Heat stress is one of the most significant and growing welfare threats to farmed animals globally. As temperatures rise with climate change, understanding and mitigating heat stress becomes increasingly urgent for the welfare of billions of animals.
Heat stress causes a spectrum of welfare harms from mild discomfort to severe physiological crisis and death. The subjective experience of sustained heat stress — inability to cool down, respiratory distress, progressive weakness — is likely to involve significant suffering in animals with the nervous system complexity to experience it. Chronic subacute heat stress, which occurs at temperatures below the threshold for obvious clinical signs, impairs immune function, gut health, and reproductive performance without triggering acute emergency management responses. This creates a large hidden welfare burden that is often not recognised as heat-related. During acute heat waves, livestock mortality events can involve thousands to millions of birds dying from hyperthermia in hours — a catastrophic welfare failure that climate change will make more frequent. The asymmetry between the economic incentive to protect high-producing dairy cows and the absence of equivalent protection for lower-value broilers and pigs reflects a welfare gap that requires regulatory intervention rather than market-driven improvement alone.