CDP (formerly the Carbon Disclosure Project) has facilitated nearly 10,000 corporate climate risk disclosures in its twenty years of operation. Despite an influx of public authority (government-led initiatives and rules) that threatens to compete with CDP, the private governor has instead expanded its “governance space,” drawing in even greater numbers of disclosures to its platform. To explain this puzzle, we argue that the consequences of public intervention in private governance, whether competitive or cooperative, are a function of the subsequent costs and benefits firms face when deciding to disclose voluntarily with private governors. Using process tracing and interview data, we find that the degree to which public interventions can crowd-in firms to private governance and expand private governance space depends on the extent to which private governors can respond to or shape public interventions in ways that maximize complementarity. This paper critically contributes to understanding the outcomes of public-private governance interactions by illustrating how private governance initiatives can co-evolve with public authority to ratchet up public rules.