By Kornelis Blok and Nicole van den Berg 
The concept of a carbon budget has become very common since the appearance of the 5th assessment report of the IPCC . The idea is simple: if we wish to stay within a certain maximum temperature increase by the end of this century, say 1.5 – 2 °C compared to pre-industrial levels, there is a maximum total volume of CO2 we can still emit until the end of the century. It turns out that maximum temperature rise in the year 2100 and carbon budget are highly correlated. The carbon budget is generally applied to the world as a whole.
But what is the carbon budget for individual countries, or for groups of countries, like the European Union? Within the arena of the international climate negotiations, not all countries are treated alike. In the UNFCCC, the framework convention on climate change adopted in 1992, the famous phrase is included that countries have common but differentiated responsibilities. In practice this has been interpreted such that developed countries should act earlier than developing countries, because of their higher per capita emissions rate, their historic responsibility and their greater ability to pay. Of course, a lot has changed since 1992 and the distinction between developed and developing countries has become less clear, but there is still a wide variety in social and economic development among the nearly 200 countries that participate in the climate negotiations.
Back to the carbon budget: if there is one global budget, how do we share that pie among countries? To share the budget, effort differentiation rules can be used. Such rules represent what can be considered a fair distribution of the emissions space among countries. In many studies, such effort differentiation rules have been applied to emissions for one specific year in the future . In a recent study , under the leadership of the Netherlands Environmental Assessment Agency (PBL), we established effort differentiation rules for the entire future carbon budget. In this weblog, we want to focus on just one region, the European Union, and contrast the calculated budget with the climate strategy of the European Commission that was published last year, and which was discussed in a previous blog.
Let’s first look at the carbon volume claimed in the EU climate strategy. The cumulative emissions are 23 – 28 gigatonnes (1 gigatonne is one billion tonnes) over the period 2018 – 2100 . This seems modest if we compare it to the emissions in the year 2017; these were 3.7 gigatonnes of CO2, which would leave us the equivalent of just about seven years of 2017 emissions. However, even in the most ambitious scenarios, the EU would emit a lot more, but that is compensated by removals of CO2 from the atmosphere through natural and technological solutions, the so-called negative emissions. See the figure below:
What budgets do the various effort differentiation rules allow? One obviously fair rule would be to share the budget on a per capita basis; however, this would be very tight for countries with high current per capita emissions. A softer approach is per capita convergence: allowances per capita converge from the current level to an equal per capita level in some year in the future. Other approaches are based on countries’ ability-to-pay and on “greenhouse development rights”. We will come back to the latter approach later.
In our paper, we considered two budgets: 400 and 1000 gigatonnes of CO2 which were by then considered as representative for pathways compatible with a maximum temperature rise of 1.5 °C and 2 °C, respectively . The outcomes for the EU carbon budget are listed in the following table . The ranges result from different parameter settings, based on the wide authorship from a diverse group of countries. For comparison, outcomes are also given for pathways that lead to maximum cost-effectiveness. Budgets are for the period 2018-2100, as in the EU climate strategy .
|Effort differentiation rule||Budget in case of a|
global 400 gigatonne budget
(in gigatonnes CO2)
|Budget in case of a|
global 1000 gigatonne budget
(in gigatonnes CO2)
|Immediate equal per capita emissions||0||+40|
|Per capita convergence||+5 to +10||+55 to +75|
|Equal cumulative per capita emissions||-70 to -25||-30 to +15|
|Ability to pay||-90 to -35||-25 to +25|
|Greenhouse development rights||-720 to -400||-565 to -290|
|For comparison: Cost-effectiveness||-15 to +155||+30 to +215|
We see that outcomes can be highly variable, depending on the effort differentiation rule. If we look at the first four rules, we see that, for a 1000 gigatonne global CO2 budget, calculated budgets are fairly close to each other. The proposed EU cumulative emissions of 23 – 24 gigatonnes of CO2 are pretty much in the middle of the range of allocated budgets. However, for a tighter global budget most budgets allocated to the EU turn negative. Reaching a negative budget of, for example, 25 gigatonnes of CO2, would require that either emissions are reduced faster, or additional CO2 is removed from the atmosphere: 0.3 gigatonnes per year averaged for this century, about 50% more than what is already included in the strategy . To summarize: the EU strategy may be generally 2 °C compatible according to most of the effort differentiation rules discussed so far, but most likely not yet 1.5 °C compatible.
