The January 2022 Hunga eruption cooled the southern hemisphere in 2022 and 2023

The 2022 Hunga volcanic eruption injected a significant quantity of water vapor into the stratosphere while releasing only limited sulfur dioxide. It has been proposed that this excess water vapor could have contributed to global warming, potentially pushing temperatures beyond the 1.5 °C threshold of the Paris Climate Accord. However, given the cooling effects of sulfate aerosols and the contrasting impacts of ozone loss (cooling) versus gain (warming), assessing the eruption's net radiative effect is essential. Here, we quantify the Hunga-induced perturbations in stratospheric water vapor, sulfate aerosols, and ozone using satellite observations and radiative transfer simulations. Our analysis shows that these components induce clear-sky instantaneous net radiative energy losses at both the top of the atmosphere and near the tropopause. In 2022, the Southern Hemisphere experienced a radiative forcing of -0.55 ± 0.05 W m⁻² at the top of the atmosphere and -0.52 ± 0.05 W m⁻² near the tropopause. By 2023, these values decreased to -0.26 ± 0.04 W m⁻² and -0.25 ± 0.04 W m⁻², respectively. Employing a two-layer energy balance model, we estimate that these losses resulted in cooling of about -0.10 ± 0.02 K in the Southern Hemisphere by the end of 2022 and 2023. Thus, we conclude that the Hunga eruption cooled rather than warmed the Southern Hemisphere during this period.