Climate change modulates the stratospheric volcanic sulfate aerosol lifecycle and radiative forcing from tropical eruptions

Explosive volcanic eruptions that inject sulfur gases into the stratosphere significantly influence climate variability. The resulting sulfate aerosols scatter sunlight, causing negative radiative forcing and surface cooling, lasting 1–3 years. Large-magnitude eruptions (VEI >5, injecting >10 Tg SO2) are infrequent but profoundly affect climate, as exemplified by Mount Tambora (1815) and Mount Pinatubo (1991). Even moderate-magnitude eruptions (VEI 3–5, injecting <1 Tg SO2) can impact Earth's radiation balance. While the climatic effects of eruptions are well-studied, the impact of climate change on these eruptions and potential feedback loops remain less understood. The study's primary focus is on how climate change might affect the three main processes that control this interaction: 1) changes in the spatial, temporal, and magnitude distribution of eruptions; 2) the effect of the background climate state on the response to eruptions; and 3) the direct impact of climate change on the volcanic sulfate aerosol cycle and associated radiative forcing. Recent advances in interactive stratospheric aerosol modeling and eruptive column models provide the necessary tools for this investigation. This study utilizes these advances to investigate how climate change will specifically affect the stratospheric volcanic sulfate aerosol lifecycle and radiative forcing, using a combined modeling approach.

Existing research has primarily focused on how changing climatic conditions affect the distribution of explosive eruptions, particularly linking increased frequency and magnitude to ice-covered volcano deglaciation. However, these feedbacks are not relevant on the timescale of current climate projections. Other proposed mechanisms impacting eruption distribution, such as changes in the hydrological cycle, remain debated. Studies investigating how the background climate state affects the climate response to volcanic eruptions have shown mixed results, with some suggesting enhanced cooling of the upper ocean and surface air and others indicating dampened surface cooling. Prior research exploring the impact of climate change on the stratospheric volcanic sulfate aerosol lifecycle has shown that the injection altitude is climate-sensitive but hasn't quantified downstream impacts on radiative forcing and surface temperatures. The impact of climate change on other processes governing stratospheric volcanic sulfate aerosol properties is largely unexplored. The development of interactive stratospheric aerosol modeling, coupled with improved eruptive column models, enables a more thorough investigation of these complexities.

This study employs a coupled modeling approach combining the UKESM1 global climate model (in an atmosphere-only configuration) with a one-dimensional (1D) eruptive column model. The UKESM1 model incorporates an interactive stratospheric aerosol module to simulate aerosol lifecycle processes and radiative effects. The 1D eruptive column model predicts SO2 injection heights, considering eruption intensity and UKESM1-simulated meteorological profiles. The study focuses on two eruption scenarios: a moderate-magnitude eruption (1 Tg SO2) and a large-magnitude eruption (10 Tg SO2), both simulated under present-day (1990–2000) and high-end future (2090–2100, SSP5-8.5) climate scenarios. Ten ensemble members are run for each scenario to account for natural variability, focusing on SO2 injection height and Quasi-Biennial Oscillation (QBO) phase. To separate the impact of injection height changes from other changes in the aerosol life cycle, additional simulations were run with future climate conditions but historical injection heights. Analysis includes examining changes in SO2 injection height, stratospheric aerosol optical depth (SAOD), radiative forcing at the top of the atmosphere (TOA) and the surface, sulfur mass mixing ratio, aerosol effective radius, Brewer-Dobson circulation speed, aerosol nucleation rate, and temperature responses in the stratosphere and troposphere. The Finite Amplitude Impulse Response (FaIR) model was used to estimate surface temperature anomalies from the TOA radiative forcing time series. Statistical significance of differences between scenarios was assessed using non-parametric Mann-Whitney U-tests.

For moderate-magnitude eruptions, the peak monthly global-mean SAOD anomaly decreased by a factor of four in the future climate scenario. This was primarily attributed to increased tropopause height, while the SO2 injection height remained unchanged. The resulting reduction in stratospheric SO2 injections led to significantly lower TOA and surface radiative forcing. In contrast, for large-magnitude eruptions, the peak global-mean SAOD increased by around 10%, with increases in both TOA and surface radiative forcing. Analysis showed that the increase in radiative forcing was a result of two competing effects: 1) decreased aerosol size and lifetime, primarily due to the accelerated Brewer-Dobson circulation; and 2) increased SO2 injection height resulting from decreased stratospheric stratification. The reduction in aerosol size led to more efficient scattering, while the faster Brewer-Dobson circulation resulted in faster aerosol removal. The enhanced radiative forcing from large-magnitude eruptions resulted in amplified stratospheric warming, tropospheric cooling, and surface cooling, with changes being most pronounced when considering changes in SO2 injection height. The study also found a faster decay of SAOD and radiative forcing in the future climate. Analysis of aerosol effective radius indicated a smaller size in the future climate, contributing to the observed changes in SAOD and radiative forcing. These changes in effective radius were linked to the accelerated Brewer-Dobson circulation, affecting aerosol transport to higher latitudes and impacting aerosol growth rates. The study also highlighted a stronger correlation between tropical sulfur burden e-folding time and aerosol effective radius, suggesting the residence time in the tropical pipe as a dominant driver of effective radius variability.

The findings show that climate change differentially affects the radiative forcing from volcanic eruptions. For moderate-magnitude eruptions, the effects are dampened due to the rise in the tropopause height. Conversely, for large-magnitude eruptions, the effects are amplified due to changes in aerosol properties and increased injection height. The acceleration of the Brewer-Dobson circulation plays a key role in these differences, altering aerosol lifetime and size distribution. The results demonstrate the importance of considering both eruptive column dynamics and aerosol lifecycle when assessing climate-volcano interactions. The amplified temperature response to large-magnitude eruptions in a warmer climate has significant implications for climate variability. This challenges the use of constant volcanic forcing in future climate projections and highlights the need for improved models that account for climate-volcano feedbacks. These results are also of interest to geoengineering research, demonstrating how small changes in aerosol lifetime can significantly affect radiative forcing.

This study shows that climate change will modulate the climatic effects of volcanic eruptions differently depending on eruption magnitude. Moderate eruptions will exhibit reduced radiative forcing, whereas large eruptions will have amplified effects. The acceleration of the Brewer-Dobson circulation is identified as a key driver of these changes. Future work should focus on improving coupled ocean-atmosphere models and incorporating a statistically realistic eruption distribution to better quantify the net effects of climate-volcano interactions across all eruption types. Similar studies investigating extra-tropical eruptions and past climate conditions are needed to further improve understanding of these complex interactions.

The study utilized atmosphere-only simulations, neglecting ocean feedbacks. The 1D eruptive column model simplified the representation of plume dynamics, and the co-injection of other volcanic products besides SO2 was not considered. The use of a simple climate model (FaIR) for surface temperature estimation presents a potential source of uncertainty. While the study incorporates ensemble simulations to account for natural variability, inherent uncertainties in model parameterizations and the complex interplay of factors affecting the aerosol cycle remain.