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Excessive exposure to ultraviolet (UV) radiation harms humans and ecosystems. The level of surface UV radiation had increased due to declines in stratospheric ozone in the late 1970s in response to emissions of chlorofluorocarbons. Following the implementation of the Montreal Protocol, the stratospheric loading of chlorine/bromine peaked in the late 1990s and then decreased; subsequently, stratospheric ozone and surface UV radiation would be expected to recover and decrease, respectively. Here, we show, based on multiple data sources, that the May–September surface UV radiation in the tropics and Northern Hemisphere mid-latitudes has undergone a statistically significant increasing trend [about 60.0 J m −2 (10 yr) −1 ] at the 2σ level for the period 2010–20, due to the onset of total column ozone (TCO) depletion [about −3.5 DU (10 yr) −1 ]. Further analysis shows that the declines in stratospheric ozone after 2010 could be related to an increase in stratospheric nitrogen oxides due to increasing emissions of the source gas nitrous oxide (N 2 O).

The stratospheric ozone layer, which prevents solar ultraviolet radiation from reaching the surface and thereby protects life on earth, is expected to recover from past depletion during this century due to the impact of the Montreal Protocol. However, how the ozone column over the Arctic will evolve over the next few decades is still under debate. In this study, we found that the ozone level in the Arctic stratosphere at 100–150 hPa during 1998–2018 exhibits a decreasing trend of − 0.12 ± 0.07 ppmv decade –1 from MERRA2, suggesting a continued depletion during this century. About 30% of this ozone depletion is contributed by the second leading mode of sea surface temperature anomalies (SSTAs) over the North Pacific with one month leading and therefore is dynamical in origin. The North Pacific SSTAs associated with this mode tend to result in a weakened Aleutian low, a strengthened Western Pacific pattern and a weakened Pacific–North American pattern, which impede the upward propagation of wavenumber-1 waves into the lower stratosphere. The changes in the stratospheric wave activity may result in decreased ozone in the Arctic lower stratosphere through weakening the Brewer-Dobson circulation. Our findings uniquely linked the recent ozone depletion in the Arctic stratosphere to the North Pacific SSTs and might provide new understanding of how dynamical processes control Arctic stratospheric ozone.

Polar stratospheric clouds (PSCs) play a crucial role in ozone depletion in the polar stratosphere. In this study, the space-based PSCs record from CALISPO and an offline three-dimensional chemical transport model (SLIMCAT) are used to analyze the PSCs in the Arctic and the Antarctic for the period 2006−2021. Observations indicate that the seasonal evolution of the Antarctic PSC area is similar from year to year. In contrast, the Arctic PSCs show large differences in seasonal variations of coverage and duration in different years. The SLIMCAT simulations effectively capture the seasonal and interannual variations of PSCs. However, the simulated PSC areas are larger than CALIPSO observations, which can be attributed to the relatively high instrumental detection threshold of CALIPSO. SLIMCAT can capture the zonal asymmetry of PSCs in both the Antarctic and Arctic, and it can reproduce a more accurate spatial distribution of PSCs when the PSC coverage area is larger. In addition, accurate simulation of HNO3 is important for PSC simulation. Because the simulation of denitrification processes is poor in SLIMCAT, which uses the thermodynamic equilibrium PSC scheme, the PSCs modeled by SLIMCAT are located at higher altitudes compared to the observation in the Antarctic, where the denitrification processes are strong. In contrast, for ice PSCs of which HNO3 is not required in calculations and the Arctic where denitrification is weak, the simulated PSC at different altitudes closely matches the observations.

The January 2022 eruption of Hunga Tonga‐Hunga Ha'apai (HTHH) injected a huge amount (∼150 Tg) of water vapor (H2O) into the stratosphere, along with small amount of SO2. An off‐line 3‐D chemical transport model (CTM) successfully reproduces the spread of the injected H2O through October 2023 as observed by the Microwave Limb Sounder. Dehydration in the 2023 Antarctic polar vortex caused the first substantial (∼20 Tg) removal of HTHH H2O from the stratosphere. The CTM indicates that this process will dominate removal of HTHH H2O for the coming years, giving an overall e‐folding timescale of 4 years; around 25 Tg of the injected H2O is predicted to still remain in the stratosphere by 2030. Following relatively low Antarctic column ozone in midwinter 2023 due to transport effects, additional springtime depletion due to H2O‐related chemistry was small and maximized at the vortex edge (10 DU in column). Plain Language Summary Around 150 Tg (150 million tons) of water vapor was injected into the stratosphere during the eruption of Hunga Tonga‐Hunga Ha'apai. Water vapor is a greenhouse gas and this increase is expected to have a warming effect in the troposphere, as well causing perturbations in stratospheric chemistry and aerosols. We use an atmospheric model to study the residence time of this excess water vapor and its impact on the recent Antarctic ozone hole. The model performance is evaluated by comparison with satellite measurements. Wintertime dehydration in the Antarctic stratosphere in 2023 is found to be an important mechanism for removal of the volcanic water from the stratosphere. However, the overall removal rate is predicted to be slow; around 25 Tg (17%) is still present in 2030. The direct impact of the excess water vapor on ozone via chemical processes in the Antarctic ozone hole in 2023 is small. Key Points Antarctic dehydration is a major removal pathway of stratospheric H2O injected from Hunga Tonga‐Hunga Ha'apai (HTHH) eruption HTHH H2O caused small (up to 10 DU) additional chemical ozone depletion in 2023 Antarctic spring Model indicates e‐folding timescale of 4 years for removal of HTHH H2O from stratosphere

