Tree-ring-based hydrological records for western Himalaya, India, since a.d. 1560

We analysed 565 increment cores from 325 Himalayan cedar [Cedrus deodara (Roxb.) G. Don] trees growing at 13 moisture-stressed, widely distributed sites in the western Himalayan region. We found a strong positive relationship between our tree-ring width chronologies and spring precipitation which enabled us to reconstruct precipitation back to a.d. 1560. This reconstruction is so far the longest in this region. The calibration model explains 40% variance in the instrumental data (1953–1997). The most striking feature of the reconstruction is the unprecedented increase in precipitation during the late twentieth century relative to the past 438 years. Both wet and dry springs occurred during the Little Ice Age. A 10-year running mean showed that the driest period occurred in the seventeenth century while the wettest period occurred in the twentieth century. Spectral analysis of the reconstructed series indicated a dominant 2-year periodicity.     

The impacts of orbital parameters on summer precipitation over China in the Holocene

The impact of orbital parameters on the climate of China in the Holocene is simulated from 11kaBP to 0kaBP with an interval of 1ka using National Center for Atmospheric Research (NCAR) Community Atmosphere Model version 2 (CAM2). The geographic distributions of summer precipitation around both 9kaBP and 4kaBP were realistically captured by CAM2, compared to the proxy data collected from 80 stations. Among all orbital parameters, the precession plays a major role in computing solar radiation, which dominates the variations of summer precipitation over China during the Holocene. The summers around 9kaBP were the wettest in China. Later on, the precipitation gradually reduced to the minimum around 0kaBP by about 10%. This tremendous change occurred from the Northeast China and the eastern Inner Mongolia extending southwestwards to the Qinghai-Tibet Plateau, especially over the Qinghai-Tibet Plateau.     

A millennial long March–July precipitation reconstruction for southern-central England

We present a millennial long dendroclimatic reconstruction of spring/summer precipitation for southern-central England. Previous research identified a significant moisture stress signal in ring-width data measured from oak trees growing in southern England. In this study, we build upon this earlier work, specifically targeting south-central England, to derive a well replicated oak ring-width composite chronology using both living and historical material. The data-set includes 352 living trees (AD 1629–2009) and 1540 individual historical series (AD 663–1925). The period expressed by at least 50 trees in any year is AD 980–2009. Calibration experiments identify the optimal seasonal predictand target as March–July precipitation (1901–2007: r² = 0.33). However, comparison with the long Kew Gardens precipitation record indicates a weakening in tree-growth/climate response from ~1800 to 1920 which we speculate may be related to smoke and sulphur dioxide (SO₂) emissions at that time which may have also contributed to a decrease in tree productivity. The time-series derived using the regional curve standardisation method to capture lower frequency information shows a mediaeval period with alternating multi-decade-long dry and wet periods, with AD 1153–1172 being the wettest reconstructed 20-year period in the whole record. Drier conditions are prevalent from ~1300 to the early sixteenth century followed by a period of increasing precipitation levels. The most recent four centuries of the record appear similar to the mediaeval period with multiple decade-long dry and wet periods. The late twentieth century is the second reconstructed wettest period. These centennial hydroclimatic trends are in broad agreement with independent regional scale hydroclimatic reconstructions from tree-ring (East Anglia), historical, speleothem and peat water level proxy archives in the United Kingdom and appear coupled with reconstructed sea surface temperature changes in the North Atlantic which in turn influence the Atlantic meridional overturning circulation and westerly airflow across the UK.     

