The pace of anthropogenic climate change shows no signs of slowing down. The latest IPCC report highlights that every compartment of the Earth System is impacted by the catastrophic consequences of anthropogenic climate change. In the ocean, one major consequence is the loss of oxygen, which has been measured for decades and raises pressing concerns about marine resources, but model projections of future ocean deoxygenation are notoriously uncertain. Fortunately, studying past occurrences of rapid climate warming and marine oxygen stress provides us with a unique mean to draw constraints on potential thresholds in the Earth system and to gain insights on model ability to reproduce the consequences of such changes. In this project, we specifically focus on a tailored episode of brutal climate warming: the Paleocene-Eocene Thermal Maximum (56 Ma). Apart from the pace of change, this event indeed shares striking similarities with the anthropogenic climate change: 1) it is a perturbation exclusively driven by carbon input in the surficial reservoirs of the Earth; 2) it is geologically instantaneous, as carbon is input in the atmosphere and ocean in less than 20 kyrs (best guess 6 kyrs); 3) estimates of added carbon are of the same magnitude as RCP8.5 projections at the 2300 horizon; 4) redox proxies document a global deoxygenation. Because of these characteristics, the PETM has fostered the recovery of numerous archives and is perhaps the most-well covered interval of pre-Quaternary climates. We propose, for the first time, to use an Earth System Model of complexity and resolution close to those used in recent CMIP exercises to investigate the transient dynamical, biogeochemical and oxygen responses to the PETM. Unique lessons about CMIP-class model’s ability to simulate rapid warming perturbations are expected to emerge from the results and to inform the next generation of models used to project our future.

Laser absorption techniques are increasingly used for isotope ratio measurements. These methods offer specific advantages such as the ability to discriminate between isobaric isotopologues (e.g., 16O-13C-16O vs 16O-12C-17O). Their precision, however, still lags behind that of modern mass spectrometers. This project aims to develop an integrated dual-inlet laser spectrometer capable of measuring isotopologue ratios in CO2 with an internal precision on the order of 0.01 permil, allowing measurements of subtle but significant isotopic anomalies in the oxygen isotope ratios (18O/16O and 17O/16O) of atmospheric CO2 and/or carbonate minerals. The corresponding “O-17 excess” (Delta-17O) is an important tracer of atmospheric chemistry and paleo-hydrology, but its measurement in CO2 (as opposed to O2 or H2O) remains challenging because of isobaric interference. Development of this new instrument will be based on close collaboration between stable isotope geochemists and laser spectroscopists from LSCE (Laboratoire des Sciences du Climat et de l'Environnement) and LIPhy (Laboratoire Interdisciplinaire de Physique). Building on our previous work and proof-of-concept experiments, we propose to build the first of a new generation of ultra-precise cavity ring-down spectrometers and to combine it with a custom dual-inlet system derived from existing IRMS devices. In order to attain the extreme precision levels quoted above, we plan to lock a DFB laser near 1.6~µm using optical feedback from a specially designed, ultra-stable "source cavity", resulting in a source with a very narrow linewidth around a highly stable center frequency, which will be injected in one or more ring-down cavities containing sample or reference gases. Design, assembly and initial testing will be performed at LIPhy, and the laser spectrometer will then be transferred to LSCE for accuracy tests, further optimizations, and initial geochemical applications. This process will take place in the context of a new PhD project which will include both instrumental development and early scientific applications, under dual LIPhy/LSCE supervision. As an added benefit, this will ensure a good transfer of theoretical and practical expertise from one lab to the other. By the end of this work, the instrument should be fully operational at LSCE, providing important new observations for paleo-climate and atmospheric studies. Technical advances from this project will provide a foundation for future implementations of this technique, and will certainly benefit a wide range of other laser spectrometric applications.

