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This PhD thesis deals with the Brazilian Proposal, that is the assessment of national contributions to anthropogenic climate change. To answer the Proposal, we have developed a compact Earth system model, named OSCAR v2.1. The carbon cycle (CO2, CH4), the atmospheric chemistry of greenhouse gases (CH4, N2O, O3, halogenated compounds), as well as aerosols and climate dynamics are included in this model. It is driven by anthropogenic emissions of active compounds, and by land-use changes. After acknowledging the ability of the model to reproduce past observations of the main climatic variables, and after exposing the fundamental principles of attribution exercises, we attribute climate change to its anthropogenic causes. We find that the climate feedback -- over both the carbon cycle and the atmospheric chemistry -- has a prominent effect that exacerbates the relative importance of each anthropogenic forcing. In decreasing order, emissions of fossil carbon dioxide, of sulfur dioxide, of methane, and land-use changes, are found to be the most important contributors to climate change in 2008. Through these forcings, the so-called developing countries are now contributing more to climate change than the so-called developed countries. It is however still the contrary on a per capita basis; but we show that such an accounting approach makes it impossible to reach equity within a less-than-two-degree warming trajectory. ; Cette thèse traite du Brazilian Proposal, c'est-à-dire de la détermination des contributions nationales au changement climatique d'origine humaine. Pour répondre à cette question, nous avons développé un modèle compact du système Terre, OSCAR v2.1. Ce modèle intègre une représentation du cycle du carbone (CO2, CH4), de la chimie atmosphérique des gaz à effet de serre (CH4, N2O, O3, composés halogénés), mais également des aérosols et de la dynamique climatique. Il est forcé en émissions anthropiques de composés actifs et en changements d'usage des sols. Après avoir constaté la bonne capacité du modèle à ...

This PhD thesis deals with the Brazilian Proposal, that is the assessment of national contributions to anthropogenic climate change. To answer the Proposal, we have developed a compact Earth system model, named OSCAR v2.1. The carbon cycle (CO2, CH4), the atmospheric chemistry of greenhouse gases (CH4, N2O, O3, halogenated compounds), as well as aerosols and climate dynamics are included in this model. It is driven by anthropogenic emissions of active compounds, and by land-use changes. After acknowledging the ability of the model to reproduce past observations of the main climatic variables, and after exposing the fundamental principles of attribution exercises, we attribute climate change to its anthropogenic causes. We find that the climate feedback -- over both the carbon cycle and the atmospheric chemistry -- has a prominent effect that exacerbates the relative importance of each anthropogenic forcing. In decreasing order, emissions of fossil carbon dioxide, of sulfur dioxide, of methane, and land-use changes, are found to be the most important contributors to climate change in 2008. Through these forcings, the so-called developing countries are now contributing more to climate change than the so-called developed countries. It is however still the contrary on a per capita basis; but we show that such an accounting approach makes it impossible to reach equity within a less-than-two-degree warming trajectory. ; Cette thèse traite du Brazilian Proposal, c'est-à-dire de la détermination des contributions nationales au changement climatique d'origine humaine. Pour répondre à cette question, nous avons développé un modèle compact du système Terre, OSCAR v2.1. Ce modèle intègre une représentation du cycle du carbone (CO2, CH4), de la chimie atmosphérique des gaz à effet de serre (CH4, N2O, O3, composés halogénés), mais également des aérosols et de la dynamique climatique. Il est forcé en émissions anthropiques de composés actifs et en changements d'usage des sols. Après avoir constaté la bonne capacité du modèle à reproduire les observations passées des principales grandeurs climatiques, et après avoir énoncé les grands principes régissant les exercices d'attribution, nous attribuons les causes anthropiques du changement climatique. Nous trouvons que la rétroaction climatique, sur le cycle du carbone et sur la chimie atmosphérique, a un effet prépondérant qui exacerbe l'importance relative de chaque forçage anthropique. Par ordre décroissant, émissions de dioxyde de carbone fossile, de dioxyde de soufre, de méthane, et usages des sols, sont trouvés comme étant les plus importants contributeurs au changement climatique en 2008. A travers ces forçages, les pays dits en développements sont dorénavant de plus grands contributeurs au changement climatique que les pays dits développés. C'est cependant toujours l'inverse si l'on résonne en contribution par tête ; mais nous montrons qu'un tel raisonnement rend incompatibles une trajectoire de réchauffement inférieur à deux degrés et équitable.

