Drought fluctuations based on dendrochronology since 1786 for the Lenglongling Mountains at the northwestern fringe of the East Asian summer monsoon region

The Lenglongling Mountains(LLM)located in northeastern part of the Tibet Plateau,belong to a marginal area of the East Asian summer monsoon(EASM)and are sensitive to monsoon dynamics.Two tree-ring width chronologies developed from six sites of Picea crassifolia in the LLM were employed to study the regional drought variability.Correlation and temporal correlation analyses showed that relationships between the two chronologies and self-calibrated Palmer Drought Severity Index(sc_PDSI)were significant and stable across time,demonstrating the strength of sc_PDSI in modeling drought conditions in this region.Based on the relationships,the mean sc_PDSI was reconstructed for the period from 1786 to 2013.Dry conditions prevailed during 1817–1819,1829–1831,1928–1931 and 1999–2001.Relatively wet periods were identified for 1792–1795 and 1954–1956.Spatial correlations with other fourteen precipitation/drought reconstructed series in previous studies revealed that in arid regions of Northwest China,long-term variability of moisture conditions was synchronous before the 1950 s at a decadal scale(1791–1954).In northwestern margin of the EASM,most of all selected reconstructions had better consistency in low-frequency variation,especially during dry periods,indicating similar regional moisture variations and analogous modes of climate forcing on tree growth in the region.     

Reconstructing the annual precipitation variation since 1899 based on tree-ring width in the western Hedong sandy land of Ningxia

Based on the analysis of the correlation between the tree-ring width of Pinus tabulaeformis and the climate factors in the western Hedong sandy land of Ningxia, a conversion equation between the annual precipitation and the tree-ring width since 1899 was reconstructed. The results of cross verification indicated that the conversion equation is stable and the reconstructed results are reliable. The result of reconstructed annual precipitation showed the remarkable fluctuation of precipitation and dry-to-wet variation before the 1940s. The smaller fluctuation and high frequent changes of precipitation occurred during the period of 1940s-1980s and after the 1980s the change trend of the precipitation became high periodic extent and low frequent. The study found that there were some coincidences with the climate change in Changling Mountains, Helan Mountains and the east of Qilian Mountains. The relatively dry periods in the beginning of 20th century, 1920s to 1930s, the end of the 20th century and 2004 to 2006 in the western Hedong sandy land of Ningxia accelerated the desertification, while the relatively humid period during the periods of the 1910s-1920s, 1930s-1940s and 1990s is favorable to prevent and control the desertification, and to weaken the climate warming and drying. The periods of annual precipitation variation in the western Hedong sandy land of Ningxia since 1899 are approximately 2-4 years, 5-7 years and 10 years.     

Tree ring based drought variability in Northwest Tajikistan since 1895 AD

Determining the mechanisms controlling the changes of wet and dry conditions will improve our understanding of climate change over the past hundred years, which is of great significance to the study of climate and environmental changes in the arid regions of Central Asia. Forest trees are ecologically significant in the local environment, and therefore the tree ring analysis can provide a clear record of regional historical climate. This study analyzed the correlation between the tree ring width chronology of Juniperus turkestanica Komarov and the standardized precipitation evapotranspiration index(SPEI) in Northwest Tajikistan, based on 56 tree ring samples collected from Shahristan in the Pamir region. Climate data including precipitation, temperature and the SPEI were downloaded from the Climate Research Unit(CRU) TS 4.00. The COFECHA program was used for cross-dating, and the ARSTAN program was used to remove the growth trend of the tree itself and the influence of non-climatic factors on the growth of the trees. A significant correlation was found between the radial growth of J. turkestanica trees and the monthly mean SPEI of February–April. The monthly mean SPEI sequence of February–April during the period of 1895–2016 was reconstructed, and the reconstruction equation explained 42.5% of the variance. During the past 122 a(1895–2016), the study area has experienced three wetter periods(precipitation above average): 1901–1919, 1945–1983 and 1995–2010, and four drier periods(precipitation below average): 1895–1900, 1920–1944, 1984–1994 and 2011–2016. The spatial correlation analysis revealed that the monthly mean SPEI reconstruction sequence of February–April could be used to characterize the large-scale dry-wet variations in Northwest Tajikistan during the period of 1895–2016. This study could provide comparative data for validating the projections of climate models and scientific basis for managing water resources in Tajikistan in the context of climate change.     

