Joint work with Matthias Ruth1, Ning Zeng1, Safa Motesharrei1, and Jorge Rivas2. Earth System Models (ESM) designed to study climate change should include a fully coupled Human model, with submodels such as Population, Energy, Agriculture and Fisheries, Water, as well as environmental sources and sinks. Specifically, fully coupled means that subcomponents of a model (e.g., the atmosphere and the ocean) are coupled in a two-way fashion: for example, the atmosphere can change the ocean and in turn is affected by the feedback of this change. The importance of having positive, negative, and delayed feedbacks is obvious: the phenomenon of El Niño-Southern Oscillation (ENSO) is the result of such feedbacks, and thus, one-way coupled ocean-atmosphere models (used until about 1990) were not able to reproduce ENSO. ESMs are currently much more comprehensive than they used to be (e.g., the Community ESM, CESM), and include fully coupled land-ocean-atmosphere components. Vegetation models, which used to influence the climate of the model but without feedbacks (one-way coupling), are now also driven by the climate (two-way coupling). However, there is an important component of the Earth System (ES) that is not included in the ESMs: the Human System, which in reality is not only strongly coupled but actually dominates many components of the ES. For example, the human appropriated portion of the Natural Primary Productivity (HANPP) is estimated to be at least 25%, and about 60% is affected by human activities. Nevertheless, population remains a taboo subject in the discussions and policies on climate change. In this talk we discuss how to develop a fully coupled ESM-Human model in order to be able to study the role of population in climate change, and the new mathematical problems that may arise. 1University of Maryland, 2University of Minnesota