By: Clément Rauch (S7 FRA)
COP27 took place this November, bringing together heads of state from all over the world; the conference is about climate change, and invites the world’s population to question the model of society to choose. Political decisions determine changes in society and the economy and have a significant impact on climate change mitigation. Which scenario is the most desirable for humanity and the planet?
The IPCC (Intergovernmental Panel on Climate Change) differentiates its forecasts according to development scenarios, which are called shared socio-economic trajectories, or SSPs. In the report published by the IPCC in 2021 , the quantified consequences announced are different depending on the scenario followed. The SSP2 scenario is an average, considering that countries behave differently with regard to climate. On the contrary, the other scenarios consider that all the countries of the world behave in the same way; they are the most interesting.
The SSP1 scenario foresees sustainable development for the planet. Clearly, this scenario is desirable; it makes solving climate challenges manageable, and people adapt easily. The scenario predicts that the world’s population increases slowly, but economic growth is strong and accompanied by significant development. Technical progress is made, but energy and resource consumption are low, and global economic cooperation is high.
Other trajectories are addressed by the IPCC such as the SSP3 and SSP5 scenarios, but they are apparently not desirable as they make solving the climate challenges very complicated. The SSP3 scenario foresees a development mainly characterised by strong competition between countries , while the SSP5 scenario foresees a development mainly characterised by the use of fossil fuels . For people, it is much easier to adapt to the SSP5 scenario than to the SSP3 scenario.
Figure 1: Future anthropogenic emissions of key drivers of climate change and warming contributions by groups of drivers for the five illustrative scenarios
Figure 2: Cumulative anthropogenic CO2 emissions taken up by land and ocean sinks by 2100 under the five illustrative scenarios
Figure 3: Selected indicators of global climate change under the five illustrative scenarios
Figure 4: Near-linear relationship between cumulative CO2 emissions and the increase in global surface temperature
- Credit (charts): IPCC, 2021: Summary for Policymakers. In: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on 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. ↑
- In the SSP3 scenario, the world’s population increases rapidly, but economic growth is low, with limited technical and human progress. Thus, the world experiences a substantial increase in energy and resource consumption, but these are depleted, and global cooperation is weak. ↑
- In the SSP5 scenario, the world’s population increases slowly, but economic growth is very strong and accompanied by significant development. Thus, the world experiences a significant increase in fossil energy demand, energy and resource consumption, and global economic coordination. ↑
- Contextual information (from the report). This Report assesses the climate response to five illustrative scenarios that cover the range of possible future development of anthropogenic drivers of climate change found in the literature. They start in 2015, and include scenarios with high and very high GHG emissions (SSP3-7.0 and SSP5-8.5) and CO2 emissions that roughly double from current levels by 2100 and 2050, respectively, scenarios with intermediate GHG emissions (SSP2-4.5) and CO2 emissions remaining around current levels until the middle of the century, and scenarios with very low and low GHG emissions and CO2 emissions declining to net zero around or after 2050, followed by varying levels of net negative CO2 emissions (SSP1-1.9 and SSP1-2.6). Emissions vary between scenarios depending on socio-economic assumptions, levels of climate change mitigation and, for aerosols and non-methane ozone precursors, air pollution controls.Panel (a): Annual anthropogenic (human-caused) emissions over the 2015–2100 period. Shown are emissions trajectories for carbon dioxide (CO2) from all sectors (GtCO2/yr, left graph) and for a subset of three key non-CO2 drivers considered in the scenarios: methane (CH4, MtCH4/yr, top-right graph); nitrous oxide (N2O, MtN2O/yr, middle-right graph); and sulphur dioxide (SO2, MtSO2/yr, bottom-right graph, contributing to anthropogenic aerosols in panel (b).Panel (b): Warming contributions by groups of anthropogenic drivers and by scenario are shown as the change in global surface temperature (°C) in 2081–2100 relative to 1850–1900, with indication of the observed warming to date. Bars and whiskers represent median values and the very likely range, respectively. Within each scenario bar plot, the bars represent: total global warming (°C; ‘total’ bar); warming contributions (°C) from changes in CO2 (‘CO2’ bar) and from non-CO2 greenhouse gases (GHGs; ‘non-CO2 GHGs’ bar: comprising well-mixed greenhouse gases and ozone); and net cooling from other anthropogenic drivers (‘aerosols and land use’ bar: anthropogenic aerosols, changes in reflectance due to land-use and irrigation changes, and contrails from aviation). The best estimate for observed warming in 2010–2019 relative to 1850–1900 is indicated in the darker column in the ‘total’ bar. ↑