A statistical analysis of the daily precipitation over Serbia: trends and indices

A statistical analysis of the daily maximum and mean monthly precipitation measured at ten meteorological stations in Serbia during the period 1949?C2007 is presented. Although the means of the daily maximum and monthly precipitation varied throughout the year, their ratio was almost uniform, with an average value of 32.6% for Serbia. The precipitation events within each year were ranked and then the trends on the ten wettest days of the year were assessed. Averaged across Serbia, the wettest day of the year produces 41.3?mm of precipitation and accounted for 6.3% of the total annual precipitation. Taken together, 35.5% (232.0?mm) of the total annual precipitation fell during the ten wettest days of the year. Over the course of the twentieth century, the average annual precipitation on the wettest day across Serbia increased by nearly 9%. Also, averaged across Serbia, statistically insignificant increasing trends were found on each of the ten wettest days of the year. Furthermore, four climate indices were analysed. Heavy precipitation indices (i.e., RR20 and R95T) increased in Serbia at the end of the twentieth century and thereafter.     

Spatial drought reconstructions for central High Asia based on tree rings

Spatial reconstructions of drought for central High Asia based on a tree-ring network are presented. Drought patterns for central High Asia are classified into western and eastern modes of variability. Tree-ring based reconstructions of the Palmer drought severity index (PDSI) are presented for both the western central High Asia drought mode (1587–2005), and for the eastern central High Asia mode (1660–2005). Both reconstructions, generated using a principal component regression method, show an increased variability in recent decades. The wettest epoch for both reconstructions occurred from the 1940s to the 1950s. The most extreme reconstructed drought for western central High Asia was from the 1640s to the 1650s, coinciding with the collapse of the Chinese Ming Dynasty. The eastern central High Asia reconstruction has shown a distinct tendency towards drier conditions since the 1980s. Our spatial reconstructions agree well with previous reconstructions that fall within each mode, while there is no significant correlation between the two spatial reconstructions.     

Intra- to multi-decadal terrestrial precipitation regimes at the end of the 20th century

Intra- to multi-decadal (IMD) variation in terrestrial precipitation during 1901–98 was evaluated here by sampling annual precipitation rankings over 6–30 year moving time windows and converting those rankings to Mann-Whitney U statistics. Those U statistics were then used to identify the most significant concentrations of wet and dry years relative to a null hypothesis that assumes stationary climate variability. This time series analysis approach served as the basis of a climate survey method used to identify IMD precipitation regimes over continental areas, and was also used to evaluate IMD variation in time series of annual precipitation spatially averaged over those areas. These methods showed a highly significant incidence of wet years over North America during 1972–98, with 8 of the 10 wettest years of 1901–98 occurring during that 27-year period. A comparably significant incidence of late century wetness was also found over a northern Europe grid region, with 7 of the 10 wettest years occurring during 1978–98. Although significant wet and dry regimes were also found over other land areas in the last decades of the 20th century, the late century North American and northern European wet periods stood out as the most statistically significant found here during 1901–98. It is suggested that these recent wet periods are actually terrestrial evidence of a single multi-decadal precipitation mode extending across the North Atlantic, and the most observable evidence of an even broader pattern of recent North Atlantic climate change.     

Differential regional responses in drought length, intensity and timing to recent climate changes in a Mediterranean forested ecosystem

The Mediterranean area is one of the regions of the world where GCMs agree the most on precipitation changes due to climate change. In this study we aim to assess the impact of recent climate change on drought features of Mediterranean ecosystems in Southern France. Regional climatic trends for the 1971–2006 period are compared to drought trends based on a water balance model accounting for soil properties, vegetation structure and functioning. Drought, defined here as periods when soil water potentials drop below ??0.5 MPa, is described in terms of intensity, duration and timing, which are integrative of both climate variability and site conditions. Temporal trends in precipitation, temperature and solar radiation lead altogether to drier and warmer conditions over the region but with a high spatial heterogeneity; for similar climatic trends, a significant increase in drought intensity was detected in the wettest areas of the region, whereas drought intensity in the driest areas did not change. Indeed, in the wettest areas, we observed an earlier onset of drought by about 1 month, but a constant end of drought. In the driest areas of the region, we observed the same earlier onset of drought but combined with an earlier end of drought, thus leading to a shift of the dry season without increasing its duration. The definition of drought features both in terms of intensity but also of seasonal timing appears relevant to capture historical or forecasted changes in ecosystem functioning. Studies concerning climate change impacts on forested ecosystems should be interpreted with caution when using climate proxies alone.     

