Indirect stratospheric moisture increase after a Pinatubo-magnitude eruption can be comparable to direct increase after 2022 Hunga

Kurzfassung

Stratospheric water vapour is an important greenhouse gas and plays a key role in stratospheric chemistry1–4. Explosive volcanic eruptions can enhance the stratospheric moisture content both directly and indirectly. In the direct pathway, moisture is directly injected into the stratosphere from within the volcanic plume leading to anomalies in water vapour concentrations that are initially very large and highly localized. For the vast majority of direct injections, the corresponding moisture increases observed in the satellite era were negligible and short-lived5,6. However, it was proposed that direct moisture injections caused by explosive eruptions that entrain sea-water could lead to substantial and immediate increases in stratospheric moisture mass7. In 2022 the eruptions of Hunga—a submarine volcano—led to a substantial and clearly observable water vapour increase in the stratosphere via the direct pathway for the first time. Estimates for the directly injected water vapour include 139 ± 88, 146 ± 59, 5010 and 70–150 Tg11, which is approximately equal to 5–10% of the stratospheric background water vapour burden. In contrast, the observed emitted sulfur dioxide (SO2) mass amounts to only 0.6–1.0 Tg SO212,13. Even when considering potential lowbiases in the observational-based SO2 estimates caused by the speedup of aerosol formation due to the increased availability of moisture within the plume14,15, the injected water vapour amount is still at least one order of magnitude larger than the SO2 amount. The interaction between sea-water and magma (phreato-magmatic eruption) also enhanced the explosivity of the Hunga eruptions, with the volcanic plume reaching a maximum altitude of 57 km16. In the first three weeks after the eruption, changes in infra-red forcing attributable to the enhanced water vapour at altitudes above 25 km led to the descent of the plume to lower altitudes in the stratosphere17,18.