Heat wave frequency up 40% since 2010, driving 15% rise in U.S. summer mortality

National Weather Service data show the number of days exceeding 95°F in major metros rose sharply after 2018. CDC mortality files record heat as a contributing cause in 3,500 deaths in 2023, up from 2,100 in 2014. Observational linkage studies in The Lancet Planetary Health attribute roughly 40% of the increase to longer event duration rather than peak temperature alone.

These patterns match findings from the 2022–2024 summers, where cardiovascular and renal admissions spiked 12–18% during multi-day events in the Southeast and Midwest. Medicare claims analyses reveal disproportionate impact on adults over 75 and those with prior diabetes or heart failure diagnoses. No randomized interventions exist; evidence remains limited to time-series and case-crossover designs.

Projections from the EPA Climate Change Indicators report indicate another 30% increase in heat-related mortality by 2035 under current emissions trajectories. Cities without cooling-center expansion or targeted home-visit programs for high-risk residents will likely see the steepest rises. Next required evidence includes city-level quasi-experimental evaluations of cooling interventions measuring both mortality and cardiorespiratory hospitalizations.