Climate Scenarios: RCPs vs. SSPs
If you’ve been reading climate-related news or papers, you might have noticed the abbreviations “RCP” and “SSP.” Here’s what they mean and how they are different!
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Overview
Climate models are tools scientists can use to see what our environment might look like in the future based on a given scenario. “RCP” and “SSP” are both sets of scenarios that describe different conditions that would affect our future environment. These terms originated from the Coupled Model Intercomparison Project (CMIP), which “creates standards and sets experimental protocols for climate models” to ensure that scientists are using climate models consistently.
Representative Concentration Pathways (RCPs)
In CMIP5, the previous version of the Coupled Model Intercomparison Project (CMIP) described above, Representative Concentration Pathways (RCPs) are used to describe climate scenarios. RCPs account only for greenhouse gas concentrations in the atmosphere.
“RCPs tell where we could end up without helping us understand the path we took to get there.”
Shared Socioeconomic Pathways (SSPs)
CMIP6, the most recent version of the Coupled Model Intercomparison Project (CMIP) described above, uses Shared Socioeconomic Pathways (SSPs) to describe climate scenarios.
“SSPs represent changes in population, economic growth, education, urbanization, and the rate of technological development that would affect future greenhouse gas emissions, providing a storyline of how we could reach certain levels of warming.”
Considering socioeconomic factors, such as population, education, and economic growth, in addition to environmental factors such as greenhouse gas concentrations allows climate scientists to form a more complete picture of what different futures might look like based on present actions.
Comparing RCPs and SSPs
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These two tables provide a great overview of the different scenarios in both RCPs and SSPs, and how the two compare.
Table Source: Fifth National Climate Assessment: Front Matter.
Full citation: Avery, C.W., A.R. Crimmins, S. Basile, A.M. Grade, and A.A. Scheetz, 2023: Front matter. In: Fifth National Climate Assessment. Crimmins, A.R., C.W. Avery, D.R. Easterling, K.E. Kunkel, B.C. Stewart, and T.K. Maycock, Eds. U.S. Global Change Research Program, Washington, DC, USA. https://doi.org/10.7930/NCA5.2023.FM.
Table Source: USDA Northwest Climate Hub.
You may also see references to the five SSP-RCP (‘SSPX-Y’) scenarios, in which the letters and numbers before the hyphen refer to SSPs and those after the hyphen refer to corresponding RCPs. These SSP-RCP scenarios are:
SSP1-1.9
SSP1-2.6
SSP2-4.5
SSP3-7.0
SSP5-8.5
Too much information? Here’s a less technical table:
What Do These Scenarios Look Like?
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Population, GDP, and Urbanization
The first three graphs show what population, gross domestic product (GDP), and urbanization might look like under a variety of SSPs. RCPs are not included in these graphs.
Based on these graphs, the human population under SSP3 only would increase from present-day until 2100. Under SSPs 1 and 5, population would peak around 2050 and decline sharply afterwards; under SSPs 2 and 4, population would peak around 2070 and decrease slowly afterwards.
GDP will increase under all SSPs, but will experience the most drastic increase under SSP5, followed by SSP1. GDP would be lowest under SSP3. GDP increase under SSP5 would likely be due to increases in technological and extractive industries, while it may be related to renewable energy under SSP1. While there is a significant difference between projected GDP under SSP5 and SSP1, notably, GDP is still second-highest under SSP1 - a scenario in which nearly all fossil fuel production is halted.
Finally, urbanization would increase most significantly under SSPs 1, 4, and 5, followed by SSP2 and then SSP3. High urbanization under SSPs 4 and 5 may be due to population increase and/ or increasing industrial production, while high urbanization under SSP1 might be caused by an increase in high-density, walkable cities.
Graphs Credit: Government of Canada, CMIP6 and Shared Socio-economic Pathways Overview
Graphs Sources listed as:
Riahi, K., van Vuuren, D. P., Kriegler, E., Edmonds, J., O’Neill, B. C., Fujimori, S., Bauer, N., Calvin, K., Dellink, R., Fricko, O., Lutz, W., Popp, A., Crespo Cuaresma, J., KC, S., Leimbach, M., Jiang, L., Kram, T., Rao, S., Emmerling, J., Ebi, K., Hasegawa, T., Havlik, P., Humpenöder, F., Aleluia Da Silva, L., Smith, S., Stehfest, E., Bosetti, V., Eom, J., Gernaat, D., Masui, T., Rogelj, J., Strefler, J., Drouet, L., Krey, V., Luderer, G., Harmsen, M., Takahashi, K., Baumstark, L., Doelman, J., Kainuma, M., Klimont, Z., Marangoni, G., Lotze-Campen, H., Obersteiner, M., Tabeau, A., & Tavoni, M. (2017). The Shared Socioeconomic Pathways and their energy, land use, and greenhouse gas emissions implications: An overview. Global Environmental Change, 42, 153-168. https://doi.org/10.1016/j.gloenvcha.2016.05.009.
