Points of no return to respect the Paris Agreement

The physical reality of the Earth system implies that there are clear conditions to respect the Paris agreement, or to limit any climate impact below a certain level. To be policy relevant, these conditions should be expressed in terms of emission targets, such as peaking date, budget, or annual value of global CO2 emissions. They have been explored by the IPCC using integrated assessment models. However, past work has focused on bottom-up scenarios, and on temperature as the only metric evaluating climate impacts, even though not all impacts are linearly related to it.

Here, we show that for these emission targets, across thousands of scenarios we have generated, there are points of no return after which limiting a given climate impact becomes geophysically infeasible. In addition to the Paris Agreement objectives (consisting of a 1.5 °C temperature target above pre-industrial (PI) era possibly overshot by no more than 0.5°C), we investigate three other climate targets:  ocean acidification, sea-level elevation rate, and Arctic sea-ice melting. We use a newly developed model called PathFinder; a reduced-form carbon-climate model that also emulates the three global climate impacts we investigate. The model is calibrated through Bayesian inference, using outputs from the state-of-the-art CMIP6 models as prior parameters, and the latest IPCC assessment and observations of the Earth system as constraints. This advanced calibration is enabled by the model’s capacity of using temperature and atmospheric CO2 concentration as inputs (instead of anthropogenic emissions and non-CO2 radiative forcing).

Thanks to this backward approach, we demonstrate that, for every emission target considered, the combination of climate impact targets is non-linear. While the Paris Agreement insists on the importance of reaching a carbon neutral world in 2050, our results show that global CO2 emissions must peak before 2030 but do not have to reach net-zero to keep all targets reachable with at least 50% chances. We also highlight the inevitable role of geoengineering technologies in reaching the Paris Agreement, as chances to keep it reachable goes from at least 69% if SRM or CDR are available to 10% if none of them is.