The proposed Trans Mountain heavy-oil pipeline from Alberta to the Pacific coast has law, economics and the Constitution on its side.

It has been approved by the National Energy Board and okayed by the federal government which, under the constitution, has the ultimate authority in such matters.

First Nations along the right of way may not all agree. But they have been consulted, which is all that the law demands.

The project would boost Alberta’s economy and, by opening up new energy markets in Asia, reduce Canada’s unhealthy reliance on the U.S.

Barring one thing, it is a no-brainer.

But that one thing — the danger posed by climate change — is the deal breaker that could and should kill this project. It trumps everything else.

By itself, the proposed pipeline expansion would not increase the greenhouse gas emissions that, according to scientists, threaten to dangerously alter the world’s climate.

But it would support Alberta’s troubled oilsands industry, which does.

While the oilsands accounted for just under 10 per cent of Canada’s greenhouse gases in 2014, they are the fastest growing source of carbon emissions in the country.

In climate terms, the most sensible course of action would be to gradually shut the oilsands down. Thanks to the growth of cheaper forms of energy, they are already headed in that direction. It wouldn’t take much of a nudge to finish the job.

Instead, both the New Democratic Party government in Alberta and the Liberal government in Ottawa have been trying to have it both ways.

On the one hand, they’ve put in place measures to reduce emissions marginally. On the other, they have been pushing pipelines, like Kinder Morgan’s Trans Mountain conduit, that would encourage expanded oilsands production and thus more emissions.

Prime Minister Justin Trudeau says this shows that fighting climate change need not threaten economic growth. What it really shows is that half measures get you only halfway.

Government action on the climate front has been so timid that Canada is not on track to meet the carbon reduction targets it agreed to in Paris in 2015 — targets that the Trudeau Liberals, when in opposition, used to dismiss as grossly inadequate.

All of this is background to the political story being played out in Alberta and British Columbia. Alberta’s NDP Premier Rachel Notley needs the Trans Mountain pipeline built if she is to have a chance of winning the next election. B.C. Premier John Horgan needs the pipeline scuppered if his Green-supported, minority NDP government is to survive.

To add to the complications, much of the opposition to the project comes not from critics who object to the pipeline itself but from those who worry that the increased tanker traffic resulting from the pipeline might foul B.C.’s pristine coastline.

As well, Horgan’s attempts to lure liquefied natural gas plants to B.C. cast some doubt on his commitment to reducing carbon emissions.

In the background, Kinder Morgan is becoming increasingly nervous about the future of its pipeline project, while the federal government tries desperately to figure out what it can do to solve the mess.

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If this dispute is to be handled by law alone, then pipeline proponents have the edge. The Constitution seems clear that Ottawa has the ultimate authority over interprovincial undertakings.

It also has the constitutional authority to declare its unilateral jurisdiction over any project that it deems to be in the national interest. It has used this power at least 470 times since Confederation.

But anti-pipeline protestors have already shown they are willing to face arrest to make their point. If this becomes a showdown over the future of the planet, law may not be enough.

Thomas Walkom appears Monday, Wednesday and Friday. appears Monday, Wednesday and Friday.

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