Rogelj, J. (2016, May 06). The Shared Socio‐Economic Pathways (SSPs): An overview [PowerPoint slides]. United Nations Framework Convention on Climate Change. https://unfccc.int/sites/default/files/part1_iiasa_rogelj_ssp_poster.pdf.
Graphs Caption: “Differences in the projections for some of the socio-economic conditions used in the SSPs. From left to right is shown population growth, Gross Domestic Product (GDP), and urbanization. Note that the SSPs shown are baselines, meaning that there is no RCP or radiative forcing associated with each SSP. Therefore, each SSP describes a future in the absence of new climate policies beyond those in place today or any new mitigation or adaptation strategies. The baseline SSP projections are shown in comparison to ranges produced from peer-reviewed scientific literature” (Government of Canada, CMIP6 and Shared Socio-economic Pathways Overview).
Greenhouse Gas Emissions
The next three graphs show what emissions of CO2 (carbon dioxide), CH4 (methane), and N2O (nitrous oxide) - all greenhouse gases - might look like under a variety of RCPs. SSPs are not included in these graphs.
Carbon dioxide emissions in 2100 are projected to be highest under SSP5, followed by SSP3 and SSP2. Surprisingly, emissions under SSP4 are second-lowest, followed by SSP1, which is due to “comparatively low fossil fuel dependence and increased deployment of non-fossil energy sources” in both SSP4 and 1 (Riahi et al., 2017).
Methane emissions are often driven by “population growth and food demand” (Riahi et al., 2017). Therefore, methane emissions would be greatest under SSP5 until approximately 2050 due to “massive expansion of the fossil fuel infrastructure;” however, by 2100, methane emissions under SSP3 would be greatest (Riahi et al., 2017). As expected, methane emissions under SSP1 would be lowest.
Nitrous oxide, a less well-known but powerful greenhouse gas, primarily comes from “agricultural soil, [fertilizer use], animal manure, sewage, industry, automobiles and biomass burning” today (Riahi et al., 2017). Therefore, nitrous oxide emissions would be highest under both SSP3 and 4 due to “high population and/ or fertilizer use” (Riahi et al., 2017). Similar to methane emissions, nitrous oxide emissions would be high under SSP5 until approximately 2040, when they would taper off. Again, as expected, nitrous oxide emissions under SSP1 would be lowest, as this scenario depicts “sustainable agriculture practices and low population assumptions” (Riahi et al., 2017).
Graphs Credit: Government of Canada, CMIP6 and Shared Socio-economic Pathways Overview
Graphs Sources listed as:
O'Neill, B. C., Tebaldi, C., van Vuuren, D. P., Eyring, V., Friedlingstein, P., Hurtt, G., Knutti, R., Kriegler, E., Lamarque, J.-C., Lowe, J., Meehl, G. A., Moss, R., Riahi, K., & Sanderson, B. M. (2016). The Scenario Model Intercomparison Project (ScenarioMIP) for CMIP6. Geoscientific Model Development, 9(9), 3461–3482. https://doi.org/10.5194/gmd-9-3461-2016.
Riahi, K., van Vuuren, D. P., Kriegler, E., Edmonds, J., O’Neill, B. C., Fujimori, S., Bauer, N., Calvin, K., Dellink, R., Fricko, O., Lutz, W., Popp, A., Crespo Cuaresma, J., KC, S., Leimbach, M., Jiang, L., Kram, T., Rao, S., Emmerling, J., Ebi, K., Hasegawa, T., Havlik, P., Humpenöder, F., Aleluia Da Silva, L., Smith, S., Stehfest, E., Bosetti, V., Eom, J., Gernaat, D., Masui, T., Rogelj, J., Strefler, J., Drouet, L., Krey, V., Luderer, G., Harmsen, M., Takahashi, K., Baumstark, L., Doelman, J., Kainuma, M., Klimont, Z., Marangoni, G., Lotze-Campen, H., Obersteiner, M., Tabeau, A., & Tavoni, M. (2017). The Shared Socioeconomic Pathways and their energy, land use, and greenhouse gas emissions implications: An overview. Global Environmental Change, 42, 153-168. https://doi.org/10.1016/j.gloenvcha.2016.05.009.
Graphs Caption: “Differences in the projections of global greenhouse gas emissions for carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) across SSPs are shown in the top three plots” (Government of Canada, CMIP6 and Shared Socio-economic Pathways Overview).
Land Use Changes
The final four graphs show what land use - divided into cropland, forest, pasture, and other natural land - might look like under a variety of RCPs. SSPs are not included in these graphs.
