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Improving the understanding of the functioning of water bodies in the urban critical zoneAn observation platform of an urban lake in the Greater Paris region
No full textInternational audienceLakes and ponds are part of the urban critical zone. Many of them have been created in the recent decades as sand-pit lakes or retention ponds. Sand-pit lakes are the result of sand and gravel extraction for the construction of towns. In order to better understand the physical and biogeochemical processes at stake and to which extent they may be affected by climate change, autonomous monitoring stations can provide long-term, high-frequency, reliable datasets. These data are also very useful for the calibration of numerical model parameters.The monitoring station implemented in Lake Creteil, in a highly urbanised area of the Greater Paris region (France) is presented. The surface area of the lake is 0.4 km2, average depth 4 m, maximum depth 6 m. The lake is fed by groundwater flowing from the Marne to the Seine and by the stormwater network of an urban catchment (1 km2). This observation platform is part of an OSU (Observatoire des Sciences de l’Univers) and is also associated to the French SNO OBSERVIL (Service National d’Observation) network.The instrumented buoy is equipped with underwater probes to measure physical and biogeochemical parameters and a weather station. Underwater measurements are performed every 15 minutes and meteorological measurements every 10 minutes. Temperature probes (CS225 Campbell) are deployed at five different depths: 0.5 m, 1.5 m, 2.5 m, 3.5 m and 4.5 m. At 1.5 m depth, a multiparameter probe (YSI Exo3) measures oxygen, conductivity, chlorophyll-a and phycocyanin. The weather station measures the wind speed and direction, air temperature, relative humidity, atmospheric pressure, rainfall height, short and longwave radiations. The results of the timeseries analysis are presented to illustrate how some physical and biogeochemical processes occurring in the lake (e.g. thermal stratification, peak of phytoplankton biomass, anoxia of the deep layers…) have been quantified. The use of the data for parameter calibration and validation of hydro-ecological numerical models is also presented
Anthropogenic climate change has increased severity of mid-latitude storms and impacted airport operations
No full textInternational audienceThe impact of extreme weather events, particularly those associated with tropical and extra-tropical cyclones (TC and ETC), on aviation can rise serious concerns in the context of the ongoing climate change. These events often lead to significant disruptions, including flight cancellations, delays, re-routing, and impacts on airport infrastructure resilience to adverse weather conditions. This study conducts an analysis of the influence of anthropogenic climate change on four recent major storm events that occurred over Europe, the USA, and East Asia, with an in-depth analysis on the Storm Eunice, a powerful ETC that affected the UK and Ireland. Using climate reanalysis data we assess the dynamics of these extreme storms and their implications for aviation operations, particularly during critical phases such as takeoff and landing. Our research underscores the growing intensity of extreme storms, particularly stronger winds, driven by human-induced climate change, and stresses the need for taking into account growing climate hazards to optimize planes and airport operations
Weakened stratification across the Eurasian Basin enables enhanced vertical spreading of Atlantic Water
No full textInternational audienceOcean stratification plays a crucial role in regulating heat fluxes from the ocean to the sea ice and atmosphere. In the Arctic Ocean, a layer of fresh and cold Polar Water near the surface isolates warm and saline Atlantic Water (AW) due to a strong halocline. While the stratification in the eastern Eurasian Basin halocline is known to have weakened since the 2000s, we here investigate the variability of stratification over the full AW depth range at basin scale. We analyze 40 years of in situ hydrographic data to track changes in stratification across the entire Eurasian Basin and introduce a conceptual dynamical model to explain the underlying mechanisms. Our results show that the most pronounced temperature and salinity variability occurs in the ∼100 m thick thermocline, which separates warm AW at depth from the colder, fresher halocline above. This variability is driven by warm, saline anomalies lasting for a few years entering through Fram Strait and propagating along the boundary current, superimposed on long-term trends. Our combined observational and modeling approach reveals that thermocline stratification has weakened by up to 50% in many regions, enhancing the potential for upward diffusive fluxes of heat and salt toward the base of the halocline. Plain Language SummaryOcean stratification plays a key role in determining the response of the Arctic Ocean to