The Alberta oil sands and climate

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The claimed economic benefits of exploiting the vast Alberta oil-sand deposits need to be weighed against the need to limit global warming caused by carbon dioxide emissions.

The US federal government recently rejected approval for TransCanada's proposed Keystone XL pipeline. But TransCanada plans to submit a revised proposal shortly. The proposed pipeline is part of a US$13 billion system aimed at connecting the bituminous oil sands in Alberta, Canada with refining capabilities in the United States, including those as far south as Texas1. There has been widespread public interest in, and opposition to the pipeline, primarily owing to environmental concerns (for example, ref. 2). Similar public opposition has arisen towards the proposed Northern Gateway pipeline in British Columbia, which aims to make the oil sands accessible to Asian markets.

The size of the Alberta oil-sand deposits is massive. Estimates for oil-in-place are 1.8 trillion barrels3, although the economically viable 'proven reserve' is estimated at only around 170 billion barrels with 26 billion barrels under active development3. For orientation, Alberta's 1.8 trillion barrels of oil-in-place is roughly seven times the size of Saudi Arabia's proven reserves4. It has been suggested that construction of the TransCanada pipeline will encourage an expansion of the area under active development2. Indeed, greenhouse-gas emissions resulting from expanding oil-sand production are Canada's fastest-growing emissions source5, and have the potential to contribute significantly to anthropogenic climate change. This is accentuated by the fact that the oil sands are more energy intensive to produce than conventional crude oil — and have a greater 'well-to-wheel' carbon footprint6 (see Supplementary Information).

Global warming

Here we quantify the carbon dioxide-induced potential for global warming contained in the Alberta oil sands in the context of global fossil-fuel resources. To estimate the potential for global warming, we exploit the fact that the carbon dioxide-induced global mean temperature change (ΔT) can be inferred directly from cumulative carbon emissions (ET) by means of the carbon–climate response7, which is equal to ΔT/ET. Observational constraints produce a carbon–climate response of 1.5 °C per trillion metric tonnes of carbon emitted (Tt C), with a range of 1.0–2.1 °C per Tt C (5th–95th percentile). The uncertainty range includes uncertainties in climate sensitivity, as well as uncertainties in 'carbon sensitivity' (carbon sinks and carbon–climate feedbacks)7. We estimate the carbon available for emissions from the oil sands based on a per-barrel carbon content multiplied by the number of barrels (see Supplementary Information).

If the entire Alberta oil-sand resource (that is, oil-in-place) were to be used, the associated carbon dioxide emissions would induce a global mean temperature change of roughly 0.36 °C (0.24–0.50 °C; Table 1).

Table 1 Oil-sand potential for global warming in the context of global fossil-fuel resources.

This potential warming is almost half of the observed warming seen in the past 100 years (0.76 °C). However, considering only the economically viable reserve of 170 billion barrels reduces this potential for warming by about tenfold (to 0.02–0.05 °C), and if only the reserve currently under active development were combusted, the warming would be almost undetectable at our significance level.

Additional emissions resulting from natural gas, diesel and electricity use during bitumen extraction, upgrading and refining have not been included here, but could increase these numbers (see Supplementary Information). Neither have we considered the potential impact of future carbon capture and storage technologies, which may decrease the stated emissions and warming. It is important to recognize that our estimates do not include greenhouse gases other than carbon dioxide and do not address other potentially deleterious environmental, health and social side effects of oil-sand production (for example, ref. 8) or its potential economic benefits3.

Carbon footprints

To have a 66% chance of limiting warming to less than the 2 °C limit put forth in the 2009 Copenhagen Accord, one carbon–climate modelling study estimated that total future global carbon emissions should be limited to less than 5.9×1017 g C (ref. 9). If this amount were to be distributed equally among the current global population, the resulting allowable per capita cumulative carbon footprint would be 85 tonnes of carbon. The eventual construction of the Keystone XL pipeline would signify a North American commitment to using the Alberta oil-sand reserve, which carries with it a corresponding carbon footprint. For comparison, by fully using only the proven reserves of the Alberta oil sands, the current populations of the United States and Canada would achieve a per capita cumulative carbon footprint of 64 tonnes of carbon.

The Government of Canada has recently announced its intent also to secure the Chinese market for oil-sand products10, indicating the potential exploitation of this resource regardless of the ultimate fate of the Keystone XL pipeline. If distributed over the current population of China, the proven Alberta oil-sand reserves would lead to a per capita cumulative carbon footprint of 16 tonnes of carbon. However, many other sources of fossil fuels will also be needed if growing Chinese, and indeed worldwide, energy demand is to be met through the exploitation of fossil fuels.

Recent estimates show that an enormous global fossil-fuel total resource base is available to meet this growing demand11 (Table 1). Resources here are defined as those fossil fuels in the Earth's crust for which “economic extraction is potentially feasible”11 — an upper limit of nature's bounty and human techno-economic ability. Coal resources have the largest potential for global carbon emissions (79% of the total), followed by unconventional gas (15%) and only then the unconventional oil of which the Alberta oil sands form a part (3%; Fig. 1). Coal's significance is due to the large tonnage available, together with its high carbon content. It is clear that the total global fossil-fuel resource base could easily yield over 2 °C of warming given sufficient global demand and a lack of international regulation.

Figure 1: Central estimate of the potential for warming of the different fossil-fuel resources in Table 1.

The red line indicates the limit of 2.0 °C warming from pre-industrial times agreed to under the Copenhagen Accord. Note, that here we only consider the effects of anthropogenic carbon dioxide. The potential for warming associated with proven Alberta oil-sand reserves is indicated as a barely visible sub-component (shown in pink) of unconventional oil (global). The potential warming of the total Alberta oil-sands oil-in-place (OIP) is shown in black. *The carbon–climate response method is not valid for emissions above about 20×1017 g C, so these figures are not valid climate change estimates, but are included for comparison.

If North American and international policymakers wish to limit global warming to less than 2 °C they will clearly need to put in place measures that ensure a rapid transition of global energy systems to non-greenhouse-gas-emitting sources, while avoiding commitments to new infrastructure supporting dependence on fossil fuels12.


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Correspondence to Andrew J. Weaver.

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