Substantial and prolonged enhancements in stratospheric water vapor (SWV) have occurred after large‐magnitude explosive tropical volcanic eruptions, with modified tropopause entry caused by aerosol‐absorptive heating. Here, we analyze the timing and longevity of heating‐driven post‐eruption SWV changes within CMIP6‐VolMIP short‐term climate‐response experiments with the UK Earth System Model (UKESM1). We find aerosol‐absorptive heating causes peak SWV increases of 17% (∼1 ppmv) and 10% (0.5 ppmv) at 100 and 50 hPa, at ∼18 and ∼23 months after a Pinatubo‐like eruption, respectively. We track the temperature response in the tropical lower stratosphere and identify the main SWV increase occurs only after the descending aerosol heating reaches the tropopause, suggesting a key role for aerosol microphysical processes (sedimentation rate). We explore how El Niño–Southern Oscillation variability modulates this effect. Post‐eruption SWV increases are ∼80% stronger for the La Nina phase compared to the ensemble mean. Tropical upwelling strongly mediates this effect. Plain Language Summary Strong volcanic eruptions, such as the 1991 eruption of Mt Pinatubo, inject a large amount of SO2 directly into the stratosphere, thereby enhancing the stratospheric aerosol layer and causing a short‐term climatic perturbation. Another substantial part of the climatic influence is the change in stratospheric water vapor (SWV), which affects the chemical processes and the radiative budget of the atmosphere. Along with near‐instantaneous injection of water vapor into the stratosphere, volcanic eruptions can indirectly enhance the entry of water vapor into the stratosphere through aerosol‐induced tropopause heating. This work analyses Earth system model experiments designed to explore how volcanic impacts combine with internal climate variability. We find that peak SWV entry mixing ratios occur only within the second post‐eruption year, consistent with the substantially lagged timing of SWV increase seen in post‐Pinatubo satellite measurements. This analysis provides a new perspective on the temporal evolution of the observed post‐Pinatubo SWV increase and an improved quantification of its impacts. Key Points Aerosol‐induced absorptive‐heating increases stratospheric water vapor (SWV) by up to 17% at 18 months post‐eruption in a Pinatubo‐like experiment Analyzing simulations by El Niño–Southern Oscillation (ENSO) variability show an 80% larger peak SWV increase occurs if an eruption is followed by a La Niña phase The timing of peak SWV increase occurs when volcanic‐aerosol‐induced heating reaches the tropopause, with ENSO modulation of tropical upwelling

Severe vortex-wide ozone loss in the Arctic would expose both ecosystems and several millions of people to unhealthy ultraviolet radiation. Adding to these worries, and extreme events as the harbingers of climate change, exceptionally low ozone with column values below 220 DU occurred over the Arctic in March and April 2020. Sporadic occurrences of low ozone with less than 220 DU at different regions of the vortex for almost 3 weeks were found for the first time in the observed history in the Arctic. Furthermore, a large ozone loss of about 2.0–3.4 ppmv triggered by an unprecedented chlorine activation (1.5–2.2 ppbv) matching the levels occurring in the Antarctic was also observed. The polar processing situation led to the first-ever appearance of loss saturation in the Arctic. Apart from these, there were also ozone-mini holes in December 2019 and January 2020 driven by atmospheric dynamics. The large loss in ozone in the colder Arctic winters is intriguing and demands rigorous monitoring of the region.

It has been proposed that two isomers of the SO dimer (cis‐ and trans‐OSSO) are candidates for the unknown UV absorber in Venus' atmosphere because they have a good spectral match with the absorber, despite the low concentrations predicted by 1D photochemical models. Here OSSO chemistry (production from SO and loss by photolysis, thermal decomposition, and reaction with O and Cl) has been included in the photochemistry scheme of a 3D planetary climate model (PCM‐Venus) along with sulfur injection due to meteoric ablation. 1D multiple scattering radiative transfer modeling is then used to predict the resulting top‐of‐the‐atmosphere reflectance produced by OSSO. The modeled OSSO concentrations are shown to be ∼3 orders of magnitude too low to explain the observed absorbance levels, and the predicted ratio of the OSSO isomers provides an unsatisfactory match to the spectral shape of the unknown absorber.

