Recurrent transitions to Little Ice Age-like climatic regimes over the Holocene

Holocene climate variability is punctuated by episodic climatic events such as the Little Ice Age (LIA) predating the industrial-era warming. Their dating and forcing mechanisms have however remained controversial. Even more crucially, it is uncertain whether earlier events represent climatic regimes similar to the LIA. Here we produce and analyse a new 7500-year long palaeoclimate record tailored to detect LIA-like climatic regimes from northern European tree-ring data. In addition to the actual LIA, we identify LIA-like ca. 100-800 year periods with cold temperatures combined with clear sky conditions from 540 CE, 1670 BCE, 3240 BCE and 5450 BCE onwards, these LIA-like regimes covering 20% of the study period. Consistent with climate modelling, the LIA-like regimes originate from a coupled atmosphere-ocean-sea ice North Atlantic-Arctic system and were amplified by volcanic activity (multiple eruptions closely spaced in time), tree-ring evidence pointing to similarly enhanced LIA-like regimes starting after the eruptions recorded in 1627 BCE, 536/540 CE and 1809/1815 CE. Conversely, the ongoing decline in Arctic sea-ice extent is mirrored in our data which shows reversal of the LIA-like conditions since the late nineteenth century, our record also correlating highly with the instrumentally recorded Northern Hemisphere and global temperatures over the same period. Our results bridge the gaps between low- and high-resolution, precisely dated proxies and demonstrate the efficacy of slow and fast components of the climate system to generate LIA-like climate regimes.Peer reviewe

Last millennium northern hemisphere summer temperatures from tree rings: Part I: The long term context

Large-scale millennial length Northern Hemisphere (NH) temperature reconstructions have been progressively improved over the last 20 years as new datasets have been developed. This paper, and its companion (Part II, Anchukaitis et al. in prep), details the latest tree-ring (TR) based NH land air temperature reconstruction from a temporal and spatial perspective. This work is the first product of a consortium called N-TREND (Northern Hemisphere Tree-Ring Network Development) which brings together dendroclimatologists to identify a collective strategy for improving large-scale summer temperature reconstructions. The new reconstruction, N-TREND2015, utilises 54 records, a significant expansion compared with previous TR studies, and yields an improved reconstruction with stronger statistical calibration metrics. N-TREND2015 is relatively insensitive to the compositing method and spatial weighting used and validation metrics indicate that the new record portrays reasonable coherence with large scale summer temperatures and is robust at all time-scales from 918 to 2004 where at least 3 TR records exist from each major continental mass. N-TREND2015 indicates a longer and warmer medieval period (∼900–1170) than portrayed by previous TR NH reconstructions and by the CMIP5 model ensemble, but with better overall agreement between records for the last 600 years. Future dendroclimatic projects should focus on developing new long records from data-sparse regions such as North America and eastern Eurasia as well as ensuring the measurement of parameters related to latewood density to complement ring-width records which can improve local based calibration substantially

Six hundred years of South American tree rings reveal an increase in severe hydroclimatic events since mid-20th century

South American (SA) societies are highly vulnerable to droughts and pluvials, but lack of long-term climate observations severely limits our understanding of the global processes driving climatic variability in the region. The number and quality of SA climate-sensitive tree ring chronologies have significantly increased in recent decades, now providing a robust network of 286 records for characterizing hydroclimate variability since 1400 CE. We combine this network with a self-calibrated Palmer Drought Severity Index (scPDSI) dataset to derive the South American Drought Atlas (SADA) over the continent south of 12°S. The gridded annual reconstruction of austral summer scPDSI is the most spatially complete estimate of SA hydroclimate to date, and well matches past historical dry/wet events. Relating the SADA to the Australia–New Zealand Drought Atlas, sea surface temperatures and atmospheric pressure fields, we determine that the El Niño–Southern Oscillation (ENSO) and the Southern Annular Mode (SAM) are strongly associated with spatially extended droughts and pluvials over the SADA domain during the past several centuries. SADA also exhibits more extended severe droughts and extreme pluvials since the mid-20th century. Extensive droughts are consistent with the observed 20th-century trend toward positive SAM anomalies concomitant with the weakening of midlatitude Westerlies, while low-level moisture transport intensified by global warming has favored extreme rainfall across the subtropics. The SADA thus provides a long-term context for observed hydroclimatic changes and for 21st-century Intergovernmental Panel on Climate Change (IPCC) projections that suggest SA will experience more frequent/severe droughts and rainfall events as a consequence of increasing greenhouse gas emissions

