What are emission scenarios?

Emission scenarios are plausible future development pathways of human greenhouse gas and aerosol emissions.

These emission scenarios for greenhouse gases and aerosols serve as the basis for working out the possible climate conditions of the future. The latest generation of emission scenarios, the so-called Representative Concentration Pathways (RCP), define trajectories representing greenhouse gas and aerosol concentrations for particular climate projections (namely, radiative forcing values in the year 2100). Each trajectory is, in turn, representative of a particular development scenario for anthropogenic emissions

The scale of future anthropogenic emissions is heavily dependent on global political decisions as well as on population growth and technological advances. These uncertainties are depicted through various emission scenarios. One emission scenario thus works on the basis of the global community agreeing to drastically reduce emissions of greenhouse gases: RCP2.6. This scenario assumes an additional radiative forcing of 2.6 Watts per square metre by the end of the 21st century. Another scenario represents a situation in which humans continue in the same way as we have to date, emitting ever more greenhouse gases: RCP8.5 This represents a radiative forcing of 8.5 Watts per square metre by the end of the century. In between these two are other scenarios that entail greater or lesser degrees of technical advancement that would lead to slight reductions in greenhouse gases. The higher the radiative forcing value, the greater the change in climate conditions.

Climate models have been run on the basis of all of these scenarios so as to demonstrate the impact of political decision-making and other factors on the climate of the future. Emission scenarios therefore represent potential courses of action. They allow the climate-related consequences of courses of action to be quantified, without making any statements about which pathway is the most likely.

Scenario	RCP Scenario	Characteristics
No mitigation	RCP8.5	No climate mitigation measures are implemented. Greenhouse gas emissions continually increase. The radiative forcing in the year 2100 stands at 8.5 W/m2 in comparison to the year 1850.
Limited mitigation	RCP4.5	Although greenhouse gas emissions are somewhat curbed, the content in the atmosphere continues to increase for another 50 years. The 2°C target is not achieved. The radiative forcing in the year 2100 stands at 4.5 W/m2 in comparison to the year 1850.
Concerted mitigation efforts	RCP2.6	Climate mitigation measures are taken. With reductions in emissions being implemented straight away, the increase in greenhouse gases in the atmosphere is halted within around 20 years. This allows the targets of the Paris Climate Agreement of 2016 to be reached. The radiative forcing in the year 2100 stands at 2.6 W/m2 in comparison to the year 1850.

Adapted from Proclim, "Brennpunkt Klima Schweiz" – a report on responding to climate change in Switzerland, published in 2016.

The Shared Socio Economic Pathways (SSPs)- A new approach

Why new scenarios for climate change research?

• Socio‐economic scenarios used to derive emissions scenarios without (baseline scenarios) and with climate policies (mitigation scenarios)
• Emissions scenarios used to derive climate change projections
• Climate change projections and socio‐economic scenarios used to evaluate climate impacts and adaptation measures

Narratives of the future

The SSPs are based on four narratives describing broad socioeconomic trends that could shape future society. These are intended to span the range of plausible futures.

• SSP1: low challenges for mitigation (resource efficiency) and adaptation (rapid development)
• SSP3: high challenges for mitigation (regionalized energy/ land policies) and adaptation (slow development)
• SSP4: low challenges for mitigation (global hightech economy), high for adapt.(regional low tech economies)
• SSP5: high challenges for mitigation (resource / fossilfuel intensive) and low for adapt.(rapid development

NarrativesinO’Neilletal.,2016,GlobEnvChange,onlinefirst)

Scenario Matrix Architecture

Combination of SSP and RCP model runs in the SSP database, with RCPs listed in order of increasing mitigation and SSPs in the (rough) order of increasing mitigation difficulty. Ratios in cells indicate the number of models that succeeded in making the scenario “work” out of the total number of models available for the SSP. Chart by Carbon Brief, adapted from Figure S1 in Rogelj et al (2018).

Each box in the figure shows the number of models that were able to successfully reach the RCP target, out of the total number of models available for a given SSP. For example, the “3/4” in the SSP5 / RCP2.6 cell means that four IAMs tried to achieve RCP2.6 in an SSP5 world, but only three of the models could find a solution. The other model could not either reduce emissions fast enough or generate sufficient negative emissions. Similarly, only SSP5 could generate scenarios that reached RCP8.5-levels of radiative forcings, while emissions were too low in other SSP baselines.

To ascertain whether the underlying socioeconomic factors in an SSP allow for the level of mitigation necessary to meet RCP targets, models used shared policy assumptions about limits to international cooperation in the short-to-medium term and the possible speed of emissions reductions.

The figure below shows the different emissions trajectories broken out by SSP, as well as indicating the level of adaptation and mitigation challenges associated with each. In general, SSP1 (bottom left box) has faster emission reductions and less negative emissions required later in the century under deeper mitigation scenarios, compared to other SSPs.

The fact that IAMs could not find a viable solution for some below-2C and below-1.5C scenarios does not necessarily mean that these scenarios are impossible. Models are necessarily imperfect and cannot foresee all of the technological or societal changes that will happen over the coming century. For example, models used to struggle to reach 2C targets before they started including large-scale negative emissions technologies – though these still largely exist only in the models, rather than in real-world deployments at scale. Similarly, what modellers consider as plausible rates of emission reduction or negative emissions may turn out to be overly conservative (or optimistic). Pathways such as SSP3, where strong mitigation scenarios cannot be modelled, should be seen as an indication that such a world of resurgent nationalism and regional divisions greatly increases the risk that the transformations required might not be achievable.