Does Subnational Climate Action Trigger a Tipping Point? Catalytic Cascading and Vertical Policy Diffusion in the Net-Zero Transition

Subnational climate commitments are widely viewed as potential catalysts of national climate ambition, yet systematic evidence on whether and under what conditions subnational action shapes national target-setting remains limited. We introduce the concept of catalytic cascading to describe a cumulative process in which expanding policy commitments change the likelihood of subsequent policy commitment, even at a different jurisdictional level, through reinforcing top-down, horizontal, and bottom-up dynamics, without necessarily following an abrupt threshold or tipping behavior. We examine this theory through the lens of vertical policy diffusion, assessing whether subnational net-zero target declarations are associated with later national net-zero adoption while accounting for country characteristics and cross-national peer influences. Using a novel global time-series dataset of national and subnational net-zero targets, we estimate discrete-time adoption models using a directed country-pair (dyad–time) event history design that incorporates dyad-specific similarity and exposure measures alongside within-country subnational coverage. Across specifications, the share of a country's population, GDP, or greenhouse gas emissions covered by subnational net-zero commitments is positively and statistically significantly associated with the probability of national target adoption. The estimated relationship is gradual and monotonic rather than characterized by a sharp inflection, suggesting that bottom-up influence operates through cumulative reinforcement rather than threshold acceleration in this setting. These findings provide empirical support for catalytic cascading as an observable bottom-up linkage within polycentric climate governance and clarify the conditions under which subnational momentum is plausibly consequential for national target-setting.