Reconstruction of hydrological changes based on tree-ring data of the Haba River,northwestern China

Reconstructing the hydrological change based on dendrohydrological data has important implications for understanding the dynamic distribution and evolution pattern of a given river. The widespread, long-living coniferous forests on the Altay Mountains provide a good example for carrying out the dendrohydrological studies. In this study, a regional composite tree-ring width chronology developed by Larix sibirica Ledeb. and Picea obovata Ledeb. was used to reconstruct a 301-year annual(from preceding July to succeeding June) streamflow for the Haba River, which originates in the southern Altay Mountains, Xinjiang, China. Results indicated that the reconstructed streamflow series and the observations were fitting well, and explained 47.5% of the variation in the observed streamflow of 1957–2008. Moreover, floods and droughts in 1949–2000 were precisely captured by the streamflow reconstruction. Based on the frequencies of the wettest/driest years and decades, we identified the 19 th century as the century with the largest occurrence of hydrological fluctuations for the last 300 years. After applying a 21-year moving average, we found five wet(1724–1758, 1780–1810, 1822–1853, 1931–1967, and 1986–2004) and four dry(1759–1779, 1811–1821, 1854–1930, and 1968–1985) periods in the streamflow reconstruction. Furthermore, four periods(1770–1796, 1816–1836, 1884–1949, and 1973–1997) identified by the streamflow series had an obvious increasing trend. The increasing trend of streamflow since the 1970s was the biggest in the last 300 years and coincided with the recent warming-wetting trend in northwestern China. A significant correlation between streamflow and precipitation in the Altay Mountains indicated that the streamflow reconstruction contained not only local, but also broad-scale, hydro-climatic signals. The 24-year, 12-year, and 2.2–4.5-year cycles of the reconstruction revealed that the streamflow variability of the Haba River may be influenced by solar activity and the atmosphere–ocean system.     

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.     

Temporal and spatial variation of annual mean air temperature in arid and semiarid region in northwest China over a recent 46 year period

We analyzed the 1961-2006 mean surface air temperature data of 138 stations in China’s northwest arid and semi-arid areas(CNASA),to measure climate change in terms of annual mean air temperature changes.We used methods of linear regression analysis,multinomial fitting,Empirical Or-thogonal Function(EOF),Rotated Empirical Orthogonal Function(REOF),Mann-Kendall,Glide T-examination,wavelet analysis and power spectrum analysis.The results show that(1) the warming rate of the annual mean air temperature in CNASA was 0.35oC/10a during the 1961-2006 study period.Some places in the west part of Xinjiang and east part of the Qinghai plateau,which is impacted by the terrain of leeward slope,exhibit smaller increasing trends.However,the majority of region has shown distinct warming in line with general global warming;(2) The standard deviation of the annual mean temperature distribution is non-uniform.The south Xinjiang and east Qinghai-south Gansu areas show relatively small standard deviations,but the inter-annual variation in annual mean air temperature in the greater part of the region is high;(3) Inner Mongolia,Shaanxi,Gansu,Ningxia and Tarim Basin are the areas where the temperature changes are most sensitive to the environment.The degree of uniformity in annual mean air temperature increase is higher in the arid and semi-arid area.From the early 1970s,the trend in tempera-ture changed from a decrease to an increase,and there was a marked increase in mean temperature in 1986.After that mean temperature went through a period of rapid increase.The entire area’s 10 hottest years all occurred in or since the 1990s,and 90% of various sub-districts’ hottest years also occurred after 1990.The process of temperature change appears to have a roughly 5-year and a 10-year cycle;(4) An-nual mean air temperature variation has regional differences.In Inner Mongolia-Xinjiang and Shaanxi-Gansu-Ningxia-Qinghai areas,the temperature variation in their northern areas was very different from that in their southern areas;(5) Using     

Long-term variations in runoff of the Syr Darya River Basin under climate change and human activities

