POGG is potentially a sledgehammer for Ottawa. Practically any action might be justified as being in the name of "peace, order, and good government" and not exclusively within provincial powers. But POGG is only one line in one section of a long constitutional text. The architecture of the Constitution Act devotes a great deal of care and attention to establishing the provinces as responsible governments, and setting out a broad range of powers only the provinces can exercise. It would not do so if the provinces and their powers could be arbitrarily erased by invocation of a single line elsewhere in the text. As a result, both the old Judiciary Committee of the Privy Council and its successor the Supreme Court of Canada have been cautious in authorizing POGG, as they should be. Provincial powers usually survive POGG challenges, as they should.
Chief Justice Richard Wagner's majority reasons, as summarized here at least, declare that since climate change is both a pressing area of national concern and one where the "failure of one or more provinces to co-operate would prevent the other provinces from successfully addressing it", there are legitimate grounds to see the question as one of the rare ones which meet the high threshold POGG requires.
It's a 6-3 decision, however, with some strong dissents. Justice Russell Brown, one of the dissenters, seems to rely on a slippery-slope argument against any application of POGG:
"Its implications go far beyond the [carbontax] act, opening the door to federal intrusion — by way of the imposition of national standards — into all areas of provincial jurisdiction, including intra-provincial trade and commerce, health, and the management of natural resources. It is bound to lead to serious tensions in the federation."
Rowe said that the POGG or national concern doctrine should be a "residual and circumscribed power of last resort."
One might think that POGG already is a circumscribed power of last resort, and that Wagner has made a cogent argument that this is precisely one of the rare situations that justify its existence, hardly the start of a movement to abolish the provinces as sovereign powers within their own sphere of jurisdiction. The constitution, it turns out, does not preclude action on climate. Score one for the constitution.
* The phrase was originally devised as "peace, welfare, and good government" by the constitution- makers at Quebec City in 1864.
Update, March 26: I see (what early press reports mostly skipped): theSCC declared that carbon pricing is a regulatory levy, not a tax. Taxes must go into general revenues; levies can be focussed for specific outcomes. So there's that, too.