This PhD thesis deals with the Brazilian Proposal, that is the assessment of national contributions to anthropogenic climate change. To answer the Proposal, we have developed a compact Earth system model, named OSCAR v2.1. The carbon cycle (CO2, CH4), the atmospheric chemistry of greenhouse gases (CH4, N2O, O3, halogenated compounds), as well as aerosols and climate dynamics are included in this model. It is driven by anthropogenic emissions of active compounds, and by land-use changes. After acknowledging the ability of the model to reproduce past observations of the main climatic variables, and after exposing the fundamental principles of attribution exercises, we attribute climate change to its anthropogenic causes. We find that the climate feedback -- over both the carbon cycle and the atmospheric chemistry -- has a prominent effect that exacerbates the relative importance of each anthropogenic forcing. In decreasing order, emissions of fossil carbon dioxide, of sulfur dioxide, of methane, and land-use changes, are found to be the most important contributors to climate change in 2008. Through these forcings, the so-called developing countries are now contributing more to climate change than the so-called developed countries. It is however still the contrary on a per capita basis; but we show that such an accounting approach makes it impossible to reach equity within a less-than-two-degree warming trajectory. ; Cette thèse traite du Brazilian Proposal, c'est-à-dire de la détermination des contributions nationales au changement climatique d'origine humaine. Pour répondre à cette question, nous avons développé un modèle compact du système Terre, OSCAR v2.1. Ce modèle intègre une représentation du cycle du carbone (CO2, CH4), de la chimie atmosphérique des gaz à effet de serre (CH4, N2O, O3, composés halogénés), mais également des aérosols et de la dynamique climatique. Il est forcé en émissions anthropiques de composés actifs et en changements d'usage des sols. Après avoir constaté la bonne capacité du modèle à reproduire les observations passées des principales grandeurs climatiques, et après avoir énoncé les grands principes régissant les exercices d'attribution, nous attribuons les causes anthropiques du changement climatique. Nous trouvons que la rétroaction climatique, sur le cycle du carbone et sur la chimie atmosphérique, a un effet prépondérant qui exacerbe l'importance relative de chaque forçage anthropique. Par ordre décroissant, émissions de dioxyde de carbone fossile, de dioxyde de soufre, de méthane, et usages des sols, sont trouvés comme étant les plus importants contributeurs au changement climatique en 2008. A travers ces forçages, les pays dits en développements sont dorénavant de plus grands contributeurs au changement climatique que les pays dits développés. C'est cependant toujours l'inverse si l'on résonne en contribution par tête ; mais nous montrons qu'un tel raisonnement rend incompatibles une trajectoire de réchauffement inférieur à deux degrés et équitable.

