Christophe Cassou

The implementation of adaptation policies requires seamless relevant information about near-term climate evolution, which remains highly uncertain due to the strong influence of internal variability. The recent development of approaches to improve near-term climate information by selecting members from large ensembles – based on their agreement with either observed or predicted sea surface temperature patterns – have shown promising results across timescales from weeks to decades. Here, we propose a new method to provide climate forecasts over Europe by combining information from both observations and decadal predictions through a two-stage member selection from ensembles of climate simulations. Several predictors are tested as observational metrics based on their influence on the European climate variability at annual to decadal timescale. A retrospective evaluation over Europe demonstrates the added value of this method in reducing the spread of uncertainty stemming from both internal climate variability and model uncertainty. This method can outperform historical simulations in 5-, 10-, and 15-year temperature forecasts of summer and winter temperature over Europe. It can also provide larger forecast added value than decadal prediction, for example for land summer temperature over WCE, using surface temperature as predictor. The optimal predictor varies by region and should be evaluated on a case-by-case basis. This improved regional climate information supports more targeted adaptation strategies for the coming decades.

Triple oxygen isotopes are powerful tracers of hydrological processes, yet their variability in atmospheric water vapor and the underlying controls remain poorly understood. We present a one-year record of triple oxygen and hydrogen isotopes of atmospheric water vapor (δ18OV, d-excessV, 17O-excessV) measured below, within and above a downy oak forest canopy at the AnaEE platform O3HP in the French Mediterranean. This vapor dataset is complemented by isotopic data of precipitation (δ18OP, d-excessP, 17O-excessP) and groundwater, as well as monthly observations of stomatal conductance and transpiration. Seasonal variations in 17O-excessV and d-excessV likely reflect changing evaporative conditions at oceanic moisture sources. d-excessP and 17O-excessP show a similar seasonal pattern, enhanced by summer rain re-evaporation. However, no clear isotopic differences were observed in vapor or precipitation derived from different oceanic source regions and weather regimes, likely due to frequent mixing of multiple moisture sources. Diurnal variations in 17O-excessV and d-excessV reflect a combination of vegetation-related processes, including local evapotranspiration. However, the impact of evapotranspiration was not evident at daily timescales. Although precipitation often deviates from isotopic equilibrium with near-surface atmospheric water vapor at the event scale due to incomplete equilibration and rain re-evaporation, equilibrium water vapor reliably approximates the near-surface isotopic composition of atmospheric water vapor at annual scale. Our results highlight the potential of 17O-excess for understanding water exchange between the land and the atmosphere, regardless of climatic and vegetation conditions. They enhance the mechanistic interpretation of precipitation isotopes, which is essential for reliable paleoclimate reconstructions.

In a rapidly changing climate, evidence-based decision-making benefits from up-to-date and timely information. We track twelve key sets of indicators of the state of the climate system, closely following Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment report (AR6) methods, to produce our fourth annual publication. One of the indicators, the Earth's energy imbalance (EEI) provides a crucial integrative measure of the overall heating of the planet and the pace of climate change – this has more than doubled since the 1976–1995 period. A newly added indicator of temperature extremes, the number of days experiencing marine heatwaves, has more than tripled between 1991 and 2025. For the 2016–2025 decade average, observed warming relative to 1850–1900 was 1.26 [1.13 to 1.36] °C, of which 1.24 [1.0 to 1.5] °C was human-induced. Human-induced warming reached 1.37 °C relative to 1850–1900 in the year 2025, increasing at a rate of 0.27 [0.2–0.4] °C per decade over 2016–2025. This high rate of warming, which matches the all-time high seen last year in the instrumental record, was caused by a combination of greenhouse gas emissions being at an all-time high of 54.6 ± 5.5 GtCO ₂ e yr ⁻¹ over the last decade (2015–2024), as well as reductions in the strength of aerosol cooling. Despite this, there is evidence that CO ₂ emission growth is slowing. The continuation of these annual updates could track decreases or increases in the rate of human influence and climatic changes presented here, reflecting the outcomes of societal choices during the critical 2020s decade. The data presented herein can provide a useful reference point for the drafting of the IPCC seventh assessment report. In total, we employ analysis from over 40 global datasets (https://doi.org/10.5281/zenodo.20499280, Smith et al., 2026a). Future monitoring of these indicators, such as ocean and satellite measurements of the Earth's energy imbalance, are threatened by geopolitical and public funding decisions. Our ability to consistently track many of the indicators requires the continuity of observation programs and coordination mechanisms, including the Global Climate Observing System (GCOS) program, that enable their effective integration and use.

