Calculation of an emissions budget for Switzerland
based on Bretschger’s (2012) methodology
April 26th, 2023

Dr. Yann Robiou du Pont,
Dr. Zebedee Nicholls
This scientific report is based on the most recent and best available science. The authors are
uninfluenced as to form or content by the exigencies of litigation.

1Dr. Yann Robiou du Pont is a researcher at Utrecht University. His current research is funded
by a Marie Curie fellowship from the European Commissions, focuses on quantifying what are
fair and ambitious contributions from national and subnational actors to align with the Paris
Agreement mitigation goals. He holds a PhD from the University of Melbourne focused on
quantifying equitable mitigation scenarios for all countries to meet the Paris Agreement goals.
His results, published in Nature Climate Change and Nature Communications, visible on the
Paris-Equity-Check.org interactive website that assesses the ambition of countries emissions
pledges. His studies are used in IPCC and UNEP reports, court cases, by diplomats at UN
climate negotiations and by national and subnational governments to set their emissions
targets (net-zero target and 2030 NDC of the UK, the Government of Victoria).
His background is in physics, with a Magistère in fundamental physics and a Master in climate
physics. Prior to his current research focus, he conducted research in physical oceanography(University of Harvard and Paris Sorbonne), hydrology (University of California, Berkeley), seaice modelling (McGill University) and cosmology (University of Oxford).
Full profile available at: https://www.uu.nl/staff/YRobiouduPont

Dr. Zebedee Nicholls is an expert from the University of Melbourne and the International
Institute for Applied Systems Analysis (IIASA) in reduced complexity climate modelling. He is
also the Modelling and Data Director at Climate Resource. His research focuses on thedevelopment, evaluation and application of reduced complexity models with a particular focus
on the Model for the Assessment of Greenhouse gas Induced Climate Change (MAGICC).
Alongside A/Prof Malte Meinshausen, he leads the Reduced Complexity Model
Intercomparison Project (RCMIP). In the IPCC’s Sixth Assessment Report, he led the writingof Cross-Chapter Box 7.1 on reduced complexity models used for scenario classification in
AR6, was a Contributing Author to WG1 Chapters 1, 4, 5, 6, 7 and Technical Summary and
WG3 Chapter 3 and Annex C. He completed his PhD at the Climate & Energy College within
the University of Melbourne’s School of Geography, Earth and Atmospheric Sciences in 2021,
and his undergraduate Masters course in Physics at St. John’s College, University of Oxford,
where his Master’s thesis was supervised by Prof Myles Allen. He is currently working on thenext phase of RCMIP, taking MAGICC open source and developing more regionally detailedemulators.
Full profile available at: https://findanexpert.unimelb.edu.au/profile/792896-zebedee-nicholls

