The Parties to the UNFCCC and Paris Agreement agreed to act "on the basis of equity" to protect the climate system. Equitable effort-sharing is an irreducibly normative matter, yet some influential studies have sought to create quantitative indicators of equitable effort that claim to be value-neutral (despite evident biases). Many of these studies fail to clarify the ethical principles underlying their indicators, some mislabel approaches which favour wealthy nations as 'equity approaches', and some combine contradictory indicators into composites we call 'derivative benchmarks'. This Perspective reviews influential climate effort sharing assessments and presents guidelines for developing and adjudicating policyrelevant (but not ethically neutral) equity research.

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The idea of a global “carbon budget”—the cumulative amount of “allowable” carbon emissions to meet a global temperature target—has become established as a central concept in climate science and policy. As a concept explicitly aimed at mediating between scientific knowledge and policymaking, the carbon budget has always been actively positioned in relation to ongoing policy debates, but the specific forms this concept has taken have varied. This article reviews key contributions to the carbon budget literature from the 1980s until today, in order to identify how scientists have positioned the concept between the worlds of science and policy. Three main shifts are identified in how the policy relevance of the carbon budget is envisaged in the scientific literature. The shifts can be related in part to developments in climate science, and in part to changes in international climate policy. The history of the carbon budget thus illustrates how science and policy interacts to shape dominant understandings of how climate change can be known and governed. This article is categorized under: Climate, History, Society, Culture > Ideas and Knowledge