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1. A probabilistic framework for quantifying the role of anthropogenic climate change in marine-terminating glacier retreats

   https://doi.org/10.5194/tc-16-2725-2022  

   Christian, John Erich; Robel, Alexander A.; Catania, Ginny (January 2022, The Cryosphere)

   Abstract. Many marine-terminating outlet glaciers have retreated rapidly in recent decades, but these changes have not been formally attributed to anthropogenic climate change. A key challenge for such an attribution assessment is that if glacier termini are sufficiently perturbed from bathymetric highs, ice-dynamic feedbacks can cause rapid retreat even without further climate forcing. In the presence of internal climate variability, attribution thus depends on understanding whether (or how frequently) these rapid retreats could be triggered by climatic noise alone. Our simulations with idealized glaciers show that in a noisy climate, rapid retreat is a stochastic phenomenon. We therefore propose a probabilistic approach to attribution and present a framework for analysis that uses ensembles of many simulations with independent realizations of random climate variability. Synthetic experiments show that century-scale climate trends substantially increase the likelihood of rapid glacier retreat. This effect depends on the timescales over which ice dynamics integrate forcing. For a population of synthetic glaciers with different topographies, we find that external trends increase the number of large retreats triggered within the population, offering a metric for regional attribution. Our analyses suggest that formal attribution studies are tractable and should be further pursued to clarify the human role in recent ice-sheet change. We emphasize that early-industrial-era constraints on glacier and climate state are likely to be crucial for such studies. 

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2. Reducing the Ionospheric Contamination Effects on the Column O/N ₂ Ratio and Its Application to the Identification of Non‐Migrating Tides

   https://doi.org/10.1029/2022JA031148  

   Krier, Christopher S.; England, Scott L.; Meier, R. R.; Frey, Harald U. (April 2023, Journal of Geophysical Research: Space Physics)

   Abstract Prior investigations have attempted to characterize the longitudinal variability of the column number density ratio of atomic oxygen to molecular nitrogen (O/N₂) in the context of non‐migrating tides. The retrieval of thermosphericO/N₂from far ultra‐violet (FUV) emissions assumes production is due to photoelectron impact excitation on O and N₂. Consequently, efforts to characterize the tidal variability inO/N₂have been limited by ionospheric contamination from O⁺ + e radiative recombination at afternoon local times (LT) around the equatorial ionization anomaly. The retrieval ofO/N₂from FUV observations by the Ionospheric Connection Explorer (ICON) provides an opportunity to address this limitation. In this work, we derive modifiedO/N₂datasets to delineate the response of thermospheric composition to non‐migrating tides as a function of LT in the absence of ionospheric contamination. We assess estimates of the ionospheric contribution to 135.6 nm emission intensities based on either Global Ionospheric Specification (GIS) electron density, International Reference Ionosphere (IRI) model output, or observations from the Extreme Ultra‐Violet imager (EUV) onboard ICON during March and September equinox conditions in 2020. Our approach accounts for any biases between the ionospheric and airglow datasets. We found that the ICON‐FUV data set, corrected for ionospheric contamination based on GIS, uncovered a previously obscured diurnal eastward wavenumber 2 tide in a longitudinal wavenumber 3 pattern at March equinox in 2020. This finding demonstrates not only the necessity of correcting for ionospheric contamination of the FUV signals but also the utility of using GIS for the correction. 

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3. Composition and Flux of Dissolved and Particulate Carbon and Nitrogen in the Lower Tocantins River

   https://doi.org/10.1029/2022JG006846  

   Neu, Vania; da S. Araújo, Maria G.; Guedes, Victor M.; Ward, Nicholas D.; Ribeiro, Maridalva M.; Richey, Jeffrey E.; Krusche, Alex V. (June 2023, Journal of Geophysical Research: Biogeosciences)

