High probability of triggering climate tipping points under current policies modestly amplified by Amazon dieback and permafrost thaw

Abstract. We investigate the probabilities of triggering climate tipping points under five Shared Socioeconomic Pathways (SSPs) and how they are altered by including the additional carbon emissions that could arise from tipping points within the Earth's carbon cycle. The crossing of a climate tipping point at a threshold level of global mean surface temperature (threshold temperature) would commit the affected subsystem of the Earth to abrupt and largely irreversible changes with negative impacts on human well-being. However, it remains unclear which tipping points would be triggered under the different SSPs due to uncertainties in the climate sensitivity to anthropogenic greenhouse gas emissions, the threshold temperatures and timescales of climate tipping points, and the response of tipping points within the Earth's carbon cycle to global warming. We include those uncertainties in our analysis to derive probabilities of triggering for 16 previously identified climate tipping points within the Earth system. To conduct our analysis, we use the reduced complexity climate model FaIR (Finite amplitude Impulse Response) which is coupled to a conceptual model of the tipping processes within the Amazon rainforest and permafrost, which are the two major tipping points within the Earth's carbon cycle. Uncertainties are propagated by employing a Monte Carlo approach for the construction of large model ensembles. We find that carbon tipping points increase the risk for high-temperature pathways, but on average their warming effect remains small, with its median staying 1 order of magnitude lower than the median anthropogenic warming for all SSPs. Therefore, they have low potential to increase the probability of triggering other tipping points. The maximum triggering probability increase from carbon tipping points among all SSPs occurs under SSP2-4.5, with a 3 percentage point increase averaged over all tipping points. The warming trajectory expected from current policies compares best to SSP2-4.5, which we find to be unsafe with regard to triggering climate tipping points. Our most conservative estimate of triggering probabilities averaged over all tipping points is 62 % under SSP2-4.5, and nine tipping points have a more than 50 % probability of getting triggered. Under SSP1-2.6 and SSP1-1.9, the risk of triggering climate tipping points is reduced significantly compared to SSP2-4.5; however, it also remains less constrained since the behaviour of climate tipping points in the case of a temperature overshoot is still highly uncertain.