By contrast, the PPI payments were, by the time of the Budget, very much a known quantity.

BBC: A welcome boost - unless you're a bank

Here the maths is unambiguous - we have insufficient budget for the carbon we are already emitting and by the time shale gas is produced in any quantity (five to 10 years), there will be no emissions space left for it.

BBC: Viewpoints: Fracking's risks and benefits