Alexandre Cauquoin

Adaptation needs local temperature projections, yet their uncertainty remains large. Historical temperature differences between observations and the model-ensemble mean contain unforced variability, substantially limiting constraints on future local warming. Whether neighboring records strengthen these constraints, and at what spatial scale, remains a key unresolved question. We show that distance-weighted averaging of observation–model differences within a fixed radius improves skill over using only the target-grid record. This improvement arises because nearby locations share forced warming signals, allowing averaging to reduce noise while retaining information about future warming at the target. We therefore develop an adaptive observational constraint that selects the neighborhood radius and weights neighboring information according to its noise and relevance to target warming. It improves skill over the target-grid constraint in 33 of 34 SSP5-8.5 models at a 50-year lead and in both lower-forcing SSPs. Applied to HadCRUT5, it lowers projected late-century local warming under SSP5-8.5 and narrows spread.

Deuterium-excess ( d-excess ) in Antarctic ice-cores has long been used to reconstruct glacial–interglacial changes in moisture-source temperature (ΔT source ) or relative-humidity. Yet temperature, relative-humidity, and wind-speed jointly and inseparably influence d-excess , an ambiguity that has undermined single-variable readings. Here we resolve it by reframing d-excess as a proxy for the oceanic evaporation rate (E): all three controls drive both evaporation and its d-excess in the same direction. We calibrate the modern spatial E– d-excess relationship against reanalysis and satellite evaporation products (R² = 0.83–0.97) and apply it to the EPICA Dome C record to produce the first continuous 800,000-year history of moisture-source evaporation. Reconstructed E couples tightly with ΔT source during cold glacial periods (≤ −3.5 °C; R² = 0.57) but decouples through the transitional (−3.5 to −1 °C; R² = 0.10) and warm (≥ −1 °C; R² = 0.23) regimes, where wind, proxied by EDC dust-flux, becomes the dominant control. Jointly, source-temperature and wind explain 81% of evaporation variance across 800 kyr, against 55% for temperature alone. The control on evaporation shifts from thermodynamic to dynamic as climate warms, implying that the response of the water cycle to future warming is governed by atmospheric-circulation as much as by temperature.

Tritium (3H or T) radionuclide with a half-life of 12.32 years is generated by cosmic rays in the upper atmosphere and becomes tritiated water (HTO) existing in all water cycle compartments across the globe. In Japan, the anthropogenic tritium was released to the atmosphere on March-April 2011 due to the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident and to the ocean with start of the Advanced Liquid Processing System (ALPS)-treated water discharge from 24th August 2023 prompting tritium monitoring in Fukushima waters. On 12-14th October 2023, 19 nested headwater catchments were sampled during baseflow for tritium measurements in this study and one river water sampling site located west of Fukushima city overlapped with the Fukushima Prefectural tritium monitoring programme, which have been sampling surface water sites twice per year to measure tritium without enrichment. In addition, monthly precipitation was collected at IER roof for tritium analysis. Using our measured tritium with a lump-parameter model, we used the long-term time-series of tritium in monthly precipitation scaled from the Tokyo to Fukushima area and combined the environmental tritium, which is generated by anthropogenic activities and cosmic rays in the upper atmosphere, with the FDNPP anthropogenic tritium release to simulate mean transit times (MTTs) from 1 to 200 years. Using available tritium measurements without enrichment allowed to exclude the short MTTs due to the FDNPP tritium input reducing the MTT range and estimated groundwater storage volume. As a result, we demonstrate that environmental tritium radioisotope is a useful tracer with developed tritium time-series in precipitation and surface water measurements to evaluate terrestrial water cycle in Fukushima. We also recommend the continuation of tritium monitoring is needed to disentangle natural levels with the ongoing tritium-related FDNPP activities for tritium measurements without and with electrolytic enrichment.