- Contextual information (from the report). The cumulative anthropogenic (human-caused) carbon dioxide (CO2) emissions taken up by the land and ocean sinks under the five illustrative scenarios (SSP1-1.9, SSP1-2.6, SSP2-4.5, SSP3-7.0 and SSP5-8.5) are simulated from 1850 to 2100 by Coupled Model Intercomparison Project Phase 6 (CMIP6) climate models in the concentration-driven simulations. Land and ocean carbon sinks respond to past, current and future emissions; therefore, cumulative sinks from 1850 to 2100 are presented here. During the historical period (1850–2019) the observed land and ocean sink took up 1430 GtCO2 (59% of the emissions).The bar chart illustrates the projected amount of cumulative anthropogenic CO2 emissions (GtCO2) between 1850 and 2100 remaining in the atmosphere (grey part) and taken up by the land and ocean (coloured part) in the year 2100. The doughnut chart illustrates the proportion of the cumulative anthropogenic CO2 emissions taken up by the land and ocean sinks and remaining in the atmosphere in the year 2100. Values in % indicate the proportion of the cumulative anthropogenic CO2 emissions taken up by the combined land and ocean sinks in the year 2100. The overall anthropogenic carbon emissions are calculated by adding the net global land-use emissions from the CMIP6 scenario database to the other sectoral emissions calculated from climate model runs with prescribed CO2 concentrations.33 Land and ocean CO2 uptake since 1850 is calculated from the net biome productivity on land, corrected for CO2 losses due to land-use change by adding the land-use change emissions, and net ocean CO2 flux. ↑
- Contextual information (from the report). The projections for each of the five scenarios are shown in colour. Shades represent uncertainty ranges – more detail is provided for each panel below. The black curves represent the historical simulations (panels a, b, c) or the observations (panel d). Historical values are included in all graphs to provide context for the projected future changes.Panel (a) Global surface temperature changes in °C relative to 1850–1900. These changes were obtained by combining Coupled Model Intercomparison Project Phase 6 (CMIP6) model simulations with observational constraints based on past simulated warming, as well as an updated assessment of equilibrium climate sensitivity. Changes relative to 1850–1900 based on 20-year averaging periods are calculated by adding 0.85°C (the observed global surface temperature increase from 1850–1900 to 1995–2014) to simulated changes relative to 1995–2014. Very likely ranges are shown for SSP1-2.6 and SSP3-7.0.Panel (b) September Arctic sea ice area in 106 km2 based on CMIP6 model simulations. Very likely ranges are shown for SSP1-2.6 and SSP3-7.0. The Arctic is projected to be practically ice-free near mid-century under intermediate and high GHG emissions scenarios.
Panel (c) Global ocean surface pH (a measure of acidity) based on CMIP6 model simulations. Very likely ranges are shown for SSP1-2.6 and SSP3-7.0.
Panel (d) Global mean sea level change in metres, relative to 1900. The historical changes are observed (from tide gauges before 1992 and altimeters afterwards), and the future changes are assessed consistently with observational constraints based on emulation of CMIP, ice-sheet, and glacier models. Likely ranges are shown for SSP1-2.6 and SSP3-7.0. Only likely ranges are assessed for sea level changes due to difficulties in estimating the distribution of deeply uncertain processes. The dashed curve indicates the potential impact of these deeply uncertain processes. It shows the 83rd percentile of SSP5-8.5 projections that include low-likelihood, high-impact ice-sheet processes that cannot be ruled out; because of low confidence in projections of these processes, this curve does not constitute part of a likely range. Changes relative to 1900 are calculated by adding 0.158 m (observed global mean sea level rise from 1900 to 1995–2014) to simulated and observed changes relative to 1995–2014.
Panel (e) Global mean sea level change at 2300 in metres relative to 1900. Only SSP1-2.6 and SSP5-8.5 are projected at 2300, as simulations that extend beyond 2100 for the other scenarios are too few for robust results. The 17th–83rd percentile ranges are shaded. The dashed arrow illustrates the 83rd percentile of SSP5-8.5 projections that include low-likelihood, high-impact ice-sheet processes that cannot be ruled out.
Panels (b) and (c) are based on single simulations from each model, and so include a component of internal variability. Panels (a), (d) and (e) are based on long-term averages, and hence the contributions from internal variability are small. ↑
- Contextual information (from the report). Top panel: Historical data (thin black line) shows observed global surface temperature increase in °C since 1850–1900 as a function of historical cumulative carbon dioxide (CO2) emissions in GtCO2 from 1850 to 2019. The grey range with its central line shows a corresponding estimate of the historical human-caused surface warming. Coloured areas show the assessed very likely range of global surface temperature projections, and thick coloured central lines show the median estimate as a function of cumulative CO2 emissions from 2020 until year 2050 for the set of illustrative scenarios (SSP1-1.9, SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5). Projections use the cumulative CO2 emissions of each respective scenario, and the projected global warming includes the contribution from all anthropogenic forcers. The relationship is illustrated over the domain of cumulative CO2 emissions for which there is high confidence that the transient climate response to cumulative CO2 emissions (TCRE) remains constant, and for the time period from 1850 to 2050 over which global CO2 emissions remain net positive under all illustrative scenarios, as there is limited evidence supporting the quantitative application of TCRE to estimate temperature evolution under net negative CO2 emissions.Bottom panel: Historical and projected cumulative CO2 emissions in GtCO2 for the respective scenarios.