Background/Question/Methods . Models simplify the complexities of reality; they would be of no use if they did not. Economists typically use but one model in any analysis, one model in particular dominates environmental and development policymaking, and consequently policymaking is a relatively simple exercise, at least in theory. Ecologists on the other hand think about ecological systems through multiple lenses. Seeing system complexity more fully, they tend to leap to meta policy recommendations such as limiting human population and consumption to reduce the broad drivers of ecosystem change. These differences in approaches to complexity can be seen in the process of assessing climate (IPCC) and ecosystem (Millennium Ecosystem Assessment) change and policy alternatives. The historical interaction of economists with natural scientists in the IPCC, and the increasing role of ecological feedbacks in our understanding of climate change is now bringing questions of climate policy closer to the position of those addressing ecosystem change. The difficulties California faces in addressing water supply and environmental management under climate change will also be used an example. Results/Conclusions . While natural scientists and policy analysts who understand complexity are clearly becoming more sophisticated, economists and the policy process still demands clear answers to complex problems.