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Guilbeault has become increasingly disillusioned in recent months by what he has called the dismantling of climate policies under Prime Minister Mark Carney.

He quit Carney's cabinet last year after Ottawa's energy deal with Alberta was made public. In November 2025, Carney and Alberta Premier Danielle Smith signed a memorandum of understanding on working toward approval of a bitumen pipeline to transport one million barrels or more per day from Alberta to Asian markets through a port on the B.C. coast.

In a media statement, Guilbeault said he strongly opposed the agreement between Ottawa and Alberta and called the proposal to exempt Alberta from clean electricity regulations "a serious mistake."

He wrote that "several elements of the climate action plan I worked on as minister of the environment have been, or are about to be, dismantled: the consumer carbon pricing, the zero-emission vehicle standard, the oil and gas sector emissions cap, the framework to eliminate fossil fuel subsidies and the clean electricity regulations."

Since then, he has publicly criticized his own government's approach to climate policy.

The Liberals currently have a slim majority, with 174 seats in the House of Commons.

MP Jonathan Wilkinson, another former environment minister, is set to leave his role to serve as Canada's next ambassador to the European Union.

In an open letter to his constituents last month, Wilkinson said he would be vacating his seat "in the coming months." He told ¹ú²úÓÕ»ó¸£Àû he'll "probably" start the role in early July.

Last week, the CBC reported that 14 Liberal MPs sent a letter to the prime minister at the end of April to express their concern about the government's planned agreement with Alberta. 

At a housing announcement Monday, Carney said that in a healthy caucus, there is always an exchange of views and suggestions. 

"There's 160 other members of that caucus," he said. "We have to take decisions that are in the interests of the country."

Responding to media reports stating Guilbeault was set to resign, federal ministers Lena Diab and Marjorie Michel showered the former environment minister with praise.

Michel, who was holding a press conference on Parliament Hill on Tuesday about the Ebola outbreak in Africa, said Guilbeault was a "fantastic" colleague.

"I really appreciate his work, his advocacy, and I'm fully respectful of his decision," she said. "At the same time, I have complete faith in what our government is doing."

Diab said in French that Guilbeault was an excellent minister and member of Parliament.

Guilbeault has represented the Montreal riding of Laurier—Sainte-Marie since 2019.

Guilbeault has often said his activism began at age five, when he climbed a tree in an attempt to stop real estate developers who were clearing forest near his home in La Tuque, Que.

In 1993, he co-founded Équiterre and helped it become the largest environmental organization in Quebec. He was its senior director from 2007 to 2018.

Before politics, he also worked with Greenpeace, Deloitte and Copticom.

Guilbeault was one of two Greenpeace activists arrested in 2001 for climbing the CN Tower in Toronto — then the world's tallest building. The two activists unfurled a banner from the observation deck in an attempt to draw attention to climate change and protest the decision by the Canadian and American governments not to ratify the 1997 Kyoto accord.

International Trade Minister Maninder Sidhu told ¹ú²úÓÕ»ó¸£Àû on Tuesday that he has "tremendous" respect for Guilbeault and his work. 

"Our conversation will continue on the environment," he said.

Guilbeault joined former prime minister Justin Trudeau's cabinet as heritage minister. Two years later, he took over the environment portfolio.

He led Canada's participation in the United Nations' climate change conference, COP29, in 2021 and spearheaded both the 2030 emissions reduction plan and the development of a national climate adaptation strategy and nature conservation plan.

Guilbeault was one of the first members of Trudeau's cabinet to publicly endorse Carney's leadership of the Liberals.

"I've known Mark for many years. We've worked together on issues of green energy, transition, fighting climate change, and the role of the financial sector in fighting climate change," he said at a cabinet retreat on Jan. 21, 2025.

He said at the time he was working with Carney to develop his climate plan. He also pivoted from his years of advocacy when he told reporters that while he believed the consumer carbon price was "one of the best tools we have to fight climate change, it's not the only one."

This report by ¹ú²úÓÕ»ó¸£Àû was first published May 26, 2026.