Carbon dioxide emissions from Western Canada's oilsands are set to increase from five per cent to 16 per cent of the national total by 2020 under current plans. (Canadian Press)

CBC News has obtained a government document that says reducing greenhouse gases from Western Canada's oilsands will be much more difficult than some politicians and the industry suggest.

The ministerial briefing notes, initially marked "Secret," say that just a small percentage of the carbon dioxide released in mining the sands and producing fuel from them can be captured.

The oilsands are the fastest-growing source of CO2 in the country, set to increase from five per cent to 16 per cent of total emissions by 2020 under current plans.

Capturing the gas and pumping it underground has been the key public strategy for reducing the oilsands industry's contribution to global warming.

The briefing notes, obtained by CBC News under freedom-of-information legislation, are based on the findings of a joint Canada and Alberta task force on carbon capture and storage.

Not concentrated enough

Little of the oilsands' carbon dioxide can be captured because most emissions aren't concentrated enough, the notes say. For efficient capture, there must be a high concentration of CO2 coming out of a smoke stack.

"Only a small percentage of emitted CO2 is 'capturable' since most emissions aren't pure enough," the notes say. "Only limited near-term opportunities exist in the oilsands and they largely relate to upgrader facilities."

The Canadian and Alberta governments are spending about $2.5 billion on developing carbon capture and storage, and the oilsands generally come up as the first reason for spending the money.

In March, when he repeated a $240-million federal commitment to a project in Saskatchewan, Prime Minister Stephen Harper said: "This new technology, carbon capture and storage, when fully commercialized ... will collect carbon dioxide emissions from oilsands operations and coal-fired electrical plants and seal them deep underground."

The briefing notes, which went to federal and provincial politicians, were produced months before Harper's announcement. The carbon capture task force issued a public version of its final report in January.

David Keith, a professor of petroleum and chemical engineering at the University of Calgary, was the lead scientist on the task force.

He says he's frustrated that politicians and the industry keep focusing on the oilsands when there are sources of greenhouse gases to capture more easily and at less cost, including coal-fired power plants.

Rational people shouldn't focus on reducing emissions in the oilsands through carbon capture and storage, Keith says.

"The actual content of the briefing note is a pretty fair summary of the technical situation we have," he told CBC News.