Table of Contents

 

This is a synthesis and explainer on the “8.5” scenarios used to explore future global warming.

It is a document for the interested.

Introduction

“8.5” refers to Watts/meter^2 (W/m^2), the average projected change, in the year 2100, of radiative energy per second per unit area due to human influences. This is often called radiative forcing or perhaps excess energy due to human influences.  Though often referred to as the change due to increasing carbon dioxide, the scenarios include other greenhouse gases, aerosols, land surface changes, and other aspects of human activities.

Over the years, there have been numerous 8.5 scenarios, in particular Representative Concentration Pathways (RCP8.5) and Shared Socioeconomic Pathways (SSP5-8.5). No distinction of the details will be made here, and they will all be called 8.5 scenarios. They are generally classified as “very high” carbon dioxide emission scenarios.

From RCP 8.5—A scenario of comparatively high greenhouse gas emissions, “RCP8.5 is a so-called ‘baseline’ scenario that does not include any specific climate mitigation target.”

An important point, here, is that there is no specific target of carbon dioxide concentrations, emissions, or temperature.

This article is motivated by the politicization of climate science and the rhetoric that has followed from two events.

  1. For the next round of simulations for the Coupled Model Intercomparison Project (CMIP7), scenarios with radiative forcing changes as high as 8.5 W/m^2 have been eliminated.
  2. The U.S. Global Change Research Program has proposed to amend the 2023 National Climate Assessment (NCA5). The purposes for the amendment include:

NCA5 was released in November 2023. The proposed amendment would clarify the interpretation and Federal use of NCA5 statements, figures, tables, traceable accounts, regional summaries, public summaries, and derivative Federal communications that rely materially on RCP8.5, SSP5-8.5, SRES A2, or analogous high-emissions pathways. The proposed amendment would not withdraw NCA5 in its entirety or disturb its compilation of observations, regional information, adaptation considerations, or appropriately described scenario-based analyses.

The proposed amendment would clarify that results dependent on these pathways should not be presented as expected, baseline, business-as-usual, likely, central, or ordinary Federal planning futures unless the same result is independently reproduced under a presently plausible scenario and evaluated against observations. Such results may remain useful as sensitivity analyses, bounding exercises, or stress tests when the applicable scenario, material assumptions, and limitations are clearly disclosed where the projected impact is communicated.

Each of this events has generated opinion-driven, political articles to support different positions, pro and con, on the need and urgency to address climate change. For example, this piece, “Factcheck: Trump’s false claims about the IPCC and ‘RCP8.5’ climate scenario,” documents a statement by President Trump that was picked up by many of his political allies. The piece also includes responses from a set of scientists. President Trump promised, “TRUTH, SCIENCE, and FACT,” and the revisions to NCA5 are presumed to be part of that truth telling.

“The Scenario Model Intercomparison Project for CMIP7 (ScenarioMIP-CMIP7)” provides a discussion of the development of scenarios for CMIP7. From the abstract:

This paper describes the scenario framework for ScenarioMIP as part of CMIP7. The design process has involved various rounds of interaction with the research community and user groups at large. The proposal covers a set of scenarios exploring high levels of climate change (to explore high-end climate risks), medium levels of climate change (anchored to current policy), and low levels of climate change (aligned with current international agreements).

The only point that I make from this paper is that discussion of whether or not the 8.5 scenarios are still plausible has been present in the scientific community for a number of years. Indeed, scientists have advocated that the scenarios be removed from the experimental protocol. The potential implausibility of the 8.5 scenarios is attributed to “trends in the costs of renewables, the emergence of climate policy and recent emission trends.”  That the highest emission scenario would be eliminated might be considered good news – a determination that changes that have taken place in the recent decades are pointing towards reduced emissions.

Most of the discussion about plausibility has evolved after the simulations for the previous CMIP phases had been initiated or completed. Therefore, removal of these scenarios from the CMIP7 protocol is the first opportunity to incorporate this new analysis.

Analysis

Introduction

From the References and Introduction, there have been nuanced and robust discussions about the scenarios used to investigate climate futures since the beginning of the process. They have been designed to span a range of outcomes from low emissions and low warming to high emissions and high warming. They include factors other than greenhouse gas emissions, such as aerosols, land-use changes, and the development of policy and technology. As the future of all of these factors are dependent on humans, they are uncertain. The span of the scenarios are designed to span uncertainty and to help manage uncertainty.

When scenarios are deemed “plausible,” it means that an effort is made to assure that internally they make sense. It suggests possibility. It does not mean that they are probable or likely.

