Timescales of Anthropogenic Perturbation to Earth System Processes

Many important questions in Earth system science require an integrated understanding on the duration of processes such as mass flows, particularly for components of the Earth system that are interconnected and are changing rapidly due to human activity. This type of questions require an integrated and general approach to study timescales of Earth system processes. A number of publications in Tellus have been fundamental to establish the basis for such a general approach. In this contribution, I present a review on how Tellus helped to define the basis to study timescales of Earth system processes, establishing the definitions of timescale metrics such as age, transit-, and turnover time. In addition, I present an overview of an expanded theory based on recent progress in hydrology and carbon cycle science where new methods to study timescales for time-dependent systems out of equilibrium have been recently developed. The new theory facilitates the study of timescale problems in which rates of mass flows are changing fast and interacting nonlinearly with other system components. I discuss the application of the theory to studying the time required to remove anthropogenic carbon dioxide from the interconnected atmosphere–land–ocean system, the time that anthropogenic reactive nitrogen spends in the natural environment, and the time that methane spends in the atmosphere. Overall, the new theory helps to clarify concepts and expands the range of scientific questions to problems on the anthropogenic perturbation of timescales of Earth system processes.