Multi-century variability in the Pacific North American circulation pattern reconstructed from tree rings

We here present a reconstruction (1725-1999) of the winter Pacific North American (PNA) pattern based on three winter climate sensitive tree ring records from the western USA. Positive PNA phases in our record are associated with warm phases of ENSO and PDO and the reorganization of the PNA pattern towards a positive mode is strongest when ENSO and PDO are in phase. Regime shifts in our PNA record correspond to climatic shifts in other proxies of Pacific climate variability, including two well-documented shifts in the instrumental period (1976 and 1923). The correspondence breaks down in the early 19th century, when our record shows a prolonged period of positive PNA, with a peak in 1800-1820. This period corresponds to a period of low solar activity (Dalton Minimum), suggesting a ‘positive PNA like' response to decreased solar irradiance. The distinct 30-year periodicity that dominates the PNA reconstruction in the 18th century and again from 1875 onwards is disrupted during this perio

Jet stream position explains regional anomalies in European beech forest productivity and tree growth

The mechanistic pathways connecting ocean-atmosphere variability and terrestrial productivity are well-established theoretically, but remain challenging to quantify empirically. Such quantification will greatly improve the assessment and prediction of changes in terrestrial carbon sequestration in response to dynamically induced climatic extremes. The jet stream latitude (JSL) over the North Atlantic-European domain provides a synthetic and robust physical framework that integrates climate variability not accounted for by atmospheric circulation patterns alone. Surface climate impacts of north-south summer JSL displacements are not uniform across Europe, but rather create a northwestern-southeastern dipole in forest productivity and radial-growth anomalies. Summer JSL variability over the eastern North Atlantic-European domain (5-40E) exerts the strongest impact on European beech, inducing anomalies of up to 30% in modelled gross primary productivity and 50% in radial tree growth. The net effects of JSL movements on terrestrial carbon fluxes depend on forest density, carbon stocks, and productivity imbalances across biogeographic regions. Here the authors show that extremes in the summer jet stream position over Europe create a beech forest productivity dipole between northwestern and southeastern Europe and can result in regional anomalies in forest carbon uptake and growth.This work was supported by Fundacio La Caixa through the Junior Leader Program (LCF/BQ/LR18/11640004) and the Universidad Politécnica de Madrid through the Programa Propio (PINV-18-SBSYN2-105-F1TXYR). The following authors acknowledge funding support. I.D.L.: Agnese N. Haury Visiting Scholar & Trainee Fellowship (Laboratory of Tree-Ring Research, University of Arizona), the Mobility Award Jose Castillejo, Ministry of Education, Spanish Government (CAS19/00331) and the Programa de Ayudas Beatriz Galindo, Secretaría de Estado de Universidades, Investigación, Desarrollo e Innovación (#BG20/00065). V.T.: National Science Foundation CAREER grant (AGS-1349942). B.A.: Spanish Ministry of Science and Innovation through the JeDiS project (RTI-2018-096402-B-I00). F.B.: project "Inside out" (#POIR.04.04.00-00-5F85/18-00) funded by the HOMING program of the Foundation for Polish Science co-financed by the European Union under the European Regional Development Fund. AB, AM, CSZ: Bavarian Ministry of Science and the Arts in the context of the Bavarian Climate Research Network (BayKliF). A.H.: PinCaR project (UHU-1266324) by ERD Funds, Andalucía Regional Government, Consejería de Economía, Conocimiento, Empresas y Universidad 2014-2020. EM-S: Swiss National Science Foundation project TRoxy (No. 200021_175888). A.S.J.: Natural Environment Research Council grants NE/V00929X/1 and NE/S010041/1. J.K., L.M., M.M.T., R.W., M.W.: research training group RESPONSE funded by the German Research Council (DFG Fi 846/8-1, DFG GRK2010). AMP: Romanian Ministry of Research, Innovation, and Digitization, Project-PN-19070506/Ctr. no. 12N/2019. I.C.P.: grant of the Romanian Ministry of Education and Research, CNCS-UEFISCDI within PNCDI III (PN-III-P4-ID-PCE-2020-2696). R.S.S.: DendrOlavide I (EQC2018-005303-P), Ministry of Science, Innovation and Universities, Spain; DendrOlavide II (IE19_074 UPO), VURECLIM (P20_00813) and VULBOS (UPO-1263216). T.L.: Slovenian Research Agency-research core funding no. P4-0107 Program research group "Forest Biology, Ecology and Technology". We thank Virgilio Gómez-Rubio for assistance and advice on the LMM development. We thank Christoph Dittmar, Wolfram Elling, and numerous students of the University of Applied Sciences Weihenstephan-Triesdorf for providing European beech tree-ring chronologies