As the model simulations are designed and executed, scenarios are not designated as likely or unlikely. They are designed to explore uncertainty, and that exploration includes policy decisions, such as mitigation, that might be made. They can be designed to achieve certain targets, such as radiative forcing, global average temperature, or specific concentrations of carbon dioxide. It is often the case that after the simulations are made public, some emerge as preferred. Some are given popular names. The 8.5 scenarios, RCP-8.5 and SSP5-8.5, have been very popular and are often called the “worst-case” and “business as usual.”

The scenarios change over time as the facts on the ground change and as we learn more. That is a fact of reality, and they should change.

As we learn more, we need to return to previous work and conclusions and re-analyze them.

Rhetoric and Motivated Reasoning

Many of the headlines in media and social media are designed to get attention. They are deliberately provocative and seeking an emotional response. They try to entice readers or listeners, or in the absence of that, deliver a message that someone wants spread.

If you read the story under the headline, sometimes the case is different. There is more analysis, more accuracy, and a more complete story.  However, most news stories are necessarily short. They strive for a simple, direct message. Scientists are frequently quoted. However, the scientists may get a phrase, which is all that survives from an hour-long interview.

In interviews, people sometimes rely on talking points that they feel are safe, or the one point to get across. The same is true with journalists. “New normal,” “climate emergency,” “avoid the worst effects,” and other catch phrases get repeated. They become meaningless. They are often inaccurate.

Some scientists, journalists, and publications have political goals they want to support.  Their stories are often motivated towards their goals.

All lobbyists, trade groups, advocates, and marketers have political goals they want to support. Their goals include influence, disruption, uncertainty, and sometimes willful ignorance.

Many industry influencers rely on their own research that is focused at finding inconsistencies  in scientific findings. They look for errors and conjecture on the possibility of errors. Occasionally, they find one, which is then mined to cast doubt on the whole process. They seek to find ethical breaches to suggest dishonestly, sloppiness, or corruption.

In short, if you are interested in what is a defensible reckoning of the evidence, you have to look beyond the headlines, the motivated reasoning, and efforts to dismiss, diminish, and discredit individuals with cherry-picked and conjectured evidence.

If what you are reading about the 8.5 scenarios is about “direst,” “worst case,” “scientists lying,” and revising scientific documents by political appointees to provide truth, science, and fact, then you are mired in rhetorical churn. You are in the world of logical flaws and motivated reasoning.

There is serious, evidence-driven, evaluated work that is not part of the churn. You need to find this, to dig deeper and find trusted sources.

Are the 8.5 scenarios “preferred?” (No)

First, I will provide a personal answer to the question, “do scientists prefer and recommend the 8.5 scenarios?”  The short answer is, “no.”

In 2021, the group I work with at University of Michigan published “A Practitioner’s Guide to Climate Model Scenarios.” In that guide we wrote

From the special box on RCP 8.5, written in 2021.

“RCP8.5 is currently the pathway with the highest amount of GHG emissions. Some practitioners have termed RCP8.5 as either a “business as usual” (BAU) or “worst case” scenario. These labels were not assigned to the RCPs by their developers; rather, they emerged by those using them. However, neither label is necessarily accurate. Each of these claims are explored next. Ultimately, practitioners will need to decide for themselves if they want to accept the underlying assumptions of RCP8.5 (including a possible socioeconomic description of it provided by SSP5) before using it in their planning.”

and

“GLISA recommends using the SSP3-7.0 scenario as an alternative BAU case when less extreme assumptions about the future are desired. If a practitioner is using CMIP5 projections, GLISA recommends exploring RCP6.0 as an alternative to the high RCP8.5 BAU case (additional recommendations on RCP6.0 below).”

This Guide did not recommend the use of the 8.5 scenarios. We were not radical in our recommendations. The document was reviewed by a panel of peers from outside institutions. The Guide was available on the now defunded, archived, and unmaintained climate.gov.

This establishes that these scientists, whose job was to recommend particular scenarios, did not recommend the 8.5 scenarios.

Next, I will address how the 8.5 scenarios are represented in the National Climate Assessment (NCA5).

I have searched the term “8.5” in the NCA5 pdf. There are 155 occurrences. In most instances, results “under a ‘very high’ scenario” are described, for example, “Under a very-high scenario (RCP8.5),” some outcome is projected.

The 8.5 scenarios are consistently listed as “very high”. Here is the summary description of the 8.5 scenarios:

Among the scenarios described here, these reflect the highest range of carbon dioxide (CO2) emissions and no mitigation. Total annual global CO2 emissions in 2100 are quadruple emissions in 2000 (RCP8.5 and SSP5-8.5). Population growth in 2100 doubles from 2000 in RCP8.5, but the SSP5-8.5 population remains relatively stable, with approximately 13% growth in 2100 from 2005. Both scenarios include fossil fuel development, but SSP5-8.5 has higher economic growth than RCP8.5.