In this study,we analyzed the hydrological and meteorological data from the Syr Darya River Basin during the period of 1930–2015 to investigate variations in river runoff and the impacts of climate change and human activities on river runoff.The Syr Darya River,which is supplied by snow and glacier meltwater upstream,is an important freshwater source for Central Asia,as nearly half of the population is concentrated in this area.River runoff in this arid region is sensitive to climate change and human activities.Therefore,estimation of the climatic and hydrological changes and the quantification of the impacts of climate change and human activities on river runoff are of great concern and important for regional water resources management.The long-term trends of hydrological time series from the selected 11 hydrological stations in the Syr Darya River Basin were examined by non-parametric methods,including the Pettitt change point test and Mann-Kendall trend tests.It was found that 8 out of 11 hydrological stations showed significant downward trends in river runof f.Change of river runoff variations occurred in the year around 1960.Moreover,during the study period(1930–2015),annual mean temperature,annual precipitation,and annual potential evapotranspiration in the river basin increased substantially.We employed hydrological sensitivity method to evaluate the impacts of climate change and human activities on river runoff based on precipitation and potential evapotranspiration.It was estimated that human activities accounted for over 82.6%–98.7%of the reduction in river runoff,mainly owing to water withdrawal for irrigation purpose.The observed variations in river runoff can subsequently lead to adverse ecological consequences from an ecological and regional water resources management perspective.     

Impact of climate change on the streamflow in the glacierized Chu River Basin,Central Asia

Catchments dominated by meltwater runoff are sensitive to climate change as changes in precipitation and temperature inevitably affect the characteristics of glaciermelt/snowmelt, hydrologic circle and water resources. This study simulated the impact of climate change on the runoff generation and streamflow of Chu River Basin(CRB), a glacierized basin in Central Asia using the enhanced Soil and Water Assessment Tool(SWAT). The model was calibrated and validated using the measured monthly streamflow data from three discharge gauge stations in CRB for the period 1961–1985 and was subsequently driven by downscaled future climate projections of five Global Circulation Models(GCMs) in Coupled Model Inter-comparison Project Phase 5(CMIP5) under three radiative forcing scenarios(RCP2.6, RCP4.5 and RCP8.5). In this study, the period 1966–1995 was used as the baseline period, while 2016–2045 and 2066–2095 as the near-future and far-future period, respectively. As projected, the climate would become warmer and drier under all scenarios in the future, and the future climate would be characterized by larger seasonal and annual variations under higher RCP. A general decreasing trend was identified in the average annual runoff in glacier(–26.6% to –1.0%), snow(–21.4% to +1.1%) and streamflow(–27.7% to –6.6%) for most of the future scenario periods. The projected maximum streamflow in each of the two future scenarios occurred one month earlier than that in the baseline period because of the reduced streamflow in summer months. Results of this study are expected to arouse the serious concern about water resource availability in the headwater region of CRB under the continuously warming climate. Changes in simulated hydrologic outputs underscored the significance of lowering the uncertainties in temperature and precipitation projection.     

Change features of time-series climate variables from 1962 to 2016 in Inner Mongolia,China

Detecting change features of climate variables in arid/semi-arid areas is essential for understanding related climate change patterns and the driving and evolution mechanism between climate and arid/semi-arid ecosystems.This paper takes Inner Mongolia of China,a unique arid/semi-arid ecosystem,as the study area.We first detected trend features of climate variables using the linear trend analysis method and then detected their trend-shift features using the breaks for additive seasonal and trend method based on the time-series of monthly precipitation and monthly mean temperature datasets from 1962 to 2016.We analyzed the different change features of precipitation and temperature on a regional scale and in different ecological zones to discover the spatial heterogeneity of change features.The results showed that Inner Mongolia has become warmer-wetter during the past 54 years.The regional annual mean temperature increased 0.4°C per decade with a change rate of 56.2%.The regional annual precipitation increased 0.07 mm per decade with a slightly change rate of about 1.7%,but the trend was not statistically significant.The warmer trend was contributed by the same positive trend in each season,while the wetter trend was contributed by the negative trend of the summer precipitation and the positive trend of the other three seasons.The regional monthly precipitation series had a trend-shift pattern with a structural breakpoint in the year 1999,while the regional monthly mean temperature series showed an increasing trend without a periodical trend-shift.After the year 2000,the warmer-wetter trend of the climate in Inner Mongolia was accelerated.The late 20th century was a key period,because the acceleration of the wetter trend in some local zones(I and II)and the alleviation of the warmer trend in some local zones(Ⅶ,Ⅷand IX)occurred simultaneously.Moreover,the change features had a strong spatial heterogeneity,the southeastern and southwestern of Inner Mongolia went through a warmer-drier trend compared with the other areas.The spatio-temporal heterogeneity of the climate change features is a necessary background for various types of research,such as regional climate change,the evolution of arid/semi-arid ecosystems,and the interaction mechanisms between climate and arid/semi-arid ecosystems based on earth-system models in Inner Mongolia.     

