Executive Summary:
Some heralded the retirement of the RCP 8.5 climate scenario as the end of the climate change hoax, while others see it as a sign of climate mitigation’s success. The truth is far more mundane yet still impactful. While the plausibility of RCP 8.5 has been hotly debated in the scientific community for several years on both plausibility and methodological grounds, its retirement reflects more the advancements in technology and shifting methodologies for modelling climate change impacts. In the much-cited report misattributed to the Intergovernmental Panel on Climate Change (IPCC) but, in reality, published by the Coupled Model Intercomparison Project (CMIP), the worst-case RCP 8.5 and best-case RCP 2.6 scenarios were retired because both were deemed no longer plausible. Instead, they were replaced with a new set of model scenarios that predict a more modest range for climate change-induced temperature rise. Rather than revealing climate change as a hoax or heralding a victory for climate change mitigation efforts, the repeal of RCP 8.5 reflects a recalibration by scientists so that their models account for changes brought about by current mitigation efforts, while still demonstrating how much further there is to go to limit the harm caused by climate change.
Key Facts:
- According to Professor Detlef Van Vuuren, RCP 8.5 was designed as a worst-case, low-probability, high-risk climate change model, which some researchers have incorrectly presented as a likely outcome if no action is taken to mitigate climate change.
- CMIP reports that RCP 8.5’s projection of a 3.2°C to 5.43°C is no longer plausible due to decreasing costs of renewable energy, declining CO2 emissions, and the emergence of global climate policy. They instead project a more limited temperature rise of between 1.5°C and 3.5°C, which eliminates both the best- and worst-case RCP scenarios.
- This hasn’t stopped public leaders like Donald Trump, from using RCP 8.5’s retirement to further the claim that climate change is a hoax. Likewise, researchers like Roger Pielke Jr, continue to question the plausibility of the assumptions underlying the new scenarios.
Background and Context:
RCP 8.5 and its other scenario counterparts 2.6, 4.5, and 6.0 were created in the early 2010s to model the change in Earth’s energy imbalance caused by human-driven greenhouse gas emissions through 2100 in order to gauge the potential long-term impacts of climate change. RCP 8.5 predicted that, with no efforts to reduce greenhouse gas emissions, expanded coal consumption, and an increasing population, global average temperatures would rise by 4.4 °C with catastrophic consequences. However, controversy arose over the model’s plausibility, with researchers Pielke, Burgess, and Ritchie arguing that the RCP 8.5 model was based on unrealistic projections of population and CO2 emission growth. Moreover, politicians in the U.S. have criticised the use of RCP 8.5’s extreme modelling to inform regulatory decisions.
Status and Trends:
While the actual contents of the CMIP report detailing the retirement of RCP 8.5 is likely to become the groundwork for new and better climate model projections, its public perception is likely to have a harmful effect on climate mitigation efforts. Political and business leaders like US President Donald Trump have already cited the report to further their argument that climate change is a hoax, and current news coverage has largely ignored much of the actual CMIP report, including their retirement of the best-case RCP 2.6 scenario. This leads to concerns about the public’s willingness to believe future climate change models and reports and opens the possibility that political and business interests may use the confusion around the CMIP report to slow or reverse climate change mitigation policies. While our world may no longer be in store for a worst-case scenario, it is also clear that there is still a much longer policy and technological road ahead in order to mitigate climate change and make a best-case 1.5 °C scenario possible.