Performance and uncertainty analysis of a short-term climate reconstruction based on multi-source data in the Tianshan Mountains region,China

Short-term climate reconstruction,i.e.,the reproduction of short-term(several decades)historical climatic time series based on the relationship between observed data and available longer-term reference data in a certain area,can extend the length of climatic time series and offset the shortage of observations.This can be used to assess regional climate change over a much longer time scale.Based on monthly grid climate data from a Coupled Model Inter-comparison Project phase 5(CMIP5)dataset for the period of 1850–2000,the Climatic Research Unit(CRU)dataset for the period of 1901–2000 and the observed data from 53 meteorological stations located in the Tianshan Mountains region(TMR)of China during the period of 1961–2011,we calibrated and validated monthly average temperature(MAT)and monthly accumulated precipitation(MAP)in the TMR using the delta,physical scaling(SP)and artificial neural network(ANN)methods.Performance and uncertainty during the calibration(1971–1999)and verification(1961–1970)periods were assessed and compared using traditional performance indices and a revised set pair analysis(RSPA)method.The calibration and verification processes were subjected to various sources of uncertainty due to the influence of different reconstructed variables,different data sources,and/or different methods used.According to traditional performance indices,both the CRU and CMIP5 datasets resulted in satisfactory calibrated and verified MAT time series at 53 meteorological stations and MAP time series at 20 meteorological stations using the delta and SP methods for the period of 1961–1999.However,the results differed from those obtained by the RSPA method.This showed that the CRU dataset produced a low degree of uncertainty(positive connection degree)during the calibration and verification of MAT using the delta and SP methods compared to the CMIP5 dataset.Overall,the calibrated and verified MAP had a high degree of uncertainty(negative connection degree)regardless of the dataset or reconstruction method used.Therefore,the reconstructed time series of MAT for the period of 1850(or 1901)–1960 based on the CRU and CMIP5 datasets using the delta and SP methods could be used for further study.The results of this study will be useful for short-term(several decades)regional climate reconstruction and longer-term(100 a or more)assessments of regional climate change.

Establishment of tree-ring chronology and climatic response of Tibetan juniper(S.tibetica) in south Tibet,western China

A tree-ring width chronology of 442 years(1567-2008) was developed from Tibetan junipers(S.tibetica) derived from south Tibet in western China.Three versions of chronology were produced according to standard dendrochronological techniques.The correlation and response analysis displays a high correlation between the standard tree ring-width chronology and observed annual mean precipitation series during the period 1961-2008.Based on a linear regression model,an annual(prior August to current July) precipitation for the past 229 years was reconstructed.This is the first well-calibrated precipitation reconstruction for the Nanggarze region,south Tibet.The results show that relatively wet years with above-average precipitation occurred in 1780-1807,1854-1866,1886-1898,1904-1949,1967-1981 and 2000-2008,whereas relatively dry years with below-average precipitation prevailed during 1808-1853,1867-1885,1899-1903,1950-1966 and 1982-1999.Common dry/wet periods during 1890s,1910s,1940s-1960s and 1980s were also identified from other moisture reconstructions of nearby regions,indicating a synchronous climatic variation in south Tibet.Abrupt change beginning in 1888 was detected,revealing a transition from wet to dry conditions in south Tibet.Power spectrum analysis reveals significant cycles of 28-year,5.5-5.6-year and 3.3-year during the past 200 years.

Two-tiered reconstruction of Late Pleistocene to Holocene changes in the freezing level height in the largest glacierized areas of the Colombian Andes

