Didier Hauglustaine

Understanding the global hydrogen (H ₂ ) budget is critical as H ₂ is expected to play an important role in future energy systems. Tropospheric H ₂ sources include direct emissions and atmospheric production via chemical reactions, while sinks are soil uptake and removal by hydroxyl radical (OH). Large uncertainties remain in quantifying the atmospheric production and loss of H ₂ due to large uncertainties in H ₂ uptake in the soil and the lack of global-scale knowledge of the abundance of OH. We use a suite of global three-dimensional Atmospheric Chemistry Models (ACM) to evaluate key reactive species involved in atmospheric production and loss – formaldehyde (HCHO), nitrogen dioxide (NO ₂ ), and carbon monoxide (CO). A box model is then used to simulate the evolution of global mean tropospheric H ₂ from pre-industrial to present day; to test different relative contributions in atmospheric production from methane and Volatile Organic Compounds (VOC); and to assess atmospheric loss with different OH concentrations. Isotopic compositions of the different sources and sinks are further used to constrain these terms and assess the potential importance of geological sources to the H ₂ budget. Models generally match each other for HCHO, though model diversity exists for NO ₂ and CO. From model evaluations and box model constraints, we estimate atmospheric H ₂ production of 37–60 Tg yr ⁻¹ , and atmospheric losses of 15–30 Tg yr ⁻¹ , suggesting that some top-down literature estimates may overestimate production. Box model results suggest an upper bound of 9 Tg yr ⁻¹ for geological sources, considerably lower than the 23 Tg yr ⁻¹ proposed previously. We recommend more isotopic observations and targeted measurement campaigns to further refine the budget.

Aviation emissions are predicted to have caused 4 % of anthropogenic warming to date. While aviation CO2 climate effects are well known, the magnitude of non-CO2 effects of aviation are highly uncertain. Nitrogen oxide (NOx) emissions from aircraft affect greenhouse gases: local production of ozone in the short term, and long-term impacts on methane, stratospheric water vapour and ozone. Ozone production is non-linear and depends on the background concentrations of NOx and volatile organic compounds (VOCs). Previous single-model studies have found an increased sensitivity of NOx-induced response to aviation emissions in high-mitigation scenarios compared to low-mitigation scenarios. Here we extend this to a multi-model study, using three models to explore the dependence of aviation NOx effects on background conditions in two future scenarios. We calculate the ozone radiative forcing from a 20 % change in aviation NOx emissions for two different future aviation emission scenarios, running each scenario in a high and low mitigation background. We find that the models agree on some features of ozone sensitivity to background scenario. We calculate a positive net NOx forcing in both future scenarios; in the high mitigation scenario in two of three models the long-term methane forcing is sufficiently negative to make the net NOx forcing negative. We estimate lower aviation net NOx RFs for high-mitigation SSP1 background compared to those for an SSP3 background. There is continued uncertainty in the climate impacts of aviation NOx, and we suggest that more model consensus is required to enable parametrisations of these NOx impacts.

Aircraft emissions of nitrogen oxides (NO_x= NO + NO₂), aerosols, and aerosol precursors provide a non-negligible contribution to the climate impact of air traffic, and the uncertainty in their climate Effective Radiative Forcing (ERF) remains significant. This study presents results from a new model intercomparison of the impact of aircraft emissions involving five state-of-the-art global models including both tropospheric and stratospheric chemistry. Aircraft NO_x increases ozone photochemical production in the free troposphere throughout the year and decreases ozone chemical loss in the high-latitude lowermost stratosphere during spring–early summer. The models generally agree on the spatial pattern of NO_x, ozone, and hydroxyl radical (OH) responses. The NO_x net ERF is systematically positive with a model mean of 18.3 mW m⁻², ranging from 9.4 to 24.5 mW m⁻² among the different models. This net NO_x forcing is reduced by 35 % and 43 % accounting for the negative forcing arising from the formation of nitrate and sulfate particles, respectively. Estimates of the aerosol direct ERF are systematically negative and range between −6.5 and −17.8 mW m⁻², compensating most of the net NO_x ERF albeit with noticeable intermodel differences arising from the diversity in aerosol parameterizations. This work shows encouraging results regarding our confidence in aviation NO_x-induced ozone response because of a good model agreement. To a lesser extent, some similarities in the results regarding aerosols are also encouraging, given the few existing model intercomparisons on this topic. However, the results also highlight areas where further modeling experiments are needed, both with more models and with dedicated sensitivity simulations to further understand the factors giving rise to the spread in model estimates of aviation emission impacts on atmospheric composition and climate.