Streamflow responses to climate change and LUCC in a semi-arid watershed of Chinese Loess Plateau

Climate change and Land Use/Cover Change(LUCC) have been identified as two primary factors affecting watershed hydrological regime. This study analyzed the trends of streamflow, precipitation, air temperature and potential evapotranspiration(PET) from 1962 to 2008 in the Jihe watershed in northwestern Loess Plateau of China using the Mann-Kendall test. The streamflow responses to climate change and LUCC were quantified independently by the elasticity method. The results show that the streamflow presented a dramatic decline with a turning point occurred in 1971, while the precipitation and PET did not change significantly. The results also show that the temperature rose markedly especially since 1990 s with an approximate increase of 1.74°C over the entire research period(1962–2008). Using land use transition matrix, we found that slope cropland was significantly converted to terrace between 1970 s and 1990 s and that forest cover increased relatively significantly because of the Grain for Green Project after 2000. The streamflow reduction was predominantly caused by LUCC and its contribution reached up to 90.2%, while the contribution of climate change to streamflow decline was only 9.8%. Although the analytical results between the elasticity method and linear regression model were not satisfactorily consistent, they both indicated that LUCC(human activity) was the major factor causing streamflow decline in the Jihe watershed from 1962 to 2008.     

Differences in response of desert plants of different ecotypes to climate warming:a case study in Minqin, Northwest China

Globally climates are warming.How do desert plants of different ecotypes respond to the climate change?This paper studied the differing responses to climate warming shown by desert plants of different ecotypes through analyzing the phenology and meteorological data of 22 desert plant species growing in Minqin Desert Botanical Garden in Northwest China during the period 1974–2009.The results indicate:(1)The temperature in the study area has risen quickly since 1974,and plants’growing periods became longer.The spring phenology of mesophytes advanced,and the autumn phenology of xerophytes was delayed;(2)The starting dates of spring phenophase of mesophytes and xerophytes differed significantly and both showed an advancing trend;(3)The spring phenology of mesophytes advanced by more days than that of xerophytes,whereas the autumn phenology of mesophytes was delayed by less days than that of the xerophytes;and(4)Mesophytes are more sensitive than xerophytes to rising temperature in spring and falling temperature in autumn.These findings are of value in plant management and regional introduction of different species.     

Drought changes and the mechanism analysis for the North American Prairie

The worst droughts in the central part of the North American Prairie in the past several hundred years have been reconstructed from tree-ring chronologies,suggesting that some drought years have exceeded the severity shown by the gauge record.A general circulation model of the Geophysical Fluid Dynamics Laboratory(GFDL) has simulated climate changes for the area during the past 250 years driven by climatic forces,providing scenarios of extreme climate that can further diagnose the mechanisms.This study refined the drought signals from the tree ring data and GFDL modeling at inter-annual and decadal time scales and analyzed the potential mechanisms driving the droughts.Results showed that drought years with summer precipitation lower than the 10 th percentiles occurred during 1777-1789,1847-1861 and 1886-1879 AD in the area.Both tree rings and model revealed that the frequency of droughts has been relatively consistent in a similar timing and frequency with climate change.Monte Carlo analysis have detected that the tree ring chronologies have recorded drought years with probabilities of 9.3%-12.8%,and the model has simulated the droughts with probabilities 5.7%-17.8%.Under CO 2 and aerosol forcing,the GFDL modeled the drought recurrences of 13 years and 25 years,which are very synchronous changes with tree rings and consistent with gauge records.The 20-a and 10-a time scale reoccurrences of droughts are very consistent with solar radiation cycles,and similar to the length of cycles in oceanic records,suggesting that terrestrial precipitation modeling is properly driven from sun-land-sea dynamics.Detected severity,variability and return periods of droughts from the present study make potential improvements in drought predictions and constructing scenarios for climate impacts and adaptation strategies.     