Water stable isotopes (δ2H and δ18O) are widely used to reconstruct past hydrological changes and paleoclimate variability. A key limitation, however, is that fossil isotope records reflect not only local temperature but also changes in circulation, moisture sources, and hydrological processes along air-mass trajectories. Although modern monthly and interannual δ2H–δ18O–temperature relationships have been proposed to calibrate paleoclimate reconstructions, their applicability to longer timescales, such as glacial–interglacial transitions, remains uncertain. Here, we use fossil groundwaters worldwide as direct archives of past annual temperature and δ2H–δ18O co-variations over the last ∼35,000 years. Our global compilation of water stable isotope and noble gas temperature records (746 samples from 39 locations) shows that, while significant relationships occur in several regions, European fossil groundwaters exhibit the most consistent δ18O–temperature correlations. In Western Europe, δ18O variations during deglaciation were primarily temperature-driven, allowing δ18O to be used as a paleothermometer on glacial–interglacial timescales. In contrast, weaker correlations in North America, Asia, and Africa likely reflect stronger influences from atmospheric circulation, convection, and/or continental moisture recycling. For Western Europe, the observed relationships are consistent with a simple Rayleigh distillation model, although sites further inland seem to reflect more complex atmospheric processes. The mean European fossil groundwater-based δ18O–temperature slope, corrected for ice-volume effects since the LGM, is 0.48 ± 0.21 ‰ °C−1 (1 SD), comparable to modern monthly relationships for precipitation (0.29 ± 0.07 ‰ °C−1, 1 SD) but lower than modern interannual estimates (1.04 ± 0.30 ‰ °C−1, 1 SD). These results provide robust constraints for interpreting isotope-based climate archives and evaluating isotope-enabled climate models.

Water stable isotopologues, particularly H₂¹⁸O and HD¹⁶O, are valuable tracers of physical and dynamical processes within the hydrological cycle. These isotopologues have been widely incorporated into isotope-enabled atmospheric general circulation models to constrain moisture sources and transport pathways. However, comprehensive evaluations of such models over the Tibetan Plateau (TP), a region whose complex orography substantially modulates South Asian monsoon dynamics, remain scarce. Here, we systematically evaluate simulations of water vapor isotopic composition (δ¹⁸O_v) from the ECHAM6-wiso model against daily in-situ observations from four high‐altitude stations (Kathmandu, Lulang, Namco, and Muztag) on the Tibetan Plateau, spanning January 2020 to November 2021. The model successfully reproduces the spatial distribution and seasonal cycle of δ¹⁸O_v; however, probability density functions reveal a systematic underestimation of isotopic depletion. Through a multi‐scale temporal decomposition of the daily δ¹⁸O_v time series, we attribute over 50% of the simulation error to deficiencies in representing large-scale atmospheric circulations (periods ≥ 30 days), while the remaining error is linked to synoptic-scale processes (3–7 days) associated with fractionation, including cloud microphysics, post‐condensation effects, and surface evaporation. The model's inability to accurately simulate terrain-induced atmospheric moisture blocking over the TP results in bias in atmospheric circulation variations, thereby amplifying the contribution of circulation-related processes to the overall error. These findings underscore the significance of atmospheric circulation in water vapor isotopic simulations and highlight the value of high‐resolution water vapor isotopic datasets for improving our understanding of moisture source attribution and water‐cycle dynamics in regions of complex topography.

