Doug Ford's government in Ontario is now insisting the province should not be expected to do more than its "fair share" to meet Canada's national commitment to reduce greenhouse gas emissions.

But Ford's definition of "fair" might present a significant challenge to the rest of the country — particularly Ford's conservative allies in Alberta and Saskatchewan.

Under Kathleen Wynne's Liberal government, Ontario was committed to reducing its emissions by 37 per cent below 1990 levels by 2030. While tearing down large portions of that government's climate agenda, Ford's Progressive Conservatives settled on a less-ambitious target: a cut of 30 per cent below 2005 by 2030.

The difference between those two numbers is approximately 30 megatonnes. (Some perspective: the provinces of Nova Scotia and New Brunswick produced a combined total of 30 Mt of emissions in 2017.)

That new provincial target matches the national target. It's also (conveniently enough) within reach already. As the Ford government now loudly notes, Ontario's emissions have declined by 22 per cent below 2005 levels.

"Today's report confirms Ontario has decreased emissions by 22 per cent since 2005, while the rest of Canada's input continues to increase," Rod Phillips, Ontario's environment minister, said last week when the federal government released the latest inventory of Canada's emissions. "Families in Ontario have already paid a significant cost for these efforts, yet the federal government continues to ask us to pay more than our fair share."

The Ford government's desire for a less-ambitious target is no doubt linked to its position on pricing carbon. At the risk of imposing equal or higher costs on the Ontario economy via alternative policies, the Progressive Conservatives likely needed to set the province's sights lower.

But in trying to make the case for doing less, the Ford government is either putting Canada's national emissions commitment in peril — or placing an interesting new burden on the rest of the country.

The Ford government highlighted an apparent disparity between Ontario — the country's second-largest provincial emitter — and the "Rest of Canada" with a handy line graph in its spring budget.

That the Ford government is willing to celebrate the emissions reductions achieved by its Liberal predecessor is at least an endearing display of bipartisanship. But that "Rest of Canada" line obscures some important distinctions.

Between 2006 and 2017, Ontario actually was one of six provinces to reduce its emissions. And two provinces achieved even steeper cuts: Nova Scotia managed a reduction of 33 per cent and New Brunswick's emissions fell by 28 per cent. The other three provinces where emissions declined were Quebec (9.8 per cent), British Columbia (1.5 per cent) and Prince Edward Island (10 per cent).

In total, those five provinces reduced their cumulative emissions by 22 Mt.

So when the Ford government says emissions have increased in the "Rest of Canada," it's ultimately talking about just four provinces: Newfoundland, Manitoba, Saskatchewan and Alberta.

Of those four, Newfoundland and Manitoba saw relatively small increases. In 2005, Newfoundland produced 9.9 Mt of emissions. In 2017, the province's emissions were 11 Mt. Manitoba went from 20 Mt to 22 Mt.

The more significant spikes were further west. In Saskatchewan, emissions grew by 14 per cent, from 68 Mt to 78 Mt. In Alberta, emissions went from 231 Mt to 273 Mt, an increase of 18 per cent. Measured in megatonnes, Alberta's increase (42 Mt) was nearly as large as Ontario's drop (45 Mt).

If Ford hopes to see the rest of the country catch up to Ontario, he might bring up those numbers the next time he calls premiers Jason Kenney and Scott Moe to discuss their joint campaign against the federal carbon price.

But in mirroring the federal target and talking about Ontario's "fair share," the Ford government is setting a potentially divisive standard that his allies surely would balk at adopting.

For the sake of hitting that national target, the federal government could do as the Ford government is doing and ask each province to achieve the same level of reductions — a 30 per cent cut below 2005 levels.

But that sort of approach would be criticized quickly for ignoring important regional differences in the sources and extent of emissions — the oil and gas sector in Alberta and Saskatchewan in particular. The fossil fuel sector is the largest and fastest-growing source of emissions in Canada and the industry is primarily based in those two provinces.

Kenney and Moe presumably would argue that the industry's success has delivered an economic boost to the entire country. They also likely would note the near impossibility of their provinces reducing emissions by 30 per cent below 2005 levels by 2030.

For Alberta, that would mean a cut of 111 Mt over the next 11 years — a reduction virtually unattainable without shutting down, or severely curtailing, the province's oil and gas industry. Some environmentalists might cheer that. Most Albertans wouldn't (and Canadians in many other provinces might not like the economic consequences).

A national climate plan focused on divvying up Canada's emissions by province would lead to disagreements over such regional disparities, both current and historical. As it is, the current arrangement can be imagined as something of a trade-off: for the sake of maintaining the economies of Western Canada, other provinces (particularly Ontario and Quebec) would make steeper reductions.

The federal Liberals' attempt to knit together a national climate plan mostly sidestepped such questions by focusing on a consistent policy — a price on carbon that would apply equally across provinces, a phase-out of coal-fired electricity and so on. That approach still required compromises, but it generally avoided stoking regional grievances.

It's almost impossible to imagine a plausible arrangement that doesn't involve Ontario reducing its emissions further and faster than the national average. Without that sort of commitment from Canada's most populous province, it's equally difficult to see Canada achieving its target for 2030.

In that way, Ontario's "fair share" is a matter of national concern.