Emissions pathways change how abnormal climatic conditions de-emerge beyond net-zero

As more and more countries set net-zero targets and progress is made on decarbonising industries, the prospect of achieving a net-zero (or even net-negative) emissions world is beginning to come into focus. Our current understanding, however, is that the climate will not immediately stabilise everywhere under such conditions. Understanding where and how the climate will change in a net-zero world is important for adaptation planning and goal setting. Climate change emergence techniques are useful for quantifying change relative to what people and ecosystems are accustomed to. However, to date these techniques have been little used to assess climate change at and beyond net-zero emissions. Whether or not aspects of the climate system “de-emerge” and return to within baseline variability remains under-explored. In this work, we use CMIP6 models to quantify climate change emergence in terms of signal-to-noise for annual- and seasonal-average temperature and precipitation, as well as strength and position of the eddy-driven jets.

Applying this framework, we calculate the rate and extent of de-emergence that occurs when carbon dioxide concentrations fall. We first combine results from multiple models participating in the Carbon Dioxide Removal Model Intercomparison Project (CDRMIP) to establish global and regional behaviour for these variables under an idealised rising/falling CO₂ scenario. We then apply the same analysis to multiple models’ results for ScenarioMIP emissions pathways with net-negative CO₂ emissions (SSP1-1.9, SSP1-2.6, SSP4-3.4, and SSP5-3.4). We find that both temperature and precipitation exhibit partial reversibility on the scale of decades to centuries, albeit with significant hysteresis due to lag effects. These patterns are clearly apparent in the CDRMIP results and less so for the SSPs. There are significant regional differences in the rate and extent of de-emergence, including a strong land-sea contrast. The jet parameters, in contrast, respond quickly to greenhouse gas and other forcings, and so do not exhibit comparable hysteresis. Those models with data extending beyond 2100 allow for better quantification of de-emergence. CO₂ peak concentrations and rates of change both influence the stable climate state, though disentangling these factors remains challenging.