Diverse climate sensitivity of Mediterranean tree-ring width and density

Understanding long-term environmental controls on the formation of tree-ring width (TRW) and maximum latewood density (MXD) is fundamental for evaluating parameter-specific growth characteristics and climate reconstruction skills. This is of particular interest for mid-latitudinal environments where future rates of climate change are expected to be most rapid. Here we present a network of 28 TRW and 21 MXD chronologies from living and relict conifers. Data cover an area from the Atlantic Ocean in the west to the Mediterranean Sea in the east and an altitudinal gradient from 1,000 to 2,500m asl. Age trends, spatial autocorrelation functions, carry-over effects, variance changes, and climate responses were analyzed for the individual sites and two parameter-specific regional means. Variations in warm season (May-September) temperature mainly control MXD formation (r=0.58 to 0.87 from inter-annual to decadal time-scales), whereas lower TRW sensitivity to temperature remains unstable over space and tim

Interannual variations in fire weather, fire extent, and synoptic-scale circulation patterns in northern California and Oregon

The Mediterranean climate region on the west coast of the United States is characterized by wet winters and dry summers, and by high fire activity. The importance of synoptic-scale circulation patterns (ENSO, PDO, PNA) on fire-climate interactions is evident in contemporary fire data sets and in pre-Euroamerican tree-ring-based fire records. We investigated how interannual variability in two fire weather indices, the Haines index (HI) and the Energy Release Component (ERC), in the Mediterranean region of southern Oregon and northern California is related to atmospheric circulation and fire extent. Years with high and low fire weather index values corresponded to years with a high and low annual area burned, respectively. HI combines atmospheric moisture with atmospheric instability and variation in HI was more strongly associated with interannual variation in wildfire extent than ERC, which is based on moisture alone. The association between fire extent and HI was also higher for fires in southern Oregon than in northern California. In terms of synoptic-scale circulation patterns, years of high fire risk (i.e., increased potential for erratic fire behavior, represented by HI and ERC) were associated with positive winter PNA and PDO conditions, characterized by enhanced regional mid-tropospheric ridging and low atmospheric moisture. The time lag we found between fire risk potential and prior winter circulation patterns could contribute to the development of long-lead fire-climate forecastin

Climate signal in tree-ring chronologies of Pinus peuce and Pinus heldreichii from the Pirin Mountains in Bulgaria

Numerous proxy climate reconstructions have been developed for Europe, but there are still regions with limited data of this kind. One region is the Balkan Peninsula, which is characterized by complex interactions between mountains and climate. We present and discuss two tree-ring chronologies—a 758-year-long one of Pinus heldreichii Christ and 340-year-long one of Pinus peuce Griseb. from treeline locations in the Pirin Mountains in Bulgaria. Climate-growth relationships were computed with bootstrap correlation functions and their consistency over time assessed by calculating the correlations over shortened periods. In addition, we reviewed and analyzed climate situations in years with unusually narrow or wide tree rings. Both species were negatively influenced by previous summer drought conditions and cold winters. Early summer temperatures were positively correlated with P. peuce radial growth, whereas P. heldreichii displayed dependence on summer precipitation. In the second half of the twentieth century, the P. heldreichii trees displayed higher sensitivity to summer drought, which was probably a result of increased summer temperatures and decreased winter precipitation. Our findings contribute to more reliable proxy climate records for the regio

Twentieth Century Redistribution in Climatic Drivers of Global Tree Growth

Energy and water limitations of tree growth remain insufficiently understood at large spatiotemporal scales, hindering model representation of interannual or longer-term ecosystem processes. By assessing and statistically scaling the climatic drivers from 2710 tree-ring sites, we identified the boreal and temperate land areas where tree growth during 19301960 CE responded positively to temperature (20.8 3.7 Mio km2; 25.9 4.6%), precipitation (77.5 3.3 Mio km2; 96.4 4.1%), and other parameters. The spatial manifestation of this climate response is determined by latitudinal and altitudinal temperature gradients, indicating that warming leads to geographic shifts in growth limitations. We observed a significant (P < 0.001) decrease in temperature response at cold-dry sites between 19301960 and 19601990 CE, and the total temperature-limited area shrunk by 8.7 0.6 Mio km2. Simultaneously, trees became more limited by atmospheric water demand almost worldwide. These changes occurred under mild warming, and we expect that continued climate change will trigger a major redistribution in growth responses to climate

