Contagious Transformations? Inter-City Spreading of Sustainability Innovations.

In the Anthropocene, socio-economic systems are an integral and highly interconnected part of Earth System. The internal dynamics of these systems will decide whether the Earth can remain in, or return to, a resilient state that resembles the Holocene. Understanding these dynamics thus represents an important aspect of Earth System Science.To prevent the irreversible crossing of Planetary Boundaries, a rapid, global societal shift towards decarbonization and sustainability is imperative. Incremental political measures have thus far proven to be insufficient to adequately address this necessity. 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.The study presented here aims to identify whether network-based contagion effects are dominant in sustainability policy adoption by cities. An attempt is made to approximate the inter-city innovation spreading network using the global air traffic network, political and trade relations, and other city-to-city connections. These networks are extracted from empirical data, and their prediction power is compared. We analyze the spreading of several municipal policies and innovations related to sustainability transformations as contagion processes on these inter-city networks. 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 by attempting to reproduce it using statistical models. Examples for investigated spreading innovations are the implementation of Bus Rapid Transit public transport systems, and membership in a sustainability organization.