Regarding the "RCP8.5" global warming scenario projection, the so-called "business as usual (no measures)" inference for the high-carbon greenhouse gas release 

1. High-carbon emission global warming "RCP8.5" scenario estimation 

Recently, discussions on climate change have become more and more intense, especially focusing on a high-emission warming projection scenario called RCP8.5. This scenario is often described as "business as usual," alluding to the possible outcomes if society does not take any steps to reduce greenhouse gas emissions. However, experts point out that the RCP8.5 scenario is not just a conservative prediction but should be seen as the worst-case scenario for high carbon emissions. The core concept of RCP8.5 is to establish a "very high baseline emissions scenario" with emission levels equivalent to 90% of the currently available no policy baseline scenarios. However, in recent years, some scholars have questioned the emission scenario adopted by RCP8.5, arguing that it assumes high emissions in the future and a significant increase in coal use. To complement this shortcoming, a new set of future projection scenarios, known as "shared socio-economic pathways" (SSPs), have emerged to provide a broader perspective on possible future Earth conditions, especially in the absence of climate policies. 

The creators of RCP8.5 emphasize that this scenario is not intended to present a similar "business as usual" outcome and is "not preferred" for any particular situation. However, as can be seen from subsequent citations, there is a communication gap between energy system modelers and the climate model community, which may lead to certain differences in the interpretation and application of RCP8.5 scenarios.

 While simulating worst-case outcomes is critical, there is also a need to focus on policy-free baseline outcomes that lead to lower emissions in the future. To understand these scenarios, one needs to evaluate various uncertainties, including the sensitivity of the climate system to greenhouse gases and the simulation of future emissions by energy system models. To address this challenge, climate research has developed a variety of different scenarios over the past few decades and applied them to various climate models to help policymakers and the public better understand possible future climate change scenarios. 

2. What is RCP (Future Climate Projection) that represents the concentration path ? 

The Representative Concentration Pathways (RCPs) are a scenario used to present future climate change, including an assessment of the sensitivity of the climate system and future emissions. The establishment of these scenarios involves many achievements accumulated by climate research over the past few decades and has become a standard version of the driving climate model research, which is widely used in policy formulation and public education. The origins of this scenario can be traced back to the IPCC 1992 scenario (IS92) of the Intergovernmental Panel on Climate Change (IPCC) Second Assessment Report (SAR), and the six emission scenarios (SRES) in the special reports presented in the third and fourth assessment reports (TAR and AR4). Subsequently, further research yielded 4 RCP scenarios in the Fifth Evaluation Report (AR5). 

Currently, nine mandatory scenarios based on Shared Socio-Economic Pathways (SSPs) are being adopted for the upcoming Sixth Assessment Report (AR6). The development of these scenarios involves the use of energy system models and integrated assessment models (IAMs) that can simulate future energy technologies and gas emissions. However, as modern climate models become more reliant on computers, the number of future emissions projections available is quite limited. In recent years, with the development of technology and socio-economic conditions, researchers have begun to feel the urgent need to update existing scenarios. Since the existing scenarios no longer fully reflect contemporary reality, after the release of IPCC AR4 in 2007, there was an urgent desire to update the old SRES scenarios developed at that time. However, due to time pressure, the researchers took a "parallel pathway", the representative concentration pathway (RCPs), as a temporary solution. 

The introduction of RCPs has somewhat compensated for the shortcomings of the socio-economic pathway that is not yet fully mature. For climate modelers and energy system modelers, the needs for future emissions scenarios are different. Energy system modelers want to explore a variety of different outcomes under different socioeconomic assumptions, such as population and economic growth. However, to effectively evaluate and compare results, climate modelers need simulation outputs that can clearly distinguish between phases of warming. Therefore, establishing these scenarios is not only a scientific exploration but also an important tool to provide a basis for future climate policy formulation. 

Over the past few decades, climate research has established a variety of different scenarios that have become the standard for driving climate model research and are widely used in policy development and public education. These key scenarios include: 

• The IPCC 1992 scenario (IS92) used in the Intergovernmental Panel on Climate Change (IPCC) Second Assessment Report (SAR) includes six different scenarios. These scenarios provided a preliminary understanding of the possibilities of climate change at that time. 

• For the six emission projection scenarios (SRES) in the IPCC Third (TAR) and Fourth Assessment Reports (AR4) Special Reports. These scenarios play a pivotal role in exploring the possibilities of climate change at different emission levels. 

•  Four representative concentration pathways (RCP) scenarios for the IPCC Fifth Assessment Report (AR5). These scenarios further deepen the understanding of future emissions impacts and provide a more comprehensive forecast of climate change. 

