The term “fossil fuels” is a very large umbrella. Coal, oil, and natural gas are the usual distinctions, but there’s also unconventional oil (such as the Alberta tar sands) and unconventional gas (such as shale gas from fracking). “Unconventional” means that the fuel is produced in a roundabout way that’s less efficient and takes more energy than regular fuel. For example, oil in northern Alberta is mixed with sand and tar that’s difficult to remove. As global supplies of conventional oil and gas decline, unconventional fuels are making up a growing segment of the petroleum market.

The different types of fossil fuels are present in different amounts in the ground. Also, for each unit of energy we get from burning them, they will release different amounts of carbon emissions. Given these variables, here’s an interesting question: how much global warming would each type of fuel cause if we burned every last bit of it?

A few weeks ago, a new study addressed this question in one of the world’s top scientific journals. Neil Swart, a Ph.D. student from the University of Victoria, as well as his supervisor Andrew Weaver, one of Canada’s top climate scientists, used existing data to quantify the warming potential for each kind of fossil fuel. Observations show the relationship between carbon emissions and temperature change to be approximately linear, so they didn’t need to use a climate model – a back-of-the-envelope calculation was sufficient. Also, since both of the authors are Canadian, they were particularly interested in how burning the Alberta tar sands would contribute to global warming.

Swart and Weaver calculated that, if we burned every last drop of the tar sands, the planet would warm by about 0.36°C. This is about half of the warming that’s been observed so far. If we only burned the parts of the tar sands proven to be economically viable, that number drops to 0.03°C. If we don’t expand drilling any further, and stick to the wells that already exist, the world would only warm by 0.01°C, which is virtually undetectable.

Conventional oil and natural gas would each cause similarly small amounts of warming, if the respective global supplies were burned completely. Unconventional natural gas would cause several times more warming – even though it’s cleaner-burning than coal and oil, there’s a lot of it in the ground.

The real game-changer, though, is coal. If we burned all the coal in the ground, the world would warm by a staggering 15°C. There’s a large uncertainty range around this number, though, because the linear relationship between carbon emissions and temperature change breaks down under super-high emission levels. The warming could be anywhere from 8°C to 25°C. In the context of previous climate changes, it’s hard to overemphasize just how dramatic a double-digit rise in average temperatures would be.

The main reason why the warming potential of coal is so high is because there’s so much of it. The Alberta tar sands are a huge resource base, but they’re tiny in comparison to global coal deposits. Also, coal is more polluting than any kind of oil: if you powered a lightbulb for one hour using coal, you would produce about 30% more CO₂ emissions than if you ran it using conventional oil.

The tar sands are more polluting than regular oil, but exactly how much more is a very difficult question to answer. The end product that goes into your car at the gas station is essentially the same, but the refining process takes more energy. You can supply the extra energy in many different ways, though: if you use coal, tar sands become much more polluting than regular oil; if you use renewable energy that doesn’t emit carbon, tar sands are about the same. The authors didn’t include these extra emissions in their study, but they did discuss them in a supplementary document, which estimated that, in an average case, tar sands cause 17% more emissions than regular oil. Taking this into account, the tar sands would cause 0.42°C of warming if they were burned completely, rather than 0.36°C.

Therefore, headlines like “Canada’s oil sands: Not so dirty after all” are misleading. Canada’s oil sands are still very dirty. There just isn’t very much of them. If we decide to go ahead and burn all the tar sands because they only cause a little bit of warming, the same argument could be used for every individual coal plant across the world. Small numbers add up quickly.

The authors still don’t support expansion of the tar sands, or construction of pipelines like the Keystone XL. “While coal is the greatest threat to the climate globally,” Andrew Weaver writes, “the tarsands remain the largest source of greenhouse gas emission growth in Canada and are the single largest reason Canada is failing to meet its international climate commitments and failing to be a climate leader.” Nationally, tar sands are a major climate issue, because they enable our addiction to fossil fuels and create infrastructure that locks us into a future of dirty energy. Also, a myriad of other environmental and social problems are associated with the tar sands – health impacts on nearby First Nations communities, threats to iconic species such as the woodland caribou, and toxic chemicals being released into the air and water.

Tar sands are slightly preferable to coal, but clean energy is hugely preferable to both. In order to keep the climate crisis under control, we need to transition to a clean energy economy as soon as possible. From this viewpoint, further development of the tar sands is a step in the wrong direction.