IT’S a green way to be bad for the environment. Canada’s tar sand oil is far more polluting than the conventional kind, but the controversial Keystone XL pipeline is the cleanest way to get it to market.

If Keystone is built, it will carry crude oil from Canada’s tar sands to refineries on the US Gulf coast. The draft assessment of the pipeline’s environmental impact, published last week by the US Department of State, compares it with carrying the oil to Oklahoma by train and then piping it the rest of the way, or shipping it the whole way. Both would produce more greenhouse gases – 8 and 17 per cent, respectively.

Scrapping Keystone, says the DoS, would only cut Canadian oil sands production by 0.4 per cent by 2030, because the oil would be exported by other routes to the US or further afield.

But the real problem is that we are exploiting tar sands at all, says Kevin Anderson of the University of Manchester in the UK. Tar sand oil is far dirtier than conventional oil, so generates between 10 and 20 per cent more greenhouse emissions.