Summary

  • RCP8.5 (Representative Concentration Pathway 8.5) is a climate change model scenario used in reports from the Intergovernmental Panel on Climate Change (IPCC).

  • RCP8.5 is widely regarded as the “worst-case” or “extreme” scenario for climate change modelling.

  • RCP8.5 predicts global warming with a temperature increase of 3.0°C to 5.1°C.

  • The IPCC has modelled more moderate climate change scenarios such as RCP2.6 and RCP4.5.

  • RCP8.5 is being used in flood risk modelling for councils around New Zealand.

  • Some councils are having flood risk modelling conducted using only RCP8.5 instead of more moderate climate change scenarios.

Computer Models and Assumptions

The Intergovernmental Panel on Climate Change (IPCC) uses a number of scenarios to create computer models of climate changes and its effects. Computer models make predictions based on the information and assumptions fed into those models. One set of scenarios used in IPCC reports are the Representative Concentration Pathways (RCPs). RCP2.6, RCP4.6, RCP 6.0, and RCP8.5 are the four common RCP scenarios with the number after RCP referring to the predicted radiative forcing in the year 2100. The number is in W/m2 as an estimate of the strength of the greenhouse effect due to the predicted levels of greenhouse gases in the Earth’s atmosphere. RCP2.6 is meant to represent a future where “stringent” mitigation policies are used to “keep global warming likely below 2°C above pre-industrial temperatures”. RCP4.6 and RCP 6.0 are called “intermediate scenarios”. RCP8.5 is called “extreme” , “worst-case”, and the “very high GHG emissions” scenario. IPCC reports claim that without additional efforts to “constrain emissions”, this will lead to a situation between RCP 6.0, and RCP8.5. These are also called “baseline scenarios” or “business-as-usual” scenarios.

“The Representative Concentration Pathways (RCPs) describe four different 21st century pathways of greenhouse gas (GHG) emissions and atmospheric concentrations, air pollutant emissions and land use. The RCPs have been developed using Integrated Assessment Models (IAMs) as input to a wide range of climate model simulations to project their consequences for the climate system. These climate projections, in turn, are used for impacts and adaptation assessment. The RCPs are consistent with the wide range of scenarios in the mitigation literature assessed by WGIII1 The scenarios are used to assess the costs associated with emission reductions consistent with particular concentration pathways.” TOPIC 2 Future Climate Changes, Risks and Impacts, Climate Change 2014 Synthesis Report Fifth Assessment Report, IPCC

Table from page 16 of the IPCC CLIMATE CHANGE 2023 Synthesis Report: Summary for Policymakers

 

How is RCP8.5 being used in New Zealand?

Local authorities often use RCP8.5 when predicting sea-level rise and flood risk. The report Kāpiti Coast Coastal Hazards Susceptibility and Vulnerability Assessment Volume 2 heavily uses the RCP8.5 scenario and also uses the RCP4.5 scenario. The use of extreme scenarios may influence building consents, insurance, council adaptation plans, and managed retreat policies.

Hamilton City Council largely used RCP8.5 as an assumption in their recent flood modelling. The results of that modelling will appear on LIM reports for property titles.

The Greater Wellington Council only refers to RCP8.5+ on their Regional flood hazard assessment webpage.

The Wairoa Cyclone Gabrielle Review document uses RCP8.5 for modelling.

Newroom has recently reported that other local councils are using assumptions based on RCP8.5 in their planning and policies. This appears to be impacting resource consents from Auckland Council.

Screenshot of part of the Hamilton City Council Flood modelling webpage

Is RCP8.5 realistic?

RCP8.5 is subject to debate concerning whether it is realistic and if we are currently tracking close to the RCP8.5 projections. RCP8.5 involves the assumption that annual coal burning will be between 7 and 10 times higher in 2100 when compared to today.

“…the RCP 8.5 is a baseline scenario with no explicit climate policy, representing the highest RCP scenario in terms of GHG emissions…

…Coal use in particular increases almost 10 fold by 2100 and there is a continued reliance on oil in the transportation sector.” K. Riahi, S. Rao, V. Krey, C. Cho, V. Chirkov, G. Fischer, G. Kindermann, N. Nakicenovic, P. Rafaj, RCP 8.5—A scenario of comparatively high greenhouse gas emissions, Climatic Change (2011)

Some scientist have harshly criticised the RCPs (especially RCP8.5) as unrealistic and implausible.

“Why does this matter? Because RCP8.5—the most commonly used RCP scenario and the one said to best represent what the world would look like if no climate policies were enacted—represents not just an implausible future in 2100, but a present that already deviates significantly from reality. We know this because we have studied RCP8.5 (as well as other climate scenarios) for years and have evaluated many of its inputs and assumptions against how the world has actually developed since 2005, where RCP8.5 begins. We have also evaluated hundreds of IPCC scenarios against near-term projections of global energy assessments. Our work (including collaborations with Matthew Burgess and other colleagues), as well as studies by other researchers published in many papers, clearly shows that most IPCC scenarios are already off track and some, like RCP8.5, significantly so.” Roger Pielke Jr., and Justin Ritchie. “How Climate Scenarios Lost Touch With Reality.”Issues in Science and Technology 37, no. 4 (Summer 2021): 74–83 

“Why, then, did the IPCC choose RCP8.5 as its only business-as-usual baseline? Not because it explicitly judged it the world’s most likely or even plausible future, although the designation implies both. Rather, it selected RCP8.5 in part to facilitate continuity with scenarios of past IPCC reports, both SRES and earlier baseline scenarios, so that results of climate modeling research across decades could be comparable. It also chose RCP8.5 to help climate modelers explore the differences between climate behavior under hypothesized extreme conditions of human-caused climate forcing and natural variability. The difference between the high (8.5 W/m2) and low (2.6 W/m2) RCP forcing pathways created, as scenario developers explained, “a good signal-to-noise ratio for evaluating the climate response in AOGCM [atmospheric-oceanic general circulation model] simulations.” The technical requirements of climate modeling, and not climate policy, drove the design of IPCC scenarios.

These decisions might be justifiable if climate models were simply scientific tools aimed at exploring a variety of conditions as a way to test hypotheses and researchers’ understanding of the climate system. But scientists, policymakers, the media, environmentalists, and the public now widely justify and interpret climate models as providing predictive information about plausible futures. By choosing RCP8.5 as one of only four forcing scenarios to be used by modelers, and compounding this choice by labeling it as the business-as-usual scenario, the IPCC promoted a scenario useful for scientific exploration but highly misleading when applied to projecting the future to inform decision-making.” Roger Pielke Jr., and Justin Ritchie. “How Climate Scenarios Lost Touch With Reality.”Issues in Science and Technology 37, no. 4 (Summer 2021): 74–83 .

“The RCP-8.5 scenario is important because it is the IPCC scenario that projects the highest amount of CO2 emissions throughout this century. Therefore, it is often used to represent a world under strong climate change, which can subsequently be contrasted with a scenario that depicts a world of aggressive climate action. Most studies about climate change effects rely on this contrast between a high-end and a low-end emission scenario, and the greater the difference between these two scenarios, the clearer the investigated effect will emerge.

For this reason, RCP-8.5 – a world of extreme climate change – is very popular within the climate modelling community and makes for most of the scenarios assessed in working groups I and II of the IPCC’s Sixth Assessment Report (AR6).” RCP-8.5: Business-As-Usual or Unrealistic Worst-Case? The contested interpretation of climate change scenarios, Climate Matters, 2011