The metrics used in environmental management are performative. That is, the tools deployed to classify and measure the natural world interact with the things they were designed to observe. The idea of performativity also captures the way these interactions shape or distort the governance activities that metrics are used to inform. The performativity of metrics reveals how mundane practices of measurement and auditing are inscribed with substantial power. This has proven particularly true for the global warming metrics, like GWP100, that are central to the management of anthropogenic climate change. Greenhouse gases are materially heterogenous, and the metrics used to commensurate their various warming impacts influence the distribution of both culpability and capital in climate policy and markets. The publication of a new warming metric, GWP* (or GWP Star), has generated a modest scientific controversy, as a diverse cast of stakeholders recognize this performativity seek to influence the metrological regime under which they live. We analyse this controversy, particularly as it unfolded in the fractious discourse around sustainable food and farming, to develop the concept of reflexive performativity: where actors are anticipatory and strategic in their engagement with the metrics that are used to govern their lives. We situate this idea in relation to, and in tentative evidential support of, the concept of reflexive modernization.