Energy and the environment are closely interconnected. In particular, energy-related carbon dioxide emissions are major contributors to climate change. To analyze options within the energy sector to curb greenhouse gas emissions, or to study alternative climate strategies such as adaptation and geoengineering measures, policy-makers can rely on mathematical decision support models, in particular E3 (economy/energy/environment) models and integrated assessment models (IAMs). This presentation reviews some of my recent contributions to climate policy design using different types of E3 models and IAMs. It also provides some thoughts on how cities (and especially smart ones) may contribute to climate change mitigation.