In recent decades, the climate in Northwest China has undergone a significant shift towards warm and humid conditions. This trend has attracted extensive attention, particularly regarding a core question: where does the additional precipitation originate from?

Climatologically, precipitation in Northwest China has long been considered to primarily depend on external moisture transport. A research team from the Northwest Institute of Eco-Environment and Resources (NIEER) of the Chinese Academy of Sciences (CAS), in collaboration with Lanzhou University and the Lanzhou Institute of Arid Meteorology of the China Meteorological Administration, indicates that although this remains true for the long-term mean state, the increase in precipitation since the late 20th century has been driven primarily by enhanced local evapotranspiration, not by strengthened external moisture inflow.

The relevant study was published in Advances of Atmospheric Sciences.

The research team used a Dynamic Recycling Model (DRM) to quantitatively assessed the relative contributions of different moisture sources to precipitation in Northwest China. The results show that, compared with the period from 1961 to 1997, the evapotranspiration increased by 10.42 mm (approximately 9.12%) each year, and the annual precipitation increased by 10.62 mm (approximately 9.18%) from 1998 to 2020 .

Notably, approximately 78% of the recent increase in precipitation is attributable to locally recycled moisture, while only about 22% is due to enhanced external moisture transport.

The study reveals pronounced spatial heterogeneity in summer precipitation changes across Northwest China. The regional pattern can be summarized as “increasing in the west and decreasing in the east." The increase in precipitation in the western subregion was significantly greater than that in the eastern part of the region—especially in areas around the Tianshan and Altun Mountains—while some parts of the eastern area even showed a declining trend.

From a temporal perspective, the late 20th century represents a clear turning point in regional precipitation variability. Around the end of the 1990s, summer precipitation shifted from a long-term decreasing trend to a sustained upward trend, marking a critical phase in the humidification of Northwest China.

These findings indicate that although more than half of the climatological mean precipitation in Northwest China originates from external water vapor input, the recent increase in precipitation since the late 20th century has been primarily driven by intensified local land-atmosphere interactions. As a result, enhanced evapotranspiration has emerged as a key driver of the recent humidification trend in Northwest China.

The study also suggests that the intensification of local evapotranspiration is associated with rising temperatures, the increased input of meltwater from glaciers and snowpack, and vegetation recovery.

However, these processes are still constrained by the availability of water resources. As glacier and snow reserves continue to decline under the ongoing warming, meltwater-supported evapotranspiration may weaken, potentially reducing the persistence of the current humidification trend.

The study also highlights the influence of large-scale oceanic variability on regional precipitation changes. Changes in the phase of the Atlantic Multi decadal Oscillation (AMO) may modulate regional precipitation patterns, thereby introducing additional uncertainty into future projections.

Professor Yu from NIEER emphasized that these results fundamentally refine our understanding of the regional hydrological cycle, “From a climatological perspective, precipitation in northwest China has long been considered heavily dependent on external moisture transport. However, our findings show that since the late 1990s, the dominant contribution to the increase in precipitation has shifted to local moisture recycling. This highlights the critical role of land–atmosphere coupling in the region’s wetting trend”.

Professor ZHANG Qiang from the Lanzhou Institute of Arid Meteorology of the China Meteorological Administration remarked, “This work provides quantitative evidence that local hydrological feedbacks have become the dominant mechanism driving recent precipitation increases in northwest China. It advances the theoretical understanding of arid-region climate change and has significant implications for drought monitoring, prediction, and water-resource management.”

Professor HUANG Jianping from Lanzhou University noted, “The warm–wet shift in Northwest China is not solely a circulation-driven phenomenon. It reflects an integrated response of the regional water cycle to warming, cryospheric changes, and ecosystem recovery. Understanding the sustainability of these drivers is essential for projecting future climate trajectories.”

Schematic illustration of the mechanisms responsible for the humidification in Northwest China

Schematic diagram of the atmospheric water budget over Northwest China. In the figure, the denominator represents the multi-year summer mean of each variable during 1998–2020 (units : 10⁵ kg s⁻¹), whereas the numerator denotes the increase relative to the 1961–1997 mean. Here, Fin denotes moisture inflow, Fout denotes moisture outflow, and Fout-a and Fout-e represent moisture outflow associated with horizontal moisture transport and local evapotranspiration, respectively. Pa denotes precipitation formed from externally advected moisture, Pe represents recycled precipitation, and P denotes total precipitation. E indicates evapotranspiration. (b) Comparison of different types of summer precipitation over the arid region of Northwest China (means for 1961–1997 and 1998–2020). Upper panel: 1961–1997; lower panel: 1998–2020. Red indicates recycled precipitation, blue indicates externally sourced precipitation, and green denotes the residual term. The numerical values represent the percentage contribution of each precipitation type to total precipitation in the corresponding period