Culture Minister Steven Guilbeault resigned from Prime Minister Mark Carney’s cabinet late Thursday to protest his government's new pact with Alberta on a proposed new pipeline.

The environmentalist and longtime climate activist was the face of the previous Liberal government's ambitious climate agenda for more than half a decade.

In a social media post Thursday, Guilbeault said he submitted his resignation to Carney that afternoon "with great sadness" — then spelled out his objections to the prime minister reversing many of the key environmental decisions made under the previous Liberal government.

"Despite this difficult economic context, I remain one of those for whom environmental issues must remain front and centre," he wrote. "That is why I strongly oppose the memorandum of understanding between the federal government and government of Alberta."

He said he intends to remain on as a Liberal MP but will no longer serve as Carney's Quebec lieutenant.

In a social media statement posted late Thursday, Carney said Guilbeault’s "leadership to advance sustainability has consistently shaped a more hopeful horizon for future generations.

"As Prime Minister, I have been deeply grateful for his counsel and contributions to our new government, which shares his fundamental commitment to climate ambition and climate competitiveness for Canada.

"While we may have differing views at times on how exactly we make essential progress, I am glad Steven will continue to offer his important perspectives as a member of Parliament in our Liberal caucus."

Guilbeault avoided TV cameras and questions from reporters for several days as news emerged of progress on an agreement between the federal government and Alberta on the energy file.

On Thursday, he made it clear that he was quitting cabinet over the pipeline deal and the government's pullback from major federal climate policies.

"Finally, over the past few months, several elements of the climate action plan I worked on as minister of the environment have been, or are about to be, dismantled," Guilbeault wrote.

He went on to cite the consumer carbon price, the electric vehicle sales mandate, the oil and gas emissions cap and the framework to eliminate fossil fuel subsidies.

"In my view, these measures remain essential to our climate action plan," Guilbeault wrote.

He added that the decision to exempt Alberta from the clean energy regulations "in exchange for stricter industrial carbon pricing rules" and the Pathways Alliance carbon capture project "is, in my view, a serious mistake."

Green Party Leader Elizabeth May said she respects Guilbeault's decision and stressed the need for MPs to be strong advocates for climate change action.

"Steven Guilbeault leaving cabinet dashes the last hope that Mark Carney is going to have a good climate record, ever," May told reporters at a news conference about the pipeline deal.

Many opposition MPs praised Guilbeault for taking a principled stand.

Bloc MP Alexis Brunelle-Duceppe told reporters Thursday that Carney is governing like former prime minister Stephen Harper and he salutes Guilbeault for taking a stand.

Conservative MP Dan Albas praised Guilbeault on Thursday after accusing him the day before of lacking the courage of his convictions because he had not resigned.

"I was wrong. He does," Albas wrote on social media. "While I don’t agree with him on policy, I will say resigning over principle is an honourable act."

B.C. NDP MP Gord Johns said he's not surprised Guilbeault quit.

"It's completely counter to what we thought Mark Carney was all about. We're in the middle of a climate emergency and all he's talking about is increase in emissions," he said.

Former Trudeau-era cabinet minister Bill Blair called Guilbeault a man of principle. He also told reporters he doesn't believe there are other longtime Liberal MPs who feel like the last decade of Liberal climate policy is being dismantled.

Liberal MP Rob Oliphant said Carney is "walking a difficult but fine line" between resource extraction and safeguarding Indigenous and provincial rights, but the prime minister must respond decisively as U.S. President Donald Trump pursues his trade war.

"Even if Mr. Trudeau were prime minister today, there would be changes," Oliphant said. "I would call myself a centre-left, a left-wing Liberal, and I'm very comfortable with Mr. Carney as he's trying to navigate these tough waters."

Earlier in the week, Liberal MPs downplayed reports of deep discontent within caucus over Ottawa's agreement with Alberta.

The memo of understanding Carney signed with Alberta Premier Danielle Smith earlier Thursday commits Ottawa and Alberta to working toward building an oil pipeline to the West Coast, throwing the future of the B.C. coastal tanker ban into question.

The document states Ottawa's commitment hinges on the pipeline being approved as a project of national interest, but affirms that changes could be made to the tanker ban to enable oil exports to Asian markets.

“Our government is very excited about this agreement and the prosperity and opportunities it will bring for Albertans and all Canadians," Smith said in a media statement. "It’s unfortunate that Mr. Guilbeault doesn’t feel the same way.”

In a series of appearances on evening TV political shows, Natural Resources Minister Tim Hodgson called the pact with Alberta a "win from an environmental perspective."

"We get an agreement on an industrial carbon price. We all know that is the most effective way to get to net-zero,” he said on CTV.

Guilbeault was federal environment minister to former prime minister Justin Trudeau for years, and served as the chief salesperson and defender of the contentious consumer carbon tax.

He quickly became a target for Conservative critics and Alberta politicians — especially Premier Smith, who branded him as a radical climate activist out to block oilsands projects and associated employment.

Guilbeault introduced Canada’s first emissions reduction plan — effectively a road map for reducing greenhouse gas emissions by 40 per cent below 2005 levels by 2030.

He came to politics as a lifelong, prominent Quebec environmental activist, and co-founder of Quebec’s largest environmental group, Équiterre, in 1993.

Two decades ago, when he was a Greenpeace activist in his early 30s, Guilbeault climbed Toronto’s CN Tower with another activist in a PR stunt, where they set up a banner that accused Canada and the U.S. of being “climate killers.”

The feat was done to protest Ottawa's failure to ratify the 1997 Kyoto Protocol climate accord. Guilbeault ultimately pleaded guilty in an Ontario court to public mischief. 

His official federal government biography celebrates the event, and points to his earliest acts of protest: at age five, he climbed a tree behind his home to protect it from real estate developers who wanted to cut it down.

Guilbeault was first elected as Liberal MP for Laurier—Sainte-Marie in 2019 and first served in cabinet as heritage minister, where he ushered in major reforms to Canadian broadcasting policy.

He moved to the environment portfolio after the 2021 election and helmed the ministry until 2025.

This report by The Canadian Press was first published Nov. 27, 2025.

— With files from Nick Murray, Catherine Morrison and David Baxter