A stable oxygen, but not carbon, isotope chronology of Callitris columellaris reflects recent climate change in north-western Australia

We investigated the dendroclimatic potential of stable carbon (δ¹³C) and oxygen (δ¹⁸O) abundances in tree rings of Callitris columellaris F. Muell. Tree-ring chronologies were constructed from the central Pilbara, north-western Australia and span 1919–1999. Variation in δ¹⁸O was more strongly related to climate than δ¹³C; ecological and physiological factors may have dampened the climate signal in the δ¹³C chronology. Tree-ring δ¹⁸O was most strongly correlated with relative humidity (RH) and rainfall (r = −0.36 and −0.39) of the wettest months of the summer period, January and February. The correlation with RH reflects its effect on evaporative enrichment of leaf water. However, tree-ring δ¹⁸O may also partly reflect the variability in ¹⁸O signatures of rainfall, which are influenced by the amount of rainfall and atmospheric humidity. From the δ¹⁸O chronology, we inferred that from 1919 to 1955 summers were relatively dry and warm, but since 1955, summers in the Pilbara region have become increasingly cooler and more humid. Since 1980, conditions have been the wettest and coolest of the last 80 years. These inferred changes in climate correspond to a measured increase in rainfall since 1980 in north-western Australia associated with a greater intensity of tropical cyclones. We conclude that δ¹⁸O abundances in tree rings of C. columellaris have significant potential for reconstructing the climate of semi-arid Australia, a region for which observational climate records are sparse.     

A 694-year tree-ring based rainfall reconstruction from Himachal Pradesh, India

We developed ring-width chronologies of Cedrus deodara [(Roxb.) G. Don] and Pinus gerardiana (Wall. Ex. Lamb) from a homogeneous moisture stressed area in Kinnaur, Himachal Pradesh. Running correlation using a 50-year window with overlap of 25 years showed strong correlations between these species chronologies during the entire common period (ad 1310–2005). Response function analysis indicated that except for January–February, precipitation has a direct relationship with growth of these species. We therefore combined both the species chronologies to develop a statistically calibrated reconstruction of March–July precipitation that spans from ad 1310–2004, and explains 46% of the variance contained in the instrumental data from the calibration period 1951–1994. In the past 694 years of the reconstruction, the wettest period was in the twentieth century (1963–1992) and the driest in the eighteenth century (1773–1802). The relationships observed between reconstructed precipitation and Indian summer monsoon on interdecadal scale, SOI, PDO and NAO indicate the potential utility of such long-term reconstructions in understanding the large-scale climate variability. Multi-taper method (MTM) spectral analysis indicated significant (p < 0.05) spectral peaks at 2–4, 6, 8, 10, 30, 33, 37 and 40–42 years in the reconstructed precipitation data.     

Temperature and Precipitation Changes in China During the Holocene

We review here proxy records of temperature and precipitation in China during the Holocene, especially the last two millennia. The quality of proxy data, methodology of reconstruction, and uncertainties in reconstruction were emphasized in comparing different temperature and precipitation reconstruction and clarifying temporal and spatial patterns of temperature and precipitation during the Holocene. The Holocene climate was generally warm and wet. The warmest period occurred in 9.6-6.2 cal ka BP, whereas a period of maximum monsoon precipitation started at about 11.0 cal ka BP and lasted until about 8.0-5.0 cal ka BP. There were a series of millennial-scale cold or dry events superimposed on the general trend of climate changes. During past two millennia, a warming trend in the 20th century was clearly detected, but the warming magnitude was smaller than the maximum level of the Medieval Warm Period and the Middle Holocene. Cold conditions occurred over the whole of China during the Little Ice Age （AD 1400-AD 1900）, but the warming of the Medieval Warm Period （AD 900-AD 1300） was not distinct in China, especially west China. The spatial pattern of precipitation showed significant regional differences in China, especially east China. The modern warm period has lasted 20 years from 1987 to 2006. Bi-decadal oscillation in precipitation variability was apparent over China during the 20th century. Solar activity and volcanic eruptions both were major forcings governing the climate variability during the last millennium.     

