Climate change is an “urgent threat to humanity” that knows no boundaries, a federal government lawyer argued Tuesday in Ontario’s carbon-tax challenge, as a judge hearing the case asked why Ottawa won’t leave the province alone.

On the second day of Ontario’s constitutional challenge to Ottawa’s carbon-pricing regime, federal lawyer Sharlene Telles-Langdon told court that harmful greenhouse gases can only be curbed by a national price on carbon.

“We know that climate change is an urgent threat to humanity,” Ms. Telles-Langdon said in her opening remarks to Ontario’s court of appeal in Toronto.

Story continues below advertisement

“Greenhouse-gas emissions are not contained within geographic boundaries. They are an interprovincial and international pollutant.”

She later added that it’s an “unquestioned fact” that pricing reduces emissions – and requires a national solution.

Ms. Telles-Langdon faced repeated questions from two members of the five-justice panel about the federal government’s jurisdiction in imposing its Greenhouse Gas Pollution Pricing Act on Ontario and other provinces.

Progressive Conservative Premier Doug Ford has said Ontario can curb greenhouse-gas emissions on its own and has already taken significant steps to do so.

Those steps, Ontario’s lawyer Josh Hunter told the court Monday, include shutting down 19 coal-fired power plants – a measure taken by the previous Liberal government – which has reduced the province’s harmful emissions by 22 per cent since 2005.

Justice James MacPherson raised the statistic during the hearing on Tuesday, asking the federal government for a response.

“We don’t care how they achieved it,” he told the federal lawyer.

Story continues below advertisement

“Why don’t you just leave them alone?”

Ms. Telles-Langdon replied that it’s not just Ontario the country needs to be worried about. “It’s Canada as a whole,” she said. “Unfortunately Ontario can do nothing about what is happening in other provinces.”

“So Ontario has to pay an extra burden because other provinces are failing?” Justice MacPherson replied.

Justice Grant Huscroft asked the federal lawyer whether the national price-based system is a “means to the end” for reducing greenhouse gases, to which she agreed.

“That’s a point against you, you shouldn’t be agreeing with it,” he said. “If Ontario or another province has an alternative means of reducing carbon, you would still force the pricing system on them.”

The judges had also questioned Ontario’s position in court on Monday, asking how Canada’s plan could be a burden to taxpayers when Ottawa has said it will return all revenue from the levy back to the province in which it was raised, including 90 per cent to households.

Story continues below advertisement

The province has argued Ottawa’s plan to reduce greenhouse-gas emissions could lead to the regulation of “all human activity,” including how people drive or where they live.

Related: Ontario warns of regulation on driving, living, under federal carbon tax

Ms. Telles-Langdon said the federal law respects provincial authority.

“The act itself intrudes minimally on provincial jurisdiction,” she said. “What it does is ensure a national system.”

Environment Minister Catherine McKenna said Tuesday that Canada is on track to meet a 2030 goal of slashing GHGs by one-third from 2005 levels, even as a new report showed emissions in 2017 edged up.

The latest national inventory report submitted to the United Nations Framework Convention on Climate Change said greenhouse gases rose about 1 per cent to 716 million tonnes of CO2 equivalent from the year before.

The report does not take into account Liberal measures to combat climate change. It attributed the increase to the resumption of oil-sands production after wildfires in 2016 and said emissions actually fell by a net 2 per cent against 2005 levels.

Story continues below advertisement

“Canada’s climate plan is working, and the overall trend in emissions is downward toward 2030,” Ms. McKenna said in a statement, noting new policies are only now coming into effect.

However, environmental groups say Ontario’s weakened targets effectively place more of a burden on Saskatchewan and Alberta to reduce emissions if Canada is to achieve its targets under the Paris accord.

The federal law that kicked in on April 1 imposes a charge on gasoline and other fossil fuels as well as on industrial polluters. The law applies only in provinces that have no carbon-pricing scheme that meets national standards – Ontario, Manitoba, Saskatchewan and New Brunswick.

Saskatchewan is awaiting the result from its own challenge in its court of appeal, backed by Ontario and New Brunswick. Manitoba recently announced it is also launching a legal challenge.

Fourteen interveners in all will get their say over the course of the hearing, which ends Thursday.

With a report from Jeff Lewis and the Canadian Press