• Nine forcing scenarios based on shared socio-economic pathways (SSPs) currently under development are expected to be applied in the future IPCC Sixth Assessment Report (AR6). These scenarios aim to provide more comprehensive and accurate predictions of future climate change to support more accurate climate policy formulation. 

The establishment and development of these scenarios represent an in-depth exploration of the impact of global climate change in the field of climate research and provide an important reference for policymakers and the public to address current and possible future climate change challenges. In future emissions scenarios, there are certain differences in the needs of climate modelers and energy system modelers. Energy system modelers work to explore multiple outcomes under different socioeconomic assumptions, such as future population and economic growth. However, to effectively evaluate and compare results, climate modelers need to be able to clearly distinguish between the outputs of the warming phase. The synergy of these two roles is crucial for crafting precise and sustainable climate policies. Since the release of IPCC AR4 in 2007, there has been a general expectation to update the old SRES scenarios developed in the late 90s to better reflect the current technological and socio-economic conditions. In a far-reaching paper published in the journal Nature in 2010, Dr. Richard Moss and colleagues pointed out that modelers hope that the new scenario will fully reflect the new economic data of the past decade, involve information on emerging technologies, and observations of environmental factors such as land use and land cover changes. 

Since the IPCC AR5 is scheduled to be launched in 2013, climate modelers need to have access to new scenarios available by 2010. Affected by time constraints, the researchers adopted a "parallel pathway", the representative concentration pathway (RCPs), as a temporary solution. The introduction of RCPs fills a gap in the immature socio-economic pathway and provides climate modelers with a reliable reference framework to conduct effective simulation analyses in a shorter timeframe. The application of this parallel pathway provides a flexible option for climate modelers to continue their research while providing more concrete and actionable data support for future climate policy formulation. Therefore, this parallel approach is not only an emergency measure but also an important tool for promoting global climate research and policy development. 

The Nature paper mentioned above provides an overview of the timeline for the development of new scenarios. The paper's content shows that researchers are closely monitoring the development of new scenarios and expect them to be integrated in tandem with the 2013 IPCC AR5 report. At the heart of this initiative is to ensure that new scenarios can be integrated into the IPCC AR5 report in a timely manner. Researchers will spare no effort to accelerate development progress to ensure that the new scenario fully reflects recent technological progress and socio-economic changes. This will help improve the accuracy and reliability of reporting and provide more specific guidance for global climate policymaking. By incorporating the new scenario into the IPCC AR5 report, researchers will be able to provide more comprehensive and detailed climate change predictions. This also means that future policymakers and stakeholders will be able to develop more targeted policies and action plans based on more accurate data and forecasts to address the increasing climate change challenge (see chart below).  

The energy system modeling community has taken a decision to first create future "radiant driver" scenarios that are not relevant to specific socioeconomic or emissions scenarios. Radiative driving forces are a measure of greenhouse gases, aerosols, and other factors that affect the climate's ability to retain additional thermal energy. Each RCP provides only one of the many possible pathways to this radiation-driven force. Those studying RCP emphasize that RCP is not meant to be a "new, fully integrated final scenario", but only focuses on future greenhouse gas concentrations and the radiative drivers of other input climate models. According to end-of-the-century radiative drives, RCP includes the following four pathways: RCP2.6 (representing an increase of 2.6 watts [W/m2] per square meter of radiative driving force relative to pre-industrial conditions), RCP4.5, RCP6.0, and RCP8.5. The choice of these four pathways depends on different priorities, including scenarios that span the predicted range of future emissions and concentrations in the scientific literature.

 However, due to the longer than expected development of the socio-economic pathway, RCPs were not integrated in time for AR5 publication. This has led to RCPs not being a useful tool for modeling different potential climate outputs due to the lack of any consistent socio-economic assumptions so that researchers can examine the likelihood of different no-policy baselines and mitigation scenarios. Dr. Glen Peters, research director of the Norwegian International Climate Research Center, said that there were many problems in the process of merging RCPs with SSPs, which also led to confusion about this work. He pointed out that RCPs are very practical in the climate modeling community and can provide a consistent path for analogy and comparison models. However, this parallel process has created considerable confusion about the relative likelihoods and assumptions of different emission scenarios that generate RCPs. This may also be one of the factors in RCP8.5 as the default "business as usual" scenario in academic literature and media. By understanding these challenges, we can better understand the significance and limitations of RCPs, leading to more precise and nuanced climate change research and decision-making.   