Standardized Precipitation Index Reconstructed from Turkish Tree-Ring Widths

May–July Standardized Precipitation Index (SPI) for the land area of most of Turkey and some adjoining regions are reconstructed from tree rings for the period 1251–1998. The reconstruction was developed from principal components analysis (PCA) of four Juniperus excelsa chronologies from southwestern and south-central Turkey and is based on reliable and replicable statistical relationships between climate and tree ring growth. The SPI reconstruction shows climate variability on both interannual and interdecadal time scales. The longest period of consecutive drought years in the reconstruction (SPI threshold ≤−1) is 2 yr. These occur in 1607–1608, 1675–1676, and 1907–1908. There are five wet events (SPI threshold ≥+1) of two consecutive years each (1330–1331, 1428–1429, 1503–1504, 1629–1630, and 1913–1914). A 5-yr moving average of the reconstructed SPI shows that two sustained drought periods occurred from the mid to late 1300s and the early to mid 1900s. Both episodes are characterized by low variability.     

Tree-ring reconstruction of summer temperature for A.D. 1475–2003 in the central Hengduan Mountains, Northwestern Yunnan, China

Development of long tree-ring records is an important task in paleoclimate studies. Here we presented a five-century long reconstruction of summer (June to August) temperature based on a tree ring-width chronology of Picea brachytyla var. complanata originating from the Hengduan Mountains of China. Climate-growth response analysis showed that summer temperature was the main climatic factor limiting tree-ring growth in the study area. The reconstructed summer temperature accounted for 47.6% of the variance in actual temperature during their common period A.D. 1958–2002. Analysis of the temperature reconstruction showed that major warm periods occurred in the A.D. 1710s–1750s, 1850s, 1920s–1950s and 1990s to present, whereas cold intervals occurred in the A.D. 1630s–1680s, 1790s–1800s, 1860s–1880s and 1950s–1980s, respectively. The low-frequency variation of the reconstruction agreed fairly well with tree-ring reconstructed temperature from nearby regions and with records of glacier fluctuations in the surrounding high mountains, suggesting that our reconstructed summer temperature was reliable, and could aid in the evaluation of regional climate variability.     

Spatial and Temporal Variability in the Growth and Climate Response of Treeline Trees in Alaska

In this study, we investigated the response of trees growing at the cold margins of the boreal forest to climate variation in the 20th century. Working at eight sites at and near alpine and arctic treeline in three regions in Alaska, we compared tree growth (from measured tree ring-widths) to historical climate data to document how growth has responded to climate variation in the 20th century. We found that there was substantial regional variability in response to climate variation. Contrary to our expectations, we found that after 1950 warmer temperatures were associated with decreased tree growth in all but the wettest region, the Alaska Range. Although tree growth increased from 1900–1950 at almost all sites, significant declines in tree growth were common after 1950 in all but the Alaska Range sites. We also found that there was substantial variability in response to climate variation according to distance to treeline. Inverse growth responses to temperature were more common at sites below the forest margin than at sites at the forest margin. Together, these results suggest that inverse responses to temperature are widespread, affecting even the coldest parts of the boreal forest. Even in such close proximity to treeline, warm temperatures after 1950 have been associated with reduced tree growth. Growth declines were most common in the warmer and drier sites, and thus support the hypothesis that drought-stress may accompany increased warming in the boreal forest.     

