The global sports industry, long celebrated for its unifying power, has in recent decades emerged as a significant and increasingly visible contributor to environmental degradation, generating substantial greenhouse gas emissions, single-use plastic waste and resource-intensive infrastructure demands. This paper traces the conceptual and operational evolution of sports sustainability through three successive tiers of ambition, charting a trajectory from carbon neutrality toward net-zero decarbonization and ultimately, the circular economy ideal of zero waste. The first tier, carbon neutrality, represents the industry's initial response to climate accountability. Grounded in the logic of measurement and offsetting, this phase saw governing bodies and event organizers quantify emissions and compensate for unavoidable impacts through carbon offset projects, exemplified by FIFA's carbon-neutral World Cup commitments and the United Nations Sports for Climate Action Framework. The second tier, net-zero decarbonization, marks a decisive shift from offsetting to abatement. Here the emphasis moves to absolute emission reductions across scopes, renewable energy procurement and efficiency retrofits, sustainable transport and mobility planning and supply-chain engagement. World Athletics' carbon-reduction guidance and the Seattle Kraken's Climate Pledge Arena illustrate how venues and federations are embedding reduction targets, monitoring, and reporting into bid, tender and contractual processes, signalling a maturation from symbolic neutrality to structural change. The third and most recent tier, zero waste, extends sustainability beyond the carbon lens to embrace the full material metabolism of sport. Rooted in circular-economy principles, this phase prioritizes waste avoidance and source reduction, reuse, recycling, and composting, and responsible disposal of residual waste, alongside plastic neutrality and the elimination of landfill diversion. These three tiers are not merely sequential milestones but a deepening hierarchy of ambition, each expanding the boundary of responsibility from atmospheric carbon to the full life cycle of materials and from organizational accounting to systemic, stakeholder-engaged transformation. In short, the evolution from carbon neutrality to zero waste signals a fundamental reconceptualization of sport's environmental role, positioning the industry not as a passive emitter requiring compensation but as an active agent of regenerative and circular practice, with significant implications for policy, governance, and the long-term resilience of the sector.