There’s still time to hold average global warming to 1.5°C in 2100, but only if global greenhouse gas emissions peak in the next few years, and if the second half of the century sees massive amounts of carbon sucked out of the atmosphere using controversial biomass with carbon capture and storage (BECCS) technology, according to a new modelling study in the journal Nature Climate Change.
The study team of 23 energy researchers used six different “integrated assessment models” (IAMs) to determine the mix of carbon reduction measures that would deliver 66% odds of avoiding warming above 1.5°C in 2100, after first overshooting the target in the course of the century. The models were grounded in Shared Socioeconomic Pathways (SSPs) that lay out assumptions for future population, economic growth, energy demand, equality, and other factors, developed for the Intergovernmental Panel on Climate Change (IPCC)’s next assessment report.
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All six of the IAMs show viable pathways to 1.5°C against a background scenario labelled “inclusive and sustainable development,” Carbon Brief reports. Four of the six show a possibility of a 1.5°C outcome with a mid-range set of assumptions that largely follow past patterns. In a world of “regional rivalry” and “resurgent nationalism”, with little international cooperation, there is no pathway to 1.5°C.
“To limit warming to below 1.5°C, all the models that the researchers examined require that global emissions peak by 2020 and decline precipitously thereafter,” Carbon Brief notes. “After 2050, the world must reduce net CO2 emissions to zero and emissions must be increasingly negative throughout the second half of the 21st century.”
And even with those measures, “all the scenarios considered still overshoot 1.5°C warming in the 2040s, before declining to around 1.3 to 1.4°C above pre-industrial levels by 2100.” Models that cut carbon faster, usually against a backdrop of “inclusive and sustainable development”, result in smaller temperature overshoots.
Carbon Brief notes that the scenarios show a remaining carbon budget of -175 to 500 gigatonnes of carbon dioxide or equivalent between 2018 and 2100, depending on emissions of non-CO2 gases like methane and nitrous oxide. “Some models with higher non-CO2 emissions have a remaining carbon budget of less than zero, requiring more CO2 to be removed from the atmosphere than added by the end of the century,” Carbon Brief notes. “In these simulations, the carbon budget for 1.5°C has already been used up.”
The study stresses the need to rapidly phase out any fossil fuels that aren’t accompanied by some form of carbon capture and storage, ramp up zero- and net-negative carbon energy sources, emphasize short-term energy efficiency gains in buildings and transportation, encourage low-energy lifestyles, introduce much higher carbon prices, and phase out most gasoline and diesel vehicles by 2060. Most of the models leave out afforestation options that could be used to sequester more carbon over the course of the century.
“The new scenarios in this study are important because they show that there are possible trajectories and technological pathways that can limit warming to below 1.5°C in 2100,” Carbon Brief notes. “However, all of the models included overshoot 1.5°C of warming in the middle of the century. Most also rely on massive amounts of still-unproven negative emissions later in the century to allow a more feasibly gradual reduction in emissions in the near term.”
