Abstract. Chemistry-climate models predict a strengthening of the Brewer-Dobson Circulation (BDC) in response to climate change, which has implications for global atmospheric composition, radiation, and climate. This predicted acceleration has not been confirmed with observations, and models also disagree about the mean stratospheric circulation and mixing strength. The BDC impacts the distribution of long-lived tracers and their empirical relationships with one another. Age of air is an important diagnostic for changes in the BDC, and it can be derived from long-lived trace gases, such as sulfur hexafluoride (SF6) and nitrous oxide (N2O). We introduce an updated technique to calculate age of air using satellite observations of N2O. We (1) compute tracer interrelationships of age of air and N2O (Age:N2O) and demonstrate that they vary with latitude, and then (2) use these relationships to calculate a new N2O-derived age timeseries that takes this latitude variability into account from 2005 to 2012. The tracer interrelationships and their variability with latitude provide a better understanding of the structure and seasonality of the BDC. In particular, latitudinally-resolved Age:N2O relationships reflect the relative importance of photochemical loss of N2O in different regions and enable hemispheric structural comparisons. The N2O-age product has more extensive spatial coverage than previous counterparts. Additionally, N2O and SF6-age compare well, showing that Age:N2O relationships are robust on seasonal and interannual time scales. While this timeseries is only 7 years long, this manuscript lays the groundwork for calculating a longer record of N2O-age to understand long-term variability and shifts of the BDC.

The deep critical zone (CZ) has long been recognized for its importance in influencing shallow landslides but was not considered feasible to include in slope stability models at the watershed scale. Here, we demonstrate that simple approximations of the CZ in a fully coupled hydrologic and soil slope stability model can effectively capture the location, timing, and likely size of shallow landslides. To achieve this, we use coupled, process-based models that incorporate the effects of 1) deep CZ structures, 2) three-dimensional transient hydrology, and 3) multidimensional slope stability, calibrated with data from an intensively monitored field site. Our results show that the hydrologically active deep CZ guides groundwater flow, influencing where it drains from or exfiltrates to the soil mantle and producing distinct patterns of soil saturation and seepage forces at the soil-bedrock boundary. A deep conductive, weathered bedrock drains the soil mantle, reducing the likelihood of destabilizing pore pressures, while the downslope thinning of the CZ forces groundwater to the surface. This pattern creates localized instability and a tendency for similar-sized landslides across the landscape. In contrast, the absence of conductive weathered bedrock results in more widespread destabilizing pore pressures, leading to larger landslides and the likelihood of landslides earlier in a storm than in landscapes underlain by a deep CZ. Our findings suggest that first-order variations of deep CZs can provide physical explanations for variations observed in the susceptibility, magnitude, and timing of shallow landslides, and that CZ structure may be inferred from patterns and timing of landsliding.

Abstract Planetary atmospheres cannot remain hydrostatic at all altitudes because they approach finite density at infinite radius, implying infinite mass. Classical treatments address this in two directions: either retain a hydrostatic structure while allowing particles in the high-velocity tail to decouple and escape in a Jeans-type manner, or promote the gas to a continuum outflow to obtain a transonic Parker-type solution. The usual criterion compares the local mean free path to the sonic point radius. If the mean free path is shorter, the atmosphere is hydrostatic with an imposed Jeans escape flux; if it is longer, the gas is hydrodynamic with Jeans escape neglected. Here, we show that hydrogen-rich atmospheres do not separate cleanly into hydrodynamic and Jeans-escape regimes. At any radius, some particles still collide and behave as a fluid, while others have already experienced their last collision and move collisionlessly on ballistic trajectories. The relative importance of these two behaviors changes smoothly with radius rather than switching at a single boundary. The hydrodynamic channel accelerates and passes through a sonic point, whereas the collisionless channel decelerates under gravity and grows with altitude, removing mass and momentum from the collisional flow. As the collisionless component grows, the bulk flow speed reaches a maximum and then decelerates thereafter, producing profiles similar to Parker breeze solutions even though escape is carried by the collisionless channel. This two-channel framework provides a first step toward a self-consistent treatment that unifies hydrodynamics and kinetics in atmospheric loss models.

