The exploitation of the tar sands in Alberta, Canada - which may contain more oil than the entire world has consumed to date – has attracted considerable controversy lately. Massive expansion of tar sands operations are planned, but this cannot happen if many new pipelines out of landlocked Alberta are not built. Two such pipelines – Enbridge Northern Gateway and Kinder Morgan Transmountain – would cross through British Columbia and have ignited such fierce opposition that they have little prospect of going forward. A third pipeline, Keystone XL, would run from Alberta to refineries on the US Gulf of Mexico coast.

Canada reneged on its Kyoto agreement, but has again promised to reduce its emissions – this time by 17 percent from the 2005 level by 2020. At the same time, the Canadian government, backed by the oil industry and its international investors, has been furiously lobbying the Obama administration to approve the Keystone XL pipeline, which is seen as vital to plans to triple production of crude oil from the tar sands between now and 2020.

These two goals – reducing greenhouse gas emissions by 17 percent and tripling tar sands operations – are simply incompatible with one another. Based on where Canada stood in 2011, emissions outside tar sand operations would need to be reduced by 14 percent in order to achieve the 2020 target even if tar sands operations were frozen at their 2011 level. This would be a difficult task even if there were a genuine effort to reduce emissions. However, there is no program in Canada to reduce its greenhouse gas emissions (what little that has been achieved to date is largely due to provincial efforts). With projected growth in tar sands emissions, the required reduction elsewhere in the economy is 23 percent. This will not happen. Thus, unless tar sands operations are at least frozen at current levels, Canada will once again break an international climate promise.

In Cancun, Mexico, the nations of the world (that includes Canada) adopted the goal of limiting global mean surface warming to no more than 2°C above the pre-industrial average and to possibly no more than 1.5°C (the average surace warming so far is about 0.8°C). The Summary for Policy Makers of the Working Group One contribution to the latest IPCC assessment gives the range of total cumulative CO2 emissions that is compatible with various targets. For the 2°C, the middle estimate is that we have already used up half of our total emission allowance. Indeed, we may have already used up 2/3 of our total allowance. Furthermore, the potential CO2 emission from the tar sands alone (200 gigatonnes of carbon) could use up all of the remaining allowance. Clearly, the overwhelming majority of tar sands bitumen will need to stay in the ground – and the same applies to offshore Arctic oil and all other unconventional sources of oil and natural gas, as conventional (and less carbon-intensive) oil and gas alone would likely use up the remaining allowance.

Ignoring the implications for its own climate promises, the Canadian government argues that approval of Keystone XL will not increase global CO2 emissions because it will simply displace equally carbon-intensive Venezuelan crude oil at the Gulf coast refineries. Not so – the Gulf coast refineries that process Venezuelan crude are owned by the Venezuelan state oil company. Rather, tar sands products are likely to displace more expensive Bakken light oil or to be exported. Fortunately, dirty Canadian (and Venezuelan) heavy oil will soon not be needed in the North American market, thanks to the doubling in the fuel efficiency in passenger vehicle fleet that will be fully phased in over the next two decades, and the potential to double the fuel efficiency of the freight vehicle fleet.

Staying within the carbon emission allowance for a 2°C warming limit will require a dramatic departure from business-as-usual. Approval of Keystone XL would lock in 40 years of expanded production of one of the most carbon-intensive oil sources on the planet, at the very time when we need to – and can – move rapidly in the opposite direction. There is no better opportunity than Keystone XL to say "no" to business-as-usual.

Danny Harvey is Professor in the Department of Geography at the University of Toronto