What is an SSP?
Shared Socioeconomic Pathways (or SSPs) were used to develop the latest generation of GHG emissions scenarios. SSPs systematically explore potential socio-economic trends over the next century and quantify the effect of these developments on greenhouse gas (GHG) concentrations. The emissions that result under these pathways serve as input to drive climate models and project resultant levels of climate change.
More information on the SSPs can be found below and here.
Why are more future scenarios (SSPs) included in CMIP6 than CMIP5?
The set of future scenarios used in CMIP6 encompass a broader range of potential greenhouse gas (GHG) emissions trajectories than those used in CMIP5.
It is possible for multiple emissions scenarios to be associated with each SSP. Due to the extensive resources required to run multiple climate model experiments, only a few SSPs are actually used to drive climate models. These are selected in an effort to span the range of possible emissions scenarios.
To enable comparison between CMIP5 and CMIP6, the three SSP scenarios which correspond to the most widely-used RCPs (2.6, 4.5, and 8.5) have been adopted by the climate modelling community (RCP2.6 – SSP1-2.6, RCP4.5 – SSP2-4.5, and RCP8.5 – SSP5-8.5). Others have been added to explore how climate may change in response to different social and economic conditions. For example, SSP1-1.9 was introduced to explore a world in which social and economic factors mean that there is a good chance of meeting the “aspirational” goal of the Paris Agreement (i.e., a global temperature increase below 1.5 °C), while SSP3-7.0 was selected to represent the medium to high end of the range of future forcing pathways.
How are SSPs different/similar to RCPs? (What are the differences between the scenarios?)
Both SSPs and RCPs are attempts to describe greenhouse gas emissions over the next century.
SSPs further refine the previous greenhouse gas concentration scenarios known as RCPs. RCPs were explicitly designed for the climate modelling community to explore the effects of different emissions trajectories or emissions concentrations (resulting in various radiative forcing values). The socio-economic characteristics used to define RCPs are not standardized, making it difficult to map societal changes like population, education, and government policies to climate targets, such as keeping global warming well below 2°C. SSPs address this by defining how societal choices can lead to changes in GHG emissions, to their concentrations in the atmosphere, and thus to radiative forcing by the end of the century (2100). As such, SSPs expand on RCPs to allow for a standardized comparison of society’s choices and their resulting levels of emissions.
If SSPs span a broader range of futures, why are we limited to only 3 on ClimateData.ca? (Why aren’t all SSPs available on ClimateData.ca?)
ClimateData.ca provides climate projections at a spatial scale more suitable for decision-making than global climate models. To achieve this, CMIP6 data must first be downscaled to a finer spatial resolution, which requires time and computing resources. At present, downscaling efforts have focused on SSPs that are comparable with the RCPs currently available on ClimateData.ca (from CMIP5). This will make it easier to compare results between the CMIP5 and CMIP6 climate model projections.
Some modelling centres did not provide runs for certain SSPs, meaning that the sample of available results is not considered to sufficiently represent the range of possible future climates resulting from that particular SSP. In instances such as this, results were not downscaled to a finer resolution.
Additional downscaled projections for other SSPs may be made available at a later date.
Why has it taken so long for SSP-based scenarios to become available?
While the SSPs were in development beginning at the same time as the RCPs, they were not ready in time for CMIP5, but have been used in CMIP6. Downscaling of the CMIP6 runs to a finer spatial resolution more suited to decision-making has also taken time.
What’s the benefit of using SSPs over RCPs?
SSPs systematically consider a wide range of factors that affect global emissions (including population growth, global levels of education and economic development), allowing for comparison of risks and impacts to society, as well as challenges to adaptation and mitigation, in a standardized manner. Importantly, mitigation actions can be superimposed within the overall socio-economic narrative of an SSP to gauge its effectiveness in reducing global emissions.
Are RCPs still valid?
Yes, RCPs are still valid.
However, SSPs provide an enhanced understanding of the relationship between socio-economic factors and climate change. Since our knowledge of the climate system is constantly evolving, using the most current set of emissions scenarios means that practitioners can ensure their work reflects the most up-to-date socio-economic and climate information available.
Which SSPs should I choose?
The complex nature of the climate system, climate models and human factors makes it difficult to determine exactly how the climate will change in future. What is known for certain is that the future climate will be different from the past. While action to mitigate climate change is essential, a certain amount of warming has already been “locked-in”. By assessing more than one possible future, planners and decision-makers can better prepare for a range of possible outcomes.
It is important to consider two questions prior to selecting SSPs: First, what components of my project are vulnerable to climate change? Second, what level of risk am I comfortable taking?
For example, the consequences of a rare but damaging environmental hazard could be very high, with the power to impact things such as local food security, national GDP, and public safety. In a situation such as this, the costs of adapting to SSP5-8.5 (the high emissions scenario) up until the end of the century may be considered worthwhile. In other circumstances, where the consequences are lower and/or the likelihood of damaging events is low, adapting to SSP5-8.5 may not be necessary or economically viable. Regardless of the project and the rationale, this question of “how much risk am I comfortable taking” is complex, undoubtedly requiring conversations with diverse partner and stakeholder groups to understand the broad range of potential impacts and implications.
A project’s planning horizon is another important consideration during this process. Over relatively short time periods (i.e. the next decade), the range of climate changes between different SSPs is small, meaning it matters less which SSP is selected. However, at mid-century the scenarios quickly diverge revealing diverse levels of climate change.
In applications not directly related to adaptation, where determining vulnerability and risk is not an important component of a project, the SSP selection process may consider additional factors. For example, SSP5-8.5 describes the most global warming and, as such, it contains the greatest “climate change signal to climate variability noise ratio”. In research applications, where the goal is to find a correlation between climate change and some other event, SSP5-8.5 might be the best pick as the climate signal is strongest under this emissions scenario.