Shengqian Chen

In arid and semi-arid mountainous regions, precipitation from autumn through early spring plays a crucial role in alleviating spring-summer drought, promoting vegetation green-up, and maintaining ecosystem stability. As such, it provides an important basis for reconstructing historical hydroclimatic variability and regional climate evolution. In this study, a tree-ring width chronology of Juniperus excelsa, collected near the timberline in the southeastern Alborz Mountains of Iran, was used to reconstruct cold-season (September-April) precipitation variability since 1650. The results show that the tree-ring width index is significantly and positively correlated with monthly precipitation from January to April, while its correlation with temperature is not statistically significant. The index is also significantly positively correlated with the Palmer Drought Severity Index (PDSI) from January to August, with the strongest relationship observed for cold-season precipitation, indicating that it is a primary factor controlling radial growth. The reconstructed precipitation series reveals pronounced dry-wet variability over the past ~380 years, including 58 drought years and 53 wet years, suggesting a slightly higher frequency of drought conditions. Prior to the 1750s and after the 1930s, precipitation fluctuated markedly around the long-term mean. In contrast, from the 1750s to the 1860s, precipitation remained predominantly below average, representing a prolonged dry period of more than a century. This was followed by a relatively wet phase from the 1870s to the 1930s, during which precipitation was generally above average. The wettest interval occurred during 1734–1747, while the most severe drought was recorded from 1995 to 2003. Compared other regions influenced by the westerlies, including West Asia, Central Asia, the northeastern Mongolian Plateau, and the Tibetan Plateau, the southeastern Alborz Mountains exhibits relatively low precipitation variability and greater overall stability. Sensitivity to interannual and decadal-scale circulation anomalies is weaker than in these regions. However, dry–wet variations show notable synchrony with patterns observed west of the Jordan River and in Central Asia. Spectral analysis indicates significant periodicities at approximately 2.2 years, 3.2–4.5 years, and 22–26 years. Overall, this study reconstructs the long-term variability of cold-season precipitation in the southeastern Alborz Mountains and provides a scientific basis for understanding regional hydrological responses to future climate warming, as well as the spatial heterogeneity of precipitation variability across westerly-influenced regions.

Stable oxygen isotopes in tree rings (δ¹⁸O) serve as important proxies for climate change and offer unique advantages for climate reconstruction in arid and semi-arid regions. We established an annual δ¹⁸O chronology spanning 1964–2023 using Juniperus excelsa tree-ring samples collected from the Alborz Mountains in Iran. We analyzed relationships between δ¹⁸O and key climate variables: precipitation, temperature, Palmer Drought Severity Index (PDSI), vapor pressure (VP), and potential evapotranspiration (PET). Correlation analysis reveals that tree-ring δ¹⁸O is highly sensitive to hydroclimatic variations. Tree-ring cellulose δ¹⁸O shows significant negative correlations with annual total precipitation and spring PDSI, and significant positive correlations with spring temperature (particularly maximum temperature), April VP, and spring PET. The strongest correlation occurs with spring PET. These results indicate that δ¹⁸O responds strongly to the balance between springtime moisture supply (precipitation and soil moisture) and atmospheric evaporative demand (temperature, VP, and PET), reflecting an integrated signal of both regional moisture availability and energy input. The pronounced response of δ¹⁸O to spring evaporative conditions highlights its potential for capturing high-resolution changes in spring climatic conditions. Our δ¹⁸O series remained stable from the 1960s to the 1990s, but showed greater interannual variability after 2000, likely linked to regional warming and climate instability. A comparison with the δ¹⁸O variations from the eastern Alborz Mountains indicates that, despite some differences in magnitude, δ¹⁸O records from the western and eastern Alborz Mountains show broadly similar variability patterns. On a larger climatic scale, δ¹⁸O correlates significantly and positively with the Niño 3.4 index but shows no significant correlation with the Arctic Oscillation (AO) or the North Atlantic Oscillation (NAO). This suggests that ENSO-driven interannual variability in the tropical Pacific plays a key role in regulating regional hydroclimatic processes. This study confirms the strong potential of tree-ring oxygen isotopes from the Alborz Mountains for reconstructing hydroclimatic conditions and high-frequency climate variability.

Located in the arid inland of Asia, the eastern part of the Silk Road is marked by certain routes being close to or even crossing large deserts, such as the Taklimakan Desert, one of the largest deserts worldwide. As a result, sand and dust activities have a considerable impact on the transport routes, the desert-oasis ecosystem, and human society along the Silk Road. However, the evolution of dust activity over the past two millennia and its relation to the changes of the Silk Road civilization remains ambiguous. Here, we present a high-resolution (~3 yr) stalagmite record from Xinjiang (northwest China) spanning the past 2,500 years, dated with 19 U/Th ages. Although the stable isotopes and trace elemental ratios of the stalagmite reveal remarkable decadal- to centennial-scale variability of the regional hydroclimate, the Mg/Ca ratio shows a quite different variation pattern compared with other geochemical proxies. Considering various factors that might influence the Mg/Ca ratio of stalagmites, our analysis reached the conclusion that the geographical location close to the desert made the imported dust likely to predominate the increase of Mg/Ca in stalagmites during many characteristic periods. For instance, we found significant increases in the Mg/Ca ratios lasting for more than two centuries during approximately 650-850 CE and 1650-1950 CE (i.e., the Little Ice Age). This generally demonstrates a pattern of reduced dust activities during the Medieval Warm Period and enhanced dust activities throughout the Little Ice Age, which is supported by evidence from the eolian sedimentary section in the southern margin of the Taklimakan Desert that directly reflects dust activity. We further found that the enhanced dust activity during the 650-850 CE might have caused the route shift of the Silk Road from south to north in the Tarim Basin. In addition, the rapid drying of Lop Nur in recent decades could also be attributed to abnormally increased dust activity, as this period was characterized by the most intense dust activity in our records over the last 2,000 years. Our findings further substantiate the argument regarding the association between societal and climatic change along the Silk Road, where the dust production from large deserts poses challenges to sustainable development in the present and the future.

Investigating the spatiotemporal variations in hydroclimate is essential for addressing the challenges posed by future climate change. The Asian drylands, encompassing Arid Central Asia (ACA) and West Asia (WA), are regions where water vapor transport is predominantly governed by the westerlies. These regions have exhibited a dipole pattern in precipitation variations over recent decades and throughout the Holocene. However, whether this pattern persisted during the past millennium remains uncertain. Our study reveals that both proxy records and PMIP4 models consistently identify a dipole precipitation pattern between ACA and WA on centennial scales over the past millennium. This pattern is attributed to inconsistent seasonal precipitation changes. During the Medieval Warm Period, ACA experienced decreased winter-spring precipitation, while WA observed increased summer rainfall. This trend reversed during the Little Ice Age. The seasonal shifts in precipitation are likely driven by the migration of the westerlies, influenced by internal variability within the Earth's system, particularly the North Atlantic Oscillation (NAO). These findings provide valuable insights for policymakers aiming to address water management challenges in Asia's drylands, offering a foundation for strategies to mitigate and adapt to the impacts of future climate change.