Cumulative emissions of CO2 largely determine global mean surface warming by the late 21st century and beyond. Most aspects of climate change will persist for many centuries even if emissions of CO2 are stopped. This represents a substantial multi-century climate change commitment created by past, present and future emissions of CO2.
Limiting the warming caused by anthropogenic CO2 emissions alone with a probability of >33 per cent, >50 per cent, and >66 per cent to less than 2°C since the period 1861–1880, will require cumulative CO2 emissions from all anthropogenic sources to stay between 0 and about 1560 GtC (33%), 0 and about 1210 GtC (50%), and 0 and about 1000 GtC (66%) since that period respectively. These upper amounts are reduced to about 880 GtC, 840 GtC, and 800 GtC (66%) respectively, when accounting for non-CO2 forcings as in RCP2.6. An amount of 531 [446 to 616] GtC, was already emitted by 2011.
(Editor's note: I added the % numbers in parentheses)
So for a two-thirds probability of not exceeding 2 degrees, the world's carbon budget is 1000 billion tonnes of carbon (gigatonnes or Gt C) less 200 Gt C for other non-CO2 emissions (like methane) and less the 531 Gt C already released as of 2011, which leaves us with 269 Gt C. Translated into CO2, that is 987 Gt CO2. For a 50 per cent chance (a coin-toss for exceeding 2 degrees), we can emit up to 1,134 Gt CO2e. For context, annual emissions globally from fossil fuel combustion are about 33 Gt CO2. So if we add emissions for 2012 and 2013, the carbon budget drops to 1,068 Gt CO2 for 50 per cent odds, and 921 Gt CO2 for 66 per cent odds.
(Editor's note: at 33Gt CO2 annually, we will use up our entire budget to have a 66% probability of not exceeding 2 degrees in 28 years. That assumes that our emissions remain stable for 28 years and then drop immediately to zero. If our emissions continue to rise then we have less than 28 years left.)
Canada’s share of that global carbon budget would be about 2.4 Gt C based on our share of the world population (for an 80% chance).
Canada’s reserves of fossil fuels are significantly larger than Canada’s fair share of a global carbon budget:
- Canada’s proven reserves of oil, bitumen, gas and coal are equivalent to 91 Gt of CO2, or 18% of the global carbon budget.
- Adding in probable reserves boosts this figure to 174 Gt, or 35% of the global carbon budget.
- A final, more speculative category including all possible reserves is 1,192 Gt — more than double the world’s carbon budget.
This means that business as usual for the fossil fuel industry is incompatible with action to address climate change that keeps global temperature increase to 2°C or less. Even at the high end of a 20 Gt carbon budget, this would imply that 78% of Canada’s proven reserves, and 89% of proven-plus-probable reserves, would need to remain underground.