Human activities have more impacts on the recent discharge reduction of the largest tributary of the Yellow River relative to last three centuries.

How to quantitatively decouple the impacts of climate change and human activities on river discharge changes is a challenge in current global change research. As the largest tributary of the Yellow River (YR), the Weihe River (WR) is a typical river whose discharge is influenced by climate change and human activities. Here, we first attempt to obtain the normal-flow season and high-flow season discharge in the lower reaches of the WR by using tree rings and historical documents, respectively. The relationship between natural discharge in the two seasons is unstable and complex since 1678. Using an innovative method, we reconstructed the natural discharge from March to October (DM-O), which explains >73 % of the variance in the observed DM-O during the modeling period 1935-1970. There were 44 high-flow years, 6 extremely high-flow years, 48 low-flow years and 8 extremely low-flow years from 1678 to 2008. The contribution of WR annual discharge to the YR is 17 % over the past three centuries, and their natural discharge changes synchronously rise and fall. Human activities, such as the construction of reservoirs and check-dams, agricultural irrigation and domestic and industrial water consumption, have more impacts than climate change on the decrease in the observed discharge. In total, 53.5 % of the discharge reduction since 1971 is due to human activities, and 46.5 % is due to climate change. In addition, this study provides an important model for how to quantify the influences of human activities and nature on discharge reduction and to reconstruct seasonal resolution climate in global change studies.

Maximum July-August temperatures for the middle of the southern Tien Shan inferred from tree-ring latewood maximum densities.

Long-term temperature reconstructions are urgently needed to prolong meteorological climatic data, which are too short to evaluate the anthropogenic effect on climate change since the Industrial Revolution. The maximum latewood chronology (MXD) of Picea schrenkiana in the middle of the southern Tien Shan was established, and it showed a strong correlation with the mean maximum temperature of the current July to August (TmaxJA), with r = 0.773 (p < 0.001, 1959-2016), which implies that a high temperature in the late growing season could increase the cell wall thickness and lead to high latewood density. Then, the TmaxJA of the middle of the southern Tien Shan was reconstructed over the period of 1720-2018. Three MXD chronologies from Kyrgyzstan significantly correlated with our TmaxJA reconstruction at the interannual scale, and they also showed similar variations on decadal scales. None of these MXD series showed a warming trend in the past century, which was also found in several MXD series from different regions of the world. Spatial correlation analysis revealed that our TmaxJA reconstruction showed significant correlations with that in eastern Asia, southern Europe, and north-western Africa, forming a teleconnection called the Silk Road Pattern. However, moving correlation analysis between our TmaxJA reconstruction and Hokkaido temperature series indicated that this teleconnection was unstable in the past 3 centuries. The volcanic eruptions from the mid-high latitudes in the Northern Hemisphere showed a stronger cooling effect than those from the Southern Hemisphere and the low latitudes of the Northern Hemisphere. The summer North Atlantic Oscillation was also shown to affect the temperature in the Tien Shan to a certain extent.

Insight into spatial-temporal patterns of hydroclimate change on the Chinese Loess Plateau over the past 250 years, using new evidence from tree rings.

The climate aridity since the mid-20th century has raised concerns about water resources on the Chinese Loess Plateau (CLP). A lack of extended observation-like precipitation records for the eastern CLP (ECLP) means that it remains unclear whether or not the current arid state of the CLP is unprecedented, and the spatial-temporal characteristics of hydroclimatic variability across the CLP over past centuries are not well understood. Here we present a regional hydrological-year precipitation reconstruction for the Heichashan Mountains, which successfully captures hydroclimate changes on the ECLP since 1773 CE. The reconstruction explains 48.72 % of the observed variance for 1957-2019 CE and reveals a wetting trend since the early 2000s and shows 2014-2020 CE to have been the second wettest period over the past 248 years. 1910-1932 CE was the longest and driest period over the past centuries. Furthermore, the 19th century was relatively wet, whereas the 20th century was dry. We demonstrate that droughts tend to occur in warm periods. Combining our new reconstruction with previously published hydroclimatic reconstructions, we find that hydroclimate has changed synchronously on the ECLP and the western CLP (WCLP) for most of the past two centuries. Some regional differences do exist, for example in the 1890s-1920s, when aridity gradually intensified across the ECLP, no similar drying is evident in records for the WCLP, although the 1920s megadrought occurred in both the ECLP and WCLP. Another difference is in the onset of the 20th-century aridity, which began in the 1950s on the ECLP, around 20 years later than it began on the WCLP. In addition to the known influences of the Asian Summer Monsoon and related large-scale circulations, this work highlights a major finding that the 1920s megadrought may be related to a regime shift in Northern Hemisphere temperature.

