Table 2 Overview of allocation rules

PCC

Per capita convergence

Equality

Converge from GF allocations over time to PC allocations.

ECPC

Equal cumulative per capita

Responsibility (and equality)

Emissions allocations converge to fully spending a rightful remaining budget (or conversely, redeeming a debt) based on historical emissions and what the country had the right to emit based on contemporary population. After convergence, a per capita allocation is issued.

AP

Ability to pay

Capability

Inversely based on GDP per capita fraction, taking into account increasing costs of marginal abatement.

PC

(Immediate) per capita

Equality

Allocations immediately proportional to current population levels without any convergence from current emissions levels. This yields a discontinuous ‘step’ between current emissions levels and future allocations.

PCB

Per capita via budget

Equality

Fraction cumulative CO₂ budget is determined on a per capita basis, then allocated over time using a linearly decreasing pathway until CO₂ is net zero. A non-CO₂ part is added separately. GHG across countries does not sum the global GHG pathway over time but only cumulatively does.

GDR

Greenhouse development rights

Responsibility and capability

Up to 2030: combination of capability and responsibility using the Responsibility–Capability Index (RCI)^44,45. After 2030: same as AP. This approach accounts for the wealth distribution within countries through a luxury threshold. The approach is generalized in the Climate Equity Reference Framework³⁴.

GF

Grandfathering

Maintain current emissions distribution

Fraction of current emissions levels is kept constant: all countries decrease emissions allowances proportionally. Preserves current emissions inequality over time.

CO

Cost optimal

Cost optimality

Output from IAMs, on a globally least-cost basis using marginal abatement curves.