Spatio-temporal variation of hydrological drought under climate change during the period 1960–2013 in the Hexi Corridor,China

In recent years, climate change has been aggravated in many regions of the world. The Hexi Corridor is located in the semiarid climate zone of Northwest China, which is particularly affected by climate change. Climate change has led to the spatial and temporal variations of temperature and precipitation, which may result in hydrological drought and water shortage. Thus, it is necessary to explore and assess the drought characteristics of river systems in this area. The patterns of hydrological drought in the Hexi Corridor were identified using the streamflow drought index(SDI) and standardized precipitation index at 12-month timescale(SPI12) from 1960 to 2013. The evolution of drought was obtained by the Mann–Kendall test and wavelet transform method. The results showed that both the mean annual SDI and SPI12 series in the Hexi Corridor exhibited an increasing trend during the study period. According to the results of wavelet analysis, we divided the study period into two segments, i.e. before and after 1990. Before 1990, the occurrence of droughts showing decreased SDI and SPI12 was concentrated in the northern part of the corridor and shifted to the eastern part of the corridor after 1990. The probability of drought after 1990 in Shule River basin decreased while increased in Shiyang River basin. The wavelet analysis results showed that Shiyang River basin will be the first area to go through the next drought period. Additionally, the relationships between drought pattern and climate indices were analyzed. The enhanced westerly winds and increased precipitation and glacier runoff were the main reasons of wet trend in the Hexi Corridor. However, the uneven spatial variations of precipitation, temperature and glacier runoff led to the difference of hydrological drought variations between the Shule, Heihe and Shiyang River basins.     

Variations of precipitation characteristics during the period 1960–2014 in the Source Region of the Yellow River,China

Precipitation, a natural feature of weather systems in the Earth, is vitally important for the environment of any region. Under global climate change condition, the characteristics of precipitation have changed as a consequence of enhanced global hydrological cycle. The source region of the Yellow River(SRYR), locating within the Qinghai-Tibet Plateau, is sensitive to the global climate change due to its complex orography and fragile ecosystem. To understand the precipitation characteristics and its impacts on the environment in the region, we studied the characteristics of rainy days and precipitation amount of different precipitation classes, such as light(0–5 and 5–10 mm), moderate(10–15, 15–20 and 20–25 mm) and heavy(≥25 mm) rains by analyzing the precipitation data of typical meteorological stations in the SRYR during the period 1961–2014, as well as the trends of persistent rainfall events and drought events. Results showed that annual average precipitation in this area had a non-significant(P>0.05) increasing trend, and 82.5% of the precipitation occurred from May to September. Rainy days of the 0–5 mm precipitation class significantly decreased, whereas the rainy days of 5–10, 10–15, and 20–25 mm precipitation classes increased and that of ≥25 mm precipitation class decreased insignificantly. The persistent rainfall events of 1-or 2-day and more than 2-day showed an increasing trend, with the 1-or 2-day events being more frequent. Meanwhile, the number of short drought periods(≤10 days) increased while long drought periods(>10 days) decreased. Since the 0–5 mm precipitation class had a huge impact on the grasslands productivity; the 5–10, 10–15, and 20–25 mm precipitation classes had positive effects on vegetation which rely on the deep soil water through moving nutrients and water into the root zone of these vegetation or through the plant-microbe interactions; the ≥25 mm precipitation class contributed to the floods; and more persistent rainfall events and fewer long drought events inferred positive effects on agriculture. Thus, these results indicate grassland degradation, less risk of floods, and the upgrading impact of climate change on agriculture. This study may provide scientific knowledge for policymakers to sustain the eco-environmental resources in the SYSR.     

Spatial and temporal change patterns of net primary productivity and its response to climate change in the Qinghai–Tibet Plateau of China from 2000 to 2015

