Where do the five new IPCC scenarios come from?
The IPCC scenarios are constantly cited when we are interested in climate and its evolution, but sometimes wrongly, and often without a clear understanding of what they imply. On the occasion of the release of the latest IPCC report, in which five new scenarios have appeared, Charlotte Vailles of I4CE explains how they were constructed and what information is available about them.
New IPCC scenarios explore a wide range of plausible futures to 2100
In its latest report, the first working group of the IPCC does not only reconstruct past climate changes and observe those that are taking place today. Crucially, it also explores possible futures. The five new scenarios used in this report present possible evolutions of the climate throughout the 21st century as a function of greenhouse gas (GHG) emissions and of the evolution of human societies. The use of scenarios – which are plausible representations of an uncertain future – enable to explore different possible evolutions of human societies and their implications for the climate. The aim of these scenarios is not to predict the future – no probability is associated with the different scenarios – but to take into account the uncertainty linked to future human activities and to inform the decisions of States and more widely of societies.
These five scenarios cover a wide range of plausible futures for GHG emissions – from a scenario in which CO2 emissions decline drastically to carbon neutrality by 2050 and are negative in the second half of the century (SSP1-1.9) to a scenario in which CO2 emissions continue to rise sharply to twice current levels in 2050 and more than three times current levels in 2100 (SSP5-8.5) (see Figure 1).
Figure 1: Future CO2 emissions in the five illustrative scenarios
Source: Sixth Assessment Report of IPCC Working Group I, 2021
These scenarios have been constructed from plausible developments in societies over the 21st century
The five scenarios are based on reference socio-economic trajectories – the SSPs (Shared Socioeconomic Pathways) – developed by the scientific community in order to create a common framework for thinking about the issues related to climate change.
Five narratives describing possible social, economic, political and technological developments by the end of the century were developed (see Table 1). These five narratives were used to model different scenarios of evolutions of the economic, energy and land use systems. Some of these scenarios were constrained by the achievement of a climate objective (these are called “transition scenarios“), while others were not (“reference scenarios“). This scenario-building was carried out by the scientific community of the IPCC third working group, which assesses solutions for mitigating climate change. Note that the socioeconomic pathway can be more or less favourable to the achievement of climate objectives, as indicated in the two columns on the right: for example, limiting global warming to 1.5°C or 2°C may prove very difficult, if not impossible, in a context of regional rivalries and inequalities described by the SSP3.
Table 1: The SSPs and their narratives
Source: I4CE, 2019, based on Riahi et al. (2017), O’Neill et al. (2015), Bauer et al. (2017)
The GHG, aerosol, and land use emission trajectories resulting from these scenarios were then processed by the scientific community to harmonize them and supplement them with other datasets -in particular, a geographic grid of emissions. In total, nine emission scenarios were obtained – representing consistent and distinct evolutions of human societies.
These scenarios are identified by a name of the form SSPx-y, where SSPx is the socioeconomic pathway used to model the scenario and y is the approximate level of radiative forcing resulting from the scenario in 2100.
Five scenarios demarcated from each other were chosen for further evaluation in this report:
- SSP1-1.9: very ambitious scenario to represent the 1.5°C goal of the Paris Agreement
- SSP1-2.6: sustainable development scenario
- SSP2-4.5: intermediate scenario
- SSP3-7.0: regional rivalry scenario
- SSP5-8.5: fossil-fuel based development
The assessment report of the IPCC first working group details the evolutions of climate for each of these five scenarios on a global scale as well as on a regional scale
These five emission scenarios have been used as inputs to climate models – which are mathematical formulations of the natural laws that govern the evolution of climate-related systems: atmosphere, ocean, cryosphere, land, biosphere, carbon cycle. These models simulate the future evolution of climate according to a given GHG emission trajectory. The models used for this sixth assessment report allow for a finer representation of certain physical, chemical and biological processes than the models previously used.
The IPCC Working Group I report details the evolution of the climate system throughout the 21st century for each of these five illustrative scenarios. The evolution of the climate system is described by a very large set of climate variables, such as temperatures, winds, precipitation.