climate change. Earlier studies have shown that the halocline, which protects sea ice from warmer water underneath, has become less stable in the eastern Eurasian Basin since the 2000s. The full range of changes down to the deeper, warmer, and saltier Atlantic Water (AW) layer has, however, been little studied. In this study, data from the past 35 years is analyzed to track stratification changes. We find that the temperature and salinity have varied the most in the so-called thermocline, which separates the warmer AW below from the colder, fresher halocline above. The changes in stratification are driven by both short-term anomalies of warm, saline water entering through Fram Strait and by a long-term warming and salinity increase. In many parts of the Eurasian Basin, the thermocline has become up to 50% less stable, making it easier for heat and salt to move upwards to the base of the halocline. Using a conceptual model, we propose a mechanism linking temperature and salinity changes to the stratification of the water column
Stochastic Optimization in Semi-Discrete Optimal Transport: Convergence Analysis and Minimax Rate
No full textInternational audienceWe investigate the semi-discrete Optimal Transport (OT) problem, where a continuous source measure is transported to a discrete target measure , with particular attention to the OT map approximation. In this setting, Stochastic Gradient Descent (SGD) based solvers have demonstrated strong empirical performance in recent machine learning applications, yet their theoretical guarantee to approximate the OT map is an open question. In this work, we answer it positively by providing both computational and statistical convergence guarantees of SGD. Specifically, we show that SGD methods can estimate the OT map with a minimax convergence rate of , where is the number of samples drawn from . To establish this result, we study the averaged projected SGD algorithm, and identify a suitable projection set that contains a minimizer of the objective, even when the source measure is not compactly supported. Our analysis holds under mild assumptions on the source measure and applies to MTW cost functions,whic include for . We finally provide numerical evidence for our theoretical results
Staying put in the face of risk? Perceptions, experiences and migration responses in global perspective
No full textThis paper investigates how perceived and experienced risks shape both long-term aspirations to migrate internationally and short-term intentions to move, using matched data from the Gallup World Poll and World Risk Poll run in 2019 across 42 countries. Both risk perceptions and direct experiences are associated with migration responses. Worries about food insecurity, water scarcity, and violent crime are consistently linked to higher migration aspirations and intentions, while climate-related concerns appear to influence aspirations only indirectly, primarily through their connection to food and water insecurity. Direct experiences of food insecurity, violent crime, and mental health problems also predict a greater desire or likelihood to move. These relationships are markedly stronger in high-income countries – particularly for international aspirations – and largely absent in lower-income settings. At the same time, risks trigger short-term intentions to move among lower-educated individuals in low-income countries. This contradiction suggests that internal mobility may remain a more feasible response to adversity in resource-constrained contexts
Teaching soil mechanics in a changing world
No full textInternational audienceGeotechnical engineers face a changing world in which the need for new approaches in the wake of climate change presents both a challenge and an opportunity for constructing and maintaining existing infrastructures. It is thus crucial to adapt teaching soil mechanics, of which the basic concepts were established in the 20th century, to the new context. At the "École nationale des ponts et chaussées", France, the Department of Civil Engineering and Construction's students can consolidate the scientific bases of various fields while learning to be flexible and adapt to any new situation. In addition to the essential and basic concepts of standard Soil Mechanics taught in traditional lectures and hands-on sessions, two projects are introduced. The first is a literature review, allowing the students a quick global view of the current problems in Geotechnical Engineering. The second is a modelling project where the students use constitutive laws to simulate some standard laboratory tests. Besides, the contents of the Soil Mechanics class and other related classes, essentially the Geotechnical Designs class, are closely coordinated. This approach would allow graduate students to quickly learn new technologies and tools and solve problems that cannot be dealt with by standard soil mechanics knowledge
Madon (Julie) – Faire durer les objets, Paris, Presses de Sciences Po, 2024 (Académique). 272 p. Bibliogr. Annexe.
