The customary way of comparing greenhouse gases masks important differences that arise from the stock nature of CO2 (and other) emissions, and the essentially flow pollutant nature of short-lived pollutants such as methane. This talk will argue that these differences may be ethically significant, especially but not only for approaches centred on polluter pays arguments. The distinction matters not only for international fairness and equity where food security concerns and pro-poor climate policy are important existing commitments, but also domestically, where issues of sectoral fairness may become significant.