Amino acids have been detected in carbonaceous chondrites, with abundances and isotopic compositions varying significantly between different meteorites as well as within individual meteorites. In this study, we assessed whether the presence and abundance of Fe-rich phases during parent body alteration can account for observed variations in amino acid concentrations and isotope composition. To test this, we examined the chemical and 13C-isotopic signatures of six amino acids─glycine, β-alanine, α-alanine, 2-aminoisobutyric acid, γ-aminobutyric acid, and isovaline─following experimental exposure to hydrothermal conditions (150 °C, 10 days) in the presence or absence of Fe-bearing materials (Fe, Fe2O3, FeS2). In the absence of Fe-rich materials, glycine and α-alanine rather withstood hydrothermal conditions, consistently with abundances reported for carbonaceous chondrites having experienced various degrees of aqueous alteration. In contrast, upon exposure to similar hydrothermal conditions, the degradation of β-alanine produced a new compound, possibly 3-aminoadipic acid, via the recombination of products of its decarboxylation and deamination, while more than 95% of γ-aminobutyric acid was converted to 2-pyrrolidone through self-cyclization. The presence of Fe-rich materials inhibited the destruction of β-alanine, 2-aminoisobutyric acid, and γ-aminobutyric acid. Fe2O3 promoted the conversion of glycine into aspartic acid, and the resulting organics interacted with Fe2O3, leading to a relatively higher organic content in the residues compared to other Fe-containing materials after the experiments. Oxides in CI chondrites may exhibit variable effects on each amino acid compound during aqueous alteration, potentially explaining the higher β-Alanine/Glycine ratios observed compared with CM chondrites. The slight changes in δ13C values of amino acids upon exposure to hydrothermal conditions, independent of the presence or absence of Fe-rich materials, could not account for the variations observed in the δ13C values of chondritic amino acids. Hence, the δ13C values of amino acids reported in CR and CM chondrites may be inherited from the preaccretion processes.

Abstract Extreme rainfall is projected to intensify as the climate warms, yet whether the greatest increases will occur in multi-day or single-day events remains uncertain. This knowledge gap is particularly pressing given recent catastrophic floods triggered by multi-day rainfall events, prompting the question of whether multi-day events could, in fact, intensify more than their daily counterparts, and by how much. This study addresses this question using an ensemble of 34 downscaled Earth System Models (ESMs) under two Shared Socioeconomic Pathways (SSP2-4.5 and SSP5-8.5), focusing on changes in extreme rainfall by the end of the century across ten regions of the contiguous United States. Our statistical framework evaluates model agreement, ensemble-mean changes, and the significance of these changes for both daily and multi-day rainfall extremes. Results show that extreme rainfall amounts are expected to increase for most regions and durations. The degree of intensification, however, depends strongly on event rarity and regional climate characteristics. Notably, in the U.S. western Gulf Coast region, very rare multi-day events (e.g., 500-year return period) are projected to intensify more than their daily counterparts, a phenomenon that could be explained by increased stalling of tropical cyclones, which can prolong heavy rainfall over multiple days. These results challenge the assumption that daily extremes dominate future risk and highlight the need to consider event duration when updating flood-hazard maps, design standards, and adaptation planning.

Stromatolite and thrombolite are two major categories of microbial carbonate, distinguished by macrofabric: laminated in stromatolite and clotted in thrombolite. Since these categories are defined at hand-specimen scale, their distinction is inherently resolution-dependent. Re-study of Aitken’s (1967) typical thrombolite, from the Cambrian of Canada, shows that—in thin-section—the individual microbial carbonate clots can be seen to be delicately laminated. Cambrian thrombolites with similar indistinctly laminated clots occur in China and the USA. Similar, but generally larger and distinctly laminated, ministromatolites (sma ll stromatolites, ≤20 mm wide) can be common in the Proterozoic. They appear to be generally scarce in the Phanerozoic but occur locally in Holocene freshwater streams and lakes. Clots in present-day near-sea domes and columns at Lake Clifton, Australia, widely regarded as thrombolite analogs and formed by calcified cyanobacterial colonies, closely resemble Aitken’s Cambrian e xample; their initial lamination can be obliterated during early diagenesis. These observations emphasize that the distinction between thrombolite and stromatolite is based on human perception at hand-specimen scale. We propose that fluctuating decline in marine microbial calcification during the Proterozoic–Phanerozoic transformed distinctly laminated ministromatolites into smaller forms whose lamination is too delicate to be evident to the naked eye—ministromatolite ‘ghosts’—and that these form the clots in Aitken’s typical Cambrian thrombolite. Proterozoic ministromatolites, Cambrian thrombolites, and their present-day non-marine analogs, reflect a ~2.5 Ga history of fluctuating marine calcification in microbial, most likely cyanobacterial, colonies.