   Abstract The Tocantins River contributes ∼5% of the total flux of water to the Amazon River plume in the Atlantic Ocean. Here, we evaluate monthly variability in the composition and abundance of carbon, nitrogen, and suspended sediment in the lower reaches of the Tocantins River from 2014 to 2016. Dissolved organic carbon concentrations generally increased during periods of high discharge and are ∼1.5 times lower than average concentrations at the mouth of the Amazon River. Dissolved inorganic carbon similarly increased during periods of high discharge. Total dissolved nitrogen and individual nitrogen species followed a similar temporal pattern, increasing during high water.predominated the dissolved inorganic nitrogen pool, followed by, and, characteristic of environments with a relatively low anthropogenic impact. Dissolved fractions represented 92% of the total carbon exported and 78% of the total nitrogen exported. The suspended particulate sediment flux was 2.72 × 10⁶ t yr⁻¹, with fine suspended sediment dominating (71.3%). Concentrations of carbon relative to nitrogen indicate a primarily terrigenous source of organic matter and CO₂derived from in situ respiration of this material during the rainy season and a primarily algal/bacterial source of organic matter during the dry season. Considering past estimates of dissolved carbon and nitrogen fluxes from the Amazon River to the Atlantic Ocean, the Tocantins River contributes 3% and 3.7% to total fluxes to the Amazon River plume region, respectively. While this contribution is relatively small, it may be influenced by future changes to the basin's land use and hydrology. 

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4. Influence of Thermal Stratification on the Structure and Evolution of the Martian Core

   https://doi.org/10.1029/2021GL095198  

   Greenwood, Sam; Davies, Christopher J; Pommier, Anne (November 2021, Geophysical Research Letters)

   Abstract The apparent end of the internally generated Martian magnetic field at 3.6–4.1 Ga is a key event in Martian history and has been linked to insufficient core cooling. We investigate the thermal and magnetic evolution of the Martian core and mantle using parameterized models and considered three improvements on previous studies. First, our models account for thermal stratification in the core. Second, the models are constrained by estimates for the present‐day areotherm. Third, we consider core thermal conductivity,, values in the range 5–40 Was suggested by recent experiments on iron alloys at Mars core conditions. The majority of our models indicate that the core of Mars is fully conductive at present with core temperatures greater than 1940 K. All of our models are consistent with the range ofW. Models with an activation volume of 6 (0)require a mantle reference viscosity of Pa s. 

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5. Deglacial Temperature and Carbonate Saturation State Variability in the Tropical Atlantic at Antarctic Intermediate Water Depths

   https://doi.org/10.1029/2023PA004674  

   Oppo, D W; Lu, W; Huang, K‐F; Umling, N E; Guo, W; Yu, J; Curry, W B; Marchitto, T M; Wang, S (September 2023, Paleoceanography and Paleoclimatology)

   Abstract Variations in the Atlantic Meridional Overturning Circulation (AMOC) redistribute heat and nutrients, causing pronounced anomalies of temperature and nutrient concentrations in the subsurface ocean. However, exactly how millennial‐scale deglacial AMOC variability influenced the subsurface is debated, and the role of other deglacial forcings of subsurface temperature change is unclear. Here, we present a new deglacial temperature reconstruction, which, with published records, helps assess competing hypotheses for deglacial warming in the upper tropical North Atlantic. Our record provides new evidence of regional subsurface warming in the western tropical North Atlantic within the core of modern Antarctic Intermediate Water (AAIW) during Heinrich Stadial 1 (HS1), an early deglacial interval of iceberg discharge into the North Atlantic. Our results are consistent with model simulations that suggest subsurface heat accumulates in the northern high‐latitude convection regions and along the upper AMOC return path when the AMOC weakens, and with warming due to rising greenhouse gases. Warming of AAIW may have also contributed to warming in the tropics at modern AAIW depths during late HS1. Nutrient andreconstructions from the same site suggest a link between AMOC intensity and the northward extent of AAIW in the northern tropics across the deglaciation and on millennial time scales. However, the timing of the initial deglacial increase in AAIW to the northern tropics is ambiguous. Deglacial trends and variability ofin the upper North Atlantic have likely biased temperature reconstructions based on the elemental composition of calcitic benthic foraminifera. 

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