The increasingly evident adverse impacts of global warming unequivocally highlight our responsibility in driving climate change. We can no longer ignore the pressing need for immediate action to safeguard the most vulnerable communities and ecosystems, as inaction is no longer an option. However, the decision on the most effective adaptation and mitigation strategies, as well as where to deploy them is a daunting task.
Over decades climate scientists relied on socio-economic and emission scenarios to characterize the complex interaction between humans and the environment. Numerous scenarios explored plausible trajectories of climate change evolution under various conditions: technological development, land and energy use, and different levels of Green House Gases (GHG) emissions (O’Neill et al., 2020). Integrated into the climate models they provide crucial information on the climate change impacts and the efficacy of different adaptation and mitigation strategies, with their associated costs and benefits.
One of the first attempts to estimate the effects of climate change was carried out in 1990. The Intergovernmental Panel on Climate Change (IPCC) employed Scenario A (SA90) in the First Assessment Report, followed by IS92 in the third IPCC report in 1992. The effort included six alternative IPCC scenarios spanning a wide array of assumptions, including projections of GHG concentration evolution in the absence of additional climate policies beyond those already in place (Girod et al., 2009). However, the development of effective and rightly placed adaptation and mitigation strategies required the ability to explore the impact of policies on climate change, prompting the birth of new scenarios. More complex and detailed, Representative Concentration Pathways (RCP) not only included the conversion of the non-CO2 gases to CO2 equivalent (not included in earlier scenarios), but they pioneered the estimation of the potential societal impacts.
The development process was led by the scientific community and designed in phases. During the first phase, scientists devised the driving forcing agents behind climate change based on the extensive literature review. They included GHG emission and concentration data, as well as the land use data. The second phase was intended to run concurrently with the first, focusing on the development of the new Shared Socioeconomic Pathways (SSP), designed to complement the RCPs. While RCPs depicted the GHG emission levels and associated global warming, SSPs aimed to assess the likelihood of emission reduction (van Vuuren et al., 2011). The development of the SSP scenarios, however, took much longer than intended and they were released only in 2017 (Raihi et al., 2017).
Representative Concentration Pathways
Future GHG emissions, pollutant concentrations, and land use projections were integrated into new climate model experiments to generate new climate prediction scenarios (van Vuuren et al., 2011). Integrated Assessment models (IAMs) explored how and if technological advancements, different socio-economic conditions and policies can sway the scenarios towards specific pathways and how they can affect the magnitude of climate change (van Vuuren et al., 2011). The primary factor applied in RCPs was the radiative forcing representing the full set of emission scenarios available in the literature at that time, from very low to exceedingly high. Overview of the existing literature provided a wide array of radiative forcing, varying from 2.5 W/m2 up to 9 W/m2 and even higher, though the majority of the scenarios covered intermediate levels (Fisher et al. 2007; Van Vuuren and Riahi 2011).
The intended purpose of the RCP scenarios was to provide data for the generation of climate models spanning both historical and future periods. One of the important aspects taken into account during the development of the RCPs was to allow exploration of slow climate processes, not reflected in previous scenarios. To assure the consistency among the scenarios, emission data were converted into concentrations using a simple carbon cycle model, and the land use data were harmonized against a common-based year and downscaled. For the year 2000 was used as a base year for emission data (Lamarque et al. 2010). Prior to 2000 due to a lack of consistent data, information was compiled from multiple emission inventories (Lamarque et al. 2010). Special attention was given to the harmonization of the land-use data due to its critical role in modeling simulations for both IAM and Climate Models (CM). Advanced climate models expanded on their input parameters beyond GHG emissions, to include other factors like reactive gases or aerosol compounds.
Following the discussion during the IPCC expert meeting (Moss et al. 2008) out of 37 scenarios aligned with set criteria, carried out by 7 international teams, 4 scenarios were selected, sufficiently distinct to represent different levels of radiative forcing. These scenarios comprise a very low scenario RCP2.6; two medium stabilization scenarios RCP4.6 and RCP6 and one very high (overshoot) scenario RCP8.5 (van Vuuren et al., 2011).
The RCP8.6 scenario designed by the team from the International Institute for Applied Systems Analysis (IIASA), Austria, devise a steady GHG emissions increase over time. Characterized by very high energy intensity, this scenario is influenced by population growth and a lack of technological development. Commonly referred to as a “business as usual scenario” in the media and the broader community, it is considered the most probable outcome if immediate action is not taken. This is, however, a misconception. The original publication by van Vuuren et al. (2011) clearly states that it is not inherently more likely than any other RCP scenario, and is merely a high-end scenario. Nonetheless, followed by its inclusion into the IPCC AR5 report as the only non-policy scenario, it has been labelled as a “business as usual” scenario, leading to the confusion among the society.
