For 70 years, crude oil has been transported via pipeline from South Portland to Montreal. Environmental activists now fear the pipeline will be reversed to carry Canadian “tar sands” crude to South Portland, for subsequent transport to North American or offshore refineries.

To pre-empt this possibility, the activists have persuaded the South Portland City Council to pass the so-called “Clear Skies Ordinance.”

The Clear Skies Ordinance is both ingenious, and disingenuous – the kind of disguised restriction on international trade that all parties, including the United States, committed to avoid in Article 3.5 of the United Nations Convention on Climate Change.

The ordinance has been carefully tailored to fit current local circumstances, banning within the city the loading, but not the unloading, of crude oil or refined petroleum products, thereby avoiding immediate job loss and delaying the demise of the port.

It also prohibits the construction or operation of related loading facilities or structures, including federally required pollution abatement equipment – the latter equipment needed during the transport of certain kinds of diluted bitumen (dilbit).

At the same time, the net has been cast widely to cover all crude oil, thereby catching not only dilbit and light crude from Canada, but American light crude from the Bakken Field.

All this is justified in terms of preventing new sources of air pollution, obstructing ocean views and preventing development that would impede the city’s land-use goals, with no mention of carbon emissions, climate change – or property values.

The environmental activists in San Francisco and Washington who helped draft the ordinance are carefully watching developments in Maine, hoping their Clear Skies Ordinance will stand.

They have reason to be concerned.

The U.S. Constitution explicitly assigns the power to regulate international and interstate commerce to Congress; crude-oil pipelines and crude-oil exports are regulated by the federal government. The ordinance will block the flow of American crude to not only foreign refineries, but American refineries along the coast.

South Portland taxpayers may not like it, but it’s time for the courts to address the constitutional collision, and define the acceptable limits of “NIMBY” laws.

As for “keeping the tar sands out of the Northeast U.S.,” supporters of the Clear Skies Ordinance should consider that roughly 85-90 percent of the greenhouse gases attributable to a barrel of oil are emitted during the refining of crude into fuel and its subsequent combustion in a vehicle – not upstream during production.

These downstream emissions occur in essentially equal amounts, whether the fuel was refined from light or heavy crude, or the crude feedstock was produced by conventional or unconventional methods.

Yes, there’s a giant carbon factory that passes through South Portland. But it’s not the Portland-Montreal crude-oil pipeline; it’s the Maine-to-Miami highway called Interstate 95.

Six countries (including the U.S.) produce crude oil blends with higher upstream greenhouse gas emissions than Canadian dilbit, and all five of these foreign crude producers served the Eastern U.S. market in 2013. Furthermore, much of the oil consumed in the region enters as refined product, primarily from the Texas Gulf Coast, where the world’s largest complex of heavy oil refineries is a major consumer of “dirty” Latin American and African crude.

The ordinance reinforces the case that it’s in Canada’s national interest to build a pipeline through Canadian territory to serve Eastern Canadian refineries and access Atlantic tidewater.

Finally, there is a way for South Portland residents to actually “clear the skies”: Drive a diesel-powered vehicle deploying the latest emission control technologies.

But there’s just one problem: The new technologies require the use of ultra-low sulfur diesel, and a preferred feedstock for such clean fuel happens to be – that’s right – bitumen from the Canadian oil sands.

It’s not easy being green.

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Peter Burn is counsel in the Ottawa, Canada, office of the global law firm, Dentons. He served as tax and trade policy counsel to Canada’s finance minister during the 1980s, and was involved in the creation of the Canada-US Free Trade Agreement, the precursor of NAFTA. More recently, he served as a senior adviser in the Greenhouse Gas Reductions Directorate in Environment Canada. He is the grandfather of three New Englanders.