Changes in compound heat–humidity over China: Insights from homogenized data

Global warming intensifies compound heat–humidity events, yet assessments of their historical trends can be affected by humidity-record inhomogeneities and metric dependence. To examine these effects, this study uses ERA5, JRA-3Q, CN05.1, and the Homogenized gridded Climate Dataset for China (HCD01) to investigate summer compound heat–humidity changes over China during 1961–2022. We evaluate differences in relative humidity (RH) trends among datasets and their impacts on heat–humidity indices, compare the spatial trend patterns of wet-bulb temperature ( T w ), simplified wet-bulb globe temperature (sWBGT), and equivalent temperature ( T e ), and analyze summer compound heat–humidity extremes (CHHEs), with trends assessed at the 5% significance level. The results show that humidity record inhomogeneities produced significant declining RH trends in non-homogenized datasets, whereas HCD01 exhibited a significant increase of 0.17 % per decade. These biases propagated to heat–humidity indices and led to underestimated increases, most clearly over Southeast China, where significant increasing trends in the three heat–humidity indices were found only in HCD01. T w , sWBGT, and T e showed broadly consistent spatial patterns, and the largest trends occurred over Northeast China and the northeastern Tibetan Plateau. Using T e , the frequency, intensity, and duration of CHHEs all increased significantly nationwide by 0.30 events, 0.31 °C, and 0.66 d per decade, respectively. The largest frequency and duration trends were concentrated over southern Xinjiang and southeastern Tibet, whereas the largest intensity trends occurred over northern Northeast China. Regional CHHEs clustered in 2001–2022, with intensity increasing most over Northeast China and frequency over the northeastern Tibetan Plateau. These findings highlight the importance of homogenized humidity records for accurately characterizing the widespread yet regionally heterogeneous intensification of CHHEs across China.