In a surprising twist, climate scientists have officially retired their worst-case scenario for future carbon emissions, but this good news has been twisted by some media outlets. The scenario, known as RCP 8.5 and its successor SSP5-8.5, envisioned a world where fossil fuel use continues unabated, leading to about 4.5°C of warming by 2100. However, due to falling renewable energy costs, climate policies, and emissions trends, this scenario is now considered implausible.
What the Change Really Means
The Intergovernmental Panel on Climate Change (IPCC) has not changed its forecasts; rather, scientists have updated the scenarios used in climate models. The new worst-case scenario assumes a rollback of mitigation policies, leading to about 3.5°C warming—still catastrophic but lower than the old 8.5 scenario. Crucially, the best-case scenario has also been revised upward, meaning the most optimistic path is now less optimistic.
Dr Andrew King from the University of Melbourne explains: "Climate scientists can't predict the future, so we construct scenarios of what could happen depending on what we do next. RCP8.5 was always a worst-case scenario. Even though we haven't done enough to tackle climate change, we have seen enough decarbonisation that we won't follow that path. But we have also dropped the most optimistic scenario, with substantially higher peak warming in the new lowest emissions scenario than before."
Misrepresentation by News Corp
Former editor of The Australian, Chris Mitchell, claimed the change "dramatically cuts the forecast warming of the planet by the end of this century," a gross mischaracterisation. In reality, the Paris Agreement goals of keeping warming well below 2°C and pursuing 1.5°C remain the targets, and we are far from achieving them. Mitchell also cited contrarian scientists like Roger Pielke Jr and Bjorn Lomborg, who have a history of downplaying climate risks.
Prof Glen Peters, an Australian climate scientist, notes: "The arguments of Mitchell and the like all falter, as they frame success as not hitting 4 to 5°C warming in 2100. The policy world has prioritised 'well below 2°C' and pursuing 1.5°C. We are far from succeeding on that dimension."
Implications for Australia
Australia's National Climate Risk Assessment did not use the 8.5 scenario, instead examining risks at 1.5°C, 2°C, and 3°C. Regional climate modelling in Queensland, New South Wales, and Victoria has also moved away from the 8.5 scenario, using lower emissions scenarios for their worst-case projections.
The retirement of the worst-case scenario is not a reason for complacency. It reflects that while we have avoided the most extreme future, our slow action has made the best-case scenario implausible. The real challenge remains: to limit warming to 1.5°C or well below 2°C, which requires rapid and deep emissions reductions.
