Geophysical Research Letters

Plain Language Summary Marine sediments represent the largest organic carbon (OC) sink on Earth. Preservation of OC in marine sediments, often associated with reactive iron oxides (FeR), has garnered considerable attention for its potential to protect OC from degradation. The majority of marine OC bound to FeR is found in continental margin sediments, where a significant part of FeR undergoes extensive sulfidization to form authigenic iron sulfide minerals. However, the stability of OC bound to FeR during the diagenetic transformation of iron phases in marine sulfidic sediments remains poorly understood. Our study shows that a 42% decrease in FeR during early diagenesis leads to an only 6.3% reduction in OC bound to FeR in sulfidic sediments. This result suggests that bonding of OC to FeR is stable in marine sulfidic sediments during sulfidization of reactive iron oxides. Such a finding implies that bonding of OC with FeR may have been a key mechanism for OC burial in past anoxic oceans, where euxinic conditions and sulfidization of reactive iron oxides occurred more commonly. Our findings thus have far‐reaching implications to enhance our understanding of the crucial role of OC bound to FeR within the geological carbon cycle.

Dipolarization fronts (DFs) have been widely reported in the Earth's magnetotail and are suggested to play an important role in energy conversion. Magnetic holes (MHs) are also usually observed near DFs, and recent spacecraft observations suggest that they can be excited by interchange instability (ICI). However, whether the MHs near DFs could contribute to energy conversion is still unknown. Here, by using the Magnetospheric Multiscale mission observations, we find a sub‐ion scale MH behind a DF. We present a two‐dimensional illustration of the MH, revealing that such an MH was generated by the ICI. Inside this MH, a significant energy conversion up to ∼2 nW/m³ (higher than typical observations near DFs) is caused by the local electron vortex current inside the MH and the background electric field on the DF. This study improves our understanding of energy injection during substorms and energy conversion near DFs.

Plain Language Summary Recent advances have demonstrated the importance of spatial patterns in tropical sea surface warming for determining how Earth's tropical energy balance responds to climate change. Sensitivity of the energy balance is higher when warming is concentrated in regions where air is generally ascending, such as the west Pacific, than in regions where air is generally descending, such as in the east Pacific. This variation in sensitivity across regions depends on the degree to which surface warming is communicated to the upper atmosphere, and subsequently whether low clouds brighten in the east Pacific. What is less well understood is how the atmospheric circulation—the movement of air—responds to these surface warming patterns, and how these circulation changes may be coupled to clouds and energy balance. Using climate simulations where only a specified patch of the tropical ocean is warmed, we demonstrate that if ascent regions are directly warmed these regions tend to contract in area. But there is little change in circulation if descent regions are warmed. We develop a simple conceptual model which provides insight into the mechanisms of these circulation changes and demonstrate that cloud changes can be decomposed into the responses from individual circulation regimes.

Plain Language Summary The space hurricane is a cyclonic auroral structure over the Earth's polar cap region, under northward interplanetary magnetic field conditions. During otherwise extremely quiet geomagnetic periods, it can inject a substantial amount of energy and particles into the polar upper atmosphere. Joule heating plays a crucial role in the energy budget, leading to significant atmospheric disturbances. Utilizing in situ observations from Defense Meteorological Satellite Program satellites and the Gravity Field and Steady‐State Ocean Circulation Explorer satellite, we conducted a statistical survey that investigates how space hurricanes disturb the polar thermosphere. Space hurricanes influence the average neutral horizontal wind, which displays a pattern similar to clockwise plasma convection. Enhanced Joule heating, caused by increased electric fields and Pedersen conductance associated with space hurricanes, generates notable disturbances in neutral density and vertical winds within the polar cap. These findings reveal the thermospheric characteristics of space hurricanes, which are of great significance for understanding polar ionosphere‐thermosphere coupling during quiet geomagnetic conditions.

Neural network emulators have become an invaluable tool for climate prediction tasks but do not have an inherent ability to produce equitable predictions (e.g., predictions which are equally accurate across different regions or groups of people). This motivates the need for explicit internal representations of fairness. To that end, we draw on methods for enforcing physical constraints in emulators and propose a custom loss function which punishes predictions of unequal quality across any prespecified regions or category, here defined using Human Development Index. This loss function weighs a standard error metric against another which captures inequity between groups, allowing us to adjust the priority of each. Our results show that emulators trained with our loss function provide more equitable predictions. We empirically demonstrate that an appropriate selection of an equity priority can minimize loss of performance, mitigating the tradeoff between accuracy and equity.

