Résumé
The critical zone (CZ) is a complex system, composed of many interacting subsystems over a wide range of space and time scales. These systems are driven by external forcing and internal processes, often non-linear (e.g. rainfall-runoff relationship), and feedback loops. However, due to these interactions and feedbacks, a perturbation in the forcing or in the internal dynamics can lead to regime shifts in some CZ elements. In such a case, they do not always return to their previous state after the perturbation ceases. These behaviours are generally due to the existence of positive feedback loop(s) in the internal dynamics of the system, which may induce instabilities leading to the existence of tipping points, and alternative states. A gradual and reversible response of, e.g. hydro-socio-systems to changes in forcing (e.g. rainfall, land management) is a common view of hydrological change. However, if tipping phenomena exist, this ”linear” vision may lead to incorrect anticipations of the system trajectories, and possibly inadequate adaptation strategies. The possible tipping of key elements of the earth system under climate change is a current concern of the scientific community. Hence, tipping elements may lie in the CZ. This specific scientific issue is being investigated in the framework of a scientific working group within the OZCAR community (French Critical Zone Observatories network). The objective is three-fold :i) identify the main feed-back loops driving the CZ and infer their impact on the functioning of the CZ; ii) evaluate if we use adapted observational setups to monitor them in the field and iii) ; evaluate if and how we should include these feedback processes in CZ models.This talk presents a state-of-the art of the inventory (still in progress) of known or speculative feedback-loops in the CZ, based on the expertise and observations gained at the diverse OZCAR observatories, both in temperate and tropical areas. Reported feedbacks include water, erosion, vegetation and sediment transport; carbon cycle (CC) and alteration (carbonates), CC and forest resilience under warming, CC and fertilizers, CC and humid areas (e.g. peat-lands); land use change (e.g. urbanization), water cycle and hydrological risk; permafrost... A first evaluation of their representation in CZ, hydrological and Earth System models is presented.