3. What is the worst situation that may occur ? 

A change in the development of RCP has been the combination of non-mitigation "baseline" scenarios with mitigation scenarios, in which climate policies drive different degrees of emission reductions. In previous simulation outcomes, such as IS92 and SRES, all scenarios were specifically designed to observe a range of possible baseline "no policy" outcomes, i.e., "reference" scenarios. This means that when the model considers different directions of future social development, it does not account for any future common climate mitigation outcomes or existing commitments, such as the Kyoto Protocol. In contrast, only RCP8.5 is a "baseline" scenario that includes no policy-driven mitigation (note: although RCP6.0 is consistent with some baseline scenarios, even though the specific IAM used by this model includes policy-driven mitigation). 

In the paper by Detlef van Vuuren et al. on the development of RCP scenarios, the authors say that it includes "one slowdown scenario resulting in a very low drive level (RCP2.6), two moderately stable scenarios (RCP4.5/RCP6.0), and a high baseline emissions scenario (RCP8.5)". They suggested: "RCP 8.5 should be considered a high emission scenario when 'RCP 6.0 can be interpreted as a medium baseline or a high mitigation case." This shows that the authors believe that there is no reason to think that RCP8.5 is more likely to be a result of 'business as usual' than RCP6.0. RCP8.5 was specifically selected as the high-end baseline scenario, not specifically portrayed as the most likely "business as usual" no-policy outcome. The researchers highlighted this in their paper, highlighting how emissions from each scenario compared to the range found in the energy simulation literature at the time. Below is a graph from their research paper showing how each RCP compares to the 90th percentile (dark gray) and 98th percentile (light gray) energy simulation scenarios in the past literature. By understanding the background and arguments of these studies, we can better understand the implications of RCP8.5 as a worst-case scenario for high emissions, while also recognizing the researchers' meticulous thinking and analysis of modeling and scenario design. According to a 2011 study by van Vurren et al., greenhouse gas emissions in the RCP scenario differ significantly from the projected range in the literature scenario. (see Chart V1) represents the 90th percentile, while light gray represents the 98th percentile. 

The chart clearly shows emissions of carbon dioxide, methane, and nitrous oxide. Particularly noteworthy among these scenarios is the difference in emissions, which reflects a high degree of uncertainty about future changes in greenhouse gas emissions. This uncertainty means that we need more in-depth research and evaluation to ensure that our predictions of climate change and greenhouse gas emissions are more accurate:  

Comparison of greenhouse gas emissions in the RCP scenario with the projection range of the literature scenario - dark gray is the 90th percentile and light gray is the 98th percentile. The chart shows carbon dioxide (left), methane (middle), and nitrous oxide (right). Source: van Vurren et al. (2011) Figure 6 The RCP8.5 scenario has attracted a lot of attention in the field of climate change research because it depicts an extreme scenario in which greenhouse gas emissions reached extremely high levels by the end of the 21st century.

 Although RCP8.5 is seen as a possible future development path in the academic literature, the researchers emphasize that it is not the only scenario that could lead to high emissions. According to expert analysis, RCP8.5's emission scenario goes beyond most non-policy-based scenarios in the literature, and its emission levels are roughly between the 90th and 98th percentile in the literature scenario, and sometimes even above the 98th percentile. In addition, the researchers noted that about 40 energy simulation scenarios have similar drive ratings to RCP8.5, indicating that there are other scenarios that could lead to high emissions. 

Although RCP8.5 is considered to describe a "business as usual" baseline scenario of high population growth and dependence on fossil fuels, there are discrepancies between this description and the details. The researchers emphasized that the formation of high-emission scenarios is influenced by a combination of factors, such as population growth, economic development levels, and energy use structures. It is worth noting that RCP8.5 is widely cited in some academic literature as a representative of the "business as usual" scenario, but there is still a lack of adequate communication in the scientific community about its high-emission characteristics.

 In fact, for more than 90% of the no policy baseline scenarios in the literature, the discharge levels are below the extreme scenarios described in RCP8.5. Overall, the interpretation of RCP8.5 should be done with caution and regarded as one of the possible future development paths, not the only result. Researchers' detailed analysis of emission scenarios helps us better understand the impact of different factors on climate change, and then formulate more effective response strategies to address the challenges posed by global climate change.  

4. New Shared Socio-Economic Pathways SSP Scenarios 

The new Shared Socio-Economic Pathways (SSPs) scenarios finally came out in 2017, five years later than Richard Moss and his team originally expected. SSPs integrate different populations, economic growth, and other socioeconomic assumptions into future emissions scenarios. They consider both a broad potential policy-free baseline scenario and possible differences in the realization of mitigation scenarios across different socioeconomic pathways. 

