Exploiting the Canadian oil sands to extract their stored hydrocarbons remains controversial. The mix of sand, clay, water, and bitumen that we call the Canadian oil sands might be the second largest oil deposit on the planet. If that didn't make it an attractive target for exploitation, the fact that it's located in a stable, friendly country that shares a land border with the US seals the deal for many. But extracting useable oil from oil sands is much more energy intensive (and expensive) than obtaining it from traditional oil fields, and concerns about the ecological impact remain.

In some ways, the name 'oil sands' is a bit misleading, since the area is not a sandy desert, but peatland, which has to be converted to open-pit surface mines. Although the various mines have to have plans for post-mining land reclamation, it's not possible to return the mines to peatland, and mine operators propose to construct forests and tailing lakes. A new study from the University of Alberta, published in PNAS, has examined the effect of this transformation on carbon sequestration. It doesn't paint a happy picture.

The study's authors compare the pre- and post mining landscapes of the 10 approved mines in Alberta. Prior to mining, 64 percent of the landscape was wetland, with only 24 percent supporting upland vegetation. Only four of the mines (Horizon, Jackpine-Phase 1, Muskeg, and Kearl) had post-mining reclamation plans detailed enough to be able to make a direct comparison.

Of these four mines, the most striking difference pre- and post-mining will be the loss of up to two thirds of the peatland, which will be replaced mainly by upland forest. This means forests of white aspen, jack pine, and trembling aspen will replace the current landscape of fens and bogs with tamarack and black spruce; an undergrowth of blueberries, cranberries and dogwood will replace the mosses.

The plans involve creating lakes from existing tailing ponds, which will be topped up by drainage from the hilly forests. Currently, these tailing ponds are filled with highly contaminated water that was used in the oil extraction process, but this approach appears to be the lesser of two evils; recreating wetlands with the contaminated water would likely increase soil contamination, since the increased surface area will enhance evaporation.

The loss of the wetlands is in part a function of policy, and partly the sheer amount of earth and mining byproducts. Alberta has no regulation requiring the replacement of lost wetlands, and the volume of mine tailings are such that hills will necessarily replace previously flat land.

In addition to the loss of existing wetland habitat, the authors are also concerned about the decreased ability of the post-mining landscape to sequester carbon. They estimate the change in landscape across the affected region would free between 11 and 48 million tons of carbon, which is currently fixed by the peat bogs. This is equal to seven years' of emissions from the mining process and, claim the authors, has not been figured into previous calculations about the impacts of oil sands exploitation.

This work comes with several caveats that should be considered. Neither Syncrude nor Suncor Energy's reclamation plans were detailed enough to include in the analysis, and these operations account for 60 percent of the oil sands being exploited. Further, no large-scale reclamation has undergone independent study, so it's uncertain how much the real projects will resemble the mining operators' proposals.

PNAS, 2012. DOI: 10.1073/pnas.1117693108 (About DOIs).