Bao Yang

A 3,500-year tree-ring record of annual precipitation on the northeastern Tibetan Plateau

Significance This paper describes the production and climatic interpretation of a tree-ring width chronology that is currently the longest, absolutely dated series produced for the northeastern Tibetan Plateau and one of the longest in the world. The method of chronology construction enables comparison of variations in precipitation totals over long timescales as well as shorter periods. Precipitation in this region during the last 50 years has been historically high—likely higher than for any equivalent length period in at least 3,500 years, even when considering the chronology and interpretational uncertainty. Notable dry periods occurred in the 4th century BCE and in the second half of the 15th century CE. An annually resolved and absolutely dated ring-width chronology spanning 4,500 y has been constructed using subfossil, archaeological, and living-tree juniper samples from the northeastern Tibetan Plateau. The chronology represents changing mean annual precipitation and is most reliable after 1500 B.C. Reconstructed precipitation for this period displays a trend toward more moist conditions: the last 10-, 25-, and 50-y periods all appear to be the wettest in at least three and a half millennia. Notable historical dry periods occurred in the 4th century BCE and in the second half of the 15th century CE. The driest individual year reconstructed (since 1500 B.C.) is 1048 B.C., whereas the wettest is 2010. Precipitation variability in this region appears not to be associated with inferred changes in Asian monsoon intensity during recent millennia. The chronology displays a statistical association with the multidecadal and longer-term variability of reconstructed mean Northern Hemisphere temperatures over the last two millennia. This suggests that any further large-scale warming might be associated with even greater moisture supply in this region.

A 622-Year regional temperature history of southeast Tibet derived from tree rings

A tree ring-width record spanning from AD 1377—1998 was developed from Tibetan juniper (Cupressus gigantea) growing at sites north of the deep gorge of the Yarlung Tsangbo River of southeast Tibet. A linear regression model between ring width and mean January—June temperature accounts for 35% of January—June temperature variance for the period 1961—1998. Based on this model, we reconstructed January—June temperature variation history for the study region during the past 622 years. Warm conditions occurred during AD 1385—1418, 1443—1466, 1482—1501, 1523—1548, 1570s, 1705—1759, 1770—1790, 1851—1888, 1910s, and 1938—1965, and periods of relatively cold years are identified for AD 1419—1442, 1470s, 1502—1522, 1550—1569, 1610—1640, 1680—1700, 1760s, 1791—1850, 1900s and 1965—1995. Spatial correlation between tree rings and observed temperatures indicates that the reconstruction is representative of temperature change for southeast Tibet. Regional cold conditions during AD 1625, 1685, 1760, 1800—1850, 1890—1930 and 1965—1995, and warm conditions during 1710, 1730—1750, 1850—1890 and 1930—1960 can be identified in the eastern Tibetan Plateau. Our reconstruction shows good agreement with modelled regional temperatures derived from an ECHO-G model simulation. Common cold periods around 1470, 1685 and 1845 corresponding to the Spörer Minimum, the Maunder Minimum and the Dalton Minimum periods of low solar activity and increased volcanic activity were identified, indicating that solar and volcanic activity played an important role in temperature change of this region. The discrepancy between modelled and reconstructed temperatures was noted around 1965, reflecting that temperature decrease in the late 20th century (1965—1995) is anomalous.

Tree ring-based annual streamflow reconstruction for the Heihe River in arid northwestern China From AD 575 and its implications for water resource management

We developed two tree ring-width chronologies (Qilian juniper, Sabina przewalskii Kom.) for the inland Heihe River Basin in arid northwest China using a large number of tree-ring samples (217 samples/92 trees) with accurate information about pith offsets based on Regional Curve Standardization (RCS) and standard dendrochronological (STD) methodologies. Two 1422-year reconstructions of annual (August–July) streamflow for the upstream region of the Heihe River are presented. The STD and RCS reconstructions account for 53.4% and 57.2% of the actual streamflow variance during the period 1958–2006, respectively. Both reconstructions display considerable low frequency (multidecadal to multicentury) fluctuations, although the RCS based reconstruction is superior to the STD based reconstruction for retention of low-frequency trends. Low-flow years in ad 818–852, 1112–1196, 1453–1495 and 1680–1710, and high-flow periods in ad 868–1000, 1056–1094, 1228–1271, 1327–1440, 1510–1583 and 1877–2006 are detected in both reconstructions. Both the STD and the RCS reconstructions testify to the fact that the 20th century witnessed intensified pluvial conditions in the upstream region of the Heihe River in the context of the last 1500 years. The streamflow reconstructions are anticipated to be useful to water resource planning and management for the Heihe River Basin in arid northwestern China.

