Transition entre modes climatiques : les leçons des âges glaciaires du passé géologique – COLORS
The aim of this proposal is to investigate the carbon-climate coupling through the study of major global transitions between icehouse and greenhouse states that Earth already experienced. Icehouse periods (i.e. times when polar continents are covered by ice sheets) have occurred only two times over the last 400 millions of years. The first one lasted from 330 to 270 million years; we currently live in the second one which began 34 million years ago. Although atmospheric CO2 level is generally proposed as the main driver of these climatic changes, the processes controlling variations of atmospheric CO2 levels remain enigmatic and require a good deal of more thinking. The two main questions of this proposal are: (1) under what circumstances do glaciations occur on Earth' (2)What are the processes leading the Earth system to move back to a greenhouse state' Answering these questions requires the conjunction of two methods of investigation. First, the accumulation of data is highly required. Today, the two main cold climatic modes of the Phanerozoic (540-0 Ma) are thoroughly documented by sedimentological, palaeogeographical, geochemical, palaeontological and isotopic data. These various datasets have been published by North American and European research groups and are easily available. Although it can be argued that these dataset are still incomplete and should be improved relatively to their time and spatial resolutions, they are nevertheless an important existing basis than can be used for the second method of investigation: the numerical modelling of the global climate and geochemical evolution. Indeed, the two above questions have not been fully explored up to now because of the lack of adequate modelling tools. The numerical tools include climate and oceanic models, ice-sheet models, and global biogeochemical cycle models. These tools are available, but must be tightly coupled. The degree of complexity of such coupled models is far above the complexity of carbon cycle models developed by isolated researcher that were used up to now to explore the geological past. They are also below the degree of complexity of the models developed by large scientific teams to explore century scale climatic transitions, such as the ongoing climatic changes. A coupled model strategy for the geological past lies somewhere in the middle of this range. They can be built up by a small team of scientists, and they will potentially bring new insights in our understanding of the Earth system as a whole. This is also why we strongly believe that the ANR call for proposal matches our needs: a post doctoral position will greatly help at exploring the distant past climatic evolution, simply because the French modelling community working on this thematic relies on 3 to 4 scientists only.
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