Implications of various effort-sharing approaches for national carbon budgets and emission pathways

The GDR* approach as implemented in the study leads to the most extreme outcomes, with large budgets or emission allowances allocated to developing countries compared to all other approaches.

“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.”

Under the GF* approach, a relatively large budget is allocated to China due to China's high current emissions.

“Under a GF* approach, a relatively large budget is also allocated, due to China's high current emissions.”

For developed countries, all effort-sharing approaches except grandfathering lead to more stringent budgets than cost-optimal budgets.

“Furthermore, for developed countries, all effort-sharing approaches except grandfathering lead to more stringent budgets than cost-optimal budgets”

The parameterisation of the AP* and ECPC* approach is especially relevant for India.

“The parameterisation of the AP* and ECPC* approach is especially relevant for India.”

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.

“The effect of including or excluding LULUCF CO 2 emissions mainly has a significant effect on countries with relatively large LULUCF CO 2 emissions, notably Brazil and India.”

For China, the budget would be 35 GtCO2 higher if GDP were based on MER.

“For China, the budget would be 35 GtCO 2 higher.”

Nevertheless, India's allocated budget under IEPC* is much smaller than their baseline cumulative emissions.

“Nevertheless, the allocated budget is much smaller than their baseline cumulative emissions.”

When only looking at 2030 targets, for developed countries, GDR still leads to the most extreme outcomes.

“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.”

For ECPC*, the starting year is more important than the applied discount rate.

“For ECPC*, the starting year is more important than the applied discount rate.”

For AP*, in terms of PPP, India's income is much higher than in terms of MER.

“For AP*, the reason is that in terms of PPP, their income is much higher than in terms of MER.”

For the USA, the EU, Japan and Russia, IEPC immediately leads to much lower allowances than current emission levels given their current high per capita emission levels.

“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.”

Combined, China and India are allocated more than 80% of the global budget in the GDR* approach.

“Combined, China and India are allocated more than 80% of the global budget in this approach.”

The AP* approach shows much smaller differences between countries than the GDR* approach.

“The AP* approach-while sharing some characteristics with the GDR* approach-shows much smaller differences between countries.”

For most countries, the cost-optimal reductions are between the GF* and PCC* allocations.

“For most countries, the cost-optimal reductions are between the GF* and PCC* allocations.”

The amount to which national budgets decrease depends on the approach.

“However, the amount to which national budgets decrease depends on the approach.”

For the USA, the allocated budget ranges from less than -300 GtCO2 to 160 GtCO2, about 30 times their current CO2 emissions.

“For the USA, for instance, the allocated budget ranges from less than -300 GtCO 2 to 160 GtCO 2 (about 30 times their current CO 2 emissions).”

For the pathways, GDR* emissions allowance allocations are negative by or soon after 2030.

“For the pathways, emissions allowance allocations are negative by or soon after 2030.”

A weighting towards capability 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.

“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.”

Of the two GDR* parameters, the weighting factor has the largest effect on the results, especially for countries with relatively high or low emissions per capita compared to their GDP per capita.

“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.”

For the EU, Japan and India, the differences between the approaches are even larger than for the USA.

“For the EU, Japan and India, the differences between the approaches are even larger.”

The GDR* approach leads to negative budgets for the USA, EU and Japan.

“The approach even leads to negative budgets for these countries.”

For all countries except India, the GDR* approach is the most sensitive to the different parametrisations applied here.

“For all countries except India, the GDR* approach is the most sensitive to the different parametrisations applied here.”

Effort-sharing approaches that calculate required reduction targets in carbon budgets (relative to baseline budgets) and/or take into account historical emissions when determining carbon budgets can lead to large negative remaining carbon budgets for developed countries.

“negative remaining carbon budgets for developed countries.”

• Also stated in: § 4 Conclusions · plain

  “In general, effort-sharing approaches that (i) calculate required reduction targets in carbon budgets (relative to baseline budgets) and/or (ii) take into account historical emissions when determining carbon budgets can lead to (large) negative remaining carbon budgets for developed countries.”

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.

“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.”

China's emission allowances are projected to rise significantly more under the GF approach compared to the IEPC approach.

“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 effect of the start year is especially important for the EU, the USA, Russia and China.

“The effect of the start year is especially important for the EU, the USA, Russia and China.”

For India, IEPC leads by far to the largest emission allocation by 2030.

“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.”

The GDR* approach has extreme outcomes relative to all other approaches.

“The most salient results are the extreme outcomes of the GDR* approach relative to all other approaches.”

For the USA, EU and Japan, all approaches except GF* lead to more stringent budgets compared to cost-optimal allocation.

“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.”