Amount of cropland under SSP3 would be much higher than under other scenarios; amount of cropland would increase significantly under SSP5, before slowly declining around 2050. After SSP3, the next highest amounts of cropland would be under SSP2, then 5, then 4, with the least amount of cropland being used under SSP1. SSP3 is projected to use the most amount of cropland, due to “massive growth of population, relatively low agricultural productivity, and little emphasis on environmental protection;” this leads to related land use changes in forests, pasture, and other natural land as demonstrated by the following graphs (Riahi et al., 2017).
Amount of forested land would be significantly highest under SSP1, presumably due to an increase in conservation and restoration efforts. SSP1 depicts a “sustainable land transformation… a reversal of historical trends, including a gradual, global-scale, and pervasive expansion of forests and other natural lands” (Riahi et al., 2017). Forested land would ultimately decline in every other scenario, with the most significant losses of forested land under SSP3.
Amounts of pasture will increase the most under SSPs 3 and 4, followed by SSP2. Amount of pastured land will decrease the most under SSP1, but will also decrease under SSP5.
“Other natural land” will decline the most under SSP3, but will also decline under SSPs 2 and 4. It will increase slightly under SSP5 and will increase the most under SSP1.
Graphs Credit: Government of Canada, CMIP6 and Shared Socio-economic Pathways Overview
Graphs Sources listed as:
O'Neill, B. C., Tebaldi, C., van Vuuren, D. P., Eyring, V., Friedlingstein, P., Hurtt, G., Knutti, R., Kriegler, E., Lamarque, J.-C., Lowe, J., Meehl, G. A., Moss, R., Riahi, K., & Sanderson, B. M. (2016). The Scenario Model Intercomparison Project (ScenarioMIP) for CMIP6. Geoscientific Model Development, 9(9), 3461–3482. https://doi.org/10.5194/gmd-9-3461-2016.
Riahi, K., van Vuuren, D. P., Kriegler, E., Edmonds, J., O’Neill, B. C., Fujimori, S., Bauer, N., Calvin, K., Dellink, R., Fricko, O., Lutz, W., Popp, A., Crespo Cuaresma, J., KC, S., Leimbach, M., Jiang, L., Kram, T., Rao, S., Emmerling, J., Ebi, K., Hasegawa, T., Havlik, P., Humpenöder, F., Aleluia Da Silva, L., Smith, S., Stehfest, E., Bosetti, V., Eom, J., Gernaat, D., Masui, T., Rogelj, J., Strefler, J., Drouet, L., Krey, V., Luderer, G., Harmsen, M., Takahashi, K., Baumstark, L., Doelman, J., Kainuma, M., Klimont, Z., Marangoni, G., Lotze-Campen, H., Obersteiner, M., Tabeau, A., & Tavoni, M. (2017). The Shared Socioeconomic Pathways and their energy, land use, and greenhouse gas emissions implications: An overview. Global Environmental Change, 42, 153-168. https://doi.org/10.1016/j.gloenvcha.2016.05.009.
Graphs Caption: “The bottom four plots demonstrate projected changes in some land-use conditions across SSPs. Projected changes are relative to 2010 with thick lines showing the baseline SSP projections, and coloured areas showing the range of projections for all possible radiative forcing combinations. CMIP5 RCPs are added for comparison purposes. Note that ‘other natural land’ includes all land-categories beyond forests, pasture, cropland, and build-up areas” (Government of Canada, CMIP6 and Shared Socio-economic Pathways Overview).
Behind the Scenes: Where Do The Scenarios Come From?
The numbers associated with SSPs and RCPs represent the change in Earth’s net energy (radiative forcing) in watts per square meter from the year 1750 to 2100*.
So, for example, RCP7.0 would indicate a change in Earth’s net energy by an increase of 7 watts per square meter over the time period of 1750 to 2100.
*1750 was chosen to create these scenarios because was pre-Industrial Revolution, when “radiative forcing was considered mostly stable” - in other words, when the Earth’s climate wasn’t changing rapidly as it is today.
How does Earth’s net energy change?
Many factors affect Earth’s net energy, including the albedo (reflectivity) of surfaces such as the ocean, sea ice, forests, and cities; the concentrations of greenhouse gases in the atmosphere; and incoming sunlight, also called solar radiation (NASA Earth Observatory; USDA Northwest Climate Hub).
Image Credit: Janelle Christensen, USDA Northwest Climate Hub.
Caption from USDA Northwest Climate Hub: “Radiative forcing is the change in amount of energy in minus the amount of energy out due to external drivers such as increased greenhouse gases and aerosols. Positive radiative forcing means the planet warms and negative radiative forcing means the planet cools. This graphic displays some of the factors that can influence this energy balance on the planet.”
The projected differences in Earth’s net energy are affected socioeconomic factors (as captured by SSPs) and environmental factors (as captured by both RCPs and SSPs).
Learn More
These resources will cover more information about the Fifth National Climate Assessment (NCA5) and the Climate Toolbox, which contains climate tools for all experience levels.
Also, learn more about the relationship between capitalism, climate, and justice in Naomi Klein’s 2015 book, “This Changes Everything.”
