Human progress is driven by a variety of causes, including human decision-making. The Intergovernmental Panel on Climate Change (IPCC) utilizes carbon dioxide (CO2) emission scenarios to envisage and assess “a range of probable futures”.
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The “Representative Concentration Pathways” (RCPs) and the “Shared Socioeconomic Pathways” (SSPs) are two scenarios used in the IPCC 5th Assessment Reports (AR5) and 6th Assessment Reports (AR6).
By 2100, the RCP/SSP range’s high-emissions end is compatible with 8.5 W m-2 radiative forcing and 4–5 °C warming compared to pre-industrial levels. By 2100, emissions are expected to be compatible with 1.9 W m-2 radiative forcing and 1.5 °C of warming or somewhat less.
The IPCC’s recent acknowledgment of scenario probability is part of a larger and fast-moving issue on scenario plausibility and its implications for research and policy.
The “plausibility” of scenarios has been discussed for years, but the IPCC has yet to assess the plausibility of scenarios that are essential to most climate research and assessment. A “plausible” scenario, according to our study, is one in which the scenario’s future fossil fuel and industrial (FFI) CO2 emission growth rates are consistent with observations and IEA Stated Policies Scenario (STEPS) near-term estimates.
This topic is furthered here by selecting the AR5 and AR6 scenarios that are most consistent with current data and near-term estimates of FFI carbon dioxide emissions. As a result, the article fills a critical vacuum in climate research with far-reaching consequences for both research and policy.
Studies of future climate consequences often use a small set of scenarios as baselines against which more or less strict 21st-century mitigation actions may be compared. It is also critical to regularly analyze scenarios for plausibility to guarantee the rigor of climate research aiming to predict credible futures.
That study found a set of possibilities that were deemed unlikely because they contradicted recent experience and IEA estimates for the near future. Humans extend and update the research by comparing growth rates in fossil fuel and industrial (FFI) CO2 emissions across IPCC and SSP baseline and policy scenarios to find situations that predict growth rates in FFI CO2 emissions consistent with recent history and IEA forecasts to 2050.
Scientists call such circumstances credible. The researchers examined what this selection of viable scenarios forecasts for future FFI CO2 emissions and related global average temperature change to 2100 after picking a subset of feasible scenarios from the full collection of accessible scenarios.
Researchers compare the anticipated FFI CO2 emissions growth rates of the 1184 AR5 scenarios and 127 SSP scenarios to observations for 2005–2020, as well as 2021–2050 estimates from the IEA’s 2021 World Energy Outlook (WEO).
The IEA STEPS model predicts that committed climate and energy policies would be implemented to varying degrees, but that no new policies will be implemented, similar to traditional “business as usual” (BAU) baselines. Previous IEA WEOs featured a second, slightly gloomier, BAU-like Current Policies Scenario (CPS) that assumed no new policies would be adopted beyond those currently in place.
The study compares the errors and divergences of AR5 and SSP scenarios, IEA STEPS, and IEA APS for each of the Kaya Identity factors: GDP per capita, population, carbon intensity (CO2 emissions/primary energy), and energy intensity (primary energy/GDP) to investigate the drivers of FFI CO2 emission divergences.
Figure 1 depicts FFI emissions in 2005–2050 scenarios that fulfill the ±0.1%/y and ±0.3%/y divergence standards.
Figure 1. The trajectories of all fossil-fuel-and-industry (FFI) CO2 emissions to 2100 among 1184 AR5 and 127 SSP scenarios, along with the IEA STEPS to 2050. Shaded regions indicate envelopes of scenarios meeting ±0.1%/y (blue) and ±0.3%/y (gray) divergence tolerances in FFI CO2 emissions (relative to observations and IEA projections). Image Credit: Pielke Jr, et al., 2022
Figure 2 shows the feasible possibilities in terms of cumulative CO2 emissions from 2000 to 2100 and related end-of-century global mean temperature increase (using the 2005–2050 STEPS filters) (relative to the pre-industrial baseline).
Figure 2. AR5 and SSP scenarios plotted in terms of cumulative FFI CO2 emissions in the 21st century, and warming by 2100, relative to the pre-Industrial baseline. Major SSP scenario ranges (dashed boxes) and marker scenarios are highlighted. Blue circles and light-blue triangles represent scenarios meeting the ±0.1%/y and ±0.3%/y divergence tolerances, respectively, in FFI CO2 emissions (relative to observations and IEA STEPS projections from 2005 to 2050). Image Credit: Pielke Jr, et al., 2022
The 2005–2019 and 2005–2020 filters, on the other hand, permit a small number of scenarios with warming more than 3 °C, while the 2005–2050 APS filter accepts scenarios with warming between 1.5 °C and 2 °C (Figure 3).
Figure 3. Projected temperature increases (2100, compared to the pre-Industrial baseline) in sets of AR5 and SSP scenarios selected by the various filters used in our analysis, in terms of divergences in FFI CO2 emission growth rates observed and projected by IEA STEPS or APS scenarios. Boxes denote the 25th to 75th percentile ranges; white line denotes the medians; and whiskers denote the minima and maxima. Image Credit: Pielke Jr, et al., 2022
The “plausible” subset of IPCC scenarios identified—most consistent with observations to 2020 and IEA estimates to 2050 (Figure 1)—is also compatible with a world that is now well-positioned for future climate policy success via ongoing adoption of policies that take the world closer to meeting or almost achieving a 2 °C target by 2100.
In recent years, however, actual decarbonization has surpassed IEA estimates. As a result, the findings emphasize the importance of continuing efforts to speed global economic decarbonization if the trajectory of currently probable scenarios is to be met or exceeded in the future.
Even if they were established in recent years and decades, the great majority of scenarios that estimate futures beyond 2100 failed the simple criterion of believability by 2020. This raises concerns about using long-term scenarios as forecasts of conceivable futures rather than exploratory tools and highlights the need for policy-relevant scenarios that are updated much more often with fresh observational data, similar to the IEA’s near-term scenarios.
The IEA’s STEPS near-term CO2 emission predictions for 2021 confirm expectations for a long-plateau in CO2 emissions over the following few decades, with projections of less than 3 °C warming by 2100. The envelope of potential situations found in the investigation (Figures 1–3) independently supports such assumptions. Of course, the future is unknown, as it is determined by policy decisions and expectations that will change as time passes.
Deep decarbonization remains a huge task, and achieving net-zero CO2 emissions by 2050—a mainstream policy goal—remains outside even the most plausible scenario paths (Figure 1). decision-makers throughout the world may choose to increase CO2 emissions, but based on evaluations such as the UNEP Emission Gap study, this appears to be very improbable.
The findings show that the world is in an advantageous position to meet the challenge of deep decarbonization, at least in comparison to where IPCC baseline scenarios and some public debate predicted the globe to be in 2021. Climate research and policy rely on the formulation and continuous updating of credible scenarios to enable sustained efforts to achieve deep decarbonization.
Pielke Jr, R., Burgess, M. G., & Ritchie, J. (2022). Plausible 2005-2050 emissions scenarios project between 2 and 3 °C of warming by 2100. Environmental Research Letters, 17. Available Online: https://iopscience.iop.org/article/10.1088/1748-9326/ac4ebf.
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