21. Introduction
This report is drafted at the request of the Verein KlimaSeniorinnen and of the four individual
applicants for the purpose of the proceedings in the case Verein KlimaSeniorinnen andOthers v. Switzerland (Application no. 53600/20).
The request made to us on 1 April 2023 was as follows:
In the context of the proceedings of the Verein KlimaSeniorinnen and others before
the European Court of Human Rights, the Verein KlimaSeniorinnen and the fourindividual applicants were provided by the Swiss State with a submission that had a
Policy Brief of Lucas Bretschger dated March 2012 titled: Climate Policy and Equity
Principles: Fair Burden Sharing in a Dynamic World as an annex. The Policy Brief
presents a methodology to divide the remaining carbon budget for holding global
warming below (the now outdated target of) 2 °C between countries based on“relevant equity principles'' described in the paper. On the basis of this methodology,
the Policy Brief presents budgets for several countries, but not for Switzerland.
For these reasons, the Verein KlimaSeniorinnen and the four single applicants areseeking an expert analysis of the carbon budget for Switzerland for holding the global
temperature increase to 1.5 °C, based on the methodology set out in the Policy Briefof Lucas Bretschger. More specifically, they ask the undersigned to provide thefollowing results based on this methodology:
1. What is the remaining budget for Switzerland as of 2022 (or an earlier year,
depending on the availability of emission data).
2. What is the year in which this budget would be depleted based on the impliedemission trajectory set by the emission reduction targets proposed by the
Swiss government (i.e. minus 34% by 2030 and net-zero by 2050).
3. What is the year that net-zero emissions would need to be achieved if
Switzerland were to remain within this carbon budget, assuming a linear
reduction as of 2022 (or an earlier year depending on the availability ofemission data).
We note that the version of Policy Brief (March 2012) provided to us in the context of thisrequest does not seem to be published online. An updated version dated June 2012 ispublished on the website of the Swiss Federal Institute of Technology (ETH) in Zurich (link).
A further updated version of the Policy Brief was published in October 2013 in the journal
Environment and Development Economics Vol. 18, No. 5, pp. 517–536 (link). Both
documents were provided to us. We found the methodology between the three versions of
the study to be identical. The main differences between the three studies are that the more
recent versions of the studies provide results for more countries. None of the versions of thestudy, however, present results for Switzerland and no results for Switzerland based on thisstudy could be retrieved elsewhere.
For this reason, within the context of this report, the original modelling as described in the
study has been replicated and updated with the Paris Agreement temperature target of 1.5°C and historical emission data.
Model descriptions are shown as in the October 2013 article in the journal Environment andDevelopment Economics. Where reference is made to “Policy Brief”, this refers to all three
versions of the study by professor Bretschger.

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2. Description of methodology used in the Policy Brief
In this report, we calculate each country’s share of the global budget following the methods
described in equation (5) of the Policy Brief. The Policy Brief1 calculates each country’s shareas:

𝑄𝑗 = ∑𝑀

𝑚𝑗 .𝐹𝑗𝑚𝑗 .𝐹𝑗𝑗=1. 𝑍,

where 𝑄𝑗 is the emissions budget allocated to a country 𝑗, 𝑚𝑗 is the country’s share of the
global population, and 𝑍 is the total budget. 𝐹𝑗 is calculated using equation (3) of the Policy
Brief:
𝐸𝑗

0.25

𝐹𝑗 = ( )𝐿𝑗,where 𝐸𝑗 is the country’s total emissions and 𝐿𝑗 is the country’s population. A country’s fractionof the global budget increases with its per capita emissions, which is favourable to countries
with high per capita emissions.
The Policy Brief proceeds to apply this formula, using data available at the time of publication,
to derive the shares for different countries. These shares are then applied to global greenhousegas (GHG) emissions budgets taken from Meinshausen et al. 20092. As mentioned above, thePolicy Brief does not present any results for Switzerland.
To consider the importance of historical responsibility, the Policy Brief uses four different
models: from ‘no responsibility’ to full inclusion of historical emissions for the period starting in
1990 and ending in the latest available reported national emissions data, which was 2008 at
the time the Policy Brief was written. When fully accounting for historical emissions, the Policy
Brief calculates emissions budgets as of 1990 (using 1990 indicators), and then subtracts fromthis budget countries’ historical emissions since 1990 to derive the remaining budget. For the
scenario of assuming ‘no responsibility’, the Policy Brief calculates the emissions budget
directly at the year of interest (that was 2008, the time of the latest available data in the Policy
Brief).

3. Brief critique of the methodology
As described in the ANNEX, based on the methodology in the Policy Brief, Switzerland's
budget as of 2016 (the year after the Paris Agreement was signed) is 0.1201% of the global
budget. We note that the budget share resulting from this methodology is larger than
Switzerland's share of the global population in 2016 which was around 0.1117%.