This PhD thesis deals with the Brazilian Proposal, that is the assessment of national contributions to anthropogenic climate change. To answer the Proposal, we have developed a compact Earth system model, named OSCAR v2.1. The carbon cycle (CO2, CH4), the atmospheric chemistry of greenhouse gases (CH4, N2O, O3, halogenated compounds), as well as aerosols and climate dynamics are included in this model. It is driven by anthropogenic emissions of active compounds, and by land-use changes. After acknowledging the ability of the model to reproduce past observations of the main climatic variables, and after exposing the fundamental principles of attribution exercises, we attribute climate change to its anthropogenic causes. We find that the climate feedback -- over both the carbon cycle and the atmospheric chemistry -- has a prominent effect that exacerbates the relative importance of each anthropogenic forcing. In decreasing order, emissions of fossil carbon dioxide, of sulfur dioxide, of methane, and land-use changes, are found to be the most important contributors to climate change in 2008. Through these forcings, the so-called developing countries are now contributing more to climate change than the so-called developed countries. It is however still the contrary on a per capita basis; but we show that such an accounting approach makes it impossible to reach equity within a less-than-two-degree warming trajectory. ; Cette thèse traite du Brazilian Proposal, c'est-à-dire de la détermination des contributions nationales au changement climatique d'origine humaine. Pour répondre à cette question, nous avons développé un modèle compact du système Terre, OSCAR v2.1. Ce modèle intègre une représentation du cycle du carbone (CO2, CH4), de la chimie atmosphérique des gaz à effet de serre (CH4, N2O, O3, composés halogénés), mais également des aérosols et de la dynamique climatique. Il est forcé en émissions anthropiques de composés actifs et en changements d'usage des sols. Après avoir constaté la bonne capacité du modèle à reproduire les observations passées des principales grandeurs climatiques, et après avoir énoncé les grands principes régissant les exercices d'attribution, nous attribuons les causes anthropiques du changement climatique. Nous trouvons que la rétroaction climatique, sur le cycle du carbone et sur la chimie atmosphérique, a un effet prépondérant qui exacerbe l'importance relative de chaque forçage anthropique. Par ordre décroissant, émissions de dioxyde de carbone fossile, de dioxyde de soufre, de méthane, et usages des sols, sont trouvés comme étant les plus importants contributeurs au changement climatique en 2008. A travers ces forçages, les pays dits en développements sont dorénavant de plus grands contributeurs au changement climatique que les pays dits développés. C'est cependant toujours l'inverse si l'on résonne en contribution par tête ; mais nous montrons qu'un tel raisonnement rend incompatibles une trajectoire de réchauffement inférieur à deux degrés et équitable.

International audience ; At last year's 26th UN Climate Change Conference of the Parties (COP26) in Glasgow, 141 countries committed to halt and reverse forest loss and land degradation by 2030 (1). It was part of one of several side deals designed to keep the objectives of the Paris agreement within reach. The UK government boasted that these nations had made a landmark pledge to end deforestation (2). Yet, one crucial detail was left out: whether that deforestation will be gross or net. The distinction matters, because differing interpretations of how countries can "end deforestation" significantly impact future carbon dioxide emissions. Put simply, ending gross deforestation would be a major step forward for the climate. But considering only net deforestation could be anecdotal and even be detrimental to biodiversity. Overall, human activities related to land use emit 4 gigatonnes (Gt) of carbon dioxide (CO 2) per year to the atmosphere (3). These are net emissions, comprising both carbon losses and carbon gains. On the loss side, the destruction of tree biomass during deforestation, for large-scale agriculture, cattle ranching, or shifting cultivation (cycles of cutting forest for agriculture, then abandoning to recover soil fertility, then returning), and the legacy emissions from harvested wood products, form the bulk of gross emissions, totaling 14 Gt CO 2 per year (3). On the gain side, secondary forest regrowth after reforestation, agricultural abandonment, or during shifting cultivation form the bulk of the gross carbon Differing interpretations as to how countries can "end deforestation" will have significant impacts on future carbon dioxide emissions. Only ending gross deforestation would be a major step forward for the climate. Image credit: Shutterstock/guentermanaus.