In a rapidly changing climate, evidence-based decision-making benefits from up-to-date and timely information. We track twelve key sets of indicators of the state of the climate system, closely following Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment report (AR6) methods, to produce our fourth annual publication. One of the indicators, the Earth's energy imbalance (EEI) provides a crucial integrative measure of the pace of climate change – this has more than doubled since the 1976–1995 period. A newly updated indicator of temperature extremes, the number of days experiencing marine heatwaves, has more than tripled between 1991 and 2025. For the 2016–2025 decade average, observed warming relative to 1850–1900 was 1.26 [1.13 to 1.36] °C, of which 1.24 [1.0 to 1.5] °C was human-induced. Human-induced warming reached 1.37 °C relative to 1850-1900 in the year 2025, increasing at a rate of 0.27 [0.2–0.4] °C per decade over 2016–2025. This high rate of warming, which matches the all-time high seen last year in the instrumental record, was caused by a combination of greenhouse gas emissions being at an all-time high of 54.6 ± 5.5 GtCO2e per year over the last decade (2015–2024), as well as reductions in the strength of aerosol cooling. Despite this, there is evidence that CO2 emission growth is slowing. The continuation of these annual updates could track decreases or increases in the rate of human influence and climatic changes presented here, reflecting the outcomes of societal choices during the critical 2020s decade. In total, we employ analysis from over 40 global datasets (https://doi.org/10.5281/ZENODO.7883757 Smith et al., 2026a) These data are threatened by geopolitical and public funding decisions that are cutting support for key satellite and in-situ observing programs critical for the monitoring of the atmosphere, ocean, cryosphere and land surface, including long-term data preservation and provision that is key to understanding our changing climate for today and for future generations. Overall, the Global Climate Observing System (GCOS) framework, which supports much of our international capability, is under threat. Our ability to monitor effectively many of the indicators presented herein is not guaranteed without concerted international action to ensure the continuity of observation programs and coordination mechanisms, including the GCOS program, that enable their effective integration and use.

Global surface air temperature reached unprecedented heights in early boreal fall 2023, surpassing the previous record for year-to-year temperature increase by a significant margin. We attribute most of this temperature jump to the onset and maturing stages of the 2023 El Niño, with some contributions from the North Atlantic. Using a process-based analysis from multiple observational datasets, we show that the uniqueness of the 2023 event arose from the La Niña-like ocean-atmosphere state on which it developed. This background favoured (1) a steep year-to-year increase of Sea Surface Temperature, particularly in mean atmospheric subsidence regions, reducing low-cloud cover and giving rise to a record-breaking change in the radiative budget; (2) anomalously sustained precipitation over high sea surface temperatures in the Western Pacific, fuelling unusual diabatic heating compared to canonical El Nino events. This altogether led to an exceptionally early increase in tropical tropospheric temperature in boreal fall, ultimately influencing the jump in temperature at the global scale.