Water stable isotopologues, particularly H₂¹⁸O and HD¹⁶O, are valuable tracers of physical and dynamical processes within the hydrological cycle. These isotopologues have been widely incorporated into isotope-enabled atmospheric general circulation models to constrain moisture sources and transport pathways. However, comprehensive evaluations of such models over the Tibetan Plateau (TP), a region whose complex orography substantially modulates South Asian monsoon dynamics, remain scarce. Here, we systematically evaluate simulations of water vapor isotopic composition (δ¹⁸O_v) from the ECHAM6-wiso model against daily in-situ observations from four high‐altitude stations (Kathmandu, Lulang, Namco, and Muztag) on the Tibetan Plateau, spanning January 2020 to November 2021. The model successfully reproduces the spatial distribution and seasonal cycle of δ¹⁸O_v; however, probability density functions reveal a systematic underestimation of isotopic depletion. Through a multi‐scale temporal decomposition of the daily δ¹⁸O_v time series, we attribute over 50% of the simulation error to deficiencies in representing large-scale atmospheric circulations (periods ≥ 30 days), while the remaining error is linked to synoptic-scale processes (3–7 days) associated with fractionation, including cloud microphysics, post‐condensation effects, and surface evaporation. The model's inability to accurately simulate terrain-induced atmospheric moisture blocking over the TP results in bias in atmospheric circulation variations, thereby amplifying the contribution of circulation-related processes to the overall error. These findings underscore the significance of atmospheric circulation in water vapor isotopic simulations and highlight the value of high‐resolution water vapor isotopic datasets for improving our understanding of moisture source attribution and water‐cycle dynamics in regions of complex topography.

Water isotopes serve as tracers of hydrological processes and as proxies for past climates archived in ice cores. The isotopic signal is acquired throughout the hydrological cycle—through evaporation over the oceans, precipitation, which occurs as moisture is transported from lower to higher latitudes, and during post-depositional processes in which isotopic exchange between snow and atmospheric moisture occurs. Owing to these multiple influences, the relationship between isotope ratios in ice and local temperature varies across Antarctica, and distinct relationships are found when evaluating isotope ratios and temperature across space (for example, in surface snow) compared with temporal correlations at the same site (for example, in precipitation). Here we report measurements of water vapour isotopic compositions from a traverse across East Antarctica, as well as at two fixed sites: the coastal station Dumont D’Urville and Dome C on the plateau. Combining snow and vapour isotopic data, we demonstrate that the temporal and spatial isotope–temperature relationships are distinct because of differences in how the rainout fraction varies across time and space. Our findings support a shift from thinking about the isotope–temperature relationship in terms of distinct temporal and spatial slopes to recognizing that the relationship varies along a continuum based on known dependencies between circulation dynamics and mean climate state. By distilling moisture along moist isentropic transport paths, we can predict the isotope–temperature relationship across either time or space using a physical understanding of large-scale moisture transport under different climatic conditions.

Stable water isotopes serve as tracers of climate processes and are useful for identifying drivers of regional and global hydrological variability. To get a precise picture of spatiotemporal changes in the water cycle, it is necessary to describe the mechanisms in a coherent way among the involved climatic reservoirs (atmosphere, land, and ocean). For example, in the Pacific Ocean, the oxygen isotopic composition (denoted δ ¹⁸ O) of surface seawater recorded in corals is widely used to reconstruct the El Niño–Southern Oscillation (ENSO). However, the influences of atmosphere–ocean feedback and ocean circulation on the δ ¹⁸ O–ENSO relationship are not fully understood. Describing water isotopes in the full climate system is one way to tackle this issue. Therefore, this study introduces the fully coupled isotopic version of the Model for Interdisciplinary Research on Climate version 6 (MIROC6‐iso). MIROC6‐iso exhibits good performance in reproducing spatial isotopic variations in precipitation, water vapor, and ocean water, and the relationships of δ ¹⁸ O with temperature and salinity against observations. We also find that the atmosphere‐ocean Coupled General Circulation Model (CGCM) captures δ ¹⁸ O _sw (seawater) variations significantly better than a configuration where an Atmospheric General Circulation Model is coupled to a 1D slab ocean model in Pacific Ocean in climatological condition and different ENSO phases. Budget analysis indicates that the better performance of the CGCM configuration is due to fully resolved oceanic vertical mixing and horizontal advection. This study shows that MIROC6‐iso is useful to reconstruct past climate changes and examine the recent changes in the water cycle.