Allocated carbon budgets and required reduction efforts
This section presents the carbon budgets and GHG emission allowances over time, based on the different effort-sharing approaches, using the results of the questionnaire on the parameters (see also Online Resource Chapters 2 and 3). When comparing the results of the emission pathways and budgets, it should be noted that carbon budgets are independent of time and represent CO2 only, while GHG emission pathways are time-dependent and include all GHG emissions (including LULUCF). Therefore, this comparison is useful for determining similarities or differences in trends, but budgets and pathways should not be compared on their absolute numbers. As there was rare consensus on the parameter settings, the sensitivity ranges of these parameter choices are presented in addition to the default values.
Results for default settings
Figure 2 summarises the results of the carbon budget approach in terms of national carbon budgets (2011–2100) relative to each country’s 2010 CO2 emissions for all effort-sharing approaches, compared to the cost-optimal national ‘budgets’ (cumulative emissions resulting from cost-optimal scenarios). Online Resource Fig. S.1 and Fig. 3 illustrate the results for the emission pathways’ approach, both in terms of GHG emission allowances over time (Fig. S.1) and in terms of GHG emission reduction targets in 2030 relative to 2010 GHG emissions (Fig. 3).
The most salient results are the extreme outcomes of the GDR* approach relative to all other approaches. The default values (i.e. 1850 as starting year and capability and responsibility given equal weight) lead to the largest budgets of all approaches for China and India and the smallest budgets for the USA, EU and Japan—both for the budgets and for the pathways. This is a result of a combination of relatively high historical emissions and high GDP per capita for the latter group of countries—resulting in high RCIs. The approach even leads to negative budgets for these countries. This is a consequence of the required reduction in the global budget being allocated to countries and deducted from their BAU budget—and the BAU emissions of the USA, EU, and Japan are projected to remain either constant or even decrease. In other words, it is a combination of relatively low BAU budgets (compared to history) and high RCI levels. For the pathways, emissions allowance allocations are negative by or soon after 2030. For China, the carbon budgets are smaller than for India, given the (much) higher GDP per capita levels and higher historical emissions. Combined, China and India are allocated more than 80% of the global budget in this approach. Note that for the pathways’ approach, GDR gradually moves to AP outcomes as RCI data was not available from 2031 onwards, which explains the change in trend for the GDR pathways in Fig. S.1.
The AP* approach—while sharing some characteristics with the GDR* approach—shows much smaller differences between countries. One reason is that responsibility is not taken into account. Another reason is that unlike the RCI approach, reductions are not a linear function of (responsibility and) capability, as we corrected for the shape of the marginal abatement cost curve in the AP* approach (for more details see Online Resource Table S.1). In practice, this means that a country with a GDP per capita income that is twice as high as the global average GDP per capita, the country’s relative reduction target is 26% larger (so if the average global reduction is 10%, the country needs to reduce 12.6%). This method leads to similar costs as the share of GDP, and so to smaller differences between countries. For China, the AP* approach leads to the smallest budget, as their GDP per capita is already above the world average and projected to still increase strongly while having a low RCI. In the emission pathways, this is visible through the AP approach resulting in a steady decrease in emission allowances to reach negative emission allowances in 2070, while GDR allows for an increase in emission allowances towards a peaking of about 40% above 2010 levels. Under a GF* approach, a relatively large budget is also allocated, due to China’s high current emissions. Even though China also has a large population share (IEPC*), they receive a larger budget based on current emissions. PCC* lies in between these values. The ECPC* has a comparable budget, and thus a larger budget compared to PCC* and GF*, due to China’s high population share and low historical emissions. As can be observed in the emission pathways, China’s emission allowances are projected to rise significantly more under the GF approach compared to the IEPC approach, for the same reasons.
The IEPC* approach gives outcomes similar to, but much less extreme than, the GDR* approach, with negative budgets for the USA and Russia and about zero budgets for the EU, Japan and Brazil (only with the default setting of including land use CO2 emissions). Together, India and China are allocated about half of the total global carbon budget in this approach, which is much less than the GDR* approach. As the IEPC approach implies immediate equal per capita emission allowances for the emission pathways, India is allocated more allowances than baseline emissions until about 2030. Nevertheless, the allocated budget is much smaller than their baseline cumulative emissions. For the USA, the EU, Japan and Russia, the opposite is the case: IEPC immediately leads to much lower allowances than current emission levels given their current high per capita emission levels.
As expected, the GF* approach leads to relatively large budgets for countries with relatively high current emissions per capita, such as the USA and Russia, and it leads to the smallest budget for India. Presented relative to 2010 emission levels, the carbon budgets and emission targets relative to 2010 emissions are the same across countries, as current emission levels determine the budgets (Figs. 2 and 3).
The PCC* approach is a combination of the IEPC* and GF* approaches, with results in the middle of these two approaches.
The ECPC* approach leads to the most extreme outcomes after GDR*: countries with relatively high historical per capita emissions (notably the USA and Russia) are allocated negative budgets, and Japan and the EU roughly zero budgets. Countries with low historical emissions per capita, notably India, are allocated relatively large budgets but still smaller than the GDR* approach.
