asked whether the US could still meet its emissions-curbing target, he replied: "I think it's possible, yes."
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