A July-August relative humidity record in North China since 1765 AD reconstructed from tree-ring cellulose δ18O.

Since the late 1970s, East Asian summer monsoon (EASM) has shown a significant weakening trend, and sustained drought has occurred across North China. Placing recent climate changes in the paleoclimatic context can better understand the EASM variations. Four δ18O sequences based on tree-ring cellulose of Chinese pine were developed from Mt. Beiwudang, North China, covering a period from 1700 to 2013. Based on a climatic response analysis, a transfer function was designed to reconstruct the relative humidity from July to August (RHJA hereafter). The RHJA spans from 1765 to 2013 and explains 49% (R2adj = 48%) of the instrumental variance during the calibration period (1961-2013, r = - 0.70, p < 0.0001). The RHJA is mainly influenced by precipitation in the summer rainy season and reflect EASM variations. Spatial representation analysis indicates that RHJA represents the dry/wet variations across North China. At the interannual scale, RHJA records many extreme dry/wet events, among which the events in 1876-1878, 1900, and the 1920s are extensive droughts. Those events correspond well to ENSO events, plus further correlation and periodicity analysis indicate that RHJA contains ENSO signals. At the interdecadal scale, RHJA shows a decreasing trend and unprecedented low values from 1981 to 2013, suggesting that the weakening of EASM since the late 1970s is unprecedented in the past 249 years. Similarly, the significantly correlating region in the spatial correlation analysis, covering the Meiyu/Baiu/Changma rainfall belt and India, have also undergone a climatic shift since the late 1970s according to previous papers.

Delayed warming in Northeast China: Insights from an annual temperature reconstruction based on tree-ring δ18O.

Global warming has had an unprecedented impact on environmental changes and thus human life in mid-high latitude regions. As one of the areas most affected by global warming, Northeast China has suffered from a series of ecological crises, including warming-induced water deficits, permafrost thaw, and extended growing seasons. The change in annual average temperature (annual T) variations in Northeast China since the Industrial Revolution are still not fully understood, mainly because of the lack of long-term instrumental data and high-resolution annual T reconstructions. Here, we present the first annual T reconstruction (r = -0.683, p < .001, n = 60) for 1818-2012 in Northeast China, which may also be the first temperature reconstruction based on tree-ring δ18O in China. The reconstruction is significantly related to temperature variations over mid-high latitude Eurasia and agree (p < .01) with several long-term hydroclimatic reconstructions in the surrounding area. When the internal variability in the reconstruction was high, the decadal to multidecadal cycles were significant. Further analysis found that the reconstruction was mainly affected by the East Asian Summer Monsoon (EASM) and North Atlantic Oscillation (NAO). The reconstruction was significantly negatively correlated with several time series of annual T in the Northern Hemisphere, which showed that there is a substantial difference in annual T between Northeast China and other regions of the Northern Hemisphere. The difference mainly existed before the 1950s. From the 1850s to the 1950s, the annual T in Northeast China decreased slightly. However, extreme warming began in the 1950s in Northeast China, and this warming has been unprecedented during the past two centuries. If the warming trend since the 1950s continues, then it will lead to devastating disasters to forest and permafrost ecosystems in Northeast China.

Recent anthropogenic curtailing of Yellow River runoff and sediment load is unprecedented over the past 500 y.