Using a decade of observations and chemistry‐climate model simulations (2014–2023), we highlight the key role of biosphere‐atmosphere interactions in driving late summer–autumn ozone pollution extremes over Southeast China during hot droughts. In the 2019 and 2022 droughts, stomatal closure in the Yangtze River Basin, caused by soil moisture deficits, led to ∼60% reductions in ozone deposition rates to vegetation, aligning with reduced photosynthesis inferred from satellite remote sensing of solar induced fluorescence. Ozone production increased due to higher isoprene emissions from heat stress, NOx‐rich airflow from North China, and enhanced solar radiation. Soil drought intensified temperatures and increased isoprene emissions by 27%, but these only had marginal impact on ozone (<5 ppbv) in South China, where ozone formation is NOx‐limited. Reduced ozone uptake by drought‐stressed vegetation played a dominant role, driving 10–20 ppbv increases in daily maximum 8‐hr average ozone concentrations and a threefold rise in events exceeding 100 ppbv.

Plain Language Summary Particles suspended in the atmosphere (aerosols) play a key role in cloud formation. These aerosol‐cloud interactions have a major but uncertain influence on climate. We compare four different ways to calculate aerosol‐cloud interactions in a numerical atmospheric model. We compare model results to observed changes in clouds measured from satellites during the Holuhraun eruption in Iceland in 2014, which released large amounts of volcanic gases forming atmospheric aerosols. We find that all four approaches reproduce the observed reduction in cloud droplet sizes during the eruption, but that they disagree on its intensity and its impacts on the Earth's energy budget. An earlier study found that aerosol‐cloud interactions did not significantly increase the amount of liquid water in the clouds; using a more recent version of the satellite observations we find that large increases are possible. We also show that the eruption's impacts on the Earth's energy budget strongly depend on non‐volcanic aerosols already present in the atmosphere: doubling non‐volcanic aerosols reduces the impacts by ∼30% ${\sim} 30\%$. Aerosol biases in climate models can be far greater, indicating that this could be a major source of uncertainty for aerosol‐cloud interactions and for understanding past, present and future climates.

In a 1.2‐m‐deep arctic permafrost pond, early‐summer bottom‐water renewal was dominated by thermal overturning circulation, rather than wind‐driven overturning or vertical turbulent mixing. Three high‐resolution current profilers measured turbulent dissipation rates. Three dense temperature logger arrays measured stratification. A turbulent surface mixed layer grew thicker with nightly cooling and thinner with daily warming. However, both day and night, turbulence was inhibited in a stratified layer that separated the surface mixed layer from the deeper pond. Nightly cooling, likely intensified in shallow regions of the pond, generated 10‐cm‐thick cold layers, which flowed down the sloping bed to renew bottom waters. A heat balance suggests sufficient flow to replace most bottom water each night. Groundwater flows were too slow to influence this circulation, but likely advected significant heat into sediments near the pond's western end. Bottom water renewal may influence greenhouse gas emissions and heat transport in the evolving permafrost landscape.

This study integrates data from all broadband seismic stations in Alaska and northwestern Canada in 1999–2022 to construct a shear‐wave velocity model for south‐central Alaska and northwesternmost Canada, using ambient noise wave propagation simulation and inversion. Our model reveals three key features, including (a) the presence of the subducting Yakutat slab with apparent velocity reductions near the trench and within its flat segment, (b) two slab segments beneath the Wrangell volcanic field, differing in steepness, depth, and seismic velocity, and aligning spatially with the northwestern and southeastern volcano clusters, and (c) the existence of slab windows between the Yakutat and Wrangell slabs and between the northwestern and southeastern portions of the Wrangell slab. Our findings reinforce that the Wrangell volcanoes are predominantly influenced by subduction‐related magmatism. Furthermore, the two slab windows could have induced asthenospheric upwelling, contributing to the volcanism in the Wrangell clustered volcanoes.

Titan's dense atmosphere, composed mainly of methane and nitrogen, maintains a methane cycle that shapes its surface. Like water on Earth, methane precipitation erodes Titan's surface, carving river networks at all latitudes, as revealed by the Cassini‐Huygens mission. On Earth, it is well known that laboratory and natural rivers exhibit a power‐law relationship between their bankfull geometry and water discharge, as described by the threshold theory. Here, we investigate this hydraulic‐geometric relationship on two rivers on Titan, one near the equator and the other at the south pole. We hypothesize that this relationship can be applied to any river, and test it for the first time on extraterrestrial rivers. Having shown that Titan's rivers are consistent with the threshold theory, we use this relationship to estimate river discharge from bankfull geometry. As a perspective, we then use these discharges to infer precipitation rates, which could help to better understand Titan's climate.

