Ontario's constitutional challenge to Ottawa's imposition of a carbon charge was left in the hands of five Appeal Court justices on Thursday after the hearing wrapped up with each side pressing the court to find in its favour.

In closing submissions, a federal lawyer warned that climate change posed an urgent threat. Ottawa, she said, has addressed an issue of national concern in a manner that respects provincial jurisdiction.

Ontario, however, stood by its view that the Greenhouse Gas Pollution Pricing Act — if deemed constitutional — would undermine co-operative federalism by allowing Ottawa to overstep the dividing line between federal and provincial spheres of authority.

"It governs a broad range of activities that are in provincial jurisdiction," Ontario lawyer Josh Hunter said Thursday in his closing comments. "It is notissues that fall squarely within provincial jurisdiction — a claim made by Ontario and its supporters.

The federal government maintains the levy in Ontario — currently four cents a litre on gasoline — is a regulatory charge designed to change behaviour in favour of lower greenhouse gas emissions. Ontario calls the charge an illegal tax — another violation of the Constitution — a position it repeated in final submissions.

During the hearing, opponents of the act raised scenarios in which Ottawa could now ban wood-burning fireplaces, limit how warm or cool people could keep their homes or when they could drive. Proponents of the law denounced those notions as "alarmist."

14 interveners took part in hearing

In all, 14 interveners — among them some provinces, Indigenous groups and environmental and business organizations