In the AP* approach, a country with a GDP per capita income twice as high as the global average has a relative reduction target 26% larger (e.g., if the average global reduction is 10%, the country needs to reduce 12.6%).

“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%).”

For most of the other approaches, a more stringent budget leads to smaller absolute differences in budget allocations between countries.

“However, for most of the other approaches, a more stringent budget leads to smaller absolute differences in budget allocations between countries.”

The implied carbon budget for the USA would be -150 GtCO2 under the pathways approach instead of -300 GtCO2 under the budget approach.

“(the implied carbon budget for the USA would be -150 GtCO 2 under the pathways approach instead of -300 GtCO 2 under the budget approach (Fig. 4).”

For Russia, a weighting towards responsibility leads to a much smaller budget given their relatively high emissions per capita and low GDP per capita.

“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.”

PCC* lies in between the GF* and IEPC* values.

“PCC* lies in between these values.”

Under the ECPC* approach, the budget of Brazil is much more stringent with, than without, land use CO2.

“Under the ECPC* approach, the budget of Brazil is much more stringent with, than without, land use CO 2 , as including their historical land use CO 2 emissions significantly increase their historical per capita emission levels.”

The equal cumulative per capita approach and especially the greenhouse development rights approach lead to large negative remaining carbon budgets for developed countries.

“This is the case for the equal cumulative per capita approach and especially the greenhouse development rights approach.”

For China, the carbon budgets under GDR* are smaller than for India, given China's higher GDP per capita levels and higher historical emissions.

“For China, the carbon budgets are smaller than for India, given the (much) higher GDP per capita levels and higher historical emissions.”

For AP*, this results in a larger budget (by 40 GtCO2) for India if the ability to pay would be based on MER.

“This results in a larger budget (by 40 GtCO 2 ) if the ability to pay would be based on MER.”

Including Brazil's historical land use CO2 emissions significantly increases their historical per capita emission levels.

“Under the ECPC* approach, the budget of Brazil is much more stringent with, than without, land use CO 2 , as including their historical land use CO 2 emissions significantly increase their historical per capita emission levels.”

Apart from grandfathering, all effort-sharing approaches considered here lead to smaller, and sometimes much smaller, carbon budgets for developed countries than the cost-optimal budget.

“Apart from grandfathering, all effort-sharing approaches considered here lead to smaller, and sometimes much smaller, carbon budgets for developed countries than the cost-optimal budget.”

• cites McCollum et al. 2018Not checked yet

  Not checked in the free run.

The variation in historical start year (1850, 1970 or 1990) created significant differences in ECPC* carbon budgets, in addition to the discount rate.

“Furthermore, the variation in historical start year (i.e.1850, 1970 or 1990) created significant differences in equal cumulative per capita* (ECPC*) carbon budgets, in addition to the discount rate.”

AP and AP*, in which the choice of GDP (MER or PPP) and the inclusion or exclusion of a participation threshold were considered, showed a large range of outcomes, which was largely due to MER or PPP.

“Most notably, AP and AP*, in which we considered the choice of GDP (MER or PPP) and the inclusion or exclusion of a participation threshold, showed a large range of outcomes, which was largely due to MER or PPP.”

Results were also very sensitive to the parameters chosen within a single approach.

“Still, results were also very sensitive to the parameters chosen within a single approach.”

For ECPC*, India and China have an especially larger budget for a starting year in 1850 compared to 1990, the difference being 60 GtCO2 for both countries.

“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 GtCO 2 for both countries).”

For some methods, value judgments on parameter settings play an important role in the results.

“We introduced novel methods to directly calculate national carbon budgets from effort-sharing approaches, and showed how for some methods, value judgments on parameter settings play an important role in the results.”

Carbon budget and emission pathway calculations differ mostly as a function of the different effort-sharing approaches.

“Carbon budget and emission pathway calculations differ mostly as a function of the different effort-sharing approaches.”

The historical start year did not influence the outcomes of GDR* as significantly.

“Yet, the historical start year did not influence the outcomes of GDR* as significantly, most likely due to the relatively lower weighting towards historical emissions within the RCI compared to ECPC* that focuses on responsibility entirely.”

A uniform global carbon price only leads to an equitable outcome based on the principle of acquired rights.

“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.”

Cost-optimal approaches do not lead to outcomes that can be regarded as fair according to most effort-sharing approaches.

“indicating that cost-optimal approaches do not lead to outcomes that can be regarded as fair according to most effort-sharing approaches.”

• Also stated in: § 4 Conclusions · hedged

  “This indicates that cost-optimal approaches do not lead to outcomes that can be regarded as fair according to most effort-sharing approaches.”

At the global scale, the carbon budget approach is based on the strong linear relationship between long-term temperature change and cumulative CO2 emissions.