Plain Language Summary Sea surface salinity (SSS) changes result from of rainfall, evaporation and processes internal to the ocean over the course of a year. Global average SSS becomes greatest in March and reaches a minimum in September, indicating that fresh water is leaving and entering the surface ocean and being exchanged with land or the ocean interior. The magnitude of this exchange is equivalent to about 3 cm of water averaged over the global ocean. Evidence collected since satellite SSS observations began in 2010 indicate that the magnitude of the annual cycle's exchange rate has increased substantially, equating to about 1.0 cm of extra water. As the Earth warms, the atmosphere holds more water and is better able to evaporate it from the ocean surface and transport it onto land. Consequently, the increasing seasonality of SSS is a direct consequence of climate change and an indicator of an accelerating global water cycle.

There is no basic explanation for soil moisture variability in the current climate, and models diverge on the sign of expected changes in a warming world. Here, we present a diagnostic physical theory for soil moisture at large scales. The theory is radically simpler than published alternatives, dependent only on precipitation and surface net radiation with no free parameters. Minor variations improve its performance. The theory answers two basic questions: (a) Why does soil moisture exhibit a W‐shaped latitudinal profile, even though precipitation over land does not? Poleward declines in net radiation resolve this discrepancy. (b) Why does soil moisture decrease with warming in some regions where precipitation increases? The theory predicts this phenomenon where fractional increases in net radiation exceed those in precipitation. Common alternative mechanisms, which invoke changes in vapor pressure deficit or plant responses to CO2 ${\text{CO}}_{2}$, are inessential to explaining first‐order changes in soil moisture with warming.

We investigated the occurrence and evolution of new particle formation (NPF) events over the southeast Atlantic. The studied region is under the influence of the long‐range transport of aerosols and gases during the southern African biomass burning season, from June to October every year. Interestingly, NPF was observed to coincide with the African biomass burning season, although wet removal of pre‐existing aerosols is needed during these NPF events. Surface and airborne measurements show that these NPF events likely occurred in the upper region of the marine boundary layer, and the newly formed aerosols were further transported to the surface via vertical air motions. Using a box model, we predicted that a large fraction of these particles could grow to sizes related to cloud condensation nuclei. Our study shows that NPF can occur over the southeast Atlantic, and the African biomass‐burning plume likely contributed to the NPF occurrence.

Plain Language Summary A critical aspect of predicting tropical cyclone (TC) intensity using numerical models is the initialization of the vortex structure. Current operational hurricane models (e.g., HWRF and HAFS) typically initialize TC vortices with inner‐core asymmetries directly derived from the previous 6‐hr forecast fields. However, these asymmetric components are often not accurate due to limitations of observations. Idealized simulations in this study demonstrate the significant impact of inner‐core wavenumber‐1 asymmetries in the initial TC structure on intensity evolution. TC intensification occurs only when the wavenumber‐1 asymmetries reach a specific configuration relative to the axisymmetric circulation. Otherwise, inaccurate or missing wavenumber‐1 asymmetries in the initial vortex require additional time to adjust the vertical structure of TC vortices, delaying the timing of intensification onset. These findings highlight the potential importance of accurately incorporating wavenumber‐1 asymmetries into initial conditions to improve TC intensity forecasts in operational hurricane models.

Plain Language Summary Calcium carbonate, also known as calcite, forms under the Antarctic ice sheet when warm waters from the Southern Ocean interact with the edges of the ice sheet. The formation ages of these rocks can be measured using ²³⁴U‐²³⁰Th geochronology and are thought to record periods of time when meltwaters from the interior of the ice sheet were brought to the edges by ice acceleration that occurred in response to ocean warming. We measured ²³⁴U‐²³⁰Th ages and other geochemical data from a collection of 38 Antarctic calcite precipitates. We used a Monte Carlo model to assess the coincidence between the calcite ages and millennial‐scale (meaning they are smaller temperature fluctuations that happen within glacial‐interglacial cycles) warm peaks in the paleoclimate record, and we found that they have a statistically significant correlation with periods of Southern Ocean warming, which strengthens the argument for the connection between ocean warming and ice acceleration. We also show that the calcite dates tend to cluster during periods when millennial‐scale temperature fluctuations are more pronounced and total global ice volume is high, which is consistent with the idea that the ice sheet could be more likely to respond to climate change under those conditions.