MULTI-ARCHIVE TEMPERATURE RECONSTRUCTION OF THE RUSSIAN ARCTIC FOR THE PAST TWO MILLENNIA

We present a multi-archival mean annual quantitative temperature reconstruction for Northeastern Europe covering the period of the past two millennia based on tree-ring, pollen, and historical data. This reconstruction was developed primarily to build up a comparative chronology of climatic and historical events in the study region. Five different calibration and verification approaches were used. A comparison of mean decadal temperature reconstruction for Northeastern Europe with those for larger regions and the Hemisphere shows that larger climatic events were visible both in the whole Northern Hemisphere and its separate regions. Less significant climatic events on a regional level may differ considerably from the overall climate signature on the hemispheric level. Highest pre-industrial mean annual temperatures in AD 981-990 were 1,0°C warmer and lowest in AD 1811-1820 were 1,3°C colder than average in AD 1951-1980. The new reconstruction shows much higher degree of variability as compared to Panarctic and hemispheric reconstructions

Direct transformation of tree-ring measurements into palaeoclimate reconstructions in three-dimensional space

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Estimation of Biases in RCS Chronologies of Tree Rings

Проводится сравнение RCS- и signal-free RCS- хронологий на нескольких примерах с реальными и модельными измерениями ширины годичных колец деревьев. Модельные измерения, содержащие известный климатический сигнал, строятся на основе реальных с сохранением структуры набора данных. Во всех экспериментах на модельных данных signal-free RCS превосходит обычный RCS. Но в то же время он менее устойчив к сокращению числа серий измерений. Предлагается метод оценки и корректировки возможных смещений в RCS- хронологиях древесных колец, связанных со структурой набора данных (длина и особенности индивидуальных серий, распределение данных во времени). Такая корректировка может проводиться перед построением реконструкций с применением стандартизации региональной кривой (RCS) и ее «очищенной от сигнала» модификации (signal-free RCS) для повышения точности этих реконструкций.We use several examples of modeled and real tree ring width measurements to compare RCS and signal-free RCS chronologies. Modeled data containing known climatic signal are designed to preserve the structure of dataset. All the experiments with modeled data showed the better ability of signal-free RCS to restore climatic signal. At the same time it is less (as compared to conventional RCS) robust to the reduction of sample depth. A method for evaluation and correction of biases connected with the structure of dataset (length and specifics of individual series, their distribution in time) is proposed. Such correction can be carried out before making climate reconstructions with conventional RCS and signal-free RCS chronologies

Estimation of Biases in RCS Chronologies of Tree Rings

Проводится сравнение RCS- и signal-free RCS- хронологий на нескольких примерах с реальными и модельными измерениями ширины годичных колец деревьев. Модельные измерения, содержащие известный климатический сигнал, строятся на основе реальных с сохранением структуры набора данных. Во всех экспериментах на модельных данных signal-free RCS превосходит обычный RCS. Но в то же время он менее устойчив к сокращению числа серий измерений. Предлагается метод оценки и корректировки возможных смещений в RCS- хронологиях древесных колец, связанных со структурой набора данных (длина и особенности индивидуальных серий, распределение данных во времени). Такая корректировка может проводиться перед построением реконструкций с применением стандартизации региональной кривой (RCS) и ее «очищенной от сигнала» модификации (signal-free RCS) для повышения точности этих реконструкций.We use several examples of modeled and real tree ring width measurements to compare RCS and signal-free RCS chronologies. Modeled data containing known climatic signal are designed to preserve the structure of dataset. All the experiments with modeled data showed the better ability of signal-free RCS to restore climatic signal. At the same time it is less (as compared to conventional RCS) robust to the reduction of sample depth. A method for evaluation and correction of biases connected with the structure of dataset (length and specifics of individual series, their distribution in time) is proposed. Such correction can be carried out before making climate reconstructions with conventional RCS and signal-free RCS chronologies

Removal of a non-climatically induced seven-year cycle from Nothofagus pumilio tree-ring width chronologies from Tierra del Fuego, Argentina for their use in climate reconstructions