Tree-ring inferred annual mean temperature variations on the southeastern Tibetan Plateau during the last millennium and their relationships with the Atlantic Multidecadal Oscillation

We present two tree-ring chronologies for the southeastern Tibetan Plateau (TP), established by applying the signal-free regional curve standardization and standard dendrochronological methodologies to a set of ring-width series of Tibetan juniper. The relationship between tree growth and climate shows that temperature variability in the previous year is the primary factor controlling tree growth at the upper portion of the forest belt. Accordingly, we developed a mean annual temperature reconstruction covering the period A.D. 984–2009 and explaining 50xa0% of the instrumental variance. The spatial correlation patterns suggest that our temperature reconstruction is a reasonable proxy for temperature change over the TP. At long time scales, the temperature reconstruction shows similar warm-cold patterns to those in temperature records from other regions of the TP, indicating that decadal and multidecadal temperature variations were generally synchronous across the TP during the past millennium. The periods 1140–1350 and 1600–1800 were common warm and cold episodes over the TP, respectively. Comparison of our reconstruction with four Northern Hemisphere (NH) temperature series indicates that temperature changes on the southeastern TP have generally followed the NH temperature patterns during the past millennium. Our results also suggest that temperature variability over the TP is affected by the Atlantic Multidecadal Oscillation (AMO), with the warm (cool) phases of the AMO associated with above-average (below-average) temperatures over the TP.

Preliminary multiproxy surface air temperature field reconstruction for China over the past millennium

We present the first millennial-length gridded field reconstruction of annual temperature for China, and analyze the reconstruction for spatiotemporal changes and associated uncertainties, based on a network of 415 well-distributed and accurately dated climatic proxy series. The new reconstruction method is a modified form of the point-by-point regression (PPR) approach. The main difference is the incorporation of the “composite plus scale” (CPS) and “Regularized errors-in-variables” (EIV) algorithms to allow for the assimilation of various types of the proxy data. Furthermore, the search radius is restricted to a grid size; this restriction helps effectively exclude proxy data possibly correlated with temperature but belonging to a different climate region. The results indicate that: 1) the past temperature record in China is spatially heterogenic, with variable correlations between cells in time; 2) the late 20th century warming in China probably exceeds mean temperature levels at any period of the past 1000 years, but the temperature anomalies of some grids in eastern China during the Medieval climate anomaly period are warmer than during the modern warming; 3) the climatic variability in the eastern and western regions of China was not synchronous during much of the last millennium, probably due to the influence of the Tibetan Plateau. Our temperature reconstruction may serve as a reference to test simulation results over the past millennium, and help to finely analyze the spatial characteristics and the driving mechanism of the past temperature variability. However, the lower reconstruction skill scores for some grid points underline that the present set of available proxy data series is not yet sufficient to accurately reconstruct the heterogeneous climate of China in all regions, and that there is the need for more highly resolved temperature proxies, particularly in the Tibetan Plateau.

Spatial and temporal patterns of variations in tree growth over the northeastern Tibetan Plateau during the period AD 1450–2001

