Neglecting future sporadic volcanic eruptions underestimates climate uncertainty

Most climate projections represent volcanic eruptions as a constant forcing based on historical averages. This constant forcing approach ignores the sporadic nature of eruptions, preventing a full quantification of uncertainties in climate projections. Here we show that the contribution of volcanic forcing uncertainty to the overall uncertainty in global mean surface air temperature projections reaches up to 49%, and is comparable or greater than that from internal variability throughout the 21st century. Furthermore, compared to a constant volcanic forcing, employing a stochastic volcanic forcing (i) reduces the probability of exceeding 1.5 ºC warming above pre-industrial level by at least 5% for high climate mitigation scenario (SSP1-1.9) in this century; (ii) enhances the probability of negative decadal temperature trends by up to 8%; and (iii) increases the likelihood of short-term surface cooling and warming events. Intermediate to higher climate mitigation scenarios are particularly sensitive to the choice of volcanic forcing implementation in climate projections. Using a stochastic volcanic forcing approach also enables assessment of the associated climate risks and socio-economic impacts. We recommend improved volcanic forcing approaches for future climate model experiments.