Temperature and Precipitation Changes in China During the Holocene

We review here proxy records of temperatare and precipitation in China during the Holocene,especially the last two millennia.The quality of proxy data,methodology of reconstruction,and uncertainties in reconstruction were emphasized in comparing different temperatare and precipitation reconstruction and clarilying temporal and spatial patterns of temperature and precipitation during the Holocene.The Holocene climate was generally warm and wet.The warmest period occurred in 9.6-6.2 cal ka BP,whereas a period of maximum monsoon precipitation started at about 11.0 cal ka BP and lasted until about 8.O-5.0 cal ka BP.There were a series of millennial-scale cold or dry events superimposed on the general trend of climate changes.During past two millennia,a warming trend in the 20th century was clearly detected,but the warming magnitude was smaller than the maximum level of the Medieval Warm Period and the Middle Holocene.Cold conditions occurred over the whole of China during the Little Ice Age (AD 1400-AD 1900),but the warming of the Medieval Warm Period(AD 900-AD 1300)was not distinct in China,especially west China.The spatial pattern of precipitation showed significant regional differences in China,especially east China.The modern warm period has lasted 20、years from 1987 to 2006.Bi-decadal oscillation in precipitation variability was apparent over China during the 20th century. Solar activity and volcanic eruptions both were major forcings governing the climate variability during the last millennium.     

Reconstructions of annual summer precipitation and temperature in north-central China since 1470 AD based on drought/flood index and tree-ring records

The understanding of the ongoing climate change needs high-resolution records of the past, which are difficult to obtain in north-central China. Historical documents are unique materials for high-resolution (up to season) climate change reconstruction. Here, we report an attempt of quantitative climate reconstruction covering the main part of north-central China, by combining historical drought/flood index and tree-ring data. The rigorous verification tests confirm the fidelity of transfer functions used in the reconstructions. The precipitation and temperature anomalies/intervals were then defined based on the reconstructions. Finally, the intensity of several big droughts recorded in historical documents was re-examined and the dominant and recessive patterns of heat/water changes within the study area were identified. We concluded that (1) the droughts, occurred during the years of 1484 AD, 1585–1587 AD, 1689–1691 AD, 1784–1786 AD and 1876–1878 AD, were the results of rainless and torrid combination; (2) the droughts, occurred during the years of 1560–1561 AD, 1599–1601 AD, 1609 AD, 1615–1617 AD, 1638–1641 AD and 1899–1901 AD, were first caused by rainless summer, and then controlled by low precipitation and/or high temperature; (3) the droughts, occurred during the years of 1527–1529 AD, 1720–1722 AD, 1813–1814 AD, 1856–1857 AD and 1926–1930 AD, were first caused by torrid summer, and then controlled by both low precipitation and high temperature; (4) the dominant climate pattern within the study area consisted of warm–dry and cold–wet alternations, and the recessive pattern consisted of cold–dry and warm–wet alternations. We also showed that the drought/flood index is a valuable climate proxy in quantitative reconstructions, especially in places where tree-ring data is not available.     

The importance of the geophysical context in statistical evaluations of climate reconstruction procedures

A portion of the debate about climate reconstructions of the past millennium, and in particular about the well-known Mann-Bradley-Hughes (“MBH” 1998, 1999) reconstructions, has become disconnected from the goal of understanding natural climate variability. Here, we reflect on what can be learned from recent scientific exchanges and identify important challenges that remain to be addressed openly and productively by the community. One challenge arises from the real, underlying trend in temperatures during the instrumental period. This trend can affect regression-based reconstruction performance in cases where the calibration period does not appropriately cover the range of conditions encountered during the reconstruction. However, because it is tied to a unique spatial pattern driven by change in radiative balance, the trend cannot simply be removed in the method of climate field reconstruction used by MBH on the statistical argument of preserving degrees of freedom. More appropriately, the influence from the trend can be taken into account in some methods of significance testing. We illustrate these considerations as they apply to the MBH reconstruction and show that it remains robust back to AD 1450, and given other empirical information also back to AD 1000. However, there is now a need to move beyond hemispheric average temperatures and to focus instead on resolving climate variability at the socially more relevant regional scale.     