International audience ; At last year's 26th UN Climate Change Conference of the Parties (COP26) in Glasgow, 141 countries committed to halt and reverse forest loss and land degradation by 2030 (1). It was part of one of several side deals designed to keep the objectives of the Paris agreement within reach. The UK government boasted that these nations had made a landmark pledge to end deforestation (2). Yet, one crucial detail was left out: whether that deforestation will be gross or net. The distinction matters, because differing interpretations of how countries can "end deforestation" significantly impact future carbon dioxide emissions. Put simply, ending gross deforestation would be a major step forward for the climate. But considering only net deforestation could be anecdotal and even be detrimental to biodiversity. Overall, human activities related to land use emit 4 gigatonnes (Gt) of carbon dioxide (CO 2) per year to the atmosphere (3). These are net emissions, comprising both carbon losses and carbon gains. On the loss side, the destruction of tree biomass during deforestation, for large-scale agriculture, cattle ranching, or shifting cultivation (cycles of cutting forest for agriculture, then abandoning to recover soil fertility, then returning), and the legacy emissions from harvested wood products, form the bulk of gross emissions, totaling 14 Gt CO 2 per year (3). On the gain side, secondary forest regrowth after reforestation, agricultural abandonment, or during shifting cultivation form the bulk of the gross carbon Differing interpretations as to how countries can "end deforestation" will have significant impacts on future carbon dioxide emissions. Only ending gross deforestation would be a major step forward for the climate. Image credit: Shutterstock/guentermanaus.

International audience ; At last year's 26th UN Climate Change Conference of the Parties (COP26) in Glasgow, 141 countries committed to halt and reverse forest loss and land degradation by 2030 (1). It was part of one of several side deals designed to keep the objectives of the Paris agreement within reach. The UK government boasted that these nations had made a landmark pledge to end deforestation (2). Yet, one crucial detail was left out: whether that deforestation will be gross or net. The distinction matters, because differing interpretations of how countries can "end deforestation" significantly impact future carbon dioxide emissions. Put simply, ending gross deforestation would be a major step forward for the climate. But considering only net deforestation could be anecdotal and even be detrimental to biodiversity. Overall, human activities related to land use emit 4 gigatonnes (Gt) of carbon dioxide (CO 2) per year to the atmosphere (3). These are net emissions, comprising both carbon losses and carbon gains. On the loss side, the destruction of tree biomass during deforestation, for large-scale agriculture, cattle ranching, or shifting cultivation (cycles of cutting forest for agriculture, then abandoning to recover soil fertility, then returning), and the legacy emissions from harvested wood products, form the bulk of gross emissions, totaling 14 Gt CO 2 per year (3). On the gain side, secondary forest regrowth after reforestation, agricultural abandonment, or during shifting cultivation form the bulk of the gross carbon Differing interpretations as to how countries can "end deforestation" will have significant impacts on future carbon dioxide emissions. Only ending gross deforestation would be a major step forward for the climate. Image credit: Shutterstock/guentermanaus.

International audience ; At last year's 26th UN Climate Change Conference of the Parties (COP26) in Glasgow, 141 countries committed to halt and reverse forest loss and land degradation by 2030 (1). It was part of one of several side deals designed to keep the objectives of the Paris agreement within reach. The UK government boasted that these nations had made a landmark pledge to end deforestation (2). Yet, one crucial detail was left out: whether that deforestation will be gross or net. The distinction matters, because differing interpretations of how countries can "end deforestation" significantly impact future carbon dioxide emissions. Put simply, ending gross deforestation would be a major step forward for the climate. But considering only net deforestation could be anecdotal and even be detrimental to biodiversity. Overall, human activities related to land use emit 4 gigatonnes (Gt) of carbon dioxide (CO 2) per year to the atmosphere (3). These are net emissions, comprising both carbon losses and carbon gains. On the loss side, the destruction of tree biomass during deforestation, for large-scale agriculture, cattle ranching, or shifting cultivation (cycles of cutting forest for agriculture, then abandoning to recover soil fertility, then returning), and the legacy emissions from harvested wood products, form the bulk of gross emissions, totaling 14 Gt CO 2 per year (3). On the gain side, secondary forest regrowth after reforestation, agricultural abandonment, or during shifting cultivation form the bulk of the gross carbon Differing interpretations as to how countries can "end deforestation" will have significant impacts on future carbon dioxide emissions. Only ending gross deforestation would be a major step forward for the climate. Image credit: Shutterstock/guentermanaus.