The Yellow River (YR) is the fifth-longest and the most sediment-laden river in the world. Frequent historical YR flooding events, however, have resulted in tremendous loss of life and property, whereas in recent decades YR runoff and sediment load have fallen sharply. To put these recent changes in a longer-term context, we reconstructed natural runoff for the middle reach of the YR back to 1492 CE using a network of 31 moisture-sensitive tree-ring width chronologies. Prior to anthropogenic interference that started in the 1960s, the lowest natural runoff over the past 500 y occurred during 1926 to 1932 CE, a drought period that can serve as a benchmark for future planning of YR water allocation. Since the late 1980s, the low observed YR runoff has exceeded the natural range of runoff variability, a consequence of the combination of decreasing precipitation and increasing water consumption by direct and indirect human activities, particularly agricultural irrigation. This reduced runoff has resulted in an estimated 58% reduction of the sediment load in the upper reach of the YR and 29% reduction in the middle reach.

Tree-ring evidence of the impacts of climate change and agricultural cultivation on vegetation coverage in the upper reaches of the Weihe River, northwest China.

Comprehending the characteristics and causes of vegetation coverage in history is of practical significance for studying ecological and environmental changes. As a typical region of the semi-arid and semi-humid climatic zone in northwest China, the upper reaches of the Weihe River have relatively fragile ecological environment. Based on tree-ring width chronologies, the vegetation coverage represented by the normalized difference vegetation index was reconstructed from 1630 to 2006 using a regression model. There were 64 years with high vegetation coverage and 56 years with low vegetation coverage over the past 377 years. At low frequencies, the coverage was relatively higher in the 1650s and from the 1880s to 1890s, while the coverage was lower in the 1720s and from the 1760s to 1770s. While precipitation and temperature had positive and negative influences on the changes of vegetation coverage, respectively, during the past several centuries, the agricultural cultivation played an important role on coverage changes. Along with the land reclamation expansion in history, the forest cover gradually declined, and vegetation coverage decreased. The vegetation coverage was lower when there were more arable lands reclaimed from woodlands. Regardless of the land reclamation policy during the historical period or the current conversion project of cropland to forest, they affected the vegetation coverage by influencing cover ranges of woodland and farmland.

Sunshine duration reconstruction in the southeastern Tibetan Plateau based on tree-ring width and its relationship to volcanic eruptions.

Sunshine is as essential as temperature and precipitation for tree growth, but sunshine duration reconstructions based on tree rings have not yet been conducted in China. In this study, we presented a 497-year sunshine duration reconstruction for the southeastern Tibetan Plateau using a width chronology of Abies forrestii from the central Hengduan Mountains. The reconstruction accounted for 53.5% of the variance in the observed sunshine during the period of 1961-2013 based on a stable and reliable linear regression. This reconstructed sunshine duration contained six sunny periods (1630-1656, 1665-1697, 1731-1781, 1793-1836, 1862-1895 and 1910-1992) and seven cloudy periods (1522-1629, 1657-1664, 1698-1730, 1782-1792, 1837-1861, 1896-1909 and 1993-2008) at a low-frequency scale. There was an increasing trend from the 16th century to the late 18th and early 19th centuries and a decreasing trend from the mid-19th to the early 21st centuries. Sunshine displayed inverse patterns to the local Palmer drought severity index on a multidecadal scale, indicating that this region likely experienced droughts under more sunshine conditions. The decrease in sunshine particularly in recent decades was mainly due to increasing atmospheric anthropogenic aerosols. In terms of the interannual variations in sunshine, weak sunshine years matched well with years of major volcanic eruptions. The significant cycles of the 2- to 7-year, 20.0-year and 35.2-year durations as well as the 60.2-year and 78.7-year durations related to the El-Niño Southern Oscillation, the Pacific Decadal Oscillation and the Atlantic Multidecadal Oscillation suggested that the variation in sunshine duration in the southeastern Tibetan Plateau was possibly affected by large-scale ocean-atmosphere circulations.