The level of oxygen in the Earth’s atmosphere is unique in our solar system and reflects the long history of life on Earth. It also leads to a singular oxidizing chemistry that regulates the levels of atmospheric reactive trace gases. This is of importance since some of these reactive gases, even if in trace amount, can strongly interact with life and climate. Indeed, reactive trace gases might have been key factors in massive life extinctions in the deep past via abrupt climate warming, the collapse of the stratospheric protective ozone layer, or intense acid rains. During the Cenozoic era (the last 66 Ma), environmental conditions have vastly varied but these changes, less abrupt or more localized than previously in the Earth history, have allowed life to diversify continuously and allowed in particular the mammalian evolution. If there were changes in the atmospheric composition, notably oxidizing capacity, during this period, those changes should therefore have been spatially limited or of moderate amplitude. Yet the Cenozoic era covers a wide variety of environmental conditions related to a gradual cooling of the climate of great amplitude (> 20 ° C) from a hot world, with a very strong greenhouse effect allowing tropical vegetation at high latitudes and very active carbon and nitrogen biogeochemical cycles, to the current glacial climate. In such varied climate and environments, one can expect modifications in the regulation of reactive compounds by atmospheric chemistry. Over the last decade, field observations and then laboratory and theoretical works have revealed chemical mechanisms involved in the pristine atmospheres, such as the recycling of radicals over forests or a new halogenated chemistry over the oceans. This has definitely changed our vision of the oxidising capacity of the atmosphere in untouched areas. In PaleOX, we aim to explore how the oxidizing capacity of the atmosphere has evolved throughout the Cenozoic era and how this has affected the lifetime of reactive short-lived climate forcers such as ozone or methane. To this end, PaleOX aims to study the atmospheric reactivity for five key periods of the Cenozoic by bridging the gap between the cutting-edge past climate modelling methodologies and the state of the art in atmospheric chemistry. A new Earth system model with up-to-dated representations of pristine atmospheric chemistry at its heart will be assembled to simulate consistently atmospheric chemical composition and climate at different stages of the Cenozoic. In parallel, unreleased samples of volcanic sulfate deposits coupled with advanced analysis of their isotopic composition will bring new information on the importance of past atmospheric oxidation pathways, bringing valuable constrains to the numerical analyses. The ultimate goal of this project is to determine how the self-cleaning capacity of the atmosphere changed during the Cenozoic era taking into account information and assumptions about the evolution of vegetation, fires and climatic constraints based on various proxies available in the literature. The fact that modelling methodologies currently used to study deep time climates tend to neglect interactions between chemical cycles of short lived climate forcers and climate will be examined. The possible role of these chemistry-climate links in modulating climate change and gradients will be explored and conditions of surfaces (e.g. UV levels, concentrations of compounds that can alter the functioning of ecosystems, acid deposition) will be characterized. The feedback loops induced by changes in chemical composition of the atmosphere will be assessed in various contexts.

The ERC proposal BOREAS-STG19 was about winter cold-spells. The goal was to determine The (potential) changes in cold spells frequency/intensity due to anthropogenic forcing. Since they are notably difficult to determine because they rely on chaotic properties of the atmospheric circulation whose dynamics is high dimensional and turbulent, BOREAS-STG19 aimed at evaluating the role of thermodynamic vs dynamical components of these events from large to small spatial scales tracking energy exchanges in space, time and scale-space. The criticisms expressed by the PE10 panel focused on three aspects: i) the definition of cold-spell was misleading as BOREAS was focusing both on cold and snowy spells with the role of snowfall being unclear, ii) the usefulness of studying cold-spells in a context of warming climate was questioned, iii) the use of low dimensional models was questioned because their resolution is not adequate to represent small scales feedback for cold events. The BOREAS project submitted to the STG20 call addresses these issues by: i) focusing the project on snowstorms instead of cold spells, ii) showing evidence of the fact that the link between average warming and occurrence of snowstorms is far from being trivial, iii) substituting low dimensional climate models with state of the art convection permitting simulations, capable to provide a detailed representation of the physical processes leading to snowstorms. The abstract submitted to the STG20 call follows: “BOREAS assesses how climate change modify the frequency and intensity of snowstorms affecting European