Climatic changes during the past 1300 years as deduced from the sediments of Lake Nakatsuna, central Japan

Limnological features and sediment characteristics were studied in Lake Nakatsuna, a mesotrophic lake in central Japan. The lake is dimictic, and is anoxic in the hypolimnion during thermal stratification from May to September. In an attempt to reconstruct paleoclimatic changes around the lake, a sediment core taken from the lake center spanning the past 1300 years was analyzed for its organic and inorganic contents. Climatic influences were examined on the variation of total organic carbon (TOC), total nitrogen (TN), and sand contents. Short- and long-term fluctuations in TOC, TN, and sand contents are evident, and variation in atmospheric temperature appears to be important for their long-term variability. The sediment record from AD 900 to 1200 indicates hot summers and warm winters with less snow accumulation, whereas the record from AD 1200 to 1950 is characterized by high variation of temperature, with three cool phases from AD 1300 to 1470, 1700 to 1760, and 1850 to 1950. The warm period from AD 900 to 1200 corresponds well to the Medieval Warm Period, and the second and third cool phases are related to the Little Ice Age. Received: February 16, 2001 / Accepted: June 7, 2001     

Development of the East Asian summer monsoon: Evidence from the sediment record in the South China Sea since 8.5 Ma

128 samples from Ocean Drilling Program (ODP) Site 1143 in the southern South China Sea were analyzed for grain size, clay minerals, biogenic opal content and quartz in order to reconstruct changes in East Asian monsoon climate since 8.5 Ma. An abrupt change of terrigenous mass accumulation rate (MAR), clay mineral assemblage, median grain size and biogenic opal MAR about 5.2 Ma suggests that between 8.5-5.2 Ma the source of terrigenous sediment was mainly in the region of surface uplift and basaltic volcanism in southern Vietnam. A simple model of East Asian summer monsoon evolution was based on the clay/feldspar ratio, kaolinite/chlorite ratio and biogenic opal MAR. The summer monsoon has two periods of maximum strength at 8.5-7.6 Ma and 7.1-6.2 Ma. Subsequently, there was a relatively stable period at 6.2-3.5 Ma, continued intensification about 3.5-2.5 Ma, and gradually weakening after 2.5 Ma. Since 1 Ma the monsoon has intensified, with remarkable high-frequency and amplitude variability. Simultaneous increase in sedimentation rates at ODP Sites 1143, 1146 and 1148, as well as in MAR of terrigenous materials, quartz, feldspar and clay minerals at ODP Site 1143 at 3.5-2.5 Ma, may be the erosional response to both global climatic deterioration and the strengthening of the East Asian summer monsoon after about 3-4 Ma.     

过去4000年中国降水与森林变化的数量关系

历史时期陆地森林覆被的变化是影响气候变化的重要因素。通过应用历史回推、理论推导和统计分析方法,研究了过去4000a中国降水与森林变化的数量关系。结果表明:近4000a来全国年均降水量由918mm,下降为20世纪70-80年代的612mm,再提高到21世纪初年的628mm。全国森林厚度约由9.38mm,减少到20世纪70年代的1.03mm,再恢复到21世纪初的1.58mm。由于森林相对农田、草原、城市等土地利用类型具有更强大的蒸散作用,加之中国东南季风的特殊气候动力条件,致使中国的森林对降水有重要影响。森林厚度每变化1mm,则影响降水变化37mm。中国20世纪70-90年代出现的罗布泊干涸、青海湖水位降至历史顶点、黄河断流、土地沙漠化扩大等气候干旱现象不是偶然的,从大尺度上说,是我国近4000a来森林资源破坏达于极点而导致气候干旱化达于极点的必然结果。历史上森林破坏时空上的不均匀性可能导致了气候变化的阶段性和突变,280-1230年,受人口南迁和人口数量增长的影响,中国南方亚热带和热带森林受到大规模破坏,致使在此期间中国气候表现为持续干旱化。可见我国南方森林影响全国尤其是北方气候变化的作用值得重视。治理中国北方地区的沙漠化在根本上要靠森林资源的增加,不仅要提高森林覆盖率,更要增加森林厚度。中国土地沙漠化加剧的主要原因在于气候的干旱化,而气候干旱化的主要原因则是中国森林资源的减少。在未来,只要中国森林的厚度增加了,全国年均降水量就会增加起来,中国的沙漠化就会得到治理,西北地区大面积土地的生产潜力就会逐渐得到挖掘和发挥。     

Tree-ring indicators of winter-spring temperature in Central China over the past 200 years

Long-term, high-resolution proxy records containing temperature signals are relatively scarce over Central China, limiting our understanding of regional climate and the potential driving forces. In this study, a robust tree-ring width chronology is developed based on Pinus massoniana woods from two sites on the northeast Wuling Mountains, spanning from 1815 to 2014. The presented chronology is used to reconstruct regional long-term winter-spring (January-May) temperature changes over the past two centuries in Central China. The reconstruction reveals that cold epochs occurred in 1830 s–1840 s, 1850 s–1860 s, 1900 s–1910 s and 1960 s–1970 s. The correlation patterns between the reconstruction and global sea surface temperature (SST) are suggestive of the influence of SST of the western North Pacific Ocean (especially the Japan Current region) and the North Atlantic Ocean on regional temperature. In addition, persistent close matches are observed between the chronology and the Atlantic Multidecadal Oscillation (AMO) reconstructions over the past two centuries, suggesting the AMO may have been a key modulator of long-term temperature variations over Central China.     

