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Abstract 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. 

Abstract On 15 January 2022, Hunga volcano erupted, creating an extensive and high-reaching umbrella cloud over the open ocean, hindering traditional isopach mapping and fallout volume estimation. In MODIS satellite imagery, ocean surface water was discolored around Hunga following the eruption, which we attribute to ash fallout from the umbrella cloud. By relating intensity of ocean discoloration to fall deposit thicknesses in the Kingdom of Tonga, we develop a methodology for estimating airfall volume over the open ocean. Ash thickness measurements from 41 locations are used to fit a linear relationship between ash thickness and ocean reflectance. This produces a minimum airfall volume estimate of$${1.8}_{-0.4}^{+0.3}$$ ${1.8}_{-0.4}^{+0.3}$km³. The whole eruption produced > 6.3 km³of uncompacted pyroclastic material on the seafloor and a caldera volume change of 6 km³DRE. Our fall estimates are consistent with the interpretation that most of the seafloor deposits were emplaced by gravity currents rather than fall deposits. Our proposed method does not account for the largest grain sizes, so is thus a minimum estimate. However, this new ocean-discoloration method provides an airfall volume estimate consistent with other independent measures of the plume and is thus effective for rapidly estimating fallout volumes in future volcanic eruptions over oceans. 

Abstract Bedrock weathering regulates nutrient mobilization, water storage, and soil production. Relative to the mobile soil layer, little is known about the relationship between topography and bedrock weathering. Here, we identify a common pattern of weathering and water storage across a sequence of three ridges and valleys in the sedimentary Great Valley Sequence in Northern California that share a tectonic and climate history. Deep drilling, downhole logging, and characterization of chemistry and porosity reveal two weathering fronts. The shallower front is ∼7 m deep at the ridge of all three hillslopes, and marks the extent of pervasive fracturing and oxidation of pyrite and organic carbon. A deeper weathering front marks the extent of open fractures and discoloration. This front is 11 m deep under two ridges of similar ridge‐valley spacing, but 17.5 m deep under a ridge with nearly twice the ridge‐valley spacing. Hence, at ridge tops, the fraction of the hillslope relief that is weathered scales with hillslope length. In all three hillslopes, below this second weathering front, closed fractures and unweathered bedrock extend about one‐half the hilltop elevation above the adjacent channels. Neutron probe surveys reveal that seasonally dynamic moisture is stored to approximately the same depth as the shallow weathering front. Under the channels that bound our study hillslopes, the two weathering fronts coincide and occur within centimeters of the ground surface. Our findings provide evidence for feedbacks between erosion and weathering in mountainous landscapes that result in systematic subsurface structuring and water routing.