The real outlier in the list of effort differentiation rules is greenhouse development rights (GDR). This rule has a long history. It has evolved from the Brazilian proposal that was put forward in the run-up to the Kyoto Protocol in 1997. The idea is that emission reduction efforts should be based on the historic responsibility for greenhouse gas emissions and the capacity to reduce emissions; for the latter the income of countries’ inhabitants above a certain threshold is taken as an indicator. The proposal stresses the long emission history of developing countries which made it possible for them to reach a high standard of living. Developing countries claim their own right to follow a similar development pathway. As we already argued in an earlier paper  it is the question whether that should be exactly the same pathway: new technology is available and leapfrogging to a low-carbon pathway may become an attractive option for newly developing nations.
The GDR rule may have a new meaning in a world where negative emissions play an important role. We have brought ourselves in a situation where we not only have to reduce our emissions, but also need to remove large volumes of CO2 from the atmosphere in this century. CO2 that we have accumulated in the atmosphere for over 200 years. It is exactly here that historic responsibility may become a key indicator for who should carry the burden. For the European Union it may not be necessary to realize huge amounts of negative emissions on its own territory. However, it is probably fair that the European Union should contribute more substantially to bearing the financial burden of negative emissions than what is set out in the current strategy document.
 Nicole van den Berg studied Industrial Ecology in Delft/Leiden and is now PhD researcher at Utrecht University.
 L. Clarke, K. Jiang, K. Akimoto, M. Babiker, G. Blanford, K. Fisher-Vanden, J.-C. Hourcade, V. Krey, E. Kriegler, A. Löschel, D. McCollum, S. Paltsev, S. Rose, P. R. Shukla, M. Tavoni, B. C. C. van der Zwaan, and D.P. van Vuuren, 2014: Assessing Transformation Pathways. In: Climate Change 2014: Mitigation of Climate Change. Contribution of Working Group III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
 See e.g. N. Höhne, M. den Elzen, D. Escalante: Regional GHG reduction targets based on effort sharing: a comparison of studies. Climate Policy 14 (2019) pp. 122–147, https://doi.org/10.1080/14693062.2014.849452.
 N.J. van den Berg, H.L. van Soest, A.F. Hof, M.G.J. den Elzen, D.P. van Vuuren, W. Chen; L. Drouet, J. Emmerling, S. Fujimori, N. Höhne, A. Köberle, D. McCollum, R. Schaeffer, S. Shekhar, S. Sudharmma Vishwanathan, Z. Vrontisi, K. Blok: Implications of various effort-sharing approaches for national carbon budgets and emission pathways, Climatic Change, 2019, https://doi.org/10.1007/s10584-019-02368-y, conducted under the CD-LINKS project.
 Source: In-depth review, previous footnote. There is another scenario in the document that also reaches net zero greenhouse gas emissions in 2050: 1.5TECH. This pathway doesn’t differ substantially from 1.5LIFE.
 Our understanding has developed since then, but this range can still be considered to span most Paris-compatible pathways.
 This table does not say anything about who pays for emission reductions – in practice, financial transfers, trading etc. are likely.
 We corrected the results from Van den Berg et al. by subtracting the 2011-2017 emissions (26 gigatonne CO2), based on EEA data.
 This is broadly consistent with the 1 gigatonne per year or 50 gigatonne over this century quoted here: O. Geden, G.P. Peters and V. Scott: Targeting carbon dioxide removal in the European Union, Climate Policy, 19 (2019) pp. 487-494, DOI: 10.1080/14693062.2018.1536600.
 N. Höhne and K. Blok: Calculating Historical Contributions to Climate Change – Discussing the ‘Brazilian Proposal’, Climatic Change 71(2005) pp. 141-173.