Growing-season precipitation since 1872 in the coastal area of subtropical southeast China reconstructed from tree rings and its relationship with the East Asian summer monsoon system

Tree rings from temperate zones of the world have provided abundant palaeo- ecological and paleo-hydroclimatic information. However, tree rings from subtropical to tropical regions remain relatively scarce, which greatly limit our fully understanding about the climate change issues. In the present work, tree-ring-width (TRW) measurements of Masson pine from Fujian province, the coastal area of subtropical southeast China were successfully crossdated and a TRW STD chronology was developed from 1854 to 2012. Significantly positive correlation was identified between the tree rings and April–November total precipitation (r = 0.71, p < 0.01). The reconstructed April–November precipitation exhibited two comparatively wet (1876–1886 and 1957–1962) and one comparatively dry (1986–2004) periods. An evident drying trend since 1959 was seen and it was mitigated after 1993. Most of the extreme low-precipitation years in the reconstruction were supported by the historical records. As revealed by the spatial correlation patterns, our precipitation reconstruction was also consistent with other hydroclimatic records along the coastal areas of southeast China, proving its ability to capture the large-scale hydrological signal in southeast China (mainly refers to the south of the middle-lower reaches of Yangtze River). The reconstructed precipitation showed significant correlation with the East Asian summer Monsoon (EASM) index. Moreover, it also indicated simultaneous variation with the monsoon precipitation in North China on a decadal scale, implying that growing season precipitation variations in both regions were influenced by the EASM strength. This work highlights the potential of using tree-ring width to reconstruct precipitation in subtropical southeast China, while the relevant issues about precipitation variation in this region is far from resolved.     

基于油松树轮重建晋东南地区近296年来1—6月降水变化

区域历史气候变化研究受观测数据限制,不利于认识长时间尺度上气候变化情况。本研究采用山西省东南部油松树轮样芯,建立采样区树轮年表,分析降水变化与树轮宽度年表之间的关联。结果表明: 树轮差值年表(RES)与1—6月的降水量变化有较好的相关性,(r=0.636,n=59,P<0.01)。采用线性回归方法分析山西省东南部区域1724—2019年1—6月的降水重建序列,方差解释量为40.4%,经检验重建方程稳定可靠。对降水重建序列特征分析表明,1724—2019年存在10个极端干旱年和6个极端湿润年,经历了6次干旱期(1742—1771、1830—1848、1872—1894、1917—1945、1961—1981和1990—2019年)、4次湿润期(1727—1741、1772—1829、1849—1871和1895—1916年)。最长干旱期为1742—1771、1990—2019年,最长湿润期为1772—1829年。对于过去296年,降水变化序列存在2.3、3.2～3.3、3.7～3.8、6.3～6.7和8.3～8.7年周期变化特征,其中2.3年的周期与“准两年脉动”周期重合,3.2～3.3、3.7～3.8和6.3～6.7年的周期可能与厄尔尼诺和南方涛动的变化存在一定关系。空间相关分析表明,重建降水序列能够较好地代表研究区域范围的降水变化。     

Strengthening of the East Asian summer monsoon revealed by a shift in seasonal patterns in diet and climate after 2-3 Ma in northwest China

To examine climate variability in northwest China in the late Cenozoic and to test hypotheses regarding the development of C₄ ecosystems and the dynamics of the Asian monsoons, the carbon and oxygen isotopic compositions of 32 bulk and 368 serial tooth enamel samples from herbivores in the Linxia Basin (Gansu Province), ranging in age from 25 Ma to the present, were determined. The results corroborate and improve the record previously obtained from the area, showing that all mammals in the Linxia Basin lived in habitats consisting primarily of C₃ vegetation prior to 2-3 Ma and that C₄ grasses did not become a significant component of local ecosystems until the Quaternary. The data also show that shifts in climate to drier and/or warmer conditions after about 14, 9.5, 7, and 2.5 Ma, as indicated by positive δ¹⁸O excursions in the bulk enamel-δ¹⁸O record, were accompanied by increased seasonality; whereas negative δ¹⁸O shifts in the bulk data after about 11, 6, and 1.2 Ma, which indicate shifts to wetter and/or cooler climate, were associated with decreases in seasonality. Intra-tooth δ¹³C and δ¹⁸O profiles reveal significant changes in the seasonal patterns of diet and climate after ~ 2-3 Ma. Prior to ~ 2-3 Ma, there was little or no seasonal variation in herbivores' diets and all herbivores fed on C₃ vegetation year around. After that time, the data show a significant seasonal variation in the diets of horses and bovids, ranging from a pure C₃ to a mixed C₃/C₄ diet (with C₄ plants accounting for up to ~ 60% of the diet). An inverse relationship (or negative correlation) between δ¹³C and δ¹⁸O values within individual teeth — a pattern characteristic of the summer monsoon regime — is observed in younger (< 2-3 Ma) horses and bovids but not in older fossils. These changes in intra-tooth isotopic patterns provide strong evidence for an enhanced monsoon climate since about 2-3 Ma.     