The vegetation ecosystem of the Qinghai–Tibet Plateau in China,considered to be the′′natural laboratory′′of climate change in the world,has undergone profound changes under the stress of global change.Herein,we analyzed and discussed the spatial-temporal change patterns and the driving mechanisms of net primary productivity(NPP)in the Qinghai–Tibet Plateau from 2000 to 2015 based on the gravity center and correlation coefficient models.Subsequently,we quantitatively distinguished the relative effects of climate change(such as precipitation,temperature and evapotranspiration)and human activities(such as grazing and ecological construction)on the NPP changes using scenario analysis and Miami model based on the MOD17A3 and meteorological data.The average annual NPP in the Qinghai–Tibet Plateau showed a decreasing trend from the southeast to the northwest during 2000–2015.With respect to the inter-annual changes,the average annual NPP exhibited a fluctuating upward trend from 2000 to 2015,with a steep increase observed in 2005 and a high fluctuation observed from 2005 to 2015.In the Qinghai–Tibet Plateau,the regions with the increase in NPP(change rate higher than 10%)were mainly concentrated in the Three-River Source Region,the northern Hengduan Mountains,the middle and lower reaches of the Yarlung Zangbo River,and the eastern parts of the North Tibet Plateau,whereas the regions with the decrease in NPP(change rate lower than–10%)were mainly concentrated in the upper reaches of the Yarlung Zangbo River and the Ali Plateau.The gravity center of NPP in the Qinghai–Tibet Plateau has moved southwestward during 2000–2015,indicating that the increment and growth rate of NPP in the southwestern part is greater than those of NPP in the northeastern part.Further,a significant correlation was observed between NPP and climate factors in the Qinghai–Tibet Plateau.The regions exhibiting a significant correlation between NPP and precipitation were mainly located in the central and eastern Qinghai–Tibet Plateau,and the regions exhibiting a significant correlation between NPP and temperature were mainly located in the southern and eastern Qinghai–Tibet Plateau.Furthermore,the relative effects of climate change and human activities on the NPP changes in the Qinghai–Tibet Plateau exhibited significant spatial differences in three types of zones,i.e.,the climate change-dominant zone,the human activity-dominant zone,and the climate change and human activity interaction zone.These research results can provide theoretical and methodological supports to reveal the driving mechanisms of the regional ecosystems to the global change in the Qinghai–Tibet Plateau.     

Temperature and precipitation trends in Minqin Desert during the period of 1961-2007

This paper analyzed the data of temperature and precipitation in Minqin,typical desert area in north-west China,during the period of 1961 2007 by linear regression.The result indicated that the increasing rate of the mean annual temperature in Minqin was higher than that of the average of China;and the temperature in February increased by 3.01oC averagely in the past 47 years.The climate in Minqin displayed an evident warming trend.However,there was no evidently increasing trend of precipitation in the past 47 years,and drought occurred during the whole growing season.     

A warmer but drier Marine Isotope Stage 11 during the past 650 ka as revealed by the thickest loess on the western Chinese Loess Plateau

Marine Isotope Stage 11(MIS 11; ca. 423–362 ka) is generally considered to be the best analogue for the present interglacial(Holocene), and investigation of it will improve our understanding of current climate variability and assist in predictions of future climate change. However, many recent studies primarily focus on the structure and duration of MIS 11. Little research has focused on climate warmth and stability recorded in the Chinese loess-paleosol sequences(LPS) during the S4 paleosol formation(equivalent to MIS 11). On the basis of previous work, this study presents a high-resolution record(ca. 75 a/cm) that spans from MIS 1 to MIS 15, as preserved in the thickest known Jingyuan loess section on the western Chinese Loess Plateau(CLP). This LPS is almost 165 m thick and was sampled from the upper part of L6 to the modern soil at 2-cm depth intervals. Measurements of magnetic susceptibility, mean grain size and 63 μm particle content, carbonate content, total organic carbon, and soil color of samples were made to reconstruct the paleoclimate variation, and a grain-size age model was used to constrain the chronological framework. The primary results show that a generally warm-humid climate dominated the S4 paleosol development, but the climate condition was extremely unstable during the whole of MIS 11. Two obviously different climate regimes controlled the MIS 11 climate variation: the early part of MIS 11 was extremely warm and stable, but the latter part was relatively cool(non-glacial) and unstable. This climate pattern was consistent with records on the central CLP and wavelet analysis suggested that it was forced by the 65°N insolation variability modulated by a quasi-100-ka cycle. In addition, a multi-proxy comparative study on the climate conditions during S0 to S5 paleosol development indicates that the period of S4 development might be the warmest interglacial of the past 650 ka. However, the climate condition during S4 development was not the most humid episode as recorded in Xifeng and Luochuan loess sections on the central CLP. On the contrary, it was drier than both the MIS 15 and the present interglacial on the western CLP, which is somewhat similar to the present climate pattern on the central CLP.     