SSPs cover a time span up to 2100 years, with radiative driving baseline scenarios ranging from 5.0 to 8.5 W/m². When the driving force is limited to 6.0, 4.5, 3.4, 2.6, and 1.9 W/m², they specifically consider the slowdown scenario. However, because computational limitations prevent scientists from testing all SSPs through each climate model, many "framers" scenarios have been selected for CMIP6 at different driving force levels. CMIP6 is a global climate simulation project in preparation for IPCC AR6. 

CMIP6 will include RCPs (8.5, 6.0, 4.5, and 2.6) at four drive levels, as well as new 1.9, 3.4, and 7.0 drive scenarios. The 8.5 and 7.0 scenarios are derived from no policy baseline emissions scenarios in the SSP database, while the others are based on specific levels of mitigation scenarios. The following graph (CMIP6 four-drive curve plot) shows CO2 emissions for four scenarios in CMIP6: 8.5 scenario (red), 7.0 scenario (orange), 6.0 scenario (yellow), and 4.5 scenario (blue), and compares it to the range of policy-free baseline scenarios (gray) in the SSP database.  

The new 8.5 scenario represents the highest policy-free baseline scenario for emissions across all SSP (Shared Socio-Economic Pathway) processes. The 7.0 scenario is close to the middle, while the 6.0 scenario is close to the lowest value identified by the model, and its lack of climate policy impetus may lead to the lowest collective mitigation effort. The 8.5 scenario is similar to the previous RCP 8.5 scenario, but by the end of the century, CO2 emissions have increased by about 20%, and other greenhouse gas emissions have decreased somewhat. IAMs (Integrated Evaluation Models) encounter some difficulties in building this scenario. Of the five socio-economic pathways tested, only SSP5 was able to generate such high emissions. In their overview of the SSP, Raihi and his colleagues suggest: "The 8.5 watt/sq meter scenario can only occur on a relatively small scale. In contrast, the intermediate baseline scenario (SSP2) can only generate a driving force signal of approximately 6.5 watts per square meter (range of 6.5-7.3 watts/square meter). There are many reasons for the difference between the old and new RCP8.5 scenarios. The former is based on a different IAM (REMIND instead of MESSAGE). The new scenario is also based on very different socio-economic assumptions. Although RCP 8.5 has a very high population growth rate and relatively low economic growth, the new 8.5 scenario has a high economic growth rate in addition to low population growth, with the global population peaking in 2050 and then falling back to its original level in 2100.

 RCP8.5 and the new SSP8.5 scenario have a special view of future coal use, which has drawn much criticism from energy researchers. Achieving CO2 emissions in these scenarios would require large-scale use of coal, making the use 6.5 times higher in 2100 than it is today. Figure (C.1) shows the major international energy mix in 2100 under different baseline scenarios in the SSP database. Coal usage varies greatly across baseline scenarios, from the same usage as today (SSP1, SSP4 scenarios) to the SSP5 baseline scenario based on REMIND IAM, where coal usage is several times higher than current. Based on data from the SSP database and research by Riahi et al. in 2017, the figure below (Chart C.1 Source/Carbon Brief) shows global primary energy use in 2100 under each Integrated Assessment Model (IAM) and benchmark scenario from SSP1 to SSP5. The y-axis is measured in 1018 joules (EJ), the x-axis represents different IAMs, and from left to right represents different SSPs. On the far left side of the chart, the current energy usage as of 2010 is shown for reference. The chart clearly shows the difference in global primary energy use in 2100 across benchmark scenarios. This difference reflects the different predictions of IAM for future energy use in different SSP scenarios. 

At the same time, the chart also highlights the high degree of uncertainty about energy use around the world in different scenarios. It is important to note that the forecast results of each SSP and IAM are influenced by various factors, including policy measures, economic development, energy technology innovation, etc. These data remind us that in energy policy formulation and sustainable development strategic planning, we should comprehensively consider the predictions of various scenarios and models, and propose specific countermeasures to address the challenges of global energy use.  

The time series chart below (Trend Chart C.2 on Coal Usage) shows the trend of coal use across models. The graph shows coal use (shown by a thin line) for all baseline scenarios and each SSP mitigation scenario, as well as a creator scenario (shown as a solid line) created using the CMIP6 model. This time series chart clearly illustrates the evolution trajectory of global coal use under different scenarios. 

Coal use trends across baseline scenarios and mitigation scenarios have diversified over the next few decades, reflecting different forecasts of demand and policy direction for coal energy. At the same time, the creator scenarios established using CMIP6 in these models provide an alternative perspective of observation, allowing us to understand possible future trends in global coal consumption more clearly through a deeper analysis of coal usage. The presentation of these data is of great significance to energy policymakers and relevant stakeholders, helping them formulate more comprehensive and specific energy development strategies to address global energy use challenges, while also providing an effective basis for addressing the impacts of climate change.  