1 Lucas Bretschger, Climate Policy and Equity Principles: Fair Burden Sharing in a Dynamic World Center for

Economic Research at ETH Zurich, Policy Brief 12/16, March 2012.
2 Meinshausen, M., Meinshausen, N., Hare, W. et al. Greenhouse-gas emission targets for limiting global warming
to 2 °C. Nature 458, 1158–1162 (2009). https://doi.org/10.1038/nature08017

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The Policy Brief approach is, in general, favourable to developed nations compared to theliterature on equitable budget-sharing approaches. Rather than the standard equity approachof giving less to countries who have emitted in the past, the Policy Brief approach gives higher
shares to countries that currently have higher per capita emissions. The capability of countries
does not affect the fair share. Approaches based purely on equity principles, including
responsibility and capability, would give Switzerland significantly smaller shares than thosepresented here.
The Policy Brief explains that it adds aspects of costs and technological feasibility to increase
the political acceptance of its results. In the academic literature on effort-sharing,considerations on the fairness of the responsibility for emission reductions (who pays) are
separated from where these reductions take place.3 Given the focus of the Policy Brief on the
inclusion of costs, technological feasibility and political acceptance, the methodology is not
appropriate to inform emission targets as a fair-share of a collective goal, which can be met
with international support, as is the case with the target communicated by Switzerland in its
NDC.

4. Applied assumptions for updated modelling
To calculate a 1.5 °C emissions budget for Switzerland based on the methodology of the Policy
Brief, we directly use the equation (5) from the Policy Brief as described in the previous section.
We updated the underlying data and parameterisation to reflect the latest available inventories
and findings on carbon budgets. In this section, we set out the most important choices andassumptions used in this report. In the ANNEX to this report, we provide a detailed step bystep description of all the modelling steps taken.
The Policy Brief used the global GHG budget as the starting point for its calculations. Here, we
use the global carbon (CO2) budget for 1.5 °C from the most recent IPCC report as a starting
point. After determining Switzerland's CO2 budget based on the methodology in the Policy
Brief, we convert the budget into a GHG budget.
The updated emissions budget for Switzerland is calculated as of 2016, instead of 2008 as in
the original Policy Brief. The date of 2016 is chosen to reflect the information available as of2015 (the year in which the Paris Agreement was signed) regarding the remaining emissions
budget, including information which could be used to set an emissions target for 2030. The
budget allocated remaining as of 2022 (the year after the most recent available historicemissions data) is the 2016 budget minus Switzerland’s emissions between 2016 and 2022.

3 Section 6.3.6.6, p. 456-461 of: Clarke L., 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 theIntergovernmental Panel on Climate Change [Edenhofer, O., R. Pichs-Madruga, Y. Sokona, E. Farahani, S.
Kadner, K. Seyboth, A. Adler, I. Baum, S. Brunner, P. Eickemeier, B. Kriemann, J. Savolainen, S. Schlömer, C.
von Stechow, T. Zwickel and J.C. Minx (eds.)]. Cambridge University Press, Cambridge, United Kingdom and
New York, NY, USA.

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Regarding the parameterisation, we only model the case that does not account for historicalemissions, which is consistent with the ‘no historical responsibility’ quantification in the Policy
Brief. This choice has been made due to the time constraints for preparing this report. A fullanalysis reflecting all the parameterisations of the Policy Brief would account for historicemissions prior to 2016. Compared to accounting for historical emissions since 1990, notaccounting for historic emissions prior to 2016 yields greater emissions budgets for countries
with higher historical per capita emissions than average, such as Switzerland. This can, forinstance, be seen in the results in the Policy Brief for countries such as Germany or the USA
that have smaller budgets when accounting for more historical emissions. The resultspresented here are thus favourable to Switzerland based on this methodology, and cantherefore be seen as a generous parameterisation of the approach presented in the Policy
Brief for Switzerland.

5. Results
Applying the Policy Brief's formula with updated data, we find an emissions budget for
Switzerland of 381 MtCO2-eq from 1st Jan 2022 onwards [answer to question 1]. The yearof depletion of this budget as requested in question 2 and 3 is dependent on the future emission
trajectory of Switzerland.
Here we present three possible emissions scenarios for Switzerland starting at current
emissions levels (43.4 MtCO2-eq / yr in 2021).
Firstly, we model a linear phase-out of emissions so that emissions reduce in a straight line
and reach zero when the remaining budget based on the Policy Brief runs out (Figure A).
Switzerland would need to reach net-zero by 2040 to stay within its budget of 381 MtCO2-eq[answer to question 3].

