Sub-diurnal asymmetric warming has amplified atmospheric dryness since the 1980s

Rising atmospheric vapor pressure deficit (VPD)-a measure of atmospheric dryness, defined as the difference between saturated vapor pressure (SVP) and actual vapor pressure (AVP)-has been linked to increasing daily mean near-surface air temperatures since the 1980s. However, it remains unclear whether the faster increases in daily maximum temperature (T_max) relative to daily minimum temperature (T_min) have contributed to rising VPD. Here, we show that the faster rise in T_max compared with T_min over land has intensified VPD from 1980 to 2023. This sub-diurnal asymmetric warming has driven a larger SVP increase than would occur under uniform temperature rise, while AVP is more strongly influenced by T_min. Using reanalysis data, we estimate that asymmetric warming has contributed an additional ~18% to the increase in global land VPD. Sub-daily station observations corroborate this pattern, with asymmetric warming accounting for ~30% of VPD intensification across all stations. Our findings indicate that sub-diurnal asymmetric warming has substantially amplified global warming's effect on atmospheric dryness over the past four decades, with significant implications for terrestrial water availability and carbon cycling.