One way of deducing vertical shifts in the altitudinal distribution of Colombian high-altitude páramo environments is by inferring fluctuations in the height of the local freezing level.In our research,we are implementing two complementary approaches to reconstruct Late Pleistocene to Holocene changes in the freezing level height(FLH)in two of the most extensively glacier-covered areas of the northern Andes.We combined remote sensing and field-based geomorphological mapping with time-series reconstruction of changes in the altitude of the 0°C isotherm.Changes in the FLH were based on alreadypublished~30 kyr paleo-reconstructions of sea surface temperatures(SSTs)of the eastern tropical Pacific and the western tropical Atlantic,as well as on reconstructed long-term sea level changes and empirical orthogonal functions of present-day(historical)Indo-Pacific and tropical Atlantic SST anomalies.We also analyzed the probability distribution of air-sea temperature differences and the spatial distribution of grid points with SSTs above the minimum threshold necessary to initiate deep convection.We considered available historical nearsurface and free air temperature data of ERA-Interim reanalysis products,General Circulation Model(GCM)simulations,weather stations,and(deployed by our group)digital sensors,to assess the normal Environmental Lapse Rates(ELRs)at the regional to local scale.The combined maps of glacial landforms and our reconstructed FLHs provided us with a wellfounded inference of potential past glacier advances,narrowing down the coarse resolution of ice margins suggested by previous research efforts.The extent of the areas with temperatures below the freezing point suggested here for the summits of our main study site exceeds in magnitude the corresponding glacier icecaps and front advances proposed by previous studies.Conversely,our average lowest altitudes of the FLH for our comparative site are consistently above the main glacier-front advances previously suggested.Our results indicate that,compared to the maximum upward changes that likely took place over the past ca.20,000 years in our two areas of interest,the observed(present-day)upward shifts of the FLH have occurred at a rate that significantly surpasses our inferred rates.Our study helps fill the gaps in understanding past climatic changes and present trends in the region of interest and provides some insights into analyzing the signals of natural and anthropogenic climate change.

Glaciological observations at Dome Argus, East Antarctica

Dome Argus （Dome A） in East Antarctica is a potentially likely site to meet one of the major objectives of the International Partnerships in Ice Core Sciences （IPICS） on the oldest ice core, and thus has aroused wide public and scientific interest. Since 2004/2005, many glaciological investigations have been conducted in this region. These have included GPS and ground-penetrating radar surveys, snow pit and ice core drilling, stake network measurements, and meteorological observations. In this article, the main results of these glaciological investigations in the Dome A region are summarized. We present details of the surface mass balance on different timescales and its spatial variability, geochemical characteristics of the surface snow, and paleo-environment reconstruction of ice cores. Finally, perspectives on the prospects for future studies are suggested.

Two Ultraviolet Radiation Datasets that Cover China

Ultraviolet（UV） radiation has significant effects on ecosystems, environments, and human health, as well as atmospheric processes and climate change. Two ultraviolet radiation datasets are described in this paper. One contains hourly observations of UV radiation measured at 40 Chinese Ecosystem Research Network stations from 2005 to 2015. CUV3 broadband radiometers were used to observe the UV radiation, with an accuracy of 5%, which meets the World Meteorology Organization＇s measurement standards. The extremum method was used to control the quality of the measured datasets. The other dataset contains daily cumulative UV radiation estimates that were calculated using an all-sky estimation model combined with a hybrid model. The reconstructed daily UV radiation data span from 1961 to 2014. The mean absolute bias error and root-mean-square error are smaller than 30% at most stations, and most of the mean bias error values are negative, which indicates underestimation of the UV radiation intensity. These datasets can improve our basic knowledge of the spatial and temporal variations in UV radiation. Additionally, these datasets can be used in studies of potential ozone formation and atmospheric oxidation, as well as simulations of ecological processes.

RECONSTRUCTION OF EL NINO EVENT CHRONOLOGY FOR THE LAST 600 YEAR PERIOD

The El Nino events for 1870—1989 were identified according to the sea surface temperature(SST)over the eastern equatorial Pacific(0—10°S,90—180°W)and Southern Oscillation Index.The chronicle of the El Nino events in histori- cal time(14708—1879)was reconstructed by using of a series of proxy data such as Australian droughts,Nile floods, Peruvian floods,numbers of landing typhoon in China,cool summers in East Asia,SOI calculated from tree ring data, and El Nino events identified mainly from meteorological and oceanographical conditions along the Ecuador and Peru coast. Finally,114 El Nino events were found for the period 1470—1989.The mean return interval was 4.5 years.The fre- quency of the events seems to have a cyclical variation of 70 years.No close relationship was found between the global warming and the frequency of El Nino events.

Characteristics of the modern pollen assemblages from different vegetation zones in Northeast China:Implications for pollen-based climate reconstruction