We analyzed spatial and temporal growth variations of Qilian Junipers over the northeastern Tibetan Plateau (TP) during the period 1450—2001 by applying the empirical orthogonal function (EOF) technique to seven moisture-sensitive tree ring-width chronologies. The first three principal components together explain 78.21% of the total variance. The first principal component (PC1) accounts for 51.58% of the total variance, and represents consistent tree-growth variations in the northeastern TP. The second EOF mode reveals an opposite structure for the southern and northern parts of the northeastern Tibetan Plateau, reflecting the effect of huge topography and associated atmospheric circulation on tree-growth anomalies. The third EOF mode represents an opposite structure for the eastern and western parts of the northeastern Tibetan Plateau, possibly indicating the influence of different branches of atmospheric circulation system in this area. The EOF analysis results of instrumental precipitation data resembl...We analyzed spatial and temporal growth variations of Qilian Junipers over the northeastern Tibetan Plateau (TP) during the period 1450—2001 by applying the empirical orthogonal function (EOF) technique to seven moisture-sensitive tree ring-width chronologies. The first three principal components together explain 78.21% of the total variance. The first principal component (PC1) accounts for 51.58% of the total variance, and represents consistent tree-growth variations in the northeastern TP. The second EOF mode reveals an opposite structure for the southern and northern parts of the northeastern Tibetan Plateau, reflecting the effect of huge topography and associated atmospheric circulation on tree-growth anomalies. The third EOF mode represents an opposite structure for the eastern and western parts of the northeastern Tibetan Plateau, possibly indicating the influence of different branches of atmospheric circulation system in this area. The EOF analysis results of instrumental precipitation data resemble those derived from the seven regional chronologies, providing further support that tree growth is a good indicator of regional precipitation variations. According to the PC1, dry conditions occurred in AD 1450—1510, 1631—1739, 1765—1833, 1866—1883 and 1921—1947, whereas periods of relatively wet years are identified for AD 1511—1630, 1740—1764, 1834—1865, 1884—1920 and 1948—2001. During the past 550 years, 28 years with extreme regional growth depressions was identified. These ring-width extremes generally occurred in century-scale dry periods. The years 1467, 1484, 1641, 1721, and 1928 with extreme tree-growth reductions in the northeastern TP are coeval with severe drought events in eastern China. In addition, the dry intervals or extreme growth depressions generally coincided with cold periods in the northeastern TP and vice versa, implying that the probability of occurrence of wet events in the northeastern TP will be higher in the future under the ongoing global warming.

Modeling the epidemiological history of plague in Central Asia: palaeoclimatic forcing on a disease system over the past millennium

BackgroundHuman cases of plague (Yersinia pestis) infection originate, ultimately, in the bacteriums wildlife host populations. The epidemiological dynamics of the wildlife reservoir therefore determine the abundance, distribution and evolution of the pathogen, which in turn shape the frequency, distribution and virulence of human cases. Earlier studies have shown clear evidence of climatic forcing on contemporary plague abundance in rodents and humans.ResultsWe find that high-resolution palaeoclimatic indices correlate with plague prevalence and population density in a major plague host species, the great gerbil (Rhombomys opimus), over 1949-1995. Climate-driven models trained on these data predict independent data on human plague cases in early 20th-century Kazakhstan from 1904-1948, suggesting a consistent impact of climate on large-scale wildlife reservoir dynamics influencing human epidemics. Extending the models further back in time, we also find correspondence between their predictions and qualitative records of plague epidemics over the past 1500 years.ConclusionsCentral Asian climate fluctuations appear to have had significant influences on regional human plague frequency in the first part of the 20th century, and probably over the past 1500 years. This first attempt at ecoepidemiological reconstruction of historical disease activity may shed some light on how long-term plague epidemiology interacts with human activity. As plague activity in Central Asia seems to have followed climate fluctuations over the past centuries, we may expect global warming to have an impact upon future plague epidemiology, probably sustaining or increasing plague activity in the region, at least in the rodent reservoirs, in the coming decades.See commentary: http://www.biomedcentral.com/1741-7007/8/108

Drought variability at the northern fringe of the Asian summer monsoon region over the past millennia

