Global warming has accelerated the hydrological cycle, leading to an increased frequency of extreme climate events. Relative humidity (RH), as a core indicator of atmospheric aridity, not only regulates ecosystem functions and environmental quality, but also directly affects human health and agricultural production. Reconstructing its historical evolution therefore holds significant scientific and practical value.

The Chinese Loess Plateau (CLP) serves as a critical ecological barrier and dryland farming region in Northwest China, where water availability is crucial for regional sustainability. However, existing dendroclimatological studies on the CLP have focused predominantly on its western and northeastern margins, with reconstructions largely limited to temperature, precipitation, and the Palmer Drought Severity Index. Notable gaps remain regarding relative humidity.

To address these gaps, researchers from Institute of Earth Environment of the Chinese Academy of Sciences, developed a tree-ring width chronology from 51 cores of 27 Pinus tabulaeformis Carr. individuals in the southeastern CLP. They found that RH from April to mid-August (RH_c10-23) represented the primary limiting factor for radial growth of P. tabulaeformis in the study area. They therefore produced a near-two-century RH_c10-23 reconstruction spanning 1844–2023, which explains 45.40% of the instrumental variance.

Over the past 180 years, the study area has exhibited a typical "warm-dry/cold-wet" hydrothermal configuration, with relatively dry periods during 1844–1846, 1898–1901, 1926–1935, 1974–1980, and 1997–2023, and relatively wet periods during 1850–1890, 1910–1916, and 1951–1966. Notably, the reconstruction reveals a persistent drying trend since 1844 and documents three major historical droughts in northern China (around 1900, the late 1920s, and 1940–1943), alongside an unprecedented persistent drought during 1997–2023.

Critically, despite its exceptional severity, the 1997–2023 drought caused comparatively limited economic losses and societal disruption, reflecting the substantially enhanced climate resilience of modern society. Furthermore, the study reveals that RH_c10-23 variability is synergistically regulated by large-scale climate systems including the Asian Summer Monsoon (ASM), Atlantic Multidecadal Variability/Oscillation (AMV/AMO), El Niño-Southern Oscillation (ENSO), and Indian Ocean Dipole (IOD).

This study advances understanding of regional hydroclimatic evolution, demonstrates the practical value of proactive human adaptation strategies in addressing climate change, and provides critical scientific support for water resource management and climate change mitigation.

This work published in the Journal of Forestry Research, was mainly supported by grants from the National Natural Science Foundation of China and the Natural Science Basic Research Program of Shaanxi.