Volatile organic compounds (VOCs) play an important role in atmospheric chemistry, influencing the cycling of peroxy and hydroxyl radicals, the formation of tropospheric ozone, hydrogen, secondary organic aerosol, and the lifetime of methane and other greenhouse gases. Their interactions shape overall atmospheric composition and air quality, with implications for both climate and human health. Given their significance, it is crucial for global atmospheric chemistry models to represent VOCs adequately for any given scientific question. In this context, we introduce the Volatile Organic Compound Model Intercomparison Project (VOCMIP) and invite atmospheric chemistry modelling groups to participate in this collaborative effort. VOCMIP aims to identify model consistencies and discrepancies, enhance the formulation of chemical mechanisms, and advance our understanding of VOC-related processes in the atmosphere. Global atmospheric chemistry model output will be compared to in situ measurements from surface stations and aircraft campaigns, plus satellite data for key VOCs. Special emphasis will be placed on formaldehyde (HCHO), examining its chemical sources and sinks given its central role as a radical source and as an intermediate in the photochemical destruction of VOCs.

Aircraft emissions of nitrogen oxides (NO_x=NO+NO₂), aerosols, and aerosol precursors provide a non-negligible contribution to the climate impact of air traffic, and the uncertainty on their Effective Radiative Forcing (ERF) of climate remains significant. This study presents results from a new model intercomparison of the impact of aircraft emissions involving five state-of-the-art global models including both tropospheric and stratospheric chemistry. Aircraft NO_x increases ozone photochemical production in the free troposphere throughout the year and decreases ozone chemical loss in the high-latitude lowermost stratosphere during spring–early summer. The models generally agree on the spatial pattern of NO_x, ozone, and hydroxyl radical (OH) responses. The NO_x net ERF is systematically positive and ranges from 7.3 to 22.1 mW m^-2 among the different models (14.1–22.1 mW m^-2 without the least sensitive model). Estimates of the aerosol direct ERF are systematically negative and range between -6.5 and -17.8 mW m^-2, with differences arising from the diversity in model aerosol parameterizations. This work shows encouraging results regarding our confidence in aviation NO_x-induced ozone response because of a better model agreement. However, results also highlight areas where further modeling experiments are needed, both with more models and with dedicated sensitivity simulations to further understand the factors giving rise to the spread in model estimates of aviation emission impacts on atmospheric composition and climate.

Aircraft emissions of nitrogen oxides (NO_x=NO+NO₂), aerosols, and aerosol precursors provide a non-negligible contribution to the climate impact of air traffic, and the uncertainty on their Effective Radiative Forcing (ERF) of climate remains significant. This study presents results from a new model intercomparison of the impact of aircraft emissions involving five state-of-the-art global models including both tropospheric and stratospheric chemistry. Aircraft NO_x increases ozone photochemical production in the free troposphere throughout the year and decreases ozone chemical loss in the high-latitude lowermost stratosphere during spring–early summer. The models generally agree on the spatial pattern of NO_x, ozone, and hydroxyl radical (OH) responses. The NO_x net ERF is systematically positive and ranges from 7.3 to 22.1 mW m^-2 among the different models (14.1–22.1 mW m^-2 without the least sensitive model). Estimates of the aerosol direct ERF are systematically negative and range between -6.5 and -17.8 mW m^-2, with differences arising from the diversity in model aerosol parameterizations. This work shows encouraging results regarding our confidence in aviation NO_x-induced ozone response because of a better model agreement. However, results also highlight areas where further modeling experiments are needed, both with more models and with dedicated sensitivity simulations to further understand the factors giving rise to the spread in model estimates of aviation emission impacts on atmospheric composition and climate.