Abstract Regional ionospheric structures characterized by localized spatial variations in plasma density significantly impact radio signals and Global Navigation Satellite System (GNSS) applications. Using multi-instrument observations in the Asian sector, this study characterizes the regional ionospheric structures and their impacts on GNSS positioning. Steep longitudinal Total Electron Content (TEC) gradients exceeding 2 Total Electron Content Unit (TECU) per degree are identified within a narrow latitudinal range (20°–30°N). We reveal the limitations of current final Global Ionosphere Models (GIMs) products in capturing these steep TEC gradients. These GIMs fail to capture these gradients due to a low longitudinal resolution of 5°, among which the Chinese Academy of Sciences final GIM (CASG) and Jet Propulsion Laboratory final GIM (JPLG) products perform better than others (demonstrated by higher consistency with observed TEC gradients) due to the incorporation of new GNSS constellations and the adoption of a three-layer ionosphere model, respectively. Further statistical analysis of Standard Point Positioning (SPP) results using different GIMs shows that the positioning errors of SPP are larger in the present of steep TEC gradients. Since the CASG and JPLG are better in capturing these TEC gradients, the positioning errors of SPP using the CASG and JPLG are smaller than those using other GIMs. By combining Rate of TEC Index (ROTI) observations with kinematic Precise Point Positioning (PPP) solutions, we present the positioning errors induced by ionospheric irregularities during two geomagnetic storms (1 December 2023 and 10 May 2024 geomagnetic storms). Ionosonde and incoherent scatter radar measurements at Sanya (109.6°E, 18.3°N) are used to demonstrate how Penetration Electric Fields (PEFs) modulate post-sunset ionospheric irregularities and ultimately influence the performance of kinematic PPP. An under-shielding PEF enhances ionospheric irregularities, degrading PPP accuracy from < 10 cm to > 1 m. In contrast, an over-shielding PEF suppresses ionospheric irregularities, thereby prevent irregularity-induced PPP errors during the geomagnetic storms. These findings highlight the critical role of regional ionospheric structures and storm-time electrodynamics in GNSS positioning.

The decomposition of organic sulfur (S) influences S availability to plants and trace metal dynamics in soils. While temperature effects on decomposition rate are known, the influence of precipitation remains less understood. We examined organic S decomposition and oxidation states in volcanic soils across two rainfall gradients on the Hawaiian Islands (mean annual precipitation [MAP]: 285-5066 mm). Higher MAP increased atmospheric sulfate deposition, which was largely converted to organic S by biological processes. However, the impact of precipitation on decomposition of organic S varied by rainfall regime. In wetter regions (MAP > 1500 mm), higher soil moisture promoted reducing conditions, resulting in markedly less oxidized and less decomposed organic S, with average oxidation state (AOS) of organic S decreasing from 4.5 to 2. In drier regions (MAP < 1500 mm), the decomposition of organic S (AOS = 4.2 ± 0.1) was high but did not correlate with MAP, likely because the soils remained sufficiently oxic for decomposition regardless of MAP and soil moisture. These trends reflect thermodynamic constraints on organic matter decomposition and suggest that soil organic matter enriched in reduced carbon tends to accumulate reduced sulfur, underscoring a strong coupling between carbon and sulfur cycles. Our findings suggest that climate change-induced shifts in precipitation and temperature could alter soil S cycling, with important consequences for nutrient availability and trace metal dynamics in terrestrial ecosystems.

Abstract We present a major update to the open-source atmospheric modeling package PICASO , designed for simulating the thermal structure and spectra of hydrogen-rich atmospheres of brown dwarfs and exoplanets. This release, PICASO 4.0 , expands upon the existing radiative-convective equilibrium model framework by incorporating several new capabilities. Key additions include the integration of Virga for self-consistent cloud modeling, new flexible treatments for rainout and cold trapping of volatile species, and support for photochemistry. We also introduce a parameterized energy injection scheme to simulate additional external or internal heating processes. These features are motivated by lessons from recent JWST observations that reveal the prevalence of nonequilibrium chemistry and clouds. We benchmark the new functionalities against previously published results in the literature, including the Sonora Diamondback grid, energy injected atmospheres, patchy cloud models, and other photochemical models of WASP-39b. PICASO continues to be actively developed as an open-source package aimed at enabling reproducible, community-driven atmospheric modeling of all substellar objects.

Abstract The existing heat index is complicated and slow. Furthermore, its complexity has obfuscated both mathematical inconsistencies (double values) and physical inconsistencies (supersaturation). This paper presents a simplified heat index that resolves these issues. The new approach results in small changes to the heat index for air temperatures below 300 K, but leaves the heat index unchanged for air temperatures above 300 K, where it is most commonly used. This simplification enhances interpretability, and a refactored algorithm accelerates the computation of the heat index by orders of magnitude. The optimized implementation is freely available in C++, R, and Python. In this article, we also clarify a long-standing ambiguity regarding “compensable” and “uncompensable” heat stress. Historically, “uncompensable” denoted conditions leading to fatal core temperatures. Recent studies, however, have applied the term to any inflection followed by a rise in core temperature. Using the heat index model, we show that such an observation does not necessarily imply lethality, because heat loss increases with core temperature and can yield a stable, non-fatal equilibrium. To avoid ambiguity, we therefore replace compensable/uncompensable with normothermic, hyperthermic, and lethal categories based on the predicted steady-state core temperature.