Friends of the Earth CEO Craig Bennett said Mr Trump - who once dismissed man-made climate change as a Chinese-inspired hoax - risked turning the U.S. into a "pariah state" unless he finally came out in support of the Agreement, which aims to stop global temperatures from rising more than two degrees centigrade and was accepted by his predecessor Barack Obama a year ago.
As the G7 talks came to an end, President Donald Trump seemed to have forged a bond with newly-elected French President Emmanuel Macron, a French official told CNN.
It was Trump's final stop on his 9-day overseas trip.
The G-7 summit began in the shadow of Trump's remarkable appearance at North Atlantic Treaty Organisation, in which he chastised some of the most loyal US allies for not paying their fair share to the alliance. He described a "very robust conversation ... a lot of give and take" in discussions that included leaders from Japan, Britain, France, Germany, Canada and Italy.
Trump tweeted that he would make a final decision next week on whether to withdraw from the pact.
The reaction was swift and critical.
This is Mr Trump's first G7 summit - during his first foreign trip. "But he stands in stark isolation".
There's just one problem: Trump was listening to the speech through an earpiece in his right ear, according to press secretary Sean Spicer. "There's a situation where it's six, if you count the European Union, seven, against one", she said.
Merkel left the summit telling reporters that the climate talks in particular were "very unsatisfactory".
"President Trump, more than anyone else, has taken on the role of spoiler-in-chief, blocking agreement on numerous key concerns that affect millions of the world's poorest people", commented Oxfam Italy Executive Director Roberto Barbieri.
Gary D. Cohn, director of the National Economic Council, insisted that the other countries understood Trump's refusal to make a decision on the accord, even if they did not support it.
That, they say, explains the confrontational rhetoric despite the administration having yet to define exactly what it plans to do on the key issues of trade and climate change.
He added: "We're all allies".
"The times in which we could completely depend on others are on the way out".
Julianne Smith, from the Center for a New American Security (CNAS), said the summit showed how team Trump appeared determined to maintain a "strategic ambiguity" whatever the cost to long-established relationships. "It's only fair to the United States".
So did the aerospace industry, steel and aluminum and how the economies in both countries are integrated at many different levels, Trudeau said, as he rhymed off what sounded like a greatest-hits list of Canada-U.S. trade irritants.
The official said this is based on Trump talking about innovation in clean technology and renewable energy, as well as demonstrating an understanding of climate change. Cohn stressed Friday that the president was simply being critical of the US trade deal with Germany.
The exit of the United States, the world's largest economy and second-largest greenhouse gas emitter after China, would not immediately dissolve the climate pact, which was legally ratified a year ago. European leaders he met with at the Group of 7 summit in Sicily have been pressuring Trump to stay in the accord, arguing that America's leadership on climate is crucial.
But that was then, when Barack Obama still occupied the White House. The latest move by Trump almost, albeit not entirely, negates that.
"The United States is considering its position in relation to these matters and what its policy is going to be, but there was no doubt around the table about how important this issue on climate change is", May said.