Climate system dynamics: understanding for more effective action

It is difficult to express the full complexity of Earth's climate system in just a few sentences. It encompasses all the interactions and exchanges between Earth's different reservoirs, the main ones being the atmosphere, the hydrosphere (oceans and rivers) and the biosphere, meaning all living organisms. Exchanges of energy but also of matter such as carbon or nitrogen are constantly taking place within the system, accelerating or slowing particular climate change processes.

Researcher Nathaelle Bouttes is from the Laboratory for Climate and Environmental Sciences (LSCE - Univ. Paris-Saclay/French National Centre for Scientific Research, CNRS/French Alternative Energies and Atomic Energy Commission, CEA/UVSQ). Her research   focuses on the carbon cycle, and notably on exchanges between the atmosphere and oceans, which on their own, play an important role in climate change. Carbon is mainly exchanged between reservoirs as carbon dioxide (CO₂), one of most abundant greenhouse gases on the planet.

"In their natural state, there is a balance between sinks, which remove CO₂ from the atmosphere, and sources which release this CO₂ back into the atmosphere," explains Nathaelle Bouttes. "For several decades, this balance has been changed by human activities that produce surplus CO₂ and disrupt the entire system." Today, according to the researcher, the oceans and terrestrial biosphere each absorb a quarter of CO₂ emissions, the remaining half staying in the atmosphere.
 

The impact of corals on the climate... and vice versa

Consequently, how can we improve our understanding of the carbon cycle and predict its development over the coming decades? This is the question that Nathaelle Bouttes, a specialist in climate modelling, is attempting to answer. Since 2016, the researcher has focused specifically on the hydrosphere and the relationships between coral, carbon and the climate. "The development of coral reefs is directly affected by rising sea levels and increased CO₂ in the atmosphere. By capturing some of the CO₂, the oceans are becoming more acidic, weakening the calcium carbonate structure of the corals," she continues. "But corals also act indirectly on the climate: by using carbonate ions to build their skeleton, they also increase the quantity of dissolved CO₂ in the ocean, which tends to release CO₂ into the atmosphere."

In order to represent these interactions more accurately, Nathaelle Bouttes and her team are developing a coral reef simulation module, designed to be implemented into Earth system models. Called iCORAL, this can estimate coral production on a fine scale based on different parameters, including temperature, salinity and even light penetration and available surface area. In 2024, the team outlined the model and compared it to field data to validate its reliability. In 2025, it used it to simulate the impact of the climate on coral reefs until 2300. Eventually, the researcher and her team aim to trace and explain variations in the CO₂ content in the atmosphere over the last 800,000 years. "It is important to compare the past and the future to understand how CO₂ has evolved over time and to estimate how long the CO2 added by humans will remain in the atmosphere and have repercussions on the entire system. These simulations show the legacy that we're leaving for future generations," comments the researcher.

Meanwhile, in order to make their model more exhaustive, Nathaelle Bouttes and her team are attempting to improve the representation of the feedback from coral to the climate. "From a carbon cycle perspective alone, the reduction in the production of calcium carbonate by coral reefs tends to help with carbon storage in the ocean. But coral also delivers crucial services for the environment and biodiversity," she explains. For the researcher, it is exactly this complexity of interactions that makes the study of the climate system so exciting. "A large number of components are interacting with each other at all levels! This makes it essential and highly valuable for the different disciplines to work together," she enthuses.
 

Predicting agricultural emissions of ammonia

Another LSCE researcher, Nicolas Vuichard is also working on complex systems modelling. Although the scientist was initially interested in the carbon cycle in terrestrial ecosystems, today he is focusing on the nitrogen cycle, another major element in understanding the climate system. "Carbon and nitrogen cycles are very closely linked Any disruption in one of these cycles has an impact on the entire system," explains Nicolas Vuichard. As the researcher details, although nitrogen is an essential element for life, livestock manure and nitrogen fertilisers emit ammonia (NH₃). This compound contributes to aerosol formation in the atmosphere and has repercussions on the climate and air quality. However, agricultural ammonia emissions and their impacts are difficult to quantify as the biological processes at work are complex and farming practices and the use of nitrogen fertilisers varies between regions.

In 2023, Maureen Beaudor, a PhD candidate at LSCE and her team of supervisors worked on creating a module that would better represent these agricultural ammonia flows. Called CAMEO, (for Calculation of ammonia emission in ORCHIDEE), this model is designed to be implemented with the global land surface model ORCHIDEE, but also more generally with the Earth system model the used by scientists at the Institut Pierre-Simon Laplace (IPSL). "Historically, this modelling has focused mainly on exchanges of energy, water and carbon between atmosphere, oceans and continental surfaces," explains Nicolas Vuichard. "Gradually, our team are integrating other cycles into it, including that of nitrogen, to produce a more exhaustive model that takes into account all interactions and feedback loops."