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Figure A | Straight-line pathway to net-zero in line with Switzerland’s calculated budget. To stay within its budget,
Switzerland would need to reach net-zero emissions by 2040.

Secondly, we model how the remaining part of the calculated budget is used from the start of2022. As described in the previous chapter, the methodology of the Policy Brief is not
appropriate to inform emission targets as a fair-share of a collective goal, which can be met
with international support.
For this reason, we compare the emissions budget to Switzerland's emissions targets of:●●●

34% emissions reduction in 2030 compared to 1990 levels (domestic emissions
reduction)
75% emissions reduction in 2040 compared to 1990 levels (pursued through domestic
emissions reduction with possible international offsets if necessary)
100% emissions reduction in 2050 compared to 1990 levels, i.e. the net-zero emissions
target (pursued through domestic emissions reduction with possible international
offsets if necessary)

Figure B shows the emissions scenario towards net-zero emissions in 2050 following straightline emissions trajectories between these reduction targets. Following this trajectory, theremaining budget for Switzerland of 381 MtCO2-eq will be all used up by the end of 2030
[answer to question 2]. Following this pathway would lead to Switzerland’s budget being
exceeded by 263 MtCO2-eq.

7

Figure B | Comparison of Switzerland’s emissions budget to a linear emissions pathway towards its domestic
targets. Following its reduction targets of 34% below 1990 levels by 2030, 75% below 1990 by 2040 and net-zeroby 2050, would result in Switzerland exceeding its budget of 381 MtCO2-eq by the end of 2030.

Thirdly, for illustrative purposes only, we also model how the remaining part of the calculated
budget is used from the start of 2022 pursuing the existing emissions targets of:●●●

50% emissions reduction in 2030 compared to 1990 levels (pursued through global
emissions reduction including international offsets)
75% emissions reduction in 2040 compared to 1990 levels (pursued through domestic
emissions reduction with possible international offset if necessary)
100% emissions reduction in 2030 compared to 1990 levels, also name net-zeroemissions target (pursued through domestic emissions reduction with possible
international offset if necessary)

Figure C shows the emissions scenario towards net-zero emissions in 2050 following straightline emissions trajectories between these reduction targets. This comparison with the overall
50% emissions reduction by 2030 below 1990 illustrates that even if the emissions budget
calculated here is used to inform a fair-share overall target of 50% reduction by 2030 (despite
the inconsistency of the methods to determine a fair-share budget), the budget would be
depleted by the end of 2033.

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Figure C | Comparison of Switzerland’s emissions budget to a linear emissions pathway towards its targets,
including international offsets. Following its pledges of emissions reductions of 50% below 1990 levels by 2030,
75% below 1990 by 2040 and net-zero by 2050, Switzerland will exceed its budget of 381 MtCO2-eq before theend of 2033. Following this pathway would lead to Switzerland’s budget being exceeded by 181MtCO2-eq.

The cumulative emissions that Switzerland could emit on the basis of its emissions targets canbe expressed as a fraction of the remaining global budget. The remaining global CO 2 budget
to be shared between countries as of 2022 is 217 GtCO2. Switzerland’s total cumulative CO2
emissions based on a linear pathway between the emission targets of 50% below 1990 levels
by 2030, 75% below 1990 by 2040 and net-zero by 2050 would be 450 MtCO2 (note that thisis a CO2 only budget, as compared to the GHG budgets used in the figures above). This
represents 0.2073% of the still available global CO2 budget as of 2022. As noted in the above,
Switzerland's share of the global population in 2016 was around 0.1117% and its population
share as of 2022 is around 0.1099%.
For illustrative purposes, Figure D shows the effect of delaying emissions reductions on future
mitigation rates. To meet its emissions budget, the longer Switzerland waits to start cutting
emissions, the faster it must cut emissions. Had Switzerland started reducing its domestic
emissions linearly in 1990, its emissions would only need to reach net-zero after 2060 to staywithin the budget calculated above. The lack of mitigation compared to a linear reduction since2016 implies that the date to reach net-zero emissions under a linear use of the calculated
emissions budget has moved a few years earlier in time, from 2044 to 2040.

