Over the last two centuries, the Human System went from having a small impact on the Earth System to becoming dominant, because both population and per capita consumption have grown extremely fast, especially since about 1950. We therefore argue that Human System Models must be included into Earth System Models through bidirectional couplings with feedbacks. In particular, population should be modeled endogenously, rather than exogenously as done currently in most Integrated Assessment Models. The growth of the Human System threatens to overwhelm the Carrying Capacity of the Earth System, and may be leading to collapse. Earth Sciences should be involved in the exploration of potential mitigation strategies including education, regulatory policies, and technological advances.

We describe a human population dynamics model developed by adding accumulated wealth and economic inequality to a predator-prey model of humans and nature. The model structure, and simulated scenarios that offer significant implications, are discussed. Four equations describe the evolution of Elites, Commoners, Nature, and Wealth. The model shows Economic Stratification or Ecological Strain can independently lead to collapse, in agreement with the historical record.

The measure ``Carrying Capacity'' is developed and its estimation is shown to be a practical means for early detection of a collapse. Mechanisms leading to two types of collapses are discussed. The new dynamics of this model can also reproduce the irreversible collapses found in history. Collapse can be avoided, and population can reach a steady state at maximum carrying capacity, if the rate of depletion of nature is reduced to a sustainable level, and if resources are distributed equitably.

Finally we present a Coupled Human-Climate-Water Model (COWA). Policies are introduced as drivers of the model so that the long-term effect of each policy on the system can be seen as we change its level. We have done a case study for the Phoenix AMA Watershed. We show that it is possible to guarantee the freshwater supply and sustain the freshwater sources through a proper set of policy choices.