Rapid increase in the United States influenza epidemics driven by anthropogenic rapid temperature variations during the autumn transition period

Recent decades have witnessed an intensification of influenza epidemics in the United States despite winter warming, highlighting a gap in understanding the climatic drivers of epidemic variability. Using influenza-like illness (ILI) records, reanalysis and station-based meteorological observations, and Coupled Model Intercomparison Project Phase 6 (CMIP6) climate model simulations, we investigated the influence of high-frequency temperature variability—measured as the temperature change between adjacent days (TCN)—on influenza intensity and assessed the contribution of anthropogenic climate forcing. The seasonal peak in ILI rates increased by 50% from 1997 to 2021, rising from ∼4% to ∼6%. Two-thirds of this increase was associated with the amplified October–November TCN. This association was most evident in the central United States, where the secular trend in TCN exceeded 0.1°C per decade. In the ensemble of CMIP6 simulations, a notable increase in TCN was pronounced along with enhanced warming driven by the reduction in anthropogenic aerosol (AA) emissions. The strengthening of the meridional temperature gradient caused by uneven changes in AA and greenhouse gas emissions favoured immune-related TCN by 0.012°C per decade, leading to the intensification of influenza epidemics eventually. These findings identify high-frequency temperature variability as a key climatic pathway linking anthropogenic forcing to epidemic intensification. Our findings address the need for more thoughtful mitigation and adaptation strategies to minimise the adverse health effects of anthropogenic climate drivers.