9Figure D | Illustrative scheme showing the linear emissions reductions scenarios starting at different dates andconsistent with a given budget. Note that, as this figure is for illustrative purposes, it uses a fixed budget and does
not recalculate the emissions budget based on the population shares that applied at each of the various starting
dates.

6. Conclusions
The effort-sharing formula presented in the Policy Brief was designed to limit fairness
considerations to fit political acceptance. As such, the approach in the Policy Brief reflects
neither the dimensions of equity described by the IPCC4 nor the principle of “common but
differentiated responsibilities and respective capabilities, in the light of different national
circumstances” and equity of the Paris Agreement. Thus, the methodology of the Policy Brief
is not appropriate to determine Switzerland's’ “fair share” of the global burden to achieve theParis Agreement warming threshold. This is particularly the case as parts of a “fair share” canbe met by complementing domestic emission reductions with international support.
Applying the Policy Brief's methodology, we find an emissions budget for Switzerland of 381
MtCO2-eq from 1st Jan 2022 onwards [answer to question 1].

4 Section 4.6.2 of Chapter 4: Fleurbaey M., S. Kartha, S. Bolwig, Y. L. Chee, Y. Chen, E. Corbera, F. Lecocq, W.

Lutz, M. S. Muylaert, R. B. Norgaard, C. Okereke, and A. D. Sagar, 2014: Sustainable Development and Equity.
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 [Edenhofer, O., R. Pichs-Madruga, Y. Sokona, E.
Farahani, S. Kadner, K. Seyboth, A. Adler, I. Baum, S. Brunner, P. Eickemeier, B. Kriemann, J. Savolainen, S.
Schlömer, C. von Stechow, T. Zwickel and J.C. Minx (eds.)]. Cambridge University Press, Cambridge, United
Kingdom and New York, NY, USA

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Even when ignoring historic responsibility prior to 2016, this emission budget is depleted by
the end of 2030 or 2033 at the latest, should Switzerland's emissions follow a straight line toits current emission targets [answer to question 2]. Based on the methodology of the Policy
Brief, Switzerland’s emissions targets for 2030, 2040 and 2050 represent insufficient emissions
reductions to stay within its 1.5 °C budget.
Staying within the calculated budget requires net-zero emissions to be achieved by 2040,
assuming that emissions follow a straight-line trajectory from 2022 onwards [answer toquestion 3]. Delaying emissions reduction implies an earlier date for reaching net-zeroemissions in order to stay within the calculated 1.5 °C budget.

Dr. Yann Robiou du Pont

Dr. Zebedee Nicholls

11

ANNEX
Detailed description of modelling choices and updated data sources
The implementation of the formulas from the Policy Brief is based on updated emissions data
as follows.
Instead of the reported population, national emissions data and global emissions budget
available at the time of publication of the Policy Brief, we use 2016 carbon emissions andpopulation data for all calculations of countries’ fair shares. The population data is from theWorld Population Prospect from the United Nations5, specifically this data file (last accessed
April 4, 2023). We use historical carbon emissions data from the peer reviewed composite
aggregation of emissions data PRIMAP-hist v2.4.2 and specifically its dataset prioritisingcountry reported data (named HISTCR)6. The data used reflects the latest available data fornational emissions reporting and remaining carbon budgets as per the IPCC. For Switzerland’s
emissions, we use the latest update to Switzerland’s national emissions inventory provided by
the Bundesamt für Umwelt7. For Switzerland’s targets and trajectories, this report uses the
Global Warming Potential of the fifth IPCC Assessment Report, consistent with the convention
used for the emissions targets of the Swiss government.
Applying the Policy Brief’s formula with updated data we find that Switzerland's budget 𝑄 fromthe start of 2016 is 0.1201% of the global budget (𝑍 in equation (5) of the Policy Brief). We
note that the budget share resulting from this methodology is larger than Switzerland’s shareof the global population in 2016 which was 0.1117%.
To calculate the global budget 𝑍, and get the budget for Switzerland, a few steps arenecessary.
We begin from the carbon budgets reported in the latest IPCC Working Group I Summary for
Policy Makers8. Specifically, we start from the budget for a 67% chance of staying below 1.5°C (defined as likely in the IPCC reports) of 400 GtCO2 from the 1st Jan 2020.
We then convert this to a budget from the 1st Jan 2016 by adding global CO2 emissions
between 2016 and 2020 i.e. 160 GtCO2. This gives a global carbon budget of 560 GtCO2 from
1st Jan 2016.