Northeast China is an essential area for studying the strength of East Asian Summer Monsoon(EASM), due to its northernmost location in EASM domain. However, the lack of sufficient modern pollen data in this region hinders an effective interpretation of fossil pollen records and quantitative vegetation/climate reconstructions. Here, 44 surface pollen samples from forest, steppe, and meadow were used to explore pollen-vegetation-climate relationships. Cluster analysis, species indicator analysis, and principal components analysis, were used to identify the discontinuous and continuous trends in pollen dataset. In addition, correlation analysis and boosted regression trees were used to investigate primary explanatory variables, while coinertia analysis and redundancy analysis to examine pollen-vegetation and pollen-climate correlations respectively. Our results show that:(1) vegetation can be well represented by surface pollen assemblages, i.e. forest is characterized by a high proportion of tree pollen(>70%) dominated by Betula(>40%) along with Alnus, Larix, and Pinus, whereas Steppe by herb pollen(>80%),dominated by Artemisia, Chenopodiaceae;(2) significant correlations exist between pollen assemblages and mean annual temperature and then mean annual precipitation;(3) pollen ratios of Artemisia/Chenopodiaceae and arboreal/non-arboreal can respectively be used as good indicators of humidity and temperature in Northeast China.

基于孢粉记录的亚洲季风湿润区6.8万年古气候变化定量重建

东亚季风降水的冰期-间冰期变率及其驱动机制一直备受关注和争议,然而在东亚季风湿润区内,具有完整的末次冰期以来古气候定量重建记录十分缺乏.本研究利用亚洲季风湿润区内的3个孢粉记录进行了古气候定量重建.结果表明6.8万年以来区域内的气候变化显著,年降雨量与年平均温度在末次冰盛期与全新世大暖期之间的差别可分别达到35%~51%与5~7℃.定量重建的降水记录揭示了H1与YD等快速气候变化期间东亚季风降水存在区域差异性,即印度季风影响下的中国西南部较为干燥的同时,中国中东部则表现为相对湿润.冰期-间冰期尺度下的降水记录综合变化趋势总体上与中国西南部和南亚地区的石笋δ^(18)O记录基本一致.气候重建结果还显示出MIS3降水变化对轨道太阳辐射的敏感性,尤其是南北半球间温度梯度对亚洲季风变率具有显著的影响.定量降水记录与瞬态模拟结果及主要气候强迫对比显示,在末次冰盛期过渡到全新世期间,降水的变化模式除了受控于太阳辐射变化外,还明显受到大西洋经向翻转环流事件的叠加影响.

Climatological Significance of Pollen Factors Revealed by Pollen-Climate Response Surface Functions in Dajiuhu,Shennongjia

Quantitative reconstructions of the mean July temperature and annual precipitation are performed based on pollen percentage contents from surface and stratum pollen samples,together with vertical meteorological observations across 700-2800 m in Dajiuhu,Shennongjia in Hubei Province of China.Canonical correspondence analysis and robust locally weighted regression of surface pollen samples are employed to investigate the relationship between plants and climate,and to build the seven pollen-climate response surface functions. Reconstructed results of the stable type assemblage exhibit the climate evolution since the Late-Glacial Period, including the B（？）lling-Aller（？）d warm episode,Younger Dryas cold episode,Climatic Optimum,and cold events in 8 and 6.5 ka BP.The ranges of the mean July temperature and annual precipitation have been about 5℃and 300 mm since the Late-Glacial Period.Analysis of temperature and humidity reveals that Climatic Optimum with high temperature and precipitation occurred during early Holocene and former mid-Holocene,corresponding to the high resolution records in the low latitude region.The results indicate that the pollen-climate response surface functions are capable to valuate the comprehensive influence of temperature and precipitation on pollen content and can be used to reconstruct the past climate from pollen data.

Spatial homogenization of soil-surface pollen assemblages improves the reliability of pollen-climate calibration-set

To make a reliable reconstruction of past climate from soil-surface modern pollen,it is necessary to reduce the sources of error.In this paper,pollen percentages of the sub-continental scale modern pollen-climate dataset from China and Mongolia(with 68%soil-surface samples)are homogenized at various spatial scales.A tailored calibration-set is then applied to lake sediment-surface pollen assemblages from north-central China to evaluate their predictive power.Results indicate that spatial homogenization of modern pollen percentages can increase the proportion of inertia explained by climatic variables in CCA and improve the model performance of leave-one-out cross-validation using WA-PLS.Soil-surface pollen assemblages can thus be employed into a calibration-set for reliable climate estimation and they perform better when the calibration-set has been locally homogenized.Small-scale(e.g.,radii 2,5,or 10 km)homogenization reduces the local noise in soil-surface pollen assemblages and improves the cross-validated performance,while broader scale homogenization(more than 20 km radius)blurs the pollen-climate relationship.Lake sediment-surface pollen assemblages from close to the shore could contain pollen grains transported by rivers or from the shore vegetation and thus fail to represent regional climate well like the assemblages from the central part and deep-water area of lake.