EDMONTON -- The climate crisis is a national and global issue that can't be fought entirely by the provinces, a lawyer for the federal government argued Tuesday.

"The context of this case is the greatest existential threat of our time," said Sharlene Telles-Langdon in her opening arguments in support of Ottawa's carbon tax.

The law that brought in the tax is being challenged this week by Alberta in the province's Court of Appeal.

Ontario and Saskatchewan have also gone to their top courts to oppose the tax, but lost. They are appealing to the Supreme Court of Canada.

Ottawa argues its authority for the tax comes from the Constitution's peace, order and good government clause. Establishing minimum national standards on greenhouse gas emissions "is a matter of national concern that only Parliament can address."

Telles-Langdon argued in court that the circumstances surrounding climate change have developed enough to make it a national concern. Much more is known about it, she said, and the severity of the threat has greatly increased.

"There has been a constitutionally important transformation," she said. "We're now in a situation where the dimensions of the problem are international and global."

The carbon tax flows from the federal government's right to sign international treaties, she added, and is part of living up to climate change accords such as the Paris Agreement.

"The treaties matter," Telles-Langdon said.

On Monday, a lawyer for the Alberta government argued that allowing the tax law to stand would give the federal government a tool it could use to repeatedly chip away at provincial powers.

Peter Gall said issues of "national concern" are rare. Greenhouse gases don't meet the test, he said, and upholding the tax law would open the door to Parliament stepping into provincial matters whenever it wanted.

Lawyers for attorneys-general in Ontario and Saskatchewan have already presented arguments in support of Alberta's challenge.

Those representing New Brunswick and British Columbia are also to speak during the hearing. Eight First Nations, non-governmental groups and Crown corporations have been granted intervener status as well.