The term Unburnable Carbon refers to fossil fuel energy sources (reserves and/or resources), which physically cannot be burned if the world is to adhere to any given temperature outcome. If burned, the associated emissions would mean exceeding the carbon budget for that temperature. The existence of this overhang of available fossil fuels, or unburnable carbon, leads to the concept of the carbon bubble.

Since 2013, Carbon Tracker has produced a series of reports themed on unburnable carbon which has stimulated a new debate around the future of energy and investment. This has prompted organisations ranging from the IEA, oil companies, NGOs, accountants, investors, the OECD, central banks and investment banks to consider the issue. Given that the IEA and even oil companies such as BP and Shell have confirmed that burning all known fossil fuels would result in more than 2°C of warming, we feel there is reasonable consensus around this issue.

Some aspects to consider:

  • Some fossil fuels may be used for other purposes which do not involve immediate combustion, e.g. as lubricants. Hence we use the term unburnable rather than unusable.
  • Different organisations may apply a range of carbon budgets, meaning the precise amount of unburnable carbon cited varies, however there is a consensus that there is a clear overhang, and the level of potential carbon emissions exceeds any reasonable carbon budget.

Useful links:


Unabated combustion of all today’s fossil fuel reserves would result in three times more CO2 emissions than the remaining CO2 budget.”


The issue of the bubble arises because the combined proven oil, gas and coal reserves currently on the books of fossil fuel companies (and governments in the case of NOCs) will produce far more than this amount of CO2 when consumed.


Existing reserves of fossil fuels – i.e. oil, gas and coal – if used in their entirety would generate somewhere in excess of 2.8 trillion tonnes of CO2, well in excess of the 1 trillion tonnes or so the scientific community consider is consistent with limiting the rise in global mean temperatures to no more than 2 degrees Centigrade. And this takes no account of the new discoveries which are being made all the time or of the vast resources of fossil fuels not yet booked as reserves.