International audience ; At last year's 26th UN Climate Change Conference of the Parties (COP26) in Glasgow, 141 countries committed to halt and reverse forest loss and land degradation by 2030 (1). It was part of one of several side deals designed to keep the objectives of the Paris agreement within reach. The UK government boasted that these nations had made a landmark pledge to end deforestation (2). Yet, one crucial detail was left out: whether that deforestation will be gross or net. The distinction matters, because differing interpretations of how countries can "end deforestation" significantly impact future carbon dioxide emissions. Put simply, ending gross deforestation would be a major step forward for the climate. But considering only net deforestation could be anecdotal and even be detrimental to biodiversity. Overall, human activities related to land use emit 4 gigatonnes (Gt) of carbon dioxide (CO 2) per year to the atmosphere (3). These are net emissions, comprising both carbon losses and carbon gains. On the loss side, the destruction of tree biomass during deforestation, for large-scale agriculture, cattle ranching, or shifting cultivation (cycles of cutting forest for agriculture, then abandoning to recover soil fertility, then returning), and the legacy emissions from harvested wood products, form the bulk of gross emissions, totaling 14 Gt CO 2 per year (3). On the gain side, secondary forest regrowth after reforestation, agricultural abandonment, or during shifting cultivation form the bulk of the gross carbon Differing interpretations as to how countries can "end deforestation" will have significant impacts on future carbon dioxide emissions. Only ending gross deforestation would be a major step forward for the climate. Image credit: Shutterstock/guentermanaus.

International audience ; At last year's 26th UN Climate Change Conference of the Parties (COP26) in Glasgow, 141 countries committed to halt and reverse forest loss and land degradation by 2030 (1). It was part of one of several side deals designed to keep the objectives of the Paris agreement within reach. The UK government boasted that these nations had made a landmark pledge to end deforestation (2). Yet, one crucial detail was left out: whether that deforestation will be gross or net. The distinction matters, because differing interpretations of how countries can "end deforestation" significantly impact future carbon dioxide emissions. Put simply, ending gross deforestation would be a major step forward for the climate. But considering only net deforestation could be anecdotal and even be detrimental to biodiversity. Overall, human activities related to land use emit 4 gigatonnes (Gt) of carbon dioxide (CO 2) per year to the atmosphere (3). These are net emissions, comprising both carbon losses and carbon gains. On the loss side, the destruction of tree biomass during deforestation, for large-scale agriculture, cattle ranching, or shifting cultivation (cycles of cutting forest for agriculture, then abandoning to recover soil fertility, then returning), and the legacy emissions from harvested wood products, form the bulk of gross emissions, totaling 14 Gt CO 2 per year (3). On the gain side, secondary forest regrowth after reforestation, agricultural abandonment, or during shifting cultivation form the bulk of the gross carbon Differing interpretations as to how countries can "end deforestation" will have significant impacts on future carbon dioxide emissions. Only ending gross deforestation would be a major step forward for the climate. Image credit: Shutterstock/guentermanaus.

International audience ; Knowing the historical relative contribution of greenhouse gases (GHGs) and short-lived climate forcers (SLCFs) to global radiative forcing (RF) at the regional level can help understand how future GHGs emission reductions and associated or independent reductions in SLCFs will affect the ultimate purpose of the Paris Agreement. In this study, we use a compact Earth system model to quantify the global RF and attribute global RF to individual countries and regions. As our evaluation, the United States, the first 15 European Union members, and China are the top three contributors, accounting for 21.9 ± 3.1%, 13.7 ± 1.6%, and 8.6 ± 7.0% of global RF in 2014, respectively. We also find a contrast between developed countries where GHGs dominate the RF and developing countries where SLCFs including aerosols and ozone are more dominant. In developing countries, negative RF caused by aerosols largely masks the positive RF from GHGs. As developing countries take measures to improve the air quality, their negative contributions from aerosols will likely be reduced in the future, which will in turn enhance global warming. This underlines the importance of reducing GHG emissions in parallel to avoid any detrimental consequences from air quality policies.