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 temperature projections reaches up to 49% and is comparable or greater than that from internal variability. Furthermore, compared to a constant volcanic forcing, employing a stochastic volcanic forcing i) reduces the probability to exceed 1.5 ºC warming above pre-industrial level by up to 10%, ii) enhances the probability of negative decadal temperature trends, and iii) increases the likelihood of short-term surface cooling and warming events. Consequently, using a stochastic volcanic forcing approach enables assessment of the associated climate risks and socio-economic impacts. We recommend improved volcanic forcing approaches for future climate model experiments.