“At the global scale, the carbon budget approach is based on the strong linear relationship between long-term temperature change and cumulative CO 2 emissions, because of the long lifetime of CO 2 and the main contribution of CO 2 to anthropogenic warming (Stocker et al. 2013).”

• cites Stocker et al. 2013Not checked yet

  Not checked in the free run.

For the GF* approach, including or excluding LULUCF CO2 has no effect by definition, because the only determinant to allocate budgets or emission allowances is population.

“For the GF* approach, this distinction has no effect by definition, because the only determinant to allocate budgets or emission allowances is population.”

The choice of coverage of GHG emissions matters, as including all Kyoto GHG emissions including LULUCF or only energy and industrial processes-related CO2 emissions could lead to significantly different outcomes.

“The choice of coverage of GHG emissions also matters, as including all Kyoto GHG emissions including land use, land-use change and forestry (LULUCF) emissions or only energy (and industrial processes)-related CO 2 emissions, could lead to significantly different outcomes.”

The GDR* approach leads to even more extreme outcomes under more stringent global budgets.

“As such, the GDR* approach leads to even more extreme outcomes under more stringent global budgets.”

The global carbon budget consistent with keeping global mean temperature below 2 °C with >66% likelihood is about 1000 GtCO2 for the period 2011-2100.

“and 2 °C with > 66% likelihood (in line with about 1000 GtCO 2 for 2011-2100) (Rogelj et al. 2016b;IPCC 2014)”

• cites Rogelj et al. 2016bNot checked yet

  Not checked in the free run.

• cites IPCC 2014Not checked yet

  Not checked in the free run.

It is possible to derive global targets for cumulative CO2 emissions tolerable over a certain period.

“As a result, it is possible to derive global targets for cumulative CO 2 emissions tolerable over a certain period.”

Differences between indirect and direct carbon budgets can be explained by variation in calculation methodologies between carbon budgets and emission pathways.

“Differences between these indirect and direct carbon budgets can be explained by the variation in calculation methodologies between carbon budgets and emission pathways.”

The global carbon budget consistent with keeping global mean temperature below 1.5 °C by 2100 with >66% likelihood is about 400 GtCO2 for the period 2011-2100.

“keeping global mean temperature below 1.5 °C by 2100 with > 66% likelihood (in line with about 400 GtCO 2 global carbon budget for the period 2011-2100)”

• cites Rogelj et al. 2016bNot checked yet

  Not checked in the free run.

• cites IPCC 2014Not checked yet

  Not checked in the free run.

For approaches that rely on projected BAU emissions, the results can be ambiguous, as excluding LULUCF CO2 emissions not only affects historical emissions but BAU emissions as well.

“For approaches that rely on projected BAU emissions, the results can be ambiguous, as excluding LULUCF CO 2 emissions not only affects historical emissions (important for GDR*) but BAU emissions as well.”

The outcomes of effort-sharing approaches depend significantly on the parameter settings within each approach.

“The literature has shown that outcomes of effort-sharing approaches depend significantly on the parameter settings within each approach.”

The extreme GDR* outcomes are a result of a combination of relatively high historical emissions and high GDP per capita for the USA, EU and Japan, resulting in high RCIs.

“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.”

For the pathways' approach, GDR gradually moves to AP outcomes as RCI data was not available from 2031 onwards.

“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 relatively lower weighting towards historical emissions within the RCI compared to ECPC* is the likely reason the historical start year did not influence GDR* outcomes as significantly.

“Yet, the historical start year did not influence the outcomes of GDR* as significantly, most likely due to the relatively lower weighting towards historical emissions within the RCI compared to ECPC* that focuses on responsibility entirely.”

Some approaches lead to extreme outcomes, which is a consequence of staying as closely as possible to the equity principle underlying the effort-sharing approach.

“Some approaches lead to extreme outcomes, which is a consequence of our staying as closely as possible to the equity principle underlying the effort-sharing approach.”

These approaches can lead to the allocation of negative emissions allowances or budgets, something which approaches that allocate budgets or emission allowances by definition cannot.

“Furthermore, they can lead to the allocation of negative emissions allowances or budgets, something which approaches that allocate budgets or emission allowances by definition cannot.”

The results illustrate the importance of parameter choices and calculation methodologies in order to represent effort-sharing approaches, which accurately depict the underlying equity principles.

“The results presented in this research illustrate the importance of parameter choices and calculation methodologies in order to represent effortsharing approaches, which accurately depict the underlying equity principles.”

Some of the outcomes are clearly impossible to achieve by domestic emission reductions alone.

“Although this represents a full range of outcomes, it means that some of the outcomes are clearly impossible to achieve by domestic emission reductions alone.”