Integrating Heat Stress into Circular Livestock Production and Life Cycle Assessment: Toward a Heat-Stress Circularity Index

Heat stress (HS) constrains livestock production under climate change, impairing health, welfare, productivity and survival. Because fixed resource use is then spread over less output, HS is also a systems-level sustainability problem. This narrative review, based on a targeted search of PubMed, Web of Science and Scopus (2013–2026), proposes a framework linking climate exposure, animal responses, resource flows, circular interventions and Life Cycle Assessment (LCA) across dairy cattle, pigs, poultry and small ruminants, and grades the evidence behind each link. A decomposition of meta-analytic data indicates that HS raises the impact per kg of milk by approximately 3–12% during heat-stress periods, mainly through non-feed burdens. Monte Carlo analysis shows that the direction of this effect is robust, whereas its magnitude depends mainly on the feed share of the total burden. A Heat-Stress Circularity Index (HSCI) is operationalized with explicit metrics, normalization bounds, a geometric aggregation rule and a minimum-welfare gate. A worked example shows that system rankings depend on the aggregation rule, and a pilot application to a commercial Mediterranean pig farm shows that heat-season piglet and weaner mortality triggers the welfare gate despite moderate productivity retention. Evidence is strong for physiological responses but absent for LCA under measured heat stress; five falsifiable hypotheses, with the corresponding validation designs and sample sizes, are therefore proposed.