5 United Nations, Department of Economic and Social Affairs, Population Division (2022). World Population

Prospects 2022, Online Edition. The specific file used can be retrived here (last accessed April 4th 2023)
https://population.un.org/wpp/Download/Files/1_Indicators%20(Standard)/EXCEL_FILES/1_General/WPP2022_G
EN_F01_DEMOGRAPHIC_INDICATORS_COMPACT_REV1.xlsx
6 Gütschow, J.; Pflüger, M. (2023): The PRIMAP-hist national historical emissions time series v2.4.2 (1750-2021).
zenodo. doi:10.5281/zenodo.7727475
7 Data retrieved in April 2023 from Schweizer Treibhausgas-Ausstoss 2021 leicht gestiegenhttps://www.bafu.admin.ch/bafu/de/home/dokumentation/medienmitteilungen/anzeige-nsb-untermedienmitteilungen.msg-id-94169.html
8 Table SPM.2 page 29 of IPCC, 2021: Summary for Policymakers. In: Climate Change 2021: The PhysicalScience Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panelon Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y.
Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield,
O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York,NY, USA, pp. 3−32, doi:10.1017/9781009157896.001

12Next, we must account for a difference in accounting conventions between countries’ nationallyreported emissions and the conventions which underpin IPCC carbon budgets. This difference
relates to how CO2 uptake by the land is accounted for in country reported emissions compared
to when it is used as the basis for a carbon budget. In short, the carbon budget is based onanthropogenic fluxes only, while countries include some natural uptake in their emissions
accounting. To account for this difference, we reduce the budget by 15 %, in line with Grassi
et al.9. This reduces the global carbon budget to 476 GtCO2.
The global carbon budget to be shared across countries should also exclude the emissions
scope that does not fall within countries’ borders and national emissions reporting, namely
emissions from international aviation and shipping. The average of the very low (SSP1-1.9)
and low (SSP1-2.6) emissions scenarios10 from the latest Coupled Model Intercomparison
Project11, which were also widely used in the Physical Science (WG1) Contribution to the latest
IPCC report (see, for example, Table SPM.1 of WG1’s Summary for Policy Makers12), indicatesthat international aviation and shipping will contribute to 46 GtCO2. Given the strong mitigation
assumed in these scenarios, this estimate may be lower than the space expected to be taken
by international aviation and shipping and may be considered conservative. Removing these46 GtCO2 brings the global carbon budget to be shared across countries to 430 GtCO 2 fromthe start of 2016. When using accounting conventions consistent with country reporting, global
emissions between 2016 and 2022 were 213 GtCO2. Given these emissions, the global carbon
budget to be shared across countries from 2022 onwards is 217 GtCO2.
The 2016 budget is now a) appropriate for division among countries and b) consistent withnational emissions reporting conventions. Multiplying the budget from 2016 of 430 GtCO2 by
Switzerland’s share of 0.1201% gives a carbon budget for Switzerland from 1st Jan 2016 of
516 MtCO2.
Next, we can convert to a Swiss budget from 1st Jan 2022 onwards (2021 is the last year forwhich historical emissions data is reported) by removing Swiss observed CO2 emissions

