| 英文摘要: | Climate policy and analysis often focus on energy production and consumption1, 2, but seldom consider how energy transportation infrastructure shapes energy systems3. US President Obama has recently brought these issues to the fore, stating that he would only approve the Keystone XL pipeline, connecting Canadian oil sands with US refineries and ports, if it ‘does not significantly exacerbate the problem of carbon pollution’4. Here, we apply a simple model to understand the implications of the pipeline for greenhouse gas emissions as a function of any resulting increase in oil sands production. We find that for every barrel of increased production, global oil consumption would increase 0.6 barrels owing to the incremental decrease in global oil prices. As a result, and depending on the extent to which the pipeline leads to greater oil sands production, the net annual impact of Keystone XL could range from virtually none to 110 million tons CO2 equivalent annually. This spread is four times wider than found by the US State Department (1–27 million tons CO2e), who did not account for global oil market effects5. The approach used here, common in lifecycle analysis6, could also be applied to other pending fossil fuel extraction and supply infrastructure. Globally, the International Energy Agency projects that nearly $700 billion per year will be invested in the upstream oil and gas sector over the next two decades7. The resulting infrastructure could contribute to carbon lock-in and further the problem of ‘carbon entanglement’8. Accordingly, it is crucial to understand the implications of fuel supply infrastructure for future greenhouse gas (GHG) emissions9. Innovations such as extraction-based carbon accounting10 have helped quantify the emissions associated with fossil fuel supply, not just consumption, as has traditionally been the focus. However, few analyses have quantified the incremental GHG emissions impact of new fossil fuel supply infrastructure. Broadly speaking, construction of fuel supply infrastructure could result in several categories of GHG impacts, including emissions associated with project construction and operation5; ‘lifecycle’ emissions associated with fuel extraction, processing and transportation5; and emissions associated with increased fuel use and combustion, due to price effects6, if the infrastructure increases global fuel supply. Furthermore, high-profile decisions such as the US government approval of Keystone XL could have indirect, political or structural effects, if they lead other decision-makers to reject new fossil fuel infrastructure on GHG grounds or, conversely, lead to a political backlash that inhibits other efforts to reduce emissions11. Although this last category may be the most significant, quantification is difficult and inherently speculative, so we do not further analyse it here. The three categories of emissions impact can be reflected, sequentially, as:
where: Emissionsconst = Emissions associated with infrastructure construction and operation, in tonnes CO2 equivalent (CO2e); ΔProduction = Increase in production of fuel handled by infrastructure project; EFproj = Emissions factor, per unit of fuel handled, lifecycle basis; EFref = Emissions factor, per unit of displaced, reference fuel, lifecycle basis; ΔConsumption = Increase in fuel consumption resulting from increased production. Factoring out the increase in production from the second two terms of equation (1) yields:
For the Keystone XL pipeline, the State Department has estimated all terms in equation (2) except the final one, a ratio that expresses the extent to which expanding oil sands production may increase global oil consumption. This term, and the effect it embodies, has not received significant attention in discussions of Keystone XL (ref. 12), and is therefore the subject of this Letter. Microeconomic theory provides the tools to examine the price effect of adding new production capacity to an existing market13. Our simple model simulates the interaction between global oil demand14 and supply15 for the year 2020, as depicted in Fig. 1.
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