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Characterizing green infrastructures multi-scale spatial configuration to better understand their ecological performances
No full textInternational audienceNature-based Solutions for climate change adaptation can be implemented at various scales, from large natural parks to small green patches in the middle of more urbanised area. Each of these solutions can be considered as part of a larger green infrastructure. The size and shape of the different green patches, as well as the connectivity and proximity among them can be gathered under the notion of the spatial configuration of this green infrastructure. As a tool for landplaning, it would be useful to better understand how this spatial configuration can play a role in the conservation of biodiversity and the providing of the different expected ecosystemic services. A set of indicators derived from landscape ecology, mathematics and signal processing were used to characterize five dimensions of the spatial configuration: evenness, core area, isolation, roughness and fragmentation. These indicators are computed at different scales on land use and land cover data from a French conurbation. First results show that these different indicators bring complementary information and can be useful to establish a new typology of green infrastructures. A multiscale analysis will bring further information on the relevance of such indicators and at which scale they are the most useful. Subsequently, these spatial configuration indicators will be correlated with the results of ecosystemic services simulations to better understand how to optimize the ecological performances of green infrastructures
Hanoi's transition to fossil fuel free streets: A bold step for sustainability
No full textPublished in Vietnamese. Traducteur: Hảo LinhTia Sáng 16, 2025-08-27, pp. 13–19.Hanoi will start banning gasoline motorcycles in July 2026, beginning in the city center and expanding to broader areas by 2030, to implement a decision formulated eight years in its 2017's masterplan. The policy aims to address severe air pollution (PM2.5 levels ten times WHO guidelines) and position Vietnam as a leader in sustainable urban transport, though achieving clean air will requires equally strong measures on coal power plants and other polluting activities. While the transition presents significant social and economic challenges-particularly for the migrant motorcycle-dependent workers-it creates opportunities for domestic manufacturers like VinFast to challenge Honda's market dominance and demonstrates Vietnam's commitment to enter a new stage of development.Hanoï commencera à interdire les motos à essence en juillet 2026, en commençant par le centre-ville et en élargissant progressivement la zone concernée d’ici 2030, afin de mettre en œuvre une décision formulée huit ans plus tôt dans le plan directeur de 2017. Cette politique vise à lutter contre la grave pollution de l'air (niveaux de PM2.5 dix fois supérieurs aux recommandations de l’OMS) et à positionner le Vietnam comme un leader du transport urbain durable. Toutefois, l’obtention d’un air véritablement pur exigera des mesures tout aussi fortes à l’égard des centrales à charbon et d’autres activités polluantes. Si la transition pose d’importants défis sociaux et économiques — notamment pour les travailleurs migrants dépendant des motos — elle crée également des opportunités pour les fabricants nationaux comme VinFast de contester la domination de Honda sur le marché, tout en démontrant l’engagement du Vietnam à entrer dans une nouvelle phase de développement
Graphon Mean-Field Backward Stochastic Differential Equations With Jumps and Associated Dynamic Risk Measures
No full textInternational audienceWe study graphon mean-field backward stochastic differential equations (BSDEs) with jumps and associated dynamic risk measures. We establish the existence, uniqueness and measurability of solutions under some regularity assumptions and provide some estimates for the solutions. We moreover prove the stability with respect to an interacting graphon particle systems, and obtain the convergence of an interacting mean-field particle system with inhomogeneous interactions to the graphon mean-field BSDE. We then provide some comparison theorems for the graphon mean-field BSDEs. As an application, we introduce the graphon dynamic risk measure induced by the solution of a graphon mean-field BSDE system and study its properties. We finally provide a dual representation theorem for the graphon dynamic risk measure in the convex case