Scientists have changed their worst-case future scenario for carbon emission effects. The worst-case scenario for a high-emissions future has been revised downward, but the best-case option is now implausible. This development has been misrepresented by some media outlets, including News Corp, as a dramatic reduction in forecast warming.
Background on Climate Scenarios
Since the 2010s, climate scientists have used a range of scenarios in models to understand potential future outcomes. These scenarios are not forecasts but a collection of 'what ifs' based on different levels of greenhouse gas emissions. One such scenario, RCP 8.5 (and its successor SSP5 8.5), represented the worst-case path, with global heating reaching about 4.5C by century's end. However, as renewable energy costs have dropped and climate policies have emerged, this scenario has become implausible.
Revision of Scenarios
In May, a group of scientists advising climate modellers wrote that the 8.5 scenario had 'become implausible, based on trends in the costs of renewables, the emergence of climate policy and recent emission trends.' They proposed six new scenarios, with the worst-case now projecting about 3.5C warming by 2100, roughly 1C lower than before.
Dr Andrew King, a climate scientist at the University of Melbourne, noted: 'Climate scientists can't predict the future so instead we construct scenarios of what could happen depending on what we do next. RCP8.5/SSP5-8.5 was always pretty much a worst-case scenario. Even though we haven't done enough to tackle climate change over the last decade, we have seen enough decarbonisation that we won't follow that path.'
However, King also pointed out that the most optimistic scenario has also been dropped, meaning the best-case future is now less attainable. 'It's important to note that we also have dropped the most optimistic scenario for the 21st century with substantially higher peak warming in the new lowest emissions scenario than previously,' he said.
Misrepresentation by News Corp
Former editor of the Australian, Chris Mitchell, claimed the development 'dramatically cuts the forecast warming of the planet by the end of this century,' a gross mischaracterisation. Mitchell argued that the 8.5 scenario had generated 'hysteria' and led to policies based on 'predictions the IPCC now accepts are way off kilter.' However, the Paris Agreement targets of keeping warming 'well below 2C' and pursuing 1.5C were never based on the worst-case scenario.
Prof Glen Peters, an Australian climate scientist at Norway's Centre for International Climate Research, stated: 'I think the arguments of Mitchell and the like all falter, as they frame success as not [hitting] 4 to 5C warming in 2100. The policy world has prioritised 'well below 2C' and pursuing 1.5C. We are far from succeeding on that dimension.'
Contrarian Voices
Mitchell also cited Dr Roger Pielke Jr, a senior fellow at the American Enterprise Institute, a conservative thinktank that has fought to slow climate progress. Pielke Jr had claimed the 8.5 scenario was 'embedded in the policies and regulations of most of the world's largest economies,' but this is not accurate. For instance, Australia's National Climate Risk Assessment did not use the scenario, and regional climate modelling in Australia has also ignored it in favour of lower emissions scenarios.
Mitchell drew on other contrarians like Bjorn Lomborg, reflecting a pattern of relying on a shallow pool of climate sceptics. These voices have been repeatedly used by News Corp to misrepresent climate science.