Plain Language Summary Previous studies have shown that the South China Sea summer monsoon onset is important on the large‐scale: signify the establishment of summer monsoon over East Asia‐Southeast Asia‐western North Pacific, the arrival of the main rainy season in these locations, and the adjustment of the atmospheric circulation from winter‐type to summer‐type. We find that the changes in atmospheric mean flow before and after monsoon onset can have a significant modulating effect on the synoptic‐scale perturbations gestated in them. Prior to the monsoon onset, the most obvious permutations are those propagating eastward near the equator. In contrast, after the monsoon onset, the northwestward propagating tropical disturbances off‐the‐equator become more active. Specifically, tropical depressions and tropical cyclones occur more frequently and begin to affect Southeast and East Asia. These distinctive features of tropical waves can be understood in terms of the energy conversion between the mean circulation and the disturbances.

The number concentration of cloud condensation nuclei (NCCN) is vital for quantifying aerosol‐cloud interactions. Estimating NCCN using aerosol optical properties is essential for obtaining continuous NCCN data. This study highlights the significant impact of relative humidity (RH) on NCCN estimation through aerosol optical data, especially at low supersaturations (SS). When RH exceeds a threshold (e.g., 60% at 0.2% SS), NCCN estimation shifts from underestimation to overestimation, with the overestimation degree increasing with RH. Including RH in the estimation formula can effectively reduce this bias, although the aerosol optical hygroscopicity parameter is found to have a minimal effect on NCCN estimation. Based on these insights, a new parameterization scheme for NCCN estimation is proposed, which can significantly reduce NCCN estimation bias when using wet aerosol optical data at high RH levels (40%–90%).

Plain Language Summary In Arctic regions, the snow plays an important role in regulating ground temperatures and influencing different earth processes. Acting like a blanket, the snow keeps the ground warm during the cold season and prevents heat from moving into ground during snowmelt. Snow depth and timing of snow accumulation and melt events can vary greatly from place to place, leading to significant differences in ground temperatures. We analyzed snow depth and ground interface temperatures collected continuously for 112 locations at two small subarctic sites in Alaska over two years. Our results showed that vegetation and topography strongly influenced snow depth, but their relationships changed over time and varied between sites. We also found that differences in late‐winter snow depth led to variable snow‐free dates, and local air temperature further complicated this. Finally, we developed a new metric to better estimate how snow insulated the ground, incorporating daily snow depth and air temperature throughout the entire cold season. This research advances our understanding of snow dynamics and insulation effects over space and time, which is vital for evaluating how well we are capturing snow and permafrost processes in Earth system models.

Mangroves are one of Earth's “blue lungs” due to their exceptional carbon‐storage capabilities amidst rapidly increasing carbon dioxide. Despite providing numerous ecological services, their global distribution and carbon‐storage capacities have severely declined over the past 35 years (1985–2020). Here, we quantify spatio‐temporal changes in global and national carbon‐stocks that include this period. We found that global mangrove area decreased from 17.35 million‐hectares in 1985 (carbon‐storage of 6.84 Pg) to 13.61 million‐hectares in 2020 (carbon‐storage of 5.72 Pg). Significant losses occurred in Saudi Arabia and Indonesia, with a global reduction of 21.6% in area and 16.5% in carbon‐stocks. Potential maximum loss of accumulated carbon‐storage in mangroves was equivalent to 4.13 Pg of CO2, accounting for 0.4% of the global cumulative fossil CO2 emissions (1,009 Pg) during 1985–2020. This study provides more comprehensive and accurate statistics, maps, and insights on estimating and reducing mangrove carbon emissions to support global and national protection policies.

The melting of glaciers by ocean waters along the ice sheet periphery is a major physical process driving glacier evolution in a changing climate. Using a fiber‐optic‐tethered Grounding line Remote Operated Vehicle, we explore the ice shelf cavity of Petermann Glacier, in Northwestern Greenland, with an interferometric multibeam sonar operating with 360° ${}^{\circ}$ viewing capability. We detect a uniform seafloor at 820 m depth, 200 m deeper than anticipated. The ice shelf base reflects spatial variations in ice melt with no apparent signature at the surface, including widespread ice terraces interrupted by 30–40 m ice cliffs connected to a smooth, central basal channel that deviates by many 10 m's from flotation and experiences differential melt along its sides. Water stratification at the base of the center channel is prone to diffusive convection instead of a fully‐developed turbulent state. The results illustrate the critical importance of exploring cavities in situ.

