Climate change and human adaptability: an integrated framework for historical insights and future capacity in Japan

Despite substantial evidence on temporal variations in temperature-mortality associations, the historical pace (direction and magnitude) of adaptation to climate change remains unclear. We employed a novel integrated framework to investigate temporal changes in threshold temperature and its uncertainties, and quantify the pace of historical climate adaptation by assessing the time-varying relationship between mean and threshold temperatures across 47 prefectures in Japan from 1972 to 2019. Using a first-stage time-series regression and second-stage meta-analysis, we observed consistent increases in both absolute threshold temperature (minimum mortality temperature, MMT) and relative threshold (MMT percentile, MMTP) at the country level. MMT increased significantly from 23.91 °C (95 % confidence interval [CI]: 23.14, 24.68) in the 1970s to 26.05 °C (95 % CI: 25.38, 26.71) in the 2010s. MMTP showed a nonsignificant increase from the 82.9th (95 % CI: 80.0, 85.7) to the 86.5th percentile (95 % CI: 84.2, 88.7) for MMTP. Nationwide coefficients for the association between mean and threshold temperatures derived from the mixed-effects meta-regression generally increased across subperiods, indicating slight adaptation [from 0.15 (95 % CI: -0.54, 0.84) to 0.64 (0.01, 1.28) for MMT, and from -2.57 (95 % CI: -5.33, 0.20) to -0.72 (95 % CI: -3.57, 2.12) for MMTP]. However, the trend exhibited a sharper increase in earlier decades, followed by a gradual decline or leveling off in subsequent decades. These patterns were largely consistent across age and sex subgroups, though the pace varied by cause of mortality. Our findings suggest that the Japanese population has somewhat adapted to the changing climate over time, but not at the same pace as temperature increase in more recent decades.