To prevent the irreversible crossing of Planetary Boundaries, a rapid, global societal shift towards decarbonization and sustainability is imperative. Social Contagion and Tipping Processes related to sustainable behavior and innovations represent some of the few promising mechanisms by which the societal and economic transformation may be achieved in the remaining window of opportunity. Such contagion processes are not limited to individual human beings; in their high political responsiveness and cultural radiance, cities may also be viewed as promising agents in the sustainability transformation. Responsible for a dis-proportionally large part of greenhouse gas emissions, and simultaneously one of the main drivers of sustainable policy innovation and implementation, cities may play a unique role in the global sustainability transformation. Learning from each other to reduce, prepare for and react to the coming environmental changes, they can be conceptualized as nodes in a globe-spanning network. Investigating such a learning network model may yield insights into the social tipping dynamics that are so urgently needed to control the human impacts on the Earth System.
We propose a methodology to identify network-based contagion effects in the sustainability policy adoption by cities. An attempt is made to approximate the inter-city innovation spreading network using the global air traffic network. We analyze the spreading of an illustrative public transport innovation as a contagion process on this inter-city network. Surrogate data methods and a dose-response-contagion approach are used to identify network-spreading-correlations. We then investigate the nature of the spreading process using generative models.