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Experimental Evidence for the Desynchronization of Ecosystem Dynamics by Global Change
No full textInternational audienceAnticipating ecosystem responses to global change requires identifying the isolated and combined effects of environmental disturbance across both space and time. Examining the coordinated responses of ecosystems has recently emerged as a powerful approach to advance this understanding. We conducted two complementary experiments to identify whether, and if so how, warming temperatures, nutrient enrichment, predator overexploitation (i.e., reduced apex predator abundance) and their combination drive coordinated responses of freshwater ecosystems by evaluating the dynamics of synchrony between control and disturbed mesocosms using high‐frequency dissolved oxygen saturation measurements, an integrative parameter of the metabolic balance of ecosystems. Nutrient enrichment desynchronized the oxygen dynamics and their component cycles between treatments, likely arising from elevated primary production. Warming and overexploitation tended to desynchronize oxygen cycles from the control, particularly at short time scales. Nutrient enrichment combined with warming dampened desynchronization between control (ambient) and treatment mesocosms, whereas desynchronization was enhanced when simultaneously subject to predator overexploitation. As one of the first experimental demonstrations of global change impacts on ecosystem synchrony, this study highlights the need—and opens new avenues—to detect alterations in ecosystem functioning across previously unexplored spatial and temporal scales
Le travail domestique au prisme des savoirs et des normes : transmissions, circulations et régulations. Introduction du numéro
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Les espaces à scénario : catalyseurs d’interactions ou miroir des inégalités ? Étude de cas dans une école maternelle en REP. Entre autonomie et étayage : l’institutionnalisation comme levier de justice langagière
No full textCe mémoire explore les espaces à scénario en maternelle REP comme outils de réduction des inégalités langagières. Catalyseurs d’interactions ou miroirs des inégalités ? Une étude auprès de 24 élèves de grande section compare un groupe ayant bénéficié d’ une institutionnalisation, avec guidage et reformulation, à un groupe en autonomie. Ancré dans des cadres sociologique, socio-didactique et interactionniste, le dispositif repose sur une triangulation : verbatims, entretiens, questionnaires à 40 enseignants. Les analyses (longueur des énoncés, connecteurs, structuration) révèlent que l’institutionnalisation favorise un langage scolaire narratif et structuré, réduisant les écarts entre élèves, tandis que l’autonomie les accentue. En REP, ces espaces deviennent des leviers de justice langagière grâce à un étayage explicite. Quatre propositions pédagogiques émergent : structuration des phases, gestes langagiers, coopération entre pairs, étayage différencié, pour une école plus équitable
Metal hydrides for energy storage and hydrogenation reactions
No full textInternational audienceHydrogen can react with many intermetallic compounds to form crystalline or amorphous hydrides, and are called metal hydrides [1]. Metals hydrides are the most compact way to store hydrogen [2]. For example, we can store up to 115 g H2/L in LaNi5, a much higher volumetric density than liquid hydrogen (42 g/L). A large variety of intermetallic compounds can store large amount of hydrogen in a reversible manner. They are the product of alloying two types of elements "A" and "B" from the periodic table. The elements "A", usually rare earth, alkali metals, alkali earth metals or the beginning of transition metals have a strong affinity to hydrogen whereas the elements "B", usually the right side of transition metals, have a lower affinity with hydrogen, and their hydrides form only under high pressure. So, the combination of these two types of elements results in a large variety of intermetallic compounds with different properties related to their crystal structures. Among these systems, the ternary AMgB4 compounds (A= Rare earth, B= Ni, Co) have been studied for their hydrogen and electrochemical properties [3, 4]. The AMgB4 alloys crystallize in a SnMgCu4-type cubic structure (F-43m space group) with the ordering of A and Mg in two different Wyckoff sites [5]. They can absorb a large amount of hydrogen, which modifies their structural properties. The Co for Ni substitution in such alloys allows to lower the plateau pressures in the pressure composition isotherm (PCI) and increases the H(D) capacity from about 3.8 to to 6 H/f.u. [6-8]. Their structural properties have been investigated by combining X-ray and neutron diffraction, in order to refine not only the metal atom positions but also the hydrogen (deuterium) position and occupation number [9]. The structure of the hydrides or deuterides is sensitive to the nature of the rare earth (A =Y, Nd , Pr, Tb), the transition metal particularly the Co/Ni ratio and the quantity of inserted hydrogen. A review of the different structural properties will be reported. In addition, these compounds can be of interest as catalyst for hydrogenation of CO or CO2 to produce selectively hydrocarbons
Cross-Hemispheric Transport of the Hunga Aerosol Plume: In Situ Evidence and Radiative Effects from the Northern Hemisphere
No full textAbstract. The Hunga Tonga–Hunga Ha’apai eruption (20° S) in January 2022 injected a substantial amount of water vapor and aerosols into the stratosphere, primarily impacting the Southern Hemisphere and tropics. Using a combination of satellite observations and in situ measurements with optical particle counters, we show that a significant portion of the aerosol plume was transported into the Northern Hemisphere (NH) mid-latitudes. This cross-hemispheric transport occurred within the tropically controlled transition zone, within the shallow branch of the Brewer–Dobson circulation. By October 2022, enhanced aerosol concentrations were observed up to 50° N, at altitudes between 17–23 km with some dense plumes at around 21–22 km. In situ observations reveal an effective radius of around 330 nm, comparable to what was observed in the Southern Hemisphere (SH). Aerosol extinction coefficients in the mid-latitudes (30–50° N) increased by ∼50 % over background levels, corresponding to an aerosol optical depth (AOD) increase of (1–2) × 10−3. These enhancements led to a modest, but not negligible, shortwave top-of-atmosphere (TOA) radiative forcing of −0.05 ± 0.01 W m−2 between October 2022 and April 2023. Our results show that the moderate aerosol impact of the Hunga eruption in the SH produced non-negligible radiative impacts in the NH, emphasizing the importance of considering both hemispheres when analysing the total impact
