Californian Climate Changes: Decrease in Cold-Related Mortalities Accompanied by an Uptick in Heat-Linked Emergency Scenarios

In a groundbreaking study published in Science Advances on July 30, 2025, a collaborative team of researchers from UC San Diego and Stanford University have unveiled the complex and often paradoxical health impacts of climate change, focusing on temperature extremes and their effects on mortality and morbidity in California.

Funded by the Robert Wood Johnson Foundation, various arms of the National Institutes of Health, and other organizations, the study titled "Temperature extremes impact mortality and morbidity differently" (Gould et al., 2025) calls for holistic surveillance systems and intervention strategies that incorporate the full spectrum of heat-sensitive conditions.

The team, comprising experts in public health, environmental social sciences, epidemiology, and economics, analysed an extensive dataset from 2006 to 2017, encompassing all recorded deaths, emergency department visits, hospitalization events, and daily temperature records across California.

The study finds that heat imposes a broader burden on health outcomes, resulting in increased morbidity across diverse age groups, while cold temperatures primarily elevate mortality risks among older adults. Heat is associated with a range of medical conditions beyond traditionally recognized heat stroke and cardiovascular stress, including poisonings, endocrine disorders, digestive system ailments, and injury rates.

Age stratification emerges as a pivotal factor in shaping vulnerability profiles under thermal extremes. Older adults are more sensitive to cold stress, while younger demographics are disproportionately affected by heat.

The socioeconomic implications of these health trends are profound. Moderate climate change scenarios forecast that California could see approximately 53,500 fewer deaths annually due to diminished cold weather hazards, translating into roughly $30 billion in healthcare savings. However, this benefit is partially negated by an upsurge in heat-driven morbidity, with an estimated 1.5 million additional heat-related emergency department visits, incurring around $52 million in extra healthcare expenditures.

The decline in cold-induced mortality, driven by a reduction in the frequency and intensity of extreme cold days due to climate change, constitutes a public health gain. However, this benefit is partially negated by an upsurge in heat-driven morbidity.

The study advocates for nuanced evidence to safeguard public health and ensure resilient, equitable healthcare infrastructures in the face of rising temperatures. It emphasizes the need for responsive public health strategies that are age-aware, morbidity-inclusive, and economically informed.

The study also highlights the significant strain heat exerts on healthcare systems and diverse populations, underscoring the need for precision targeting in public health interventions, moving beyond generic warnings towards tailored strategies that consider demographic-specific vulnerabilities and healthcare resource allocations.

The study was not without international collaboration, involving researchers from institutions such as the University of Exeter and funded by organizations including the UK Medical Research Council and the Natural Environment Research Council. The authors of the study declared no conflicts of interest.

In conclusion, the study provides a comprehensive overview of the complex health impacts of climate change, offering valuable insights for policymakers, healthcare providers, and the general public alike. As we continue to grapple with the challenges posed by accelerating climate change, studies like this one serve as a crucial reminder of the interconnectedness of our health, environment, and society.

References: Gould, C.F., Heaney, A.K., Heft-Neal, S., Bendavid, E., Callahan, C.W., Kiang, M.V., Burke, M., & Zivin, J.G. (2025). Temperature extremes impact mortality and morbidity differently. Science Advances. https://doi.org/10.1126/sciadv.adr3070