large populated areas in winter time. Anthropogenic emissions are responsible for temperature increase. Nonetheless, we still observe winter snowstorms. This apparent contradiction comes from the subtle effects of climate change on extreme events driven by the atmospheric circulation. Indeed, snowstorms are associated to extratropical cyclones which propagate in a southward– rather than eastward- direction as a result of the meandering of the mid-latitude jet-stream. In the present climate, the combination of higher surface winter temperatures with the advection of lower atmospheric cold air associated with those extratropical cyclones enhances evaporation and convective precipitations, often leading to more intense snowstorms than in the past. In a future climate it is unclear whether the effect of temperature increase will prevail on that of potential changes in the jet stream and convective feedback. BOREAS will clarify this by: 1) Tracking the changes in jet meandering amplitude in future emissions scenarios by projecting the jet dynamics in CMIP6 simulations on a subset of instantaneous indicators that track waviness, predictability and persistence of circulation patterns. This will assess the abundance of weather patterns possibly leading to snowstorms in the future. 2) Evaluating the capability of local convective feedback to enhance snowfalls despite increasing temperatures via a storyline approach based on simulations of past & present snowstorms with convection-permitting models, forced with future boundary conditions (e.g. higher sea-surface temperatures, different land-use & vegetation). Simulations will provide a comprehensive overview on future European snowstorms.” Based on these improvements, I request the ANR-Tremplin the funding to start the analysis of large scale drivers of snowstorms in existing climate simulations by evaluating the modifications in the tracks of extratropical cyclones travelling from polar regions to mid-latitudes in wintertime. The goal will be to assess changes in the frequency of polar extratropical cyclones travelling to mid-latitudes by evaluating the changes in the zonality of mid-latitude jet, projecting its dynamics on dynamical indicators and measuring shifts in waviness of mid-latitude patterns between present and future scenarios.

The main aim of this project is to study the uncertainties of statistical regionalization (or downscaling) of precipitation (PR) and temperature (T) at high resolution (HR, here 0.25°x0.25°) and their hydrological impacts in the Mediterranean region. Indeed, this region has geographical and environmental specificities (e.g., mountains, ocean and sea, high density of population) making necessary but particularly complex the downscaling of precipitation and its HR modeling. Moreover, this region has been identified as a “hot-spot” by IPCC (2007) for future climate changes. Indeed, while the signs of the trends seem defined, strong uncertainties remain in the intensity, the patterns and distributions of those changes. In this context, the StaRMIP project aims at intercomparing the whole set of the main statistical approaches for the regionalization of PR ant T (still too often considered as “black boxes”) in their uses and conceptual differences. Ensembles of HR statistically downscaled PR and T will be generated and the simulated fields will be integrated into hydrological models in order to evaluate their quality in space and time via sensitivity analyses. Based on those evaluations, a new statistical downscaling model will be developed to fill in the most important weaknesses and gaps of the state-of-the-art models. The HR projections for future, plugged into the hydrological models, will also serve to forecast future water availability under the constraint of various climatic projections. Hence, this project will provide: (i) Ensembles of control (CTRL, 1989-2010) and future (2021-2050 and 2051-2080) HR statistical simulations of PR and T according to different climate models and scenarios over the Mediterranean region; (ii) Guidelines to their relevant uses and interpretations; (iii) Indicators quantifying the quality (in space, time and extreme representation) of the HR simulations; (iv) An as-generic-as-possible statistical downscaling model improving the state-of-the art models; (v) Retrospective and prospective hydrological scenarios of water availability on several Mediterranean catchments; ands (vi) Uncertainty assessment of the climate and hydrological simulations, using well dedicated statistical models. The proposed intercomparisons and studies will also be extended to a part of the datasets of the dynamical simulations available from the Med-CORDEX project. The results brought by the different studies will provide theoretical and practical “guidelines” for the applications of the statistical regionalization approaches, whose relative strengths, weaknesses, and potential improvements are still not well known while they are more and more applied. Moreover, the statistical downscaling algorithms, the high-resolution simulations, as well as the evaluation techniques of those, will be freely provided to the scientific community, and a website will be created to promote the various deliverables.