To summarise, the GDR* approach—as implemented in our study—leads to the most extreme outcomes, with large budgets or emission allowances allocated to developing countries compared to all other approaches. The ECPC* approach leads to the same trend in outcomes, but less extreme. For most countries, the cost-optimal reductions are between the GF* and PCC* allocations. For the USA, EU and Japan, all approaches except GF* lead to more stringent budgets compared to cost-optimal allocation—suggesting that a uniform global carbon price only leads to an equitable outcome based on the principle of acquired rights. The differences between the approaches are very large for countries with very different GDP per capita or per capita emission levels compared to the world average (USA, EU, Japan, India), but are smaller for Brazil, China and Russia. For the USA, for instance, the allocated budget ranges from less than −300 GtCO2 to 160 GtCO2 (about 30 times their current CO2 emissions). For the EU, Japan and India, the differences between the approaches are even larger. It should be noted, however, that for the GDR* approach, we kept RCI values constant after 2030, as no projection was available beyond this year. As RCI values are likely to converge further after 2030, this leads to an overestimation of the budget allocations for developing countries and underestimation for developed countries. In the pathways’ calculations, we corrected for this by assuming a linear convergence towards the AP results. This still leads to GDR resulting in the most extreme results, but less extreme than under the budget approach (the implied carbon budget for the USA would be –150 GtCO2 under the pathways approach instead of −300 GtCO2 under the budget approach (Fig. 4). When only looking at 2030 targets, the results are slightly different. For developed countries, GDR still leads to the most extreme outcomes, but for India, IEPC leads by far to the largest emission allocation by 2030. Given India’s current low per capita emissions, in the short term, IEPC leads to larger allocations than projected baseline emissions, which is by definition not possible under the GDR approach.
In this research, we also calculated the indirect carbon budgets by subtracting remaining cumulative non-CO2 GHGs (based on the cost-optimal reduction of the different GHGs) from the total cumulative GHG emissions resulting from the pathways’ approach (see Fig. 4). Differences between these indirect and direct carbon budgets can be explained by the variation in calculation methodologies between carbon budgets and emission pathways. Firstly, in the GF*, IEPC* and PCC* methodology for carbon budgets, instead of a convergence year as used in the emission pathway calculations, a weighting factor was chosen, as carbon budget allocations are independent of time. In addition, instead of reducing from current emissions, the global carbon budget was allocated based on a share irrespective of the country’s current emissions. Secondly, for the GDR* and AP* approach, instead of a BAU pathway as used in the pathways’ calculations, an average baseline over the whole period (2010–2100) was used to calculate the entitled national carbon budget. Thirdly, for all approaches in the carbon budget calculations, instead of a global pathway to 2 °C or 1.5 °C, the global carbon budget was used. Finally, for the GDR approach, we assumed a linear convergence towards the AP approach in the pathways’ calculations from 2030 onwards, leading to less extreme outcomes compared to the budget calculations.
The general sensitivities analysed include the global ambition level (apart from the default 1000 GtCO2 budget, we included runs with 400 GtCO2 budget) and including (default) or excluding land use CO2. The sensitivity values used and results are summarised in the Online Resource Table. S.3. and the Online Resource Fig. S.2. Obviously, a more stringent global target leads to more stringent budgets or pathways for all countries. However, the amount to which national budgets decrease depends on the approach. For instance, under the GDR* approach, the difference in carbon budgets between India and the USA is 820 GtCO2 (with India allocated the largest budget) under a global budget of 1000 GtCO2 and 925 GtCO2 under a global carbon budget of 400 GtCO2. As such, the GDR* approach leads to even more extreme outcomes under more stringent global budgets. However, for most of the other approaches, a more stringent budget leads to smaller absolute differences in budget allocations between countries.
The effect of including or excluding LULUCF CO2 emissions mainly has a significant effect on countries with relatively large LULUCF CO2 emissions, notably Brazil and India. Under the ECPC* approach, the budget of Brazil is much more stringent with, than without, land use CO2, as including their historical land use CO2 emissions significantly increase their historical per capita emission levels. For approaches that rely on projected BAU emissions, the results can be ambiguous, as excluding LULUCF CO2 emissions not only affects historical emissions (important for GDR*) but BAU emissions as well. For the GF* approach, this distinction has no effect by definition, because the only determinant to allocate budgets or emission allowances is population.
For all countries except India, the GDR* approach is the most sensitive to the different parametrisations applied here. We analysed two different sensitivities for GDR*, i.e. start year of the responsibility factor and the weighting towards either capability or responsibility. Of those two parameters, the weighting factor has the largest effect on the results, especially for those countries with relatively high or low emissions per capita compared to their GDP per capita. A weighting towards capability, for instance, leads to much smaller budgets for especially the EU and Japan, both of which have relatively low emissions per capita given their GDP per capita level. For Russia, the opposite is the case: a weighting towards responsibility leads to a much smaller budget given their relatively high emissions per capita and low GDP per capita. This is also the case for Brazil, where high LULUCF CO2 emissions also play an important role. The effect of the start year is especially important for the EU, the USA, Russia and China. For the first three countries, starting in 1850 leads to much smaller budgets than starting in 1990, while the opposite is the case for China. In numbers, the EU and the USA are allocated about 90–100 GtCO2 more and Russia 33 GtCO2 more if the starting year is 1990 instead of 1850, while China would be allocated 95 GtCO2 and India 35 GtCO2 less.
The parameterisation of the AP* and ECPC* approach is especially relevant for India. For AP*, the reason is that in terms of PPP, their income is much higher than in terms of MER. This results in a larger budget (by 40 GtCO2) if the ability to pay would be based on MER. For China, the budget would be 35 GtCO2 higher. Likewise, the USA (by 35 GtCO2), the EU (by 48 GtCO2) and Japan (by 15 GtCO2) would be allocated smaller budgets if GDP were to be based on MER. For ECPC*, the starting year is more important than the applied discount rate. Similar reasoning as for GDR* holds here, with an especially larger budget for India and China for a starting year in 1850 compared to 1990 (the difference being 60 GtCO2 for both countries).