Aviation emissions are predicted to have caused 4 % of anthropogenic warming to date. While aviation CO₂ climate effects are well known, the magnitude of non-CO₂ effects of aviation are highly uncertain. Nitrogen oxide (NOx) emissions from aircraft affect greenhouse gases: local production of ozone in the short term, and long-term impacts on methane, stratospheric water vapour and ozone. Ozone production is non-linear and depends on the background concentrations of NOx and volatile organic compounds (VOCs). Previous single-model studies have found an increased sensitivity of NOx-induced response to aviation emissions in high-mitigation scenarios compared to low-mitigation scenarios. Here we extend this to a multi-model study, using three models to explore the dependence of aviation NOx effects on background conditions in two future scenarios. We calculate the ozone radiative forcing from a 20 % change in aviation NOx emissions for two different future aviation emission scenarios, running each scenario in a high and low mitigation background. We do not find a consistent sensitivity of ozone response to NOx background between the models used. The inter-model variability in ozone response is larger than the effect of different background scenarios. We calculate a positive net NOx forcing in both future scenarios; in the high mitigation scenario in two of three models the long-term methane forcing is sufficiently negative to make the net NOx forcing negative. There is continued uncertainty in the climate impacts of aviation NOx, and we suggest that more model consensus is required to enable parametrisations of these NOx impacts into simplified models.

Hydrogen (H2) is expected to be a crucial substitute for fossil fuels in the ongoing energy transition. However, atmospheric H2 is recognized as an indirect greenhouse gas, as it can contribute to global warming through its coupling to the atmospheric oxidative capacity, including interactions with methane, ozone and stratospheric water vapour. Yet, large uncertainties still remain in the atmospheric H2 budget.Within the framework of the HYway Horizon Europe project, we quantify and diagnose key chemical drivers of uncertainty in simulated H2 by focusing on two components: chemical production linked to formaldehyde (HCHO) and chemical destruction controlled by the hydroxyl radical (OH). HCHO photolysis is a major source of atmospheric H2, accounting for more than half of the global total. We firstly assess the HCHO chemical production, loss, and global burden across multiple HYway models. The global HCHO burden range from 0.69 to 1.03Tg, indicating a wide inter-model difference of 50%. Therefore, we evaluate the simulated HCHO by ground- and satellite-based observations. The contribution of biogenic hydrocarbons emissions to the HCHO budget is discussed. Oxidation by OH is the second-largest sink of atmospheric H2 after soil uptake. However, the modeled OH is typically overestimated by global chemical models. Here, we examine uncertainty in H2 chemical loss by characterizing the range of OH simulated across HYway models, and by comparing simulated OH with observation-constraint OH fields. Finally, we present a free-running H2 simulation (driven by emissions rather than prescribed surface H2 concentrations) and show that the resulting H2 fields are consistent with observations. Overall, this work provides an integrated evaluation of the chemical controls on H2 in global models and a basis for improving H2 projections under future emission scenarios.

Green ammonia (NH3) is produced based on green hydrogen (H2) and has recently gained wide interest due to its potential to decarbonize ammonia production, and as a carbon-free solution for energy storage and transportation. However, the production and use of ammonia come with other climate and environmental challenges due to its alteration of the Earth’s nitrogen cycle. Before introducing new ammonia technologies on a large scale, it is important to thoroughly understand current atmospheric impacts of anthropogenic reactive nitrogen (Nr), mainly the impacts of ammonia, nitrogen oxides (NOX) and nitrous oxide (N2O) emissions. The present work addresses pre-industrial (1850) to present-day (2019) climate effects of Nr, and has been published in Hodnebrog et al. (2025, Nature, https://doi.org/10.1038/s41586-025-09337-9).We use five independent latest-generation atmospheric chemistry models (OsloCTM3, CESM2, GISS ModelE, GFDL-AM4.1 and LMDZ-INCA), and find that the change over the industrial era of nitrate and sulfate aerosol abundances owing to Nr emissions varies greatly across the models. Consequently, the direct aerosol radiative forcing (RF) differs widely by model, even in sign. The positive ozone and negative methane RF due to Nr emissions also vary widely between models. While all five models show a net negative RF (i.e., cooling) due to historical anthropogenic Nr emissions, the net climate effect is the sum of several terms that vary in sign and are associated with substantial uncertainties. Future research is clearly needed, both to better define and narrow the uncertainties on the climate effects given here and to quantify climate effects for processes for which estimates do not yet exist (for example, aerosol-cloud interactions due to Nr emissions).