Post-1980 shifts in the sensitivity of boreal tree growth to North Atlantic Ocean dynamics and seasonal climate

The mid-20th century changes in North Atlantic Ocean dynamics, e.g. slow-down of the Atlantic meridional overturning thermohaline circulation (AMOC), have been considered as early signs of tipping points in the Earth climate system. We hypothesized that these changes have significantly altered boreal forest growth dynamics in northeastern North America (NA) and northern Europe (NE), two areas geographically adjacent to the North Atlantic Ocean. To test our hypothesis, we investigated tree growth responses to seasonal large-scale oceanic and atmospheric indices (the AMOC, North Atlantic Oscillation (NAO), and Arctic Oscillation (AO)) and climate (temperature and precipitation) from 1950 onwards, both at the regional and local levels. We developed a network of 6876 black spruce (NA) and 14437 Norway spruce (NE) tree-ring width series, extracted from forest inventory databases. Analyses revealed post-1980 shifts from insignificant to significant tree growth responses to summer oceanic and atmospheric dynamics both in NA (negative responses to NAO and AO indices) and NE (positive response to NAO and AMOC indices). The strength and sign of these responses varied, however, through space with stronger responses in western and central boreal Quebec and in central and northern boreal Sweden, and across scales with stronger responses at the regional level than at the local level. Emerging post-1980 associations with North Atlantic Ocean dynamics synchronized with stronger tree growth responses to local seasonal climate, particularly to winter temperatures. Our results suggest that ongoing and future anomalies in oceanic and atmospheric dynamics may impact forest growth and carbon sequestration to a greater extent than previously thought. Cross-scale differences in responses to North Atlantic Ocean dynamics highlight complex interplays in the effects of local climate and ocean-atmosphere dynamics on tree growth processes and advocate for the use of different spatial scales in climate-growth research to better understand factors controlling tree growth

Long-term decrease in Asian monsoon rainfall and abrupt climate change events over the past 6,700 years.

Asian summer monsoon (ASM) variability and its long-term ecological and societal impacts extending back to Neolithic times are poorly understood due to a lack of high-resolution climate proxy data. Here, we present a precisely dated and well-calibrated tree-ring stable isotope chronology from the Tibetan Plateau with 1- to 5-y resolution that reflects high- to low-frequency ASM variability from 4680 BCE to 2011 CE. Superimposed on a persistent drying trend since the mid-Holocene, a rapid decrease in moisture availability between ∼2000 and ∼1500 BCE caused a dry hydroclimatic regime from ∼1675 to ∼1185 BCE, with mean precipitation estimated at 42 ± 4% and 5 ± 2% lower than during the mid-Holocene and the instrumental period, respectively. This second-millennium-BCE megadrought marks the mid-to late Holocene transition, during which regional forests declined and enhanced aeolian activity affected northern Chinese ecosystems. We argue that this abrupt aridification starting ∼2000 BCE contributed to the shift of Neolithic cultures in northern China and likely triggered human migration and societal transformation

Old World megadroughts and pluvials during the Common Era

Climate model projections suggest widespread drying in the Mediterranean Basin and wetting in Fennoscandia in the coming decades largely as a consequence of greenhouse gas forcing of climate. To place these and other “Old World” climate projections into historical perspective based on more complete estimates of natural hydroclimatic variability, we have developed the “Old World Drought Atlas” (OWDA), a set of year-to-year maps of tree-ring reconstructed summer wetness and dryness over Europe and the Mediterranean Basin during the Common Era. The OWDA matches historical accounts of severe drought and wetness with a spatial completeness not previously available. In addition, megadroughts reconstructed over north-central Europe in the 11th and mid-15th centuries reinforce other evidence from North America and Asia that droughts were more severe, extensive, and prolonged over Northern Hemisphere land areas before the 20th century, with an inadequate understanding of their causes. The OWDA provides new data to determine the causes of Old World drought and wetness and attribute past climate variability to forced and/or internal variability