The report describes the average change in climate variables in each of the scenarios, such as the increase in surface temperature – ranging from 1.4°C to more than 4.5°C by 2100 in the different scenarios (see Figure 2). The report also describes the evolution of extreme events – such as the frequency and intensity of heat waves, droughts, torrential rains, cyclones… It also describes the evolution of the cryosphere and the oceans – with for example information on the melting of ice floes, permafrost, glaciers or the rise in sea level and ocean acidification in the different scenarios.
Figure 2: Increase in surface temperature in each scenario relative to 1850-1900 levels
Source: Sixth Assessment Report of the IPCC Working Group I, 2021
In this figure, the lines represent the average value obtained by all the climate models used. The coloured areas represent the values obtained in 90% of the simulations for SSP3-7.0 and SSP1-2.6. This range is not represented for the other scenarios for the sake of readability.
These developments are described at the global scale, but also at a regional level. This new report comes with an interactive online atlas that allows users to explore the evolution of numerous climate variables observed or simulated according to the scenarios, at different time horizons and geographical scales. This regionalized information can be used by public and private decision-makers to understand the climate changes they are facing.
These scenarios are more precise than the scenarios previously used by the IPCC and explore different trajectories over the 21st century
The previous IPCC assessment report was based on other scenarios, the RCPs – for Representative Concentration Pathways. RCPs are trajectories of the evolution of emissions and concentrations of greenhouse gases and aerosols, named according to the radiative forcing they reach by 2100. Five in number, they were developed to form a representative set of the multiple GHG emission trajectories of existing scenarios in the literature. The RCPs were previously used as inputs to climate models.
The new SSP scenarios differ from the RCPs in several respects: first, they provide a much higher level of precision and detail for climate model inputs. Second, they allow for the exploration of combinations that were not covered by the RCPs, such as the combination of low mitigation efforts and low air pollution control – and thus high aerosol emissions (now studied in SSP3-7.0).
Both RCPs and SSPs are identified by the approximate level of radiative forcing achieved in 2100, but they are not directly comparable for the same radiative forcing. The distribution of emissions over time and the proportion of different GHGs and aerosols differ. For example, SSP5-8.5 has higher CO2 concentrations than RCP 8.5, but lower methane concentrations. As another example, the ambitious SSP scenarios describe a later peak in emissions than in the ambitious RCPs: this is because actual emissions have not yet followed the trajectory of the ambitious scenarios.
RCPs are cited in this report: on some topics – such as sea level rise or regional scale projections – the scientific literature still largely uses modelling results based on RCPs.
The results of the five scenarios call for immediate action on adaptation and mitigation
The findings of this report leave no excuse for delaying action on adaptation and mitigation. Indeed, in all the scenarios considered, warming exceeds the 1.5°C limit in the next twenty years. Immediate efforts are therefore required for adaptation, and this report helps us to identify unavoidable climate changes so that we can better prepare for them. In this report, the IPCC also reminds us that the objective of limiting warming to 1.5°C in the long term is not lost, via the SSP1-1.9 scenario in which warming is limited to 1.4°C at the end of the century after temporarily exceeding 1.5°C. However, this requires drastic and immediate decisions for mitigation.
This information will be completed by reports to be published: the impacts of climate change on human societies and the solutions to be implemented to limit global warming will be assessed in detail respectively in the reports of the second and third working groups, to be both published in 2022.
Ahead of COP26, this first report already sends a clear message to decision-makers on the importance of reducing our emissions more drastically than ever to seize the last chance to limit warming below 1.5°C, but also on the need to start adapting to the impacts of climate change now.
To better understand these different terms – transition scenario, reference scenario, climate model, SSP, RCP, … – do not hesitate to (re)read the publication “Understanding transition scenarios – eight steps to read and interpret these scenarios“, which explains the key concepts around climate-related scenarios, presents the main families of scenarios and the questions that these scenarios can answer!