Characterizing regional precipitation-driven lake area change in Mongolia

Lake area is an important indicator for climate change and its relationship with climatic factors is critical for understanding the mechanisms that control lake level changes. In this study, lake area changes and their relations to precipitation were investigated using multi-temporal Landsat Thermatic Mapper(TM) and Enhanced Thermatic Mapper plus(ETM+) images collected from 10 different regions of Mongolia since the late 1980 s. A linear-regression analysis was applied to examine the relationship between precipitation and lake area change for each region and across different regions of Mongolia. The relationships were interpreted in terms of regional climate regime and hydromorphological characteristics. A total of 165 lakes with areas greater than 10 hm2 were identified from the Landsat images, which were aggregated for each region to estimate the regional lake area. Temporal lake area variability was larger in the Gobi regions, where small lakes are densely distributed. The regression analyses indicated that the regional patterns of precipitation-driven lake area changes varied considerably(R2=0.028–0.950), depending on regional climate regime and hydromorphological characteristics. Generally, the lake area change in the hot-and-dry Gobi regions showed higher correlations with precipitation change. The precedent two-month precipitation was the best determining factor of lake area change across Mongolia. Our results indicate the usefulness of regression analysis based on satellite-derived multi-temporal lake area data to identify regions where factors other than precipitation might play important roles in determining lake area change.     

Phenology of desert plants from an arid gravel plain in eastern United Arab Emirates

Phenological events for desert plants were recorded and rainfall and temperature data were gathered over a three-year time scale at a gravel plain in the eastern region of the United Arab Emirates. Variations of phenological periods were analyzed and correlations between phenological periods and climate factors were discussed. The study showed that the growth and flowering of therophytes were significantly correlated with air temperature. The timing and abundance of rainfall came to be another factor significantly correlated with the onset and duration of chamaephyte flowering as well as the duration of therophyte growth and flowering. The variation in rainfall did not affect the onset of flowering in phanerophytes. Peak growing seasons were from November to March and flowering from November to December(also February to March depending on the availability of rainfall). Flowering in phanerophytes and chamaephytes occurs 4–6 weeks and in therophytes 4–8 weeks after rain. The results showed that variations in growth and phenology between species were correlated with environmental factors, such as temperature and rainfall or, maybe, differences in life form and photosynthetic pathways, each being adapted for utilizing a particular phase of the seasonally and yearly variable rainfall. The sequence of flowering for the species under study was more or less constant despite the difference in the amount of rainfall. The fruiting patterns of most of the species were also found parallel to their flowering patterns. Our results emphasized the need to study multiple species at many sites for the understanding and forecast of regional changes in phenology.     

Spatio-temporal patterns of satellite-derived grassland vegetation phenology from 1998 to 2012 in Inner Mongolia,China

Spatio-temporal variations of vegetation phenology, e.g. start of green-up season(SOS) and end of vegetation season(EOS), serve as important indicators of ecosystems. Routinely processed products from remotely sensed imagery, such as the normalized difference vegetation index(NDVI), can be used to map such variations. A remote sensing approach to tracing vegetation phenology was demonstrated here in application to the Inner Mongolia grassland, China. SOS and EOS mapping at regional and vegetation type(meadow steppe, typical steppe, desert steppe and steppe desert) levels using SPOT-VGT NDVI series allows new insights into the grassland ecosystem. The spatial and temporal variability of SOS and EOS during 1998–2012 was highlighted and presented, as were SOS and EOS responses to the monthly climatic fluctuations. Results indicated that SOS and EOS did not exhibit consistent shifts at either regional or vegetation type level; the one exception was the steppe desert, the least productive vegetation cover, which exhibited a progressive earlier SOS and later EOS. Monthly average temperature and precipitation in preseason(February, March and April) imposed most remarkable and negative effects on SOS(except for the non-significant impact of precipitation on that of the meadow steppe), while the climate impact on EOS was found to vary considerably between the vegetation types. Results showed that the spatio-temporal variability of the vegetation phenology of the meadow steppe, typical steppe and desert steppe could be reflected by the monthly thermal and hydrological factors but the progressive earlier SOS and later EOS of the highly degraded steppe desert might be accounted for by non-climate factors only, suggesting that the vegetation growing period in the highly degraded areas of the grassland could be extended possibly by human interventions.