The RCP6, devised by the team at the National Institute for Environmental Studies (NIES) in Japan depicts GHG emissions stabilization around 2100, prompted by technological and strategical development (Fujino et al. 2006; Hijioka et al 2008). The RCP4.5 developed at the Pacific Northwest National Laboratory’s Joint Global Change Research Institute (JGCRI) in the United States is regarded as the most probable baseline scenario. In comparison to RCP6, the GHG concentrations are expected to peak around 2040 (as opposed to 2080), and decline thereafter. This scenario does not incorporate any policy intervention. Both intermediate scenarios depict steady levels of fossil fuel utilization, with a growing role of renewable energy.
The most stringent scenario, RCP2.6 was developed by the modeling team of the PBL Netherlands Environmental Assessment Agency and represents the lowest levels of GHG emissions based on the literature review at that time. It integrates the utilization of biofuel and Carbon Capture and Storage (CCS) technologies that allow it to reach negative carbon emissions.
The RCP scenarios undergo continuous updates following the release of new relevant studies and method improvements, while their fundamental assumptions remain unchanged. It is important to realize that although each RCP maintains consistent internal assumptions, as a collective set they are not consistent among each other as they encompass diverse socio-economical and technological assumptions.
To provide additional information on the long-term climate and ocean responses the RCP scenarios were extended beyond 2100, reaching up to 2300, termed extended RCP. The extended RCPs omit socio-economic projections due to increasing uncertainties regarding driving mechanisms controlling emissions.
Shared Socioeconomic Pathways
Shared Socioeconomic Pathways (SSPs) represent a novel set of pathways developed by collaboration among climate scientists, economists, and modelers of energy systems (O’Neil et al., 2020). These pathways, consisting of five scenarios, integrate various assumptions regarding population growth, economic growth, and other socioeconomic factors into future emissions scenarios, showing how societal choices can affect GHG emissions (Z. Hausfather, 2018). Currently used for a new IPCC assessment report AR6, the new SSPs offer a number of scenarios without climate policies and cover a range of possible futures based on different mitigation strategies keeping the global warming below 1.5 degrees to above 3.5 C°. Five SSPs span a broad range of socio-economic development trends, from an economy heavily reliant on fossil fuels in SSP5 to an economy oriented towards more sustainable energy sources in SSP1 (Raihi et al., 2017). The SSP narratives also include a wide range of economic progress reflecting various educational efforts aimed to foster a more sustainable and equitable future in less developed countries. Unlike previous scenarios used in the 4th and 5th IPCC assessment reports, each SSP has a baseline scenario without any mitigation efforts, while all low-end scenarios limiting the warming below 2 degrees C incorporate a certain degree of bioenergy with CCS. The introduction of the new SSP scenarios prompted the expansion of the existing RCPs, resulting in the addition of RCP1.9; RCP3.4, and RCP 7.0. The RCP1.9 aims at limiting the current temperature warming to 1.5 degrees, whereas RCP3.4 serves as an intermediate between a very stringing RCP2.6 scenario and RCP4.5. RCP7.0 offers an alternative scenario to a very high RCP8.5, representing a medium to high-end range of GHG emissions.
The release of SSPs broadens the spectrum of no policy scenarios available for the research community, extending the range of options compared to the RCP8.5.
The word of Caution
A lot of misconception persists among the general public and the media regarding the significance of different RCP scenarios. It is important to understand that neither RCPs nor SSPs are a forecast of a plausible future, but merely a prediction intended to support an informed policy decision-making process. None of the pathways is more likely to happen than the other.
Developed independently by different modeling groups, they do not constitute a comprehensive set of scenarios. Differences observed within each pathway may not be directly interpreted as a result of different policies or socio-economic developments. Climate impacts in each pathway can not be directly attributed to the policy effect in each scenario, but rather are the results of different initial model assumptions, underlying each RCP and SSP. A broader application of the SSP–RCP framework in the future will improve the comparability of assessments across different models.
Furthermore, it is imperative to keep in mind that uncertainties in carbon-cycle feedbacks are not incorporated into the RCPs, implying that the range of plausible future CO2 emissions might be considerably wider than reported. This consideration has to be adopted in future mitigation strategies (Hausfather and Betts, 2020).
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Dr. Alexandra Filippova
Dr. Filippova is a marine geochemist with an extensive and diverse scientific background in geoscience, natural management, polar marine research and climate science. Over a decade she studied processes that affect ocean circulation in the past and how they could compare to the modern day situation. One of the key questions of her studies included the role of climate induced melt water inputs in ocean circulation and climate changes on short and long term scale. Beyond academia, she truly enjoys volunteering with Non-Profit Organizations, where she advocates for diversity and inclusion of caregivers in all STEMM fields (Mothers in Science) and works on development of sustainable projects that aim at preserving nature and biodiversity (Viable Community).