The Portland City Council was right to put off voting on creation of an anti-tar sands policy for city purchases.

After a public hearing Wednesday, city officials felt they had too many questions to move forward.

Like what is the definition of “tar-sands oil” and would the city be able to acquire heating oil, gasoline, asphalt or even tires that it could be sure were not made from oil drilled in the vast sand oil fields of Alberta.

Before they vote, councilors should ask the reason behind the policy in the first place.

Is it because tar sands oil, as many environmentalists say, is a dirty product, more likely to corrode pipelines and leak into the environment? Or is it because exploiting the Canadian tar sands would flood the market with inexpensive oil, set back conservation and energy alternative programs and release more carbon into an already warming planet?

Because if it’s the second one, changing the city’s purchasing preferences won’t help much.

Even if Portland could find an alternative to tar sands derived products, it would still be burning heating oil and gasoline. Even if tar sands oil were dirtier than other oil, none of it is clean. Would the planet be better off if the city bought its oil from the Gulf of Mexico? The Nile Delta? Russia?

And if cities and businesses and individuals all over America stopped buying tar sands derived gasoline, the crude would be shipped to someone who would use it, and it would be burned in China or India.

If the concern is pipeline leaks, Mainers might be better off without the oil, but if it’s climate change, it doesn’t matter where the oil is used. The whole planet gets warmer.

Portland would be better off coninuing its efforts to use less oil and gas, regardless of where it came out of the ground. The city has made strides building green schools, converting buildings from oil to gas heat and conserving energy. As a result it has already been able to lower its carbon footprint by 34 percent.

There is room for improvement, and the city should do more. It should also continue to make the kinds of planning decisions and infrastructure investments that let residents and visitors get around without needing to use their cars.

If the problem is climate change, the answer is for Portland and the rest of the world to use less oil, not to use different oil.