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

ABSTRACT 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.