Over the last few years, there's been a growing chorus of concern about the tar sands (or oilsands) in Canada. Tar sands are a kind of oily deposit that require more complex and energy-intensive extraction methods than "conventional" oil in order to convert them into a usable fuel. This means that until recently they were too expensive to be seen as a major energy source. However, as the rest of the world's cheap and easy oil has now pretty much been discovered, rising oil prices have made the tar sands a more attractive prospect for profit-hungry oil companies. Shell, for example, have staked about a third of their future profits on tar sands extraction; BP are in the process of building their first extraction facility.
Even without considering the local environmental destruction, groundwater pollution and violation of Indigenous treaty rights that are necessary to rip or boil the tar sands out of the ground, the potential greenhouse gas emissions associated with the Canadian reserves alone have been described by campaigners as a "carbon bomb". Representing the third-largest stash of oil in the world (after Saudi Arabia and Venezuela), the threat of the expansion of Canada's tar sands was enough to spark one of the biggest environmental protests in US history last year, with the (at least partially successful) campaign against the Keystone XL pipeline. Respected climate scientist James Hansen has said that if we carry on with tar sands extraction then it will be "game over" for the climate.
But then, in February 2012 the waters were muddied by a University of Victoria study which concluded that burning all the recoverable Canadian tar sands would raise global temperatures by 0.03 degrees Celsius. This led to a whoop of delight from many pro-tar sands commentators, who gleefully pointed out that this was a pretty titchy amount of warming compared to, say, burning the world's remaining coal (14.8 degrees) or natural gas (2.9 degrees).
This was, of course, all pretty misleading, because 0.03 degrees is a lot for a single industrial project. Comparing just the tar sands to ALL of the world's coal or gas isn't a fair comparison at all - it's like saying "hey, maybe my Rottweiler has killed a few local kids, but compared to all the dog attacks in China every year that's really not so bad". More importantly, that 0.03 degrees figure doesn't come with any context about how much warming we're already committed to, and exactly where the tar sands would bring us in relation to the point of no return.
To try to clear this up, I've decided to come at it from another angle. According to researchers at the University of Oxford, we can only emit around 710 billion more tonnes of carbon dioxide in all of humanity's future if we want a decent (75%) chance of avoiding runaway climate change . Just burning the Canadian tar sands would release around 110 billion tonnes . That means that the tar sands would wipe out a seventh of our remaining atmospheric space all by themselves, just to power a small fraction (the IEA reckons 10% by 2035) of the world's vehicle fleet. That's before we start thinking about coal, gas, and all the rest of the oil in the world.
So yes, it's true that there's enough coal and gas out there to cook us several times over, but that doesn't make the tar sands any less deadly - just the Canadian deposits would take us 14% of the way towards the point of no return. Next time someone tells you we don't need to worry about the tar sands because other fuel sources are potentially worse, you can tell them that they're wrong. We need to stop burning all of these fuels, fast - and the relative isolation of tar sands in a few places, combined with its carbon intensity, local destructiveness and human rights impacts - make it a prime contender for one of the first things to shut down.
A world without fossil fuels is possible - in fact, I'm part of a project at the moment to illustrate this in an interesting and engaging new way, watch this space for more details!
 The figures on the Oxford University "trillionth tonne" website are in tonnes of carbon, not tonnes of CO2. I've converted the approximately 194 billion tonnes of carbon standing between us and a two degree rise into 710 billion tonnes of carbon dioxide.
 There are an estimated 170 billion barrels of recoverable oil in the Canadian tar sands, according to the Canadian Association of Petroleum Producers. According to official research accepted by the EU, oil from the tar sands releases 107 grams of CO2 per MJ. One barrel of oil represents approximately 5861.5 MJ, which means that extracting and burning all of Canada's tar sands would release 170 x 107 x 5861.5 / 100000 = 106.6 billion tonnes.