In order to obtain the most realistic representation possible, scientists populate the model with input data related to soil condition, but also data such as the herd size of different animal categories or the use of nitrogen fertiliser on crops and managed pastures. For the first time, Nicolas Vuichard and his team are incorporating the biomass produced by crops when calculating organic fertiliser inputs, in order to make their model more accurate and self-sufficient. "Previously, fertiliser inputs were viewed exclusively as external inputs, although agricultural systems also produce organic fertilisers through the biomass produced and ingested by the animals," the researcher points out. Today, the model takes account of both synthetic fertiliser inputs and organic inputs, which vary depending on the type of livestock farming.

In 2025, after the CAMEO model had been tested and validated, the researcher and his team simulated ammonia emissions up to 2100 under different socio-economic scenarios. This breakthrough was made possible by an original model they developed to estimate livestock densities between 2015 and 2100. Their results suggest an increase of 30% to 50% in total agricultural ammonia emissions in 2100, 20% of which can be directly attributed to climate change. In addition, the scientists identified the African continent as the largest contributor to the world's ammonia emissions in 2100, within a context of increasing animal production. To complement these results, Nicolas Vuichard plans to explore other socio-economic scenarios, particularly those related to changing diets. This is intended to predict the impact of these different consumption patterns on climate change.
 

Chromium: an emerging pollution risk for freshwater

For climate modellers, even the most optimistic scenarios today still point to continued global warming. This warming, with its numerous consequences, is contributing in particular to more severe droughts and wildfires, the environmental consequences of which remain poorly understood. Cécile Quantin is a lecturer at the Paris-Saclay Geosciences laboratory (GEOPS - Univ. Paris-Saclay/CNRS). Along with her colleagues, she is studying the consequences of these wildfires on the soils of New Caledonia, a French island in the Pacific Ocean where an average of 50,000 hectares are lost to fire annually. The island's soils are very diverse and some are naturally rich in metals including nickel, cobalt and chromium which, in its trivalent form, is poorly soluble and harmless to living organisms, including humans.

For a better understanding of the influence of wildfires on soils and the metals they contain, Gaël Théry, a PhD candidate at the GEOPS laboratory, supervised by Cécile Quantin and her colleagues, collects different soil samples and heats them in the laboratory. He then analyses the effects on chromium. The first results, published in 2024, show that the soil's trivalent chromium partially oxidises into hexavalent chromium in all soils studied, at temperatures of 400°C and above. For Cécile Quantin, these first results reflect an emerging risk for water quality: "Hexavalent chromium is highly toxic to human health, recognised as carcinogenic, and is highly mobile with a tendency to contaminate freshwater sources, and so to pollute water with metals," she explains. Although varying amounts of chromium are naturally present in soils all over the world, these conclusions are particularly alarming in an island context, where freshwater reserves are limited. The scientist and her team are calling for a more detailed global assessment of the risks caused by wildfires on freshwater pollution by metals. In the meantime, the research team is in the process of publishing another study on nickel in New Caledonia.

Cécile Quantin and her colleagues are also working in other regions of the world, including in mining areas in Brazil and India and even in China to observe the transport of suspended matter in the Yellow River. In cities, the scientist and her colleague Alexandra Courtin are exploring the impact of mining waste and the potential risks for human health. "In the Allier River in the Auvergne-Rhone-Alpes region, mining waste has been stored directly on the ground and without any protection for years. When it rains, the metals in this waste, including lead, arsenic and even antimony, become soluble, seep into the soil and are found in the rivers and even in drinking water. This is harmful to inhabitants in central France." Cécile Quantin hopes that her research in the region will improve the management of mining waste, in view of its impact on human health and the environment.
 

Wetlands to limit the transfer of pesticides into the environment

In light of these points, one question naturally springs to mind: how can the transfer of material from polluted soils into waterways be reduced? Since 1998, this question has been central to Julien Tournebize's research at the Continental Hydrosystems - Resources, Risk, Restoration laboratory (HYCAR – Univ. Paris-Saclay/French National Research Institute for Agriculture, Food and Environment, INRAE). Initially specialising in the transfer of nitrates from nitrogen fertilisers in the vineyards of Alsace, the hydrologist has since turned his attention to pesticides and nature-based solutions - or buffer zones - able to limit pesticide transfer. "I work on everything that can intercept water on its path between agricultural production areas and aquatic environments. Today, rural areas have been heavily homogenised in order to maximise agricultural production. My work focuses on studying and reintroducing various landscape elements that not only encourage biodiversity, but also act as a barrier to the transfer of contaminants."

