There are a number of different ones. One of the two main ones would be the power over matters of criminal law, and that's what underlies the Canadian Environmental Protection Act. Most of the climate regulations that have come in have been brought in under the criminal law power, including back in the Harper years.
As of the 2021 decision on the Greenhouse Gas Pollution Pricing Act, the federal government also has some authority under the “Peace, Order, and good Government” power of the Constitution—and that's the one I'm speaking mainly to for this act—particularly, as the court said, to deal with “serious” cross-border pollution. That's the main authority that arises under peace, order and good government. I would add I've been speaking with a number of constitutional law professors and experts, and there's general agreement on that point.