The risk posed by today’s fossil fuel industry to the climate comes down to simple arithmetic using just three numbers, or “doing the math’, as campaigner Bill McKibben puts it.

The first number is 2C, the amount of global warming the world’s governments have set as a limit. Beyond 2C, the world’s scientists project “severe, widespread, and irreversible impacts” on people. The 2C number began life in 1995 as a round number that was useful in political negotiations. But since then scientists have warned that even 2C of warming will bring damaging impacts.

The second number is 565 billion tonnes (GT) of carbon dioxide, which scientists estimate is the maximum amount that can be produced by future fossil fuel burning if we are to have an 80% chance of keeping global warming under 2C. If you use a lower chance, say 50% or 66%, the amount of allowable CO2 gets a bit bigger.

But whatever the amount, they are all dwarfed by the third number. This is the total CO2 that would be released if today’s proven reserves of coal, oil and gas are burned: 2,795 GT. That is fossil fuel identified and ready to extract.

So here’s the arithmetic. To stay under 2C, only 565 GT of CO2 can be emitted, but there is already 2795 GT - fives times more - ready to burn.

This simple analysis has global implications. If climate change is to be tamed, most existing fossil fuel reserves must be kept in the ground. That means continued exploration should be pointless and that assets worth trillions of dollars should become worthless. And that shows the scale of the challenge presented by climate change.

Damian Carrington