Grassi, G., Stehfest, E., Rogelj, J. et al. Critical adjustment of land mitigation pathways for assessing countries’
climate progress. Nat. Clim. Chang. 11, 425–434 (2021). https://doi.org/10.1038/s41558-021-01033-6
10 Riahi, K., Van Vuuren, D.P., Kriegler, E., Edmonds, J., O’neill, B.C., Fujimori, S., Bauer, N., Calvin, K., Dellink,
R., Fricko, O. and Lutz, W., 2017. The Shared Socioeconomic Pathways and their energy, land use, andgreenhouse gas emissions implications: An overview. Global environmental change, 42, pp.153-168.
Van Vuuren, D.P., Stehfest, E., Gernaat, D.E., Doelman, J.C., Van den Berg, M., Harmsen, M., de Boer, H.S.,
Bouwman, L.F., Daioglou, V., Edelenbosch, O.Y. and Girod, B., 2017. Energy, land-use and greenhouse gas
emissions trajectories under a green growth paradigm. Global Environmental Change, 42, pp.237-250.
O’Neill, B.C., Kriegler, E., Ebi, K.L., Kemp-Benedict, E., Riahi, K., Rothman, D.S., Van Ruijven, B.J., Van Vuuren,
D.P., Birkmann, J., Kok, K. and Levy, M., 2017. The roads ahead: Narratives for shared socioeconomic pathways
describing world futures in the 21st century. Global environmental change, 42, pp.169-180.
11
Tebaldi, C., Debeire, K., Eyring, V., Fischer, E., Fyfe, J., Friedlingstein, P., Knutti, R., Lowe, J., O'Neill, B.,
Sanderson, B. and Van Vuuren, D., 2021. Climate model projections from the scenario model intercomparison
project (ScenarioMIP) of CMIP6. Earth System Dynamics, 12(1), pp.253-293.
O'Neill, B.C., Tebaldi, C., Van Vuuren, D.P., Eyring, V., Friedlingstein, P., Hurtt, G., Knutti, R., Kriegler, E.,
Lamarque, J.F., Lowe, J. and Meehl, G.A., 2016. The scenario model intercomparison project (ScenarioMIP) for
CMIP6. Geoscientific Model Development, 9(9), pp.3461-3482.
12 IPCC, 2021: Summary for Policymakers. In: Climate Change 2021: The Physical Science Basis. Contribution ofWorking Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [MassonDelmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M.
Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K.Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou(eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 3−32,
doi:10.1017/9781009157896.001.
9

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between 2016 and 2021. These emissions amount to 211 MtCO2, leaving Switzerland with acarbon budget of 305 MtCO2 from 1st Jan 2022 onwards.
Finally, we convert from the CO2-only budget to a GHG budget. This is different from themethodology in the Policy Brief, which determined GHG budgets for individual countries basedon a global GHG budget. For physical reasons, the former methodology is more scientificallysound, and it is also in line with the IPCC that provides CO2 rather than GHG budgets. Global
warming is proportional to cumulative CO2 emissions, but not GHG emissions that can have ashorter lifetime in the atmosphere. For that reason, CO2 budgets can be associated with given
warming thresholds, unlike GHG budgets that could yield various warming responses
depending on the timing of emissions over the century. This is the reason why the IPCC only
provides CO2 budgets associated with given temperature thresholds (including the 1.5 °C
threshold with a 67% likelihood used here) and not GHG budgets. To convert the CO2 budget
calculated for Switzerland to a GHG budget we assume that CO2 makes up 80% of the GHG
budget, consistent with the assumptions used in the Third Party Intervention by Prof. Dr. Sonia
I. Seneviratne and Prof. Dr. Andreas Fischlin in the proceedings in the case Verein
KlimaSeniorinnen and others v Switzerland (Application no. 53600/20). This gives anemissions budget for Switzerland of 381 MtCO2-eq from 1st Jan 2022 onwards.
The assumption used in the Third Party Intervention was also used to determine the cumulative
CO2 emissions implied by the pledged GHG emission targets of 50% below 1990 levels by
2030, 75% below 1990 by 2040 and net-zero by 2050.

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