It is stated in the text:

The scenarios do not have relative likelihoods assigned and are all plausible futures.

NCA5 authors were advised to assess the full range of scenarios available. While use of specific scenarios was not mandated across the report, authors were encouraged to report impacts under more than one scenario in order to describe a range of possible outcomes.

I note that from CMIP5 and CMIP6, a “very low” scenario was added to the protocol.

It is also stated in NCA5 that:

Some recent studies, however, have argued that the highest scenario, SSP5-8.5, is no longer plausible without a reversal of current trends in the adoption of renewables and energy efficiency. (emphasis in the original document).

and

To understand impacts at the century scale and beyond, authors could evaluate potential climate system changes based on applied laws of physics, Earth system dynamics, and extrapolation of climate system change indicators. Additionally, very high climate scenarios, such as RCP8.5 and SSP5-8.5, can be useful tools when considering a range of plausible greenhouse gas concentrations past 2100.

In Chapter 24 of NCA5 on the U.S. Midwest it is stated.

While there are many studies linking climate change to impacts on health and community well-being, only a small number of these studies have been conducted directly or solely on Midwest populations. Furthermore, to capture the breadth of potential impacts, these studies could consider a range of scenarios, not just the highest or worst-case scenarios.

In NCA5, the 8.5 scenarios are presented as part of the portfolio of scenarios to investigate a range of outcomes. They are consistently described as “very high.” The discussions on the plausibility of the 8.5 scenarios are mentioned. Since the NCA5 is largely limited to outcomes in 2100, the potential plausibility of the 8.5 scenarios beyond 2100 is mentioned. It is, also, explicitly stated that studies should consider a range of scenarios, not just worst-case scenarios.

The use of the 8.5 scenario is consistently qualified in NCA5. As it was part of the protocol of the CMIP design, elimination of the 8.5 scenarios would have been a political or resource decision. Scientifically, it would have been limiting and reduced the ability to analyze the simulations for very high emissions and compare to peer simulations.

Should the National Climate Assessment (NCA5) be amended?

The short answer is that the NCA5 document should not be altered. If updates, errors, or contextualizations are needed, then they should be developed and presented with standard protocols of transparency, authorship, review, and comment.

  1. The NCA5 was produced using a specific protocol for determination of writing teams, review, and public comment. NCA5 is the fifth in the series of assessments mandated by congress. To alter the document outside of this process would damage the integrity of document and the process. It would diminish its effectiveness for all stakeholders. There are methods of standard practice for updating, addressing errors, and providing ancillary documents, which should be followed. The existing process provides for updates as one assessment follows another.
  2. A stated high-level priority of the amendment is “The proposed amendment would clarify that results dependent on these (8.5) pathways should not be presented as expected, baseline, business-as-usual, likely, central, or ordinary Federal planning futures unless the same result is independently reproduced under a presently plausible scenario and evaluated against observations.”  The current NCA5 document does not propose that the 8.5 scenarios be used for planning purposes. In fact, it mentions that the plausibility of the 8.5 scenarios was an ongoing matter of discussion at the time of publication. It, specifically, uses the scenarios to explore sensitivities. The 8.5 scenarios are not stated as likely. The term “business as usual” is not used to describe any scenario, and a reference that business as usual is a misleading concept is cited more than once. Since the NCA5 did not propose the 8.5 scenarios for planning, no changes are needed to NCA5 for clarification to meet this high priority goal.
  3. There is scientific justification to explore very high scenarios. We have not, globally, actually started a reduction of emissions, and future reductions are based on economic and policy outcomes that are yet to be realized. Current energy policy in the U.S. includes a focus on fossil fuels, which is, referring to NCA5, “a reversal of current trends in the adoption of renewables and energy efficiency.” This was one of the assumptions of challenging the plausibility of the 8.5 scenarios. The 8.5 scenarios explicitly include “fossil fuel development.” Presently, AI data centers are increasing demands for fossil fuels. Also, the warming observed from 2001-2025 suggests that it is premature to eliminate simulations with high climate sensitivity, that is, with higher warming.  Also noted, the NCA5 results are often focused on 2100, and beyond 2100 the higher emission scenarios cannot be eliminated. In addition, the very high emission scenarios document model performance at high emissions, and contribute to the evaluation and robustness of simulations.
  4. NCA5 does not provide guidance on scenario choice for planning. There are existing guidance documents that specifically recommend against using the 8.5 scenarios, even for high emission cases. For planning over the next 40 – 50 years, the results are relatively insensitive to choice of scenario. That is, other sources of uncertainty are more important.  Any policy guidance on scenario choices for planning should be made as an ancillary document.

References

Responses to elimination to the “8.5” scenarios