This course offers a broad overview of microbial functions and ecosystems studied in the specific context of anthropogenic climate change. Based on recent research in the field, it delineates how microbial life and its perturbations influence climate or modulate the consequences of its change, and reciprocally how microorganisms respond to global changes. It aims at providing concrete tools to understand why microbial diversity needs to be taken into account as a primordial, though unseen, ingredient of climate diagnosis, policies and action.