OTTAWA -- Prime Minister Justin Trudeau kept a "back pocket" closing statement at last year's stormy G7 summit in case U.S. President Donald Trump blocked the usual united communique, the top Canadian organizer has revealed.

Sen. Peter Boehm was Trudeau's sherpa, the senior public servant who organized the Canadian-hosted G7 leaders' meeting at Charlevoix, Que., and describes the unprecedented efforts to manage Trump's potential disruption at the meeting in an article for the French foreign policy journal, "politique etrangere."

Boehm retired from the public service three months after the summit, which ended with Trump unleashing a tirade of Twitter insults at Trudeau and his withdrawal of support for the summit's final communique.

Last September, Trudeau appointed Boehm to the Senate as an independent after a long career as a diplomat that included ambassadorships and deputy minister level offices in Ottawa.

It was no secret in the lead-up to last year's G7 that the government fretted that Trump would be a disruptive force.

Trudeau is expected to attend this year's leaders meeting later this month in France, and French officials are scrambling to have a successful summit -- with or without Trump's co-operation.

Boehm argues in his 11-page essay of the need for a successful G7 to tackle global problems with a united consensus among its member countries that includes Britain, Germany, Japan and Italy.

"The single greatest downside of using this method in a body like the G7 is that one unco-operative member can spoil desired outcomes worked toward and advocated for by the rest," he writes.

Ordinarily, the final communique, a statement that sherpas carefully craft for their leaders, is the main way for the G7 to show "unity and leadership to the world," says Boehm.

But Canada was taking nothing for granted at last year's summit.

"The choice for the Canadian summit at Charlevoix was whether to undertake the effort of negotiating a communique at all," says Boehm.

That's because there were "deep differences" between the Americans -- Boehm never names Trump directly -- the other six countries and the European Union (also a G7 member) on several issues, he says. Among the issues were climate change, the international rules-based order, protectionism in trade, and the nuclear deal between Iran and the Western powers, which Trump has since pulled the U.S. out of.

Because of those differences, Boehm says, "the concern was that a communique could have resulted in a 'lowest common denominator' consensual effort that would have not meant much nor passed public muster."

Eventually, the decision was taken to forge on with a communique, but with what amounted to a Plan B.

"It was decided among sherpas, and agreed to by Prime Minister Trudeau as host, that an effort at negotiating a communique should be undertaken, with the issuance of a chair's summary being a fallback position should the search for consensus prove futile. The Canadians did keep a 'back pocket' chair's statement on hand -- just in case."

The leaders mounted a decisive final effort, alongside their sherpas, and after a marathon overnight session, "a solidly substantive communique (with an expected division on climate change) was agreed to," says Boehm.

The hard work unravelled after Trump, who left early, tweeted from Air Force One that Trudeau was "very dishonest and weak" after the prime minister reitirated his criticism of U.S. steel and aluminum tariffs during his closing press conference. Trump also said he was withdrawing his support for the communique.

"Attempting to break consensus, ex post facto following the release of the Charlevoix Summit documents by using Twitter to demonstrate pique directed at the host was also a new, not to mention unwelcome, phenomenon," writes Boehm.

John Kirton, a University of Toronto expert on G7 summitry, said he doesn't expect Trump to pose the same threat to French President Emmanuel Macron in Biarrritz, France this month.

There will be no consensus on climate change because Macron, as host, won't agree to any watering down of support on the Paris climate accord that Trump has disavowed, Kirton said. The U.S. and the rest of the G7 will simply have to agree to disagree, he said.

But Trump may be more conciliatory on other topics so he has allies ready to help him deal with the nuclear crises involving Iran and North Korea, as well as his trade war with China, said Kirton.

Moreover, the United States will be assuming the presidency of the G7 in 2020 and Kirton said he's encouraged by recent reports Trump is considering hosting the gathering at Trump National Doral, his luxury golf resort in Miami.

Kirton expects Trump to move the G7 up to the spring, perhaps May, to get ahead of the Republican and Democratic national conventions in the summer as the U.S. presidential election campaign heats up.

"I think he wants to be seen leading the world at home, and, of course, I predict that in the mind of Donald Trump it will be the best G7 summit in world history," said Kirton.

"He's got an incentive to make it a success. You can't trash Macron and expect him to make your summit next year a success. You've got that kind of reciprocity at work already."