Coal is a major fossil fuel that has long played an important role in global energy production and consumption. According to historical data, the use of coal continues to grow, reflecting the continuous human demand and dependence on energy. However, with the increasing focus on sustainable energy, the status of coal is gradually being questioned. Judging from the scenario of the SSP (Shared Socioeconomic Pathways) database, there are predictions about future coal use trends. These scenarios consider the different development trajectories of human society and economy, and provide a variety of possible future development directions through the analysis of different factors such as population growth, economic development, and technological innovation. The study of these scenarios provides us with an important basis for predicting and planning future energy use. The CMIP6 (Coupled Model Intercomparison Project Phase 6) driving force scenario selects specific creator models that can simulate and predict future trends in global climate change. 

These models not only consider the impact of coal use on energy generation but also the impact of greenhouse gas emissions from coal combustion on the global climate. Therefore, these models have become important tools for researchers to assess the possibility of global climate change under different coal use scenarios. The historical data of global coal use, together with the SSP database scenario and the creator model selected by the CMIP6 driver scenario, provide a comprehensive understanding of future energy use and global climate change trends, which provides an important reference for formulating sustainable energy policies and addressing climate change. Global coal use has begun to decline slowly since peaking in 2014, and even in the absence of new climate policies, a scenario of a sharp increase in global coal use in the future seems unlikely. This trend is even more plausible considering the decline in the cost of other energy technologies in recent years. The upcoming "expert inspiration", in which energy experts are asked to evaluate various possible outputs, states that the RCP8.5 scenario has only a 5% probability of coming true across all no policy baseline scenarios. However, we need to be aware that there is a high degree of uncertainty in forecasting future emissions. For example, a recent study co-authored by Nobel laureate in economics Bill Nordhaus and others pointed out that before this century, there was a 35% chance of exceeding the RCP 8.5 scenario globally. 

Although most energy experts consider RCP8.5 emissions to be less achievable, it is not entirely impossible. Simulations of high-driving force scenarios, such as RCP8.5, can provide important scientific results. These scenarios can detect drastic changes in the climate system more effectively, with a high signal-to-noise ratio. In other words, because global temperatures rise more significantly in these scenarios, it is easier for researchers to distinguish between climate change signals in simulated scenarios. This is important for studying climate impacts, identifying anthropogenic warming effects, and distinguishing between natural variables. 

The RCP8.5 scenario is also used to ensure consistency as it has been included in past IPCC modelling work (CMIP5) and is similar to the scenarios included in previous IPCC reports (e.g. A2 and A1F1 in SRES scenarios). There is also significant uncertainty in the carbon cycle, and even in integrated assessment models, relatively low emissions can cause higher drivers than expected. To make it easier to compare different models, the same driving force settings are used in the CMIP climate model experimental design, limiting the model's ability to account for carbon cycle feedback. When the researchers focused on the use of RCP8.5 emissions scenarios in climate models, rather than the fixed drivers of CMIP, they found that the average CO2 concentration was 44 ppm higher and the radiative driver was about 0.25 watts/m² higher than the CMIP scenario. Simply put, given RCP8.5 emissions, the model ends up producing an average radiated driving force of 8.75 watts/square meter instead of 8.5 watts/square meter. This means that the warming impact caused by anthropogenic emissions is more severe than expected by the integrated assessment model. When the CMIP6 model is paired with better physical representations of carbon cycle feedback, such as the melting of Arctic permafrost, these carbon cycle feedbacks may be enhanced. As van Vuuren told Carbon Brief, "It's critical to have a high-end baseline scenario that explores future possibilities. 

In addition, a drive force rating of 8.5 watts/m² is not entirely impossible. In addition to the socioeconomic factors contributing to RCP8.5, strong feedback from greenhouse gases, such as methane released from tundra, can also lead to high driving force levels. At the same time, relatively sensitive climate systems may experience climate impacts consistent with RCP8.5. Therefore, for the sake of transparency, RCP8.5 should be considered a high-end scenario to explore the possibilities of high-end scenarios. The basic "no policy" scenario can serve as a useful counterfactual in climate change research, giving people an idea of what a world without climate policy will face. But at the same time, based on the reality of rapid technological development, it is not simple to understand these scenarios. While continued growth in global coal use in the early 21st century may seem plausible, in 2019 in the absence of new climate policies, it seems unlikely that this will go beyond "business as usual".