Detection and attribution of vegetation greening trend in China over the last 30 years

The reliable detection and attribution of changes in vegetation growth is a prerequisite for the development of strategies for the sustainable management of ecosystems. This is an extraordinary challenge. To our knowledge, this study is the first to comprehensively detect and attribute a greening trend in China over the last three decades. We use three different satellite‐derived Leaf Area Index (LAI) datasets for detection as well as five different process‐based ecosystem models for attribution. Rising atmospheric CO₂ concentration and nitrogen deposition are identified as the most likely causes of the greening trend in China, explaining 85% and 41% of the average growing‐season LAI trend (LAI_GS) estimated by satellite datasets (average trend of 0.0070 yr^?1, ranging from 0.0035 yr^?1 to 0.0127 yr^?1), respectively. The contribution of nitrogen deposition is more clearly seen in southern China than in the north of the country. Models disagree about the contribution of climate change alone to the trend in LAI_GS at the country scale (one model shows a significant increasing trend, whereas two others show significant decreasing trends). However, the models generally agree on the negative impacts of climate change in north China and Inner Mongolia and the positive impact in the Qinghai–Xizang plateau. Provincial forest area change tends to be significantly correlated with the trend of LAI_GS (P < 0.05), and marginally significantly (P = 0.07) correlated with the residual of LAI_GS trend, calculated as the trend observed by satellite minus that estimated by models through considering the effects of climate change, rising CO₂ concentration and nitrogen deposition, across different provinces. This result highlights the important role of China's afforestation program in explaining the spatial patterns of trend in vegetation growth.     

Temporal changes in soil C‐N‐P stoichiometry over the past 60 years across subtropical China

Controlled experiments have shown that global changes decouple the biogeochemical cycles of carbon (C), nitrogen (N), and phosphorus (P), resulting in shifting stoichiometry that lies at the core of ecosystem functioning. However, the response of soil stoichiometry to global changes in natural ecosystems with different soil depths, vegetation types, and climate gradients remains poorly understood. Based on 2,736 observations along soil profiles of 0–150 cm depth from 1955 to 2016, we evaluated the temporal changes in soil C‐N‐P stoichiometry across subtropical China, where soils are P‐impoverished, with diverse vegetation, soil, and parent material types and a wide range of climate gradients. We found a significant overall increase in soil total C concentration and a decrease in soil total P concentration, resulting in increasing soil C:P and N:P ratios during the past 60 years across all soil depths. Although average soil N concentration did not change, soil C:N increased in topsoil while decreasing in deeper soil. The temporal trends in soil C‐N‐P stoichiometry differed among vegetation, soil, parent material types, and spatial climate variations, with significantly increased C:P and N:P ratios for evergreen broadleaf forest and highly weathered Ultisols, and more pronounced temporal changes in soil C:N, N:P, and C:P ratios at low elevations. Our sensitivity analysis suggests that the temporal changes in soil stoichiometry resulted from elevated N deposition, rising atmospheric CO₂ concentration and regional warming. Our findings revealed that the responses of soil C‐N‐P and stoichiometry to long‐term global changes have occurred across the whole soil depth in subtropical China and the magnitudes of the changes in soil stoichiometry are dependent on vegetation types, soil types, and spatial climate variations.     

Vegetation history with implication of climate changes and human impacts over the last 9000 years in the Lake Nanyi area,Anhui Province,East China

Palynological analyses in combination with radiocarbon dating on a Holocene borehole from the Lake Nanyi, Anhui Province, East China demonstrate a well-documented local vegetation evolution since 9000 cal BP, which is the first record of Holocene climate change and human impact in this region. Since 9000 cal BP a mixed evergreen and deciduous broad-leaved forest dominated by Cyclobalanopsis and Quercus developed in this area, indicating a warm climate condition with enhanced insolation. A mixed evergreen and deciduous broad-leaved forest was fully developed between 6600–4500 cal BP, which corresponds to the Holocene Climate Optimum with the strong influence of East Asian summer monsoon (EASM). After 3000 cal BP the broad-leaved forest decreased rapidly, while land herbs and ferns increased. It seems that the climate condition in East China was similar to the present after Holocene Climate Optimum. Pollen results show a potential interface between environment changes and human activities. Pollen diagram demonstrates that human impacts on the natural vegetation remained weak at the early stage but significantly enhanced upwards. The distinctive fluctuations of the pollen contents among AP (trees and shrubs), and the possible agriculture indicators might infer the potential human behaviors for environment changes. Due to the enlargement of organized farming and increase in population, natural forest was eventually replaced by farmland since 3000 cal BP. This study would increase our knowledge of Holocene vegetation transition related to the monsoon dynamics on a long timescale in East China and provide an environmental background for more detailed studies on cultural developments in the middle and lower reaches of the Yangtze River region.