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This paper assessed Japan’s “fair” carbon budget consistent with the 2 °C target between 1990 and 2100 under three frequently referred effort-sharing approaches: Converging Per Capita Emissions (CPE); Common but Differentiated Convergence (CDC); and Greenhouse Development Rights (GDRs), as well as the consequent GHG emission pathways up to 2100 for the country to stay within the budget. The calculated carbon budgets were compared with Japan’s projected cumulative greenhouse gas (GHG) emissions in case of adhering to the existing national mitigation targets committed for 2020 and 2050 (“Nationally Committed Amount”: NCA). The global carbon budget for 1990-2100 was assumed to be 1800 Gt-CO2e excluding LULUCF.

The remaining carbon budgets for Japan under CPE and CDC (23 Gt-CO2e and 20 Gt-CO2e) were found to be about 50%-60% of the amount the country would emit up to 2100 under the reference NCA (39 Gt-CO2e). If Japan continues to emit GHG at current levels (1.34 Gt-CO2e/yr in 2012), the budget under CPE and CDC approaches will run out in the early 2030s. Japan’s carbon budget under GDRs was found to be negative for 1990-2100, due to the very high responsibility and capability that is determining the results under this approach.

Moreover, the average emissions reduction rate for 2020-2050 based on the government’s current mitigation targets was found to be similar to that required under the CPE approach with immediate actions from 2014. If Japan is to consider the carbon budget concept, strengthening the current 2020 target, rather than adhering to the current targets, would allow mitigation rates to remain within a more realistic range and would limit the need to buy international offsets.