We use the 3‐D Whole Atmospheric Community Climate Model to investigate stratospheric ozone depletion due to the launch of small satellites (e.g., CubeSats) with an iodine propulsion system. The model considers the injection of iodine from the satellites into the Earth's thermosphere and suggests a 4‐yr timescale for transport of the emissions down to the troposphere. The base case scenario is 40,000 small satellite launches per year into low orbit (100–600 km), which would inject 8 tons I yr−1 above 120 km as I+ ions and increase stratospheric inorganic iodine by ∼0.1 part per trillion (pptv). The model shows that this scenario produces a negligible impact on global stratospheric ozone (∼0.05 DU column depletion). In contrast, a 100‐fold increase in the launch rate, and therefore thermospheric iodine injection, is predicted to result in modeled ozone depletion of up to 14 DU (approximately 2%–7%) over the polar regions. Plain Language Summary Iodine has the potential to cause stratospheric ozone depletion. Small satellites (<10 kg) in low Earth orbit require electric propulsion to prolong their time in orbit, and there is strong interest in replacing the rare gas propellant (Xe or Kr) with I2. Here we estimate the potential impact of thermosphere iodine injection from such satellites on stratospheric ozone. Since the demand for small satellites could increase greatly in the future, it is important to understand the potential risks to ozone depletion if iodine propulsion systems are used. Assuming a scenario in which 40,000 small satellites are launched each year into relatively low orbits and each satellite is assumed to contain 200 g I2, this could inject 8 tons/year of iodine ions into the upper atmosphere (above 120 km). Using a 3‐D atmospheric chemistry‐climate model, we show that the perturbation to the total column ozone is very small, decreasing by 0.05 DU (<0.02%) globally and 0.2 DU (<0.1%) for September/October in the Antarctic polar region. However, a 100‐fold increase in the number of launches and therefore mass of iodine emitted into near‐Earth orbit is predicted to cause significant ozone depletion up to 14 DU (5.6%) in the Antarctic spring. Key Points The effect of injecting iodine into the atmosphere from I2‐propelled satellites has been modeled using an atmosphere chemistry‐climate model Injection of 8 tons I yr−1, based on the expected launch rate, is predicted to cause negligible O3 depletion (0.05 DU) globally O3 depletion increases near‐linearly with iodine mass injected, reaching up to 14 DU in the polar regions for 800 tons I yr−1

The Montreal Protocol has succeeded in limiting major ozone-depleting substance emissions, and consequently stratospheric ozone concentrations are expected to recover this century. However, there is a large uncertainty in the rate of regional ozone recovery in the Northern Hemisphere. Here we identify a Eurasia-North America dipole mode in the total column ozone over the Northern Hemisphere, showing negative and positive total column ozone anomaly centres over Eurasia and North America, respectively. The positive trend of this mode explains an enhanced total column ozone decline over the Eurasian continent in the past three decades, which is closely related to the polar vortex shift towards Eurasia. Multiple chemistry-climate-model simulations indicate that the positive Eurasia-North America dipole trend in late winter is likely to continue in the near future. Our findings suggest that the anticipated ozone recovery in late winter will be sensitive not only to the ozone-depleting substance decline but also to the polar vortex changes, and could be substantially delayed in some regions of the Northern Hemisphere extratropics. Climate change can exert a significant effect on the ozone recovery. Here, the authors show that the Arctic polar vortex shift associated with Arctic sea-ice loss could slow down ozone recovery over the Eurasian continent.

The growth in atmospheric methane (CH4) concentrations over the past 2 decades has shown large variability on a timescale of several years. Prior to 1999 the globally averaged CH4 concentration was increasing at a rate of 6.0 ppb yr−1, but during a stagnation period from 1999 to 2006 this growth rate slowed to 0.6 ppb yr−1. From 2007 to 2009 the growth rate again increased to 4.9 ppb yr−1. These changes in growth rate are usually ascribed to variations in CH4 emissions. We have used a 3-D global chemical transport model, driven by meteorological reanalyses and variations in global mean hydroxyl (OH) concentrations derived from CH3CCl3 observations from two independent networks, to investigate these CH4 growth variations. The model shows that between 1999 and 2006 changes in the CH4 atmospheric loss contributed significantly to the suppression in global CH4 concentrations relative to the pre-1999 trend. The largest factor in this is relatively small variations in global mean OH on a timescale of a few years, with minor contributions of atmospheric transport of CH4 to its sink region and of atmospheric temperature. Although changes in emissions may be important during the stagnation period, these results imply a smaller variation is required to explain the observed CH4 trends. The contribution of OH variations to the renewed CH4 growth after 2007 cannot be determined with data currently available.