Extraire la Gazette de l’histoire de son public : pour une histoire de l’activité d’informer (v. 1680-v. 1795)
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ExoPhoto: a data base of temperature-dependent photodissociation cross-sections
No full textInternational audienceWe present the ExoPhoto database www.exomol.com/exophoto, an extension of the ExoMol database, specifically developed to address the growing need for high-accuracy, temperature-dependent photodissociation cross section data towards short-UV wavelengths. ExoPhoto combines theoretical models from three major computational databases (ExoMol, UGAMOP and PhoMol) and experimental datasets from two experimental groups, providing extensive wavelength and temperature coverage. ExoPhoto currently includes photodissociation data for 20 molecules: AlH, HCl, HF, MgH, OH, NaO, MgO, O2, AlCl, AlF, CS, HeH+, CO, CO2, H2O, SO2, C2H2, C2H4, H2CO, and NH3, derived from theoretical models and supported by experimental data from 5 databases.ExoPhoto also includes detailed data on branching ratios and quantum yields for selected datasets. The data structure of ExoPhoto follows the ExoMol framework, with a consistent naming convention and hierarchical JSON-based organization. Photodissociation cross sections are stored in a set of .photo files which provide data as a function of wavelength with one file for each target molecule temperature. Future developments aim to include more photodissociation cross section data and to provide data for molecules in non-local thermodynamic equilibrium (non-LTE). These will expand the utility of ExoPhoto for advanced astrophysical, planetary modeling and industrial applications
The Impact of Extended CO<sub>2</sub> Cross Sections on Temperate Anoxic Planet Atmospheres
No full textInternational audienceOur interpretation of terrestrial exoplanet atmospheric spectra will always be limited by the accuracy of the data we use as input in our forward and retrieval models. Ultraviolet molecular absorption cross sections are one category of these essential model inputs; however, they are often poorly characterized at the longest wavelengths relevant to photodissociation. Photolysis reactions dominate the chemical kinetics of temperate terrestrial planet atmospheres. One molecule of particular importance is CO2 , which is likely present in all terrestrial planet atmospheres. The photolysis of CO2 can introduce CO and O, as well as shield tropospheric water vapor from undergoing photolysis. This is important because H2O photolysis produces OH, which serves as a major reactive sink to many atmospheric trace gases. Here, we construct CO2 cross-section prescriptions at 195 K and 300 K extrapolated beyond 200 nm from measured cross sections. We compare results from the implementation of these new cross sections to the most commonly used CO2 prescriptions for temperate terrestrial planets with Archean-like atmospheres. We generally find that the observational consequences of CO2 dissociation beyond 200 nm are minimal so long as our least conservative (highest opacity) prescription can be ruled out. Moreover, implementing our recommended extended CO2 cross sections does not substantially alter previous results that show the consequential photochemical impact of extended H2O cross sections
Médecine, transports, technologies numériques… Et si on arrêtait d’inventer de nouveaux matériaux ?
No full texthttps://theconversation.com/medecine-transports-technologies-numeriques-et-si-on-arretait-dinventer-de-nouveaux-materiaux-267560Developing new materials always seems necessary to meet urgent needs in medicine or in the field of ecological transition. However, the extraction of the raw materials needed to manufacture them and their poor recyclability have a very heavy environmental impact. How can this dilemma be resolved? “One of the worst health scandals in decades,” “a family of 10,000 eternal pollutants,” “France poisoned for life,” “the endless pollution of PFAS”: these are some of the headlines in the press about PFAS, which stands for per- and polyfluoroalkyl substances. Gone are the days when these new chemical compounds were admired and developed for their many properties: non-stick, fire-retardant, stain-resistant, waterproof, etc. Today, it is their toxicity that is causing concern. This obviously reminds us of the story of asbestos, a very good insulator, but one that proved to be highly toxic. Faced with these repeated scandals, the question arises: what if we stopped inventing new materials?Développer des matériaux nouveaux semble toujours nécessaire pour répondre à des besoins urgents en médecine ou dans le registre de la transition écologique. Pourtant, l'extraction des matières premières nécessaires à leur fabrication et leur mauvaise capacité de recyclage engendrent des impacts environnementaux très lourds. Comment résoudre ce dilemme ? « L'un des pires scandales sanitaires depuis des décennies », « une famille de 10 000 polluants éternels », « la France empoisonnée à perpétuité », « la pollution sans fin des PFAS » : voilà ce que titrait la presse à propos des PFAS, acronyme anglais de per-and polyfluoroalkyl substances. Il est loin le temps où ces nouveaux composés chimiques étaient admirés et développés pour leurs nombreuses propriétés : antiadhésifs, ignifuges, antitaches, imperméabilisants… Aujourd'hui, c'est plutôt leur toxicité qui inquiète. Cela nous rappelle évidemment l'histoire de l'amiante, un très bon isolant, mais qui s'est révélé hautement toxique. Face à ces scandales à répétition, la question se pose : et si on arrêtait d'inventer de nouveaux matériaux
