Petroleum's Other Emissions

To make room for biofuel crops, farmers slash rainforests and other vegetation that absorbs carbon dioxide from the atmosphere. And that's a problem for the climate. But a new study points out that conventional oil development also requires changes to the land that destroy CO₂ sinks and produce their own greenhouse gas emissions (Environ. Sci. Technol., DOI: 10.1021/es1013278).

Previous research has suggested that land use associated with biofuel production, such as clearing grasslands to plant corn for ethanol, could lead to producing more net CO₂ than it saves from not burning fossil fuel.

"But no one had calculated the land-use emission from fossil fuels, so they weren't comparable on an apples-to-apples basis," says Sonia Yeh, a researcher at the Institute of Transportation Studies, at the University of California, Davis.

So Yeh and colleagues decided to quantify greenhouse gas emissions from two types of oil production: conventional development from oil fields in California and Alberta and development from oil sands in Canada, which contain the viscous petroleum mixture called bitumen. Engineers extract bitumen either by in-situ processes, in which they heat it with steam and then pump it to the surface, or by surface mining, in which they burn down arboreal forests and dig up overlying peat deposits to reach oil sands.

The researchers accounted for land-use emissions from several factors, such as greenhouse gases released when burning vegetation or the loss of natural CO₂ sinks. One especially harmful consequence of surface mining is peat removal. Peat absorbs huge amounts of CO₂ from the atmosphere, and when it's destroyed, its decomposition releases methane, a much more potent greenhouse gas than CO₂.

Not surprisingly, the researchers found that surface mining leads to the greatest emissions of the production methods: For every hectare of disturbed land, the process produces about 3,600 tons of greenhouse gases. Meanwhile, in-situ oil-sands production emits about 200 tons of greenhouse gas per hectare, and conventional oil development in California and Alberta releases about 70 and 160 tons per hectare, respectively.

In comparison to emissions associated with biofuels production, surface mining's only rival is palm biodiesel production in the Indonesian and Malaysian rainforest, which releases about 3,500 tons of greenhouse gases per hectare, according to a 2008 study in Science . But surface mining only produces about 11% of the lifetime emissions from oil sands, with the rest coming from transport and combustion of the developed oil.

Also, when Yeh's team controlled for the energy output of each fuel, all four fossil fuel production methods emitted significantly less than biofuel production. For example, compared to surface mining, palm biodiesel production leads to about 140 times more greenhouse gas emissions per megajoule of energy created.

Still, the fossil fuel numbers underscore the need to minimize carbon losses, especially during surface mining, says Eddy Isaacs, a chief environmental officer at Alberta Innovates, an agency funded by the Alberta government: "It gets us thinking about land-use emissions, especially during the exploration phase."