Since 2005, Julien Tournebize and his colleague Cédric Chaumont from the HYCAR research unit have worked on a project at the Rampillon site in Seine-et-Marne. Here, a wetland was developed in 2010 to limit the transfer of nitrates and pesticides into the water tables. In this "living laboratory", they carry out long-term monitoring of water quality, biodiversity surveys and ecotoxicological tests. The objective is to understand just how effective these zones are in trapping compounds, but also to assess the impact of pesticide exposure on the biodiversity present. Between 2019 and 2023, Julien Tournebize and his team integrated the study of the Rampillon site into the PESTIPOND project, developed in partnership with a research team from the University of Strasbourg. The project focuses on natural and artificial water reservoirs and their pesticide retention and degradation mechanisms, which have yet to be fully explored. The data gathered through this partnership feed into mathematical models that can predict and quantify the transport of pesticides from agricultural plots to water reservoirs on a larger scale. The model is particularly useful to land managers and farmers who want to reduce pesticide pollution on their land, for example, by estimating the required area for a buffer zone depending on the desired effectiveness. The model produced a classification of pesticides based on their absorption and persistence in wetlands.

Julien Tournebize, who provides scientific advice within ministries, is now working on the carbon footprint of wetlands which, after several years, become significant carbon sinks. However, he regrets that nature-based solutions are still largely unrecognised and seldom incorporated into land use planning policies. The researcher also reflects on the importance of local work, closer to local stakeholders. "The area in which we live is a critical zone where everything interacts. Each local action influences the major matter trajectories and large-scale work can anticipate the local consequences. It's a complementary effort, combining knowledge and sharing it," he says.
 

Rethinking carbon storage in Europe

Claire Chenu is a researcher at the Functional Ecology and Ecotoxicology of Agroecosystems laboratory (Ecosys – Univ. Paris-Saclay/INRAE/AgroParisTech). She is part of a group of scientists exploring the climate system on a European scale. Her research focuses on soils, which contain two to three times more carbon than the atmosphere, according to estimates by the scientific community. "Soil organic matter, comprised mainly of carbon, plays a vital role in soil health and therefore in its ability to deliver ecosystem services, such as biomass supply through plant growth and even water cycle regulation. However, around the world, reserves of soil organic carbon are falling due to changes in use and unsustainable practices such as deforestation and land take," explains Claire Chenu.

The scientist is now leading a European research programme on climate-smart and sustainable management of agricultural soils. This concept includes both climate change mitigation and agro-ecosystem adaptation and in Europe, is mainly reflected by maintaining and increasing carbon storage in soils. Called the European Joint Program (EJP) SOIL, it brings together 800 research staff from 24 European countries and 46 research organisations or universities. With the support of the European Commission, the scientific team is launching several research projects to respond to "a need for development, transfer, sharing and standardisation of knowledge at the European level." In 2024, the team published an analysis of soil organic carbon (SOC) monitoring strategies in five European countries in order to illustrate the potential for harmonisation, along with a comparison of national soil monitoring systems in Europe. In the same year, the research team published two further studies on the subject. One focused on soil quality indicators and actionable levers to reverse their degradation; the second assessed the effectiveness of practices to enhance carbon storage by soils, such as agroecology and agroforestry.

While the EJP SOIL project ended in 2025, Claire Chenu continues to be involved at a European level through the Mission Soil. In particular, she works with the French representation in Brussels and the European Union to deliver the EJP SOIL conclusions. However, she notes that "the effect of climate change is a variable that is still too seldom taken into account in scientific studies." Indeed, as Nicolas Vuichard explains, "many nature-based mitigation solutions assume that the storage capacity of various reservoirs will remain unchanged. However, current thinking is that the capacity of soil carbon sinks will be reduced rather than increased." For Nathaelle Bouttes too, "nothing guarantees that the sink capacity of reservoirs, including the oceans, will remain effective over the long term." Locally, Julien Tournebize is investigating the future and long-term consequences of pesticides that accumulate in agricultural soils and do not break down.

Despite these growing uncertainties and extreme climate events that are increasingly difficult to anticipate, scientists are calling for a greater consideration of the impact of human activities on climate system dynamics. For Claire Chenu, "at the current rate, Europe is heading for a trajectory of +4°C by 2050. The climate situation is such that all possible levers must be actioned to limit its consequences." Nicolas Vuichard confirms: "Today, there are broadly enough studies and proposed solutions. It is no longer possible to justify inaction due to a lack of knowledge or too many uncertainties."

References:

• Beaudor M., Vuichard N., et al. Future Trends of Global Agricultural Emissions of Ammonia in a Changing Climate. Journal of Advances in Modeling Earth Systems, 2025.
• Bouttes N., et al. Implementing the iCORAL (version 1.0) coral reef CaCO3 production module in the iLOVECLIM climate model. Geoscientific Model Development, 2024.
• Meurer K., Chenu C., et al. How does national SOC monitoring on agricultural soils align with the EU strategies? An example using five case studies. European Journal of Soil Science, 2024.
• Théry G., Quantin C., et al. Heating effect on chromium speciation and mobility in Cr-rich soils: A snapshot from New Caledonia. Science of The Total Environment, 2024.
• Tournebize J., et al. PESTIPOND: A descriptive model of pesticide fate in artificial ponds: II. Model application and evaluation. Ecological Modelling, 2023.