Supplementary Information
Scenarios covered in the current literature range
The original database underlying the IPCC AR4 assessment (1, 2) has been extended for the purpose of this study by the scenarios developed for EMF-22 (3), the ADAM project (4) and a MESSAGE/IMAGE scenario project (5). While this does not necessarily cover all new scenarios published since AR4, the three studies mentioned represent model comparison projects with global participation of many global modelling groups and are therefore very representative of current literature. All scenarios were characterized into one of the six IPCC scenario categories (see Table 1) on the basis of reported concentration targets.
The EMF-22 project was a multi-model study (10 models) looking at scenarios that reach long-term radiative forcing targets (2.6, 3.7, 4.5 W/m2 by 2100) under different assumptions with respect to participation of different parties and the allowance of an initial overshoot of the target. All models were able to meet the two higher radiative forcing targets. The lowest target could only be met by the subset of the models and under specific conditions. The ADAM project looked at low stabilisation targets (400 and 550 ppm CO2 eq), using 6 different models. All models were able to meet both targets. The last study (MESSAGE/IMAGE) looked specifically at the attainability of the 2.6 and 2.9/3.0 W/m2 targets – comparing the results from two modelling systems. In all cases, for the different scenarios, only energy- and industry-related CO2 emissions were considered. Excluded were scenarios that either 1) reduced emissions before 2010, 2) reduced emissions by more than 50% in a single decade before 2050, or 3) showed cumulative emission over the 2000-2100 period that clearly were inconsistent with the corresponding target in the IPCC category. In total, 8 of the 139 scenarios were not included.
Correlation of CO2 and CO2 eq concentration targets
The different IPCC categories were defined both in terms of CO2 and CO2 eq concentration levels, allowing the categorising of both multigas and CO2-only studies. Figure S1 shows the cut-off points in the various IPCC categories. The correlation between the CO2 and CO2 eq concentration levels was based on the literature available at that time. New studies published since then show that the correlation between CO2 and CO2 eq targets is somewhat model-dependent, based on assumptions for 1) non-CO2 emission reduction, 2) assumed carbon cycle, and 3) ability to reduce CO2 emissions (BECCS). It is important to take this additional uncertainty into account when studying the IPCC categorisation. Although it was outside the scope of this study, it may be useful to re-evaluate the original IPCC categories.
Figure S1. Correlation of CO2-eq (ppm) and CO2 (ppm) concentrations from studies that report both values. Results are compared to the categorization used by IPCC AR4. Source: Van Vuuren et al. (6), O’Neill et al. (7) and various IMAGE model runs.
Energy and industrial emissions and full CO2 emissions
Some of the models in the literature only calculate scenarios for CO2 emissions from energy use and industrial processes. This forms the most important source of greenhouse gas emissions – but there are also other sources (about 85% of CO2 emissions stem from energy and industrial processes). In order to include as many scenarios as possible, the IPCC report focused on emission trends for CO2 emissions from energy and industry, as does this study.
We have checked whether different results would be obtained if, instead, the focus would be on total CO2 emissions. Figure S2 shows the emission average, and the 15 to 85% interval, for the scenarios included in the lowest category, for energy and industrial emissions only and total emissions. As the figure shows, emission levels are somewhat higher early in the century for the graph depicting total emissions – but the trends are very comparable. The conclusions drawn in the main text, with respect to emissions reductions for 2020 and 2050 and the peak year hold for both the selected emission source and the total emissions.
Figure S2. CO2 emissions from energy use and industrial processes (left) and total CO2 emissions (right) in CategoryI. Figure shows the category mean (line) as well as the 15 to 85% interval.
Scenarios with and without negative emissions
Figure S3 shows the emission scenarios included in category I, II and III. The figure distinguishes between scenarios with and without negative emissions. Scenarios with negative emissions are generally on the high end of the range for the 2020-2050 period (and by definition on the low end of the range by the end of the century).
Figure S3. CO2 emissions from energy use and industrial processes in the different IPCC categories. Figure shows the 15 to 85% interval of all scenarios, and the individual studies (both with and without negative emissions).
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