Forçages climatiques naturels volcanique et solaire – VOLSOL

The current major challenge for climate science is to provide policymakers with better constrained scenarios of the Earth climatic evolution over the 21st century and beyond. This requires improving our understanding of both the climate forcings in the future and the climate sensitivity. This project focuses on the climate sensitivity, that is, the response of the climate to a given forcing. Over the 21st century, the forcings are anthropogenic and natural. Although very different in nature, they all affect the radiative budget of the Earth surface and thus can be expressed as a radiative forcing (in W.m-2). The climate sensitivity is then defined as the global temperature change due to both the direct radiative forcing and the indirect climatic feedbacks. Although the climate sensitivity is calculated from General Circulation Model simulations, these models only approximate the real world, hence a direct estimation is essential by reconstructing both forcings and climate response in the past. This project focuses on the last two millennia, during which the forcings have been similar to the modern ones, the climatic variations are well documented and quantified, and the archives available with a high resolution. The goal of this project is to produce calibrated time series of the most important natural climate forcings over the last 2000 years: solar irradiance variations from the beryllium 10 (a cosmogenic isotope) and volcanic atmospheric loading from sulphur and oxygen stable isotopes. These proxies will be measured in ice originating from several and contrasted Antarctic sites. Ice cores constitute the most appropriate material to conduct our study. They preserve past solar and volcanic proxies with the required annual to sub-annual resolution to precisely study mechanisms. The originality of our approach of the solar forcing is (1) to proceed to 10Be measurements from two contrasted Antarctic ice cores that cover the last 2000 years (2) to document the deposition modes and effects of the 10Be in Antarctica (3) to model the 10Be data in order to propose an irradiance reconstruction over the last 2000 years. For the volcanic forcing study, the originality of the project is (1) to develop an analytical method for measuring continuously sulfate and sodium concentrations on ice cores, (2) to develop a new method for measuring the sulfur isotopic composition of sulfate, and (3) to study all the volcanic signals recorded in the Dome C glaciological archives over the last 2000 years to identify stratospheric volcanic eruptions, and then (4) to propose a reconstruction of the volcanic forcing over the last 2000 years with a modelling approach. These improved solar and volcanic forcings are expected to be used in climate simulations in order to test the capacity of the models to reproduce the climatic variations and to refine the climatic sensitivity. Also, these archives will document the base activity of the Sun during so called minima, which is of more fundamental interest for astrophysicists.