Canadian Prime Minister Justin Trudeau on Tuesday is reportedly expected to approve a $5.5 billion expansion of the Trans Mountain tar sands pipeline, a move environmentalists warned would make an "absolute mockery" of the House of Commons' vote to declare a climate emergency just hours earlier.

The vote Monday night made Canada the third nation to declare a national climate emergency, but critics said purely rhetorical acknowledgements of the planetary crisis are meaningless without concrete action.

"It's great that more countries and regions are doing this," tweeted 16-year-old Swedish climate activist Greta Thunberg after the vote. "But remember: The fossil fuels must stay in the ground. Forget 'climate neutral' and clever accounting. Our emissions must start their way to zero. Now."

Trudeau's government purchased the Trans Mountain pipeline from Kinder Morgan for $4.5 billion last May, sparking outrage from environmental groups and indigenous tribes.

Critics warned that approval of the pipeline expansion would represent a major blow to efforts slash carbon emissions to avert disastrous warming.

As the Washington Post reported, the Trans Mountain expansion project "would nearly triple the amount of Canadian crude oil the pipeline transports each day from Edmonton, Alberta, to the port in Burnaby, B.C., to be loaded onto tankers bound for Asia and other lucrative markets."

Cam Fenton, communications and strategy manager for 350.org Canada, tweeted that the Trans Mountain pipeline expansion is "out of line with a plan to meet a 1.5°C warming limit" by the end of the century.

In a report published last October, the U.N.'s Intergovernmental Panel on Climate Change (IPCC) warned that warming beyond 1.5°C could spark a global and "irreversible" climate catastrophe.

In a statement ahead of Trudeau's decision, the Green Party of British Columbia said approval of the Trans Mountain pipeline expansion would be "reckless."

"As elected leaders, we must act immediately and with courage to make the urgent transition away from fossil fuels and toward a clean, sustainable economy," the statement read. "We can make the changes demanded of us and create widespread prosperity as we do so. Canada's global reputation, and the future we leave for our children, are on the line."