The January 2022 Hunga Eruption Cooled the Southern Hemisphere in 2022 and 2023

The 2022 Hunga volcanic eruption injected a massive amount of water vapor into the stratosphere, with less sulfur dioxide than past eruptions. It has been proposed that this eruption could have warmed the Earth due to enhanced water vapor, possibly pushing temperatures past the 1.5°C Paris Climate Accord. However, considering the cooling impact of sulfate aerosols and ozone loss, and the warming effect of ozone gain is crucial to understand the eruption's effects. We combined satellite observations with radiative transfer simulations to show that the perturbed stratospheric water vapor, sulfate aerosol (with their sizes in the 0.27-0.30 µm radius range), and ozone associated with the Hunga eruption caused a combined clear-sky instantaneous net radiative energy loss of -0.38 ± 0.04 Wm^-2 and -0.01 ± 0.01 Wm^-2 at the top of the atmosphere in the Southern Hemisphere for the first and second year following the eruption, respectively. Using a two-layer energy balance model emulator, we estimated that this energy loss led to a cooling of ~-0.05 ± 0.02 K in the Southern Hemisphere at the end of 2022 and 2023, respectively. Therefore, we conclude that the Hunga eruption did not warm but rather cooled the Southern Hemisphere in 2022 and 2023.