Triple oxygen isotopes are a powerful tracer of hydrological processes, yet their variability in atmospheric water vapor and the processes driving them remain poorly understood. We present a one-year record of triple oxygen and hydrogen isotopes of atmospheric water vapor (V) measured at four heights below and above a downy oak forest canopy at the AnaEE platform O₃HP in the French Mediterranean. This vapor dataset is complemented by isotope data from rainfall and groundwater, as well as monthly measurements of stomatal conductance and transpiration. Our results demonstrate that ¹⁷O-excess_V is principally driven by evaporation processes. Seasonal variations in ¹⁷O-excess_V ranging from 33 ± 9 per meg in winter to 25 ± 6 per meg in summer, reflect evaporative conditions in oceanic moisture sources. Diurnal variations, particularly pronounced in summer, with daytime maxima around 33 ± 6 per meg and nighttime minima around 16 ± 7 per meg, are linked to local evapotranspiration and isotope exchange between leaf waters and the atmosphere. On a monthly scale, precipitation is generally close to isotope equilibrium with atmospheric water vapor, except in summer when rain re-evaporation occurs. At event scale, large deviations from isotope equilibrium can occur due to raindrop evaporation and incomplete re-equilibration. Our findings enhance the mechanistic basis for interpreting precipitation isotopes in paleoclimate context, improves the robustness of isotope-based model evaluation, and highlights the potential of ¹⁷O-excess for better understanding of land-atmosphere water exchange across diverse climate and vegetation contexts.

Triple oxygen isotopes are a powerful tracer of hydrological processes, yet their variability in atmospheric water vapor and the processes driving them remain poorly understood. We present a one-year record of triple oxygen and hydrogen isotopes of atmospheric water vapor (V) measured at four heights below and above a downy oak forest canopy at the AnaEE platform O₃HP in the French Mediterranean. This vapor dataset is complemented by isotope data from rainfall and groundwater, as well as monthly measurements of stomatal conductance and transpiration. Our results demonstrate that ¹⁷O-excess_V is principally driven by evaporation processes. Seasonal variations in ¹⁷O-excess_V ranging from 33 ± 9 per meg in winter to 25 ± 6 per meg in summer, reflect evaporative conditions in oceanic moisture sources. Diurnal variations, particularly pronounced in summer, with daytime maxima around 33 ± 6 per meg and nighttime minima around 16 ± 7 per meg, are linked to local evapotranspiration and isotope exchange between leaf waters and the atmosphere. On a monthly scale, precipitation is generally close to isotope equilibrium with atmospheric water vapor, except in summer when rain re-evaporation occurs. At event scale, large deviations from isotope equilibrium can occur due to raindrop evaporation and incomplete re-equilibration. Our findings enhance the mechanistic basis for interpreting precipitation isotopes in paleoclimate context, improves the robustness of isotope-based model evaluation, and highlights the potential of ¹⁷O-excess for better understanding of land-atmosphere water exchange across diverse climate and vegetation contexts.

The implementation of adaptation policies requires seamless relevant information about near-term climate evolution, which remains highly uncertain due to the strong influence of internal variability. The recent development of approaches to improve near-term climate information by selecting members from large ensembles – based on their agreement with either observed or predicted sea surface temperature patterns – have shown promising results across timescales from weeks to decades. Here, we propose a new method to provide climate forecasts over Europe by combining information from both observations and decadal predictions through a two-stage member selection from ensembles of climate simulations. Several predictors are tested as observational metrics based on their influence on the European climate variability at annual to decadal timescale. A retrospective evaluation over Europe demonstrates the added value of this method in reducing the spread of uncertainty stemming from both internal climate variability and model uncertainty. This method can outperform both historical simulations and decadal prediction in 5- 10- and 15-year temperature forecasts of winter MED, as well as summer NEU and WCE. Significant skill improvements are visible for 10- and 15-year forecasts of winter Mediterranean surface temperature over land, when using the North Atlantic Oscillation or the Atlantic Multidecadal Variability as predictors in the first selection. The optimal predictor varies by region and should be evaluated on a case-by-case basis. This improved regional climate information supports more targeted adaptation strategies for the coming decades.