A pronounced seven-year cycle is one of the most prominent features of many Fuegian and southern Patagonian Nothofagus pumilio tree-ring width chronologies. However, the source of this variability is still unclear. Here we provide multiple lines of evidence that suggest this phenomenon comes from non-climatically induced ecological factors, most probably triggered by insect outbreaks. To test this hypothesis we removed the seven-year cycle, by means of Singular Spectrum Analysis, from a network of 44 tree-ring width chronologies, from the eastern Tierra del Fuego Island in Argentina. The chronologies that were filtered this way showed a stronger climatic response, and were more successfully modeled using the Vaganov-Shashkin-Lite (VS-Lite) process-based forward model of tree growth. We conclude that the removed periodicity does not have a climatic source. Additional evidence from direct observation of affected and non-affected trees during defoliation, due to Ormiscodes sp. outbreak in 2010, is provided. We also tested the hypothesis of probable masting effects on the growth of N. pumilio at a site with a 12-yr record of seed production, but this hypothesis was not confirmed. Taking into account the non-climatic nature of this signal, we explore the utility of corrected chronologies for reconstruction of climatic parameters in Tierra del Fuego, especially for summer temperature and frost day frequency. This study is the first systematic analysis of the pronounced high-frequency cyclicity in N. pumilio tree growth in southernmost South America.Fil: Matskovsky, Vladimir. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; Argentina. Russian Academy of Sciences. Institute of Geography; RusiaFil: Roig Junent, Fidel Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; Argentina. Universidad Nacional de Cuyo. Facultad de Ciencias Agrarias; Argentina. Universidad Mayor; Chile. Universidade de Sao Paulo; BrasilFil: Martínez Pastur, Guillermo José. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Austral de Investigaciones Científicas; Argentin

Tree-ring-based hydroclimatic reconstruction for the northwest Argentine Patagonia since 1055 CE and its teleconnection to large-scale atmospheric circulation

State-of-the-art climate models project droughts of stronger intensity and longer persistence in many arid and semi-arid regions such as northern Patagonia, which constitutes a serious concern worldwide. Moisture availability has a significant influence on the dynamic, stability and function of terrestrial ecosystems. In this study, we used wood samples from 260 Austrocedrus chilensis trees growing in northwestern Patagonia to reconstruct the Standardized Precipitation Evapotranspiration Index (SPEI) for the last millennium (1055–2014). Our reconstruction explained 41.6% of the variance contained in the November–December SPEI at a 1-month scale for the period 1930–2013. The SPEI reconstruction has provided a long record of extreme pluvial (1060s, 1090s, 1200s, 1300s, 1360 s2, 1390s, 1400s, 1550s, 1580s, 1580s, 1630s, 1940s, 1960s, and 2000s) and drought events (1070s, 1150s, 1170s, 1180s, 1270s, 1310s, 1430s, 1450s, 1570s, 1600s, 1620s, and 1950s) for northwest Argentine Patagonia. Although the SPEI reconstruction indicates that the frequency of extreme events has increased since 1950, our record indicates that current levels have not exceeded those previously reached, particularly when compared to those recorded around the suggested periods for the Medieval Warm and towards the end of the Little Ice Age. The spatial and temporal relationships associated with the South Annular Mode and the Pacific Sea Surface Temperature variability as expressed by the Tripole Index indicated that the temporal variability observed in the SPEI reconstruction is modulated by hemispheric-scale atmospheric circulation dynamics. These climate forcings are likely responsible for the intensity and the rate of occurrence of extreme weather events in northwestern Patagonia. Furthermore, the SPEI reconstruction showed a spatial and temporal pattern similar to that observed in previous PDSI-based reconstructions. This study provides robust evidence of hydroclimatic variations for extratropical sectors of South America, improving our knowledge of the climate dynamics during the last millennium and allowing us to review the recently observed increase in wet and dry events in a long-term historical context.Fil: Hadad, Martín Ariel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Centro de Investigaciones de la Geosfera y Biosfera. Universidad Nacional de San Juan. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones de la Geosfera y Biosfera; ArgentinaFil: González Reyes, Álvaro. Universidad Mayor.; ChileFil: Roig Junent, Fidel Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; Argentina. Universidad Mayor.; ChileFil: Matskovsky, Vladimir. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; Argentina. Russian Academy Of Sciences. Institute Of Geography; RusiaFil: Cherubini, Paolo. Swiss Federal Research Institute. Forest Dynamics Research Unit; Suiza. University of British Columbia; Canad