Current integrated assessment models (IAMs) are by and large making equilibrium and linearity assumptions in modeling both the climatic and the economic systems, as well as the interaction of the two. In this chapter, we review the intrinsic and endogenous climatic and economic variability neglected by these assumptions. As an example of disequilibrium and nonlinear human-natural dynamics, we address the problem of commodity price variability. To illustrate the effects of disequilibrium and nonlinearity, we use a simple data-driven coupled model of the El Niño-Southern Oscillation and the macroeconomy. The model’s success at matching key features of world commodity price variability motivates the need to further develop the approach herein and apply it to more detailed IAMs, as a necessary complement to current IAM practice.