The January 2022 Hunga eruption cooled the southern hemisphere in 2022

Abstract

Abstract Explosive volcanic eruptions can profoundly cool Earth's climate by injecting sulfate aerosols into the stratosphere1. However, the submarine explosive eruption of Hunga Volcano in 2022 was unusual in that it injected into the stratosphere a massive amount of water vapor2,3, which warms the climate, and a much smaller amount of sulfur dioxide4 than previous explosive eruptions of similar magnitude. It has therefore been proposed that the Hunga eruption produced a net warming effect due to enhanced stratospheric water vapor, thereby increasing the chances that Earth’s temperature would temporarily breach the 1.5° C threshold specified in the Paris Climate Accord5. However, accounting for the cooling produced by sulfate aerosols is crucial in understanding the effects of Hunga eruption6. Here, we combine satellite observations of stratospheric composition with idealized radiative transfer model simulations to show that the Hunga eruption produced a net instantaneous clear-sky radiative energy loss of -0.48 ± 0.04 Wm-2 at the top-of-atmosphere in the southern hemisphere, resulting from its effects on stratospheric water vapor, aerosols, and ozone. Using an emulator of a two-layer energy balance model7-8, we estimate that this energy loss resulted in a cooling of -0.1 K in the southern hemisphere at the end of 2022 following the eruption. We find that the cooling produced by sulfate aerosols due to the scattering of sunlight overwhelmed the warming by stratospheric water vapor. This occurred in part because the sulfur dioxide turning into sulfate aerosols affecting optical depth was unusually efficient compared to previous subaerial eruptions. We also find that the decreased stratospheric ozone led to a cooling effect that nearly balanced the warming caused by increased stratospheric water vapor. We thus conclude that the Hunga eruption did not warm5, but rather cooled the planet with a strong hemispherical asymmetry.