Tree ring-based winter temperature reconstruction for Changting, Fujian, subtropical region of Southeast China, since 1850: linkages to the Pacific Ocean

Until recently, there have been very few tree-ring studies in southeast China due largely to the scarcity of old trees and the complexity of relationships between tree growth and climate in subtropical regions of China. Recent studies on the conifers in southeast China revealed that tree ring-based climate reconstructions are feasible. Here, we describe a reconstruction (AD 1850–2009) of November–February maximum temperatures for Changting, Fujian, southeast China based on tree ring width data of Pinus massiniana which considerably extends the available climatic information. Calibration and verification statistics for the period 1956–2009 show a high level of skill and account for a significant portion of the observed variance (32.9%) irrespective of which period is used to develop or verify the regression model. Split sample validation supports our use of a reconstruction model based on the full period of reliable observational data (1956–2009). Warm periods occurred during 1854–1859, 1868–1880, 1885–1899, 1906–1914, 1920–1943, 1964–1975 and 1994–present; while the periods of AD 1850–1853, 1860–1867, 1881–1884, 1900–1907, 1915–1919, 1944–1963 and 1976–1993 were relatively cold. The climate correlation analyses with gridded temperature dataset and SST revealed that our season temperature reconstruction contains the strong large-scale climate signals. Our results suggest that some warm winters of Changting are coincident with El Ni?o events over the past 150?years. In addition, several severely cold winters coincide with major volcanic eruptions.     

三组火山强迫资料差异及其对模式模拟结果的影响

Gao2008、Crowley2013和Sigl2015火山强迫资料,均基于极地冰芯重建.由于每组重建使用的冰芯数据和分析方法等不同,因此结果存在差异,从而影响气候模式应用.文中详细梳理三组火山强迫资料在原始冰芯数据、信号识别提取和沉积通量计算等方面的差异;介绍重建中涉及的对未知火山事件发生季节、纬度及从极地硫酸盐沉积...     

Tree-Ring Reconstructed Maize Yield in Central Mexico: 1474–2001

Maize was domesticated more than 6,000 years ago in central Mexico, and remains a vital staple food and cultural symbol in Mesoamerica. Maize yield in the central highlands is strongly dependant on adequate rainfall early in the growing season (April–June) because late maturation of the crop may result in damage from autumn frost. Climate-induced crop failures with profound socioeconomic impacts have punctuated Mexican history. However, reliable records of maize harvest have not been available until very recently, and historical records of crop yield and price are discontinuous and can be difficult to interpret. We have developed a continuous, exactly dated, tree-ring reconstruction of maize yield variability in central Mexico from 1474 to 2001 that provides new insight into the history of climate and food availability in the heartland of the Mesoamerican cultural province. The reconstruction indicates that seven of the most severe agricultural crises in Mexican history occurred during decadal-scale episodes of reconstructed maize shortfalls.     

Hydrology of Prairie Pothole Wetlands during Drought and Deluge: A 17-Year Study of the Cottonwood Lake Wetland Complex in North Dakota in the Perspective of Longer Term Measured and Proxy Hydrological Records

From 1988 to 1992 the north-central plains of North America had a drought that was followed by a wet period that continues to the present (1997). Data on the hydrology of the Cottonwood Lake area (CWLA) collected for nearly 10 years before, and during, the recent dry and wet periods indicate that some prairie pothole wetlands served only a recharge function under all climate conditions. Transpiration from groundwater around the perimeter of groundwater discharge wetlands drew water from the wetlands by the end of summer, even during very wet years.Long-term records of a climate index (Palmer Drought Severity Index), stream discharge (Pembina River), and lake level (Devils Lake) were used to put the 17-year CWLA record into a longer term perspective. In addition, proxy records of climate determined from fossils in the sediments of Devils Lake were also used. These data indicate that the drought of 1988-92 may have been the second worst of the 20th century, but that droughts of that magnitude, and worse, were common during the past 500 years. In contrast, the present wet period may be the wettest it has been during the past 130 years, or possibly the past 500 years.