Faits marquants 2020

Climate models are the major tools to study the climate system and its evolutions in the future. However, climate simulations often present statistical biases: even for the past climate, the statistical attributes of modeled variables (e.g., the mean or spatial correlations) can differ from those calculated based on observations (or reanalysis). Therefore, it is often needed to adjust climate simulations before using it, for instance, for climate change impact studies. Recently, some multivariate bias correction methods have been proposed in the literature to restore inter-variable and spatial correlations of climate simulations. However, these methods present different specificities in terms of applicability and assumptions which are not yet fully apprehended by researchers and practitioners. In this study, we intercompared 4 existing multivariate bias correction methods in order to help practitioners in the choice of methods for their applications [1]. Although the 4 methods showed a great ability to adjust the desired multivariate statistical properties, caution has to be taken before applying theses methods. It has indeed been noted that instability can be observed for some methods when considering a high number of variables to correct. Moreover, none of the multivariate bias correction methods considered in this study are designed to adjust the temporal properties of climate simulations, which leads to obtain corrected data with potential unexpected temporal behaviors. Developing new multivariate bias correction methods that are able to correct, on the one hand, inter-variable and spatial correlations and, on the other hand, temporal properties of climate simulations is an interesting perspective on which the ESTIMR team is actively working.
 

[1] Bastien François, Mathieu Vrac, Alex J. Cannon, Yoann Robin and Denis Allard. Multivariate bias corrections of climate simulations: Which benefits for which losses? Earth System Dynamics, 2020.

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Dès lors qu'un avis lui est demandé par un décideur ou un porteur d'enjeux, le scientifique coiffe sa casquette d'expert et sort à la fois de sa zone de compétence propre et de sa zone de confort pour se confronter aux décisions publiques alors que les enjeux et les pressions peuvent être considérables.
Pourtant, la posture du "sachant", si elle est attendue, souvent implicitement, lors de la sollicitation, est risquée à plus d'un titre : s'exprimer sur des champs scientifiques qui ne relèvent pas de sa spécialité propre, risque d'occulter des incertitudes signifiantes mais en dehors du paradigme habituel de travail dans son propre champ de spécialité, responsabilité engagée en cas d'erreur de priorisation du discours ou de mauvaise compréhension. Le chercheur est également soumis à une stratégie personnelle de carrière, à des enjeux de valorisation de son propre collectif de recherche, à des biais psychologiques comme le biais de confirmation ou la minoration des processus ou résultats objets de polémiques. Pour ces raisons, le discours d'un expert individuel n'engage que lui et ne saurait être normatif.

The Surface Water and Ocean Topography (SWOT) space mission will map surface area and water level changes in lakes at the global scale. Such new data are of great interest to better understand and model lake dynamics as well as to improve water management. In this study, we used the large-scale SWOT simulator developed at the French Space National Center (CNES) to estimate the expected measurement errors of the water level of different water bodies in France. These water bodies include five large reservoirs of the Seine River and numerous small gravel pits located in the Seine alluvial plain of La Bassée upstream of the city of Paris. The results show that the SWOT mission will allow to observe water levels with a precision of a few tens of centimeters (10 cm for the largest water reservoir (Orient), 23 km²), even for the small gravel pits of size of a few hectares (standard deviation error lower than 0.25 m for water bodies larger than 6 ha). The benefit of the temporal sampling for water level monitoring is also highlighted on time series of pseudo-observations based on real measurements perturbed with the simulated noise errors. Then, the added value of these future data for the simulation of lake energy budgets is shown using the FLake lake model through sensitivity experiments. Results show that the SWOT data will help to model the surface temperature of the studied water bodies with a precision better than 0.5 K and the evaporation with an accuracy better than 0.2 mm/day. These large improvements compared to the errors obtained when a constant water level is prescribed (1.2 K and 0.6 mm/day) demonstrate the potential of SWOT for monitoring the lake energy budgets at global scale in addition to the other foreseen applications in operational reservoir management.

Authors: Catherine Ottlé, Anthony Bernus, Thomas Verbeke, Karine Pétrus, Zun Yin, Sylvain Biancamaria, Anne Jost, Damien Desroches, Claire Pottier, Charles Perrin, Alban de Lavenne, Nicolas Flipo and Agnès Rivière

Ref.: Remote Sensing 2020, 12(18), 2911; https://doi.org/10.3390/rs12182911

To respect the Paris agreement targeting a limitation of global warming below 2°C by 2100, and possibly below 1.5°C, drastic reductions of greenhouse gas emissions are mandatory but not sufficient. Large‐scale deployment of other climate mitigation strategies is also necessary. Among these, increasing soil organic carbon (SOC) stocks is an important lever because carbon in soils can be stored for long periods and land management options to achieve this already exist and have been widely tested. However, agricultural soils are also an important source of nitrous oxide (N₂O), a powerful greenhouse gas, and increasing SOC may influence N₂O emissions, likely causing an increase in many cases, thus tending to offset the climate change benefit from increased SOC storage. Here we review the main agricultural management options for increasing SOC stocks. We evaluate the amount of SOC that can be stored as well as resulting changes in N₂O emissions to better estimate the climate benefits of these management options. Based on quantitative data obtained from published meta‐analyses and from our current level of understanding, we conclude that the climate mitigation induced by increased SOC storage is generally overestimated if associated N₂O emissions are not considered but, with the exception of reduced tillage, is never fully offset. Some options (e.g. biochar or non‐pyrogenic C amendment application) may even decrease N₂O emissions.

Authors: Bertrand Guenet, Benoit Gabrielle, Claire Chenu, Dominique Arrouays, Jérôme Balesdent, Martial Bernoux, Elisa Bruni, Jean‐Pierre Caliman, Rémi Cardinael, Songchao Chen, Philippe Ciais, Dominique Desbois, Julien Fouche, Stefan Frank, Catherine Henault, Emanuele Lugato, Victoria Naipal, Thomas Nesme, Michael Obersteiner, Sylvain Pellerin, David S. Powlson, Daniel P. Rasse, Frédéric Rees, Jean‐François Soussana, Yang Su, Hanqin Tian, Hugo Valin, Feng Zhou

Ref.: Global Change Biology, 2020.

En 2019, le Thème Cycles biogéochimiques et Transferts dans l'environnement a initié un cycle d'animations "projets" biannuelles  sous forme de présentations suivies d'un échange. Ces courtes présentations donnent des éléments de compréhension sur le projet (question scientifique, consortium, défis ... résultats et perspectives si on est proche de la fin du projet) et sur les implications (missions longue durée/longue distance, transfert de matériel lourd, temps de calcul, recrutement ...).

La séance du 29 juin 2020 a été filmée avec les présentations des projets :
EMME-CARE (Philippe Ciais) : présentation du projet EMME CARE
IMMILA (Olivier Evrard) : Qualification et quantification des flux de la mine au lagon
EUREC4A (Julien Totems) : EUREC4A, une campagne aéroportée internationale pour l'étude des cumulus superficiels en zone de subsidence tropicale, rôle du LSCE
CAMS (Frédéric Chevallier/Philippe Peylin) : "Les activités du service européen Copernicus Atmosphère sur les gaz à effet de serre" / "The activities of the European Copernicus Atmosphere Monitoring Service about greenhouse gases"