Upcoming imaging missions—NASA's LEXI and ESA/CAS's SMILE—will target solar wind charge exchange X‐ray (SWCX) emission from Earth's magnetosheath. This emission is generated by highly charged ions colliding with neutrals in Earth's exosphere. Accurate SWCX models require data on exospheric neutral densities, as well as solar wind flux and composition. The Advanced Composition Explorer (ACE) Solar Wind Ionic Composition Spectrometer (SWICS) provided the needed solar wind composition data from 1998 until an instrument anomaly in 2011 limited its outputs. To address this, we developed empirical functions using ion ratios (O7+/O6+,O8+/O6+,C6+/C5+ ${\mathrm{O}}^{7+}/{\mathrm{O}}^{6+},{\mathrm{O}}^{8+}/{\mathrm{O}}^{6+},{\mathrm{C}}^{6+}/{\mathrm{C}}^{5+}$) still available from ACE, partially compensating for missing composition data. The results underscore the need for a new mission to measure solar wind composition and support future SWCX analysis efforts.

Plain Language Summary The polar ionosphere is filled with non‐uniform plasma densities. In addition to higher plasma density regions, many depleted structures, such as ionization troughs, polar holes, and auroral cavities, are observed at the F‐region of the polar ionosphere. In the summer hemisphere, most of the high‐latitude ionosphere is under the sunlit ionization. However, large area with decreased total electron content (TEC) is found in the summer hemisphere. Based on an 11‐year Global Navigation Satellite System TEC data, for the first time, we investigated the spatial distribution of the polar ionospheric TEC at high latitudes in the northern hemisphere. This decreased TEC region occurs mainly in regions above 70° magnetic latitude for moderate and high solar activity. The lower‐TEC region is predominantly located in the dawn and midnight sectors. The depth of the decreased TEC region is deeper for higher Kp than for low Kp. The result is expected for further understanding of the summer polar ionosphere.

Numerous studies demonstrated a huge potential for ionospheric total electron content (TEC) measurements to be included in earthquake and tsunami risk assessments. However, up to now, only a few methodologies or tools can be used in near‐real‐time (NRT) for these purposes. For the first time, this study presents a method allowing for fast simulations of Co‐Seismic Ionospheric Disturbances (CSID) or ionoquakes. The new analytical simulation code models the propagation of acoustic‐gravity waves (AGWs) generated by a co‐seismic uplift of the ground/seafloor, by resolving the governing equations in the time‐altitude‐horizontal domain. The method models the near‐field CSID in about 60 s of simulation time, that is, before the waves are detected in the ionospheric TEC measurements. The developed method is, therefore, among the first most promising products in Ionospheric Seismology that can be used for NRT applications, such as tsunami early warning.

Induced earthquakes are occasionally associated with stress oscillations resulting from periodic industrial activities. Yet, the effects of stress oscillations on fault friction under realistic subsurface conditions are not fully clear. We conducted normal stress oscillations experiments using simulated fault gouges derived from the major reservoirs and caprocks of the Changning shale gas field under in situ conditions. Our experimental results reveal that most gouges are velocity‐strengthening under quasi‐static loading. Interestingly, after applying normal stress oscillation, they show similar shear stress evolution with different oscillation amplitudes, frequencies and load‐point velocity, suggesting a negligible effect of the rock composition and the applied P‐T conditions. We successfully reproduce our experimental results using an extended CNS (Chen‐Niemeijer‐Spiers) model and the model proposed by Linker and Dieterich (1992, https://doi.org/10.1029/92jb00017). Therefore, these experimental observations and friction models can be reliably extrapolated to more heterogeneous field cases and other regions with similar lithology distribution.

River channel bifurcations are crucial for distributing water and sediment on floodplains and deltas, but estimating discharge ratios between branches remains challenging. Using satellite imagery and in‐situ discharge data, we demonstrate that bifurcate channel widths can estimate discharge ratios at 23 of 27 bifurcations in 11 rivers worldwide, with good accuracy (R² = 0.80) in 26 of 33 measurements. An empirical width‐discharge equation derived from 5,740 United States Geological Survey gauging stations further improves accuracy (R² = 0.82). For best results, branch widths should be measured within one channel width of the bifurcation. The method is ineffective in cases influenced by tributaries, avulsion, or multiple branches. We conclude that channel width is effective for estimating discharge ratios, especially when paired with an empirical width‐discharge equation, potentially enhancing river discharge estimates from the Surface Water and Ocean Topography satellite mission, which currently lacks flow partitioning capabilities for bifurcations.