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) relative to daily minimum temperature (T) have contributed to rising VPD. Here, we show that the faster rise in T compared with T 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. 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.