The northern fringe of the Asian summer monsoon region (NASM) in China refers to the most northwestern extent of the Asian summer monsoon. Understanding the characteristics and underlying mechanisms of drought variability at long and short time-scales in the NASM region is of great importance, because present and future water shortages are of great concern. Here, we used newly developed and existing tree-ring, historical documentary and instrumental data available for the region to identify spatial and temporal patterns, and possible mechanisms of drought variability, over the past two millennia. We found that drought variations were roughly consistent in the western (the Qilian Mountains and Hexi Corridor) and eastern (the Great Bend of the Yellow River, referred to as GBYR) parts of the NASM on decadal to centennial timescales. We also identified the spatial extent of typical multi-decadal GBYR drought events based on historical dryness/wetness data and the Monsoon Asia Drought Atlas. It was found that the two periods of drought, in AD 1625–1644 and 1975–1999, exhibited similar patterns: specifically, a wet west and a dry east in the NASM. Spatial characteristics of wetness and dryness were also broadly similar over these two periods, such that when drought occurred in the Karakoram Mountains, western Tianshan Mountains, the Pamirs, Mongolia, most of East Asia, the eastern Himalayas and Southeast Asia, a wet climate dominated in most parts of the Indian subcontinent. We suggest that the warm temperature anomalies in the tropical Pacific might have been mainly responsible for the recent 1975–1999 drought. Possible causes of the drought of 1625–1644 were the combined effects of the weakened Asian summer monsoon and an associated southward shift of the Pacific Intertropical Convergence Zone. These changes occurred due to a combination of Tibetan Plateau cooling together with more general Northern Hemisphere cooling, rather than being solely due to changes in the sea surface temperature of the tropical Pacific. Our results provide a benchmark for comparing and validating paleo-simulations from general circulation model of the variability of the Asian summer monsoon at decadal to centennial timescales.

Tree-ring derived millennial precipitation record for the south-central Tibetan Plateau and its possible driving mechanism

Knowledge of Asian monsoon variability remains limited because of sparse instrumental data available only for short time series. Here, an updated tree-ring width record covering the period ad 1037–2009 was developed for the south-central Tibetan Plateau (TP). Correlation analysis revealed a significant relationship (r = 0.71) between the tree-ring index and annual (previous July to current June) precipitation series for the instrumental period 1963–2008, which accounts for 50.41% of the rainfall variability. Based on a linear regression model, the longest available regional precipitation history was reconstructed. Spatial correlation between tree ring width and annual precipitation data from previous July to current June indicates that the reconstruction is representative of precipitation changes on the south-central TP. Regional wet conditions occurred during ad 1095–1161, 1376–1403, 1414–1446, 1518–1537, 1549–1572, 1702–1757, 1848–1878 and 1891–1913, while dry periods were identified during ad1189–1242, 1256–1314, 1329–1357, 1470–1491, 1573–1623, 1636–1686, 1761–1821, 1823–1847, 1879–1890 and 1931–1985. The negative correlation between our reconstructed precipitation and India monsoon rainfall series indicates the seesaw pattern over northern and southern monsoon Asia. It is suggested that solar radiation-induced sea surface temperature (SST) anomalies over the tropical Pacific influence regional rainfall patterns. The degree of this influence has been stable at the multidecadal scale during the past 1000 years.

Intra-annual stem radial increment response of Qilian juniper to temperature and precipitation along an altitudinal gradient in northwestern China

Key messageSpring temperature is a major limiting factor at the beginning of the growing season, the timing of growth initiation can increase by about 7xa0days/°C. During the growing season, impacts of climate variables on radialxa0growth are similar along an altitudinal gradient.AbstractAltitude is considered as an important factor affecting tree growth in mountain forest ecosystems. In this paper, the results of a 2-year field study along an altitudinal gradient in the cold and arid central Qilian Mountains, northwestern China, are reported. Twelve Qilian juniper trees (Sabina przewalskii Kom.) were monitored with high-resolution dendrometers at three altitudes ranging from 2,865 to 3,550xa0m. At each altitude, a local weather station was installed close to the studied trees. We identified correlations between intra-annual growth patterns derived from the Gompertz equation with local air temperature and precipitation data. The timing of growth initiation became earlier and the growing season duration increased with decreasing altitude. The onset of radial growth occurred between early May and early June, and the growing season terminated between mid-July and late August, resulting in a growing season duration that decreased from 107 to 41xa0days as elevation increased. June is the most important growth period at each altitude. Spring temperature, which is strongly associated with elevation, is a critical factor determining the initiation of radial growth. The timing of growth initiation was delayed by 3–4xa0days per 100xa0m elevation. When associated with the modeled altitudinal spring temperature lapse rate of −0.48xa0°C/100xa0m, the onset of the growing season increased by about 7xa0days/°C. However, during the growing season, daily stem radial increments showed a positive correlation with precipitation and a negative correlation with daily maximum air temperature at all altitudes. Our study provides new data revealing the basic growth processes of Qilian juniper trees and provides significant information to quantify the responses of tree growth to future global warming.