In 2015, at COP21 in Paris, world leaders agreed to

(a) aim for a peaking of global greenhouse gas (GHG)emissions as soon as possible,

(b) achieve GHG neutrality (‘‘a balance between anthropogenic emissions and removals by sinks”) in the second half of this century, and

(c) hold global warming wellbelow 2°C(UNFCCC, 2015).

This will require strong mitigationefforts (IPCC, 2014). Cumulative carbon dioxide (CO2) emissionsand global temperature change are near-linearly related and inde-pendent of the timing of CO2emissions.

A globalcumulative ‘‘carbon budget” in line with the 2-degree limit has been proposed to be more robust and easier to implement as a policy target than emissions-rate or concentration targets.