Climate change and cities are deeply connected. Cities are one of the greatest contributors and likely best opportunities for addressing climate change. Cities are also one of the most vulnerable parts of the human society to the effects of climate change, and likely one of the most important solutions for reducing the environmental impact of humans. The UN projects that 68% of the world population will live in urban areas by 2050. In the year 2016, 31 mega-cities reported having at least 10 million in their population, 8 of which surpassed 20 million people. However, secondary cities - small to medium size cities are rapidly increasing in number and are some of the fastest growing urbanizing areas in the world further contributing to climate change impacts. Cities have a significant influence on construction and transportation—two of the key contributors to global warming emissions. Moreover, because of processes that create climate conflict and climate refugees, city areas are expected to grow during the next several decades, stressing infrastructure and concentrating more impoverished peoples in cities.

A zero-carbon city is a goal of city planners that can be variously defined. In a narrower sense of energy production and use, a zero-carbon city is one that generates as much or more carbon-free sustainable energy as it uses. In a broader sense of managing greenhouse gas emissions, a zero-carbon city is one that reduces its carbon footprint to a minimum by using renewable energy sources; reducing all types of carbon emissions through efficient urban design, technology use and lifestyle changes; and balancing any remaining emissions through carbon sequestration. Since the supply chains of a city stretch far beyond its borders, Princeton University's High Meadows Environmental Institute suggests using a transboundary definition of a net-zero carbon city as "one that has net-zero carbon infrastructure and food provisioning systems".