Legal authority of cities to enact policies to reduce GHG emissions

What is the legal authority of cities to enact policies to reduce GHG emissions or adapt to climate change? What are the practical (physical/tech/historical) reason that cities are positioned to enact policies that can create significant reductions in GHG emissions or adapt to climate change? What are the values-based reasons for cities to enact policies that can create significant reductions in GHG emissions or adapt to climate change? Are there any limitations to cities enacting climate policy? What are they and how can they be overcome?