A matter of divergence: Tracking recent warming at hemispheric scales using tree ring data

No current tree ring (TR) based reconstruction of extratropical Northern Hemisphere (ENH) temperatures that extends into the 1990s captures the full range of late 20th century warming observed in the instrumental record. Over recent decades, a divergence between cooler reconstructed and warmer instrumental large-scale temperatures is observed. We hypothesize that this problem is partly related to the fact that some of the constituent chronologies used for previous reconstructions show divergence against local temperatures in the recent period. In this study, we compiled TR data and published local/regional reconstructions that show no divergence against local temperatures. These data have not been included in other large-scale temperature reconstructions. Utilizing this data set, we developed a new, completely independent reconstruction of ENH annual temperatures (1750–2000). This record is not meant to replace existing reconstructions but allows some degree of independent validation of these earlier studies as well as demonstrating that TR data can better model recent warming at large scales when careful selection of constituent chronologies is made at the local scale. Although the new series tracks the increase in ENH annual temperatures over the last few decades better than any existing reconstruction, it still slightly under predicts values in the post-1988 period. We finally discuss possible reasons why it is so difficult to model post-mid-1980s warming, provide some possible alternative approaches with regards to the instrumental target and detail several recommendations that should be followed in future large-scale reconstruction attempts that may result in more robust temperature estimates

Precise date for the Laacher See eruption synchronizes the Younger Dryas

The Laacher See eruption (LSE) in Germany ranks among Europe’s largest volcanic events of the Upper Pleistocene. Although tephra deposits of the LSE represent an important isochron for the synchronization of proxy archives at the Late Glacial to Early Holocene transition, uncertainty in the age of the eruption has prevailed. Here we present dendrochronological and radiocarbon measurements of subfossil trees that were buried by pyroclastic deposits that frmly date the LSE to 13,006 ± 9 calibrated years before present (bp; taken as ad 1950), which is more than a century earlier than previously accepted. The revised age of the LSE necessarily shifts the chronology of European varved lakes relative to the Greenland ice core record, thereby dating the onset of the Younger Dryas to 12,807 ± 12 calibrated years bp, which is around 130 years earlier than thought. Our results synchronize the onset of the Younger Dryas across the North Atlantic–European sector, preclude a direct link between the LSE and Greenland Stadial-1 cooling, and suggest a large-scale common mechanism of a weakened Atlantic Meridional Overturning Circulation under warming condition

Testing for tree-ring divergence in the European Alps

Evidence for reduced sensitivity of tree growth to temperature has been reported from multiple forests along the high northern latitudes. This alleged circumpolar phenomenon described the apparent inability of temperature-sensitive tree-ring width and density chronologies to parallel increasing instrumental temperature measurements since the mid-20th century. In addition to such low-frequency trend offset, the inability of formerly temperature-sensitive tree growth to reflect high-frequency temperature signals in a warming world is indicated at some boreal sites, mainly in Alaska, the Yukon and Siberia. Here, we refer to both of these findings as the \u2018divergence problem\u2019 (DP), with their causes and scale being debated. If DP is widespread and the result of climatic forcing, the overall reliability of tree-ring-based temperature reconstructions should be questioned. Testing for DP benefits from well-replicated tree-ring and instrumental data spanning from the 19th to the 21st century. Here, we present a network of 124 larch and spruce sites across the European Alpine arc. Tree-ring width chronologies from 40 larch and 24 spruce sites were selected based on their correlation with early (1864\u20131933) instrumental temperatures to assess their ability of tracking recent (1934\u20132003) temperature variations. After the tree-ring series of both species were detrended in a manner that allows low-frequency variations to be preserved and scaled against summer temperatures, no unusual late 20th century DP is found. Independent tree-ring width and density evidence for unprecedented late 20th century temperatures with respect to the past millennium further reinforces our results

Notes towards an optimal sampling strategy in dendroclimatology

Though the extraction of increment cores is common practice in tree-ring research, there is no standard for the number of samples per tree, or trees per site needed to accurately describe the common growth pattern of a discrete population of trees over space and time. Tree-ring chronologies composed of living, subfossil and archaeological material often combine an uneven distribution of increment cores and disc samples. The effects of taking one or two cores per tree, or even the inclusion of multiple radii measurements from entire discs, on chronology development and quality remain unreported. Here, we present four new larch (Larix cajanderi Mayr) ring width chronologies from the same 20 trees in northeastern Siberia that have been independently developed using different combinations of core and disc samples. Our experiment reveals: i) sawing is much faster than coring, with the latter not always hitting the pith; ii) the disc-based chronology contains fewer locally absent rings, extends further back in time and exhibits more growth coherency; iii) although the sampling design has little impact on the overall chronology behaviour, lower frequency information is more robustly obtained from the disc measurements that also tend to reflect a slightly stronger temperature signal. In quantifying the influence of sampling strategy on the quality of tree-ring width chronologies, and their suitability for climate reconstructions, this study provides useful insights for optimizing fieldwork campaigns, as well as for developing composite chronologies from different wood sources

Tree-ring-based reconstruction of larch budmoth outbreaks in the central italian alps since 1774 CE

The larch budmoth (Zeiraphera diniana Gn. – LBM) offers a unique example of cyclic fluctuations in insect populations. During regular LBM mass outbreaks, defoliation of European larch (Larix decidua Mill.) subalpine trees results in distinct ring-width reductions in the host trees. Although several observations, reconstructions and models suggest that LBM outbreaks travel from the southwest to the northeast along the Alpine arc, gaps in the underlying data still hamper our mechanistic understanding of the spatio-temporal system dynamics. Evidence of historical LBM outbreaks before 1964 is generally associated with uncertainty and is particularly scarce for the Central Italian Alps. Here, we introduce four new larch ring-width chronologies from Val di Sole in the Central Italian Alps and use time-series analyses and comparisons with non-host trees (Picea abies Karst.) to reconstruct LBM mass outbreaks. We identify distinct fingerprints of 15 spatially-synchronized LBM events that occurred between 1774 and 1964 CE. Our results are important for improving qualitative space-time models to simulate travelling wave dynamics of insect populations, and for correcting ring-width-based summer temperature reconstructions from this part of the Alpine arc

Five centuries of Central European temperature extremes reconstructed from tree-ring density and documentary evidence

Future climate change will likely influence the frequency and intensity of weather extremes. As such events are by definition rare, long records are required to understand their characteristics, drivers, and consequences on ecology and society. Herein we provide a unique perspective on regional-scale temperature extremes over the past millennium, using three tree-ring maximum latewood density (MXD) chronologies from higher elevations in the European Alps. We verify the tree-ring-based extremes using documentary evidences from Switzerland, the Czech Republic, and Central Europe that allowed the identification of 44 summer extremes over the 1550-2003 period. These events include cold temperatures in 1579, 1628, 1675, and 1816, as well as warm ones in 1811 and 2003. Prior to 1550, we provide new evidence for cold (e.g., 1068 and 1258) and warm (e.g., 1333) summers derived from the combined MXD records and thus help to characterize high-frequency temperature variability during medieval times. Spatial coherence of the reconstructed extremes is found over Switzerland, with most signatures even extending across Central Europe. We discuss potential limitations of the tree-ring and documentary archives, including the (i) ability of MXD to particularly capture extremely warm temperatures, (ii) methodological identification and relative definition of extremes, and (iii) placement of those events in the millennium-long context of low-frequency climate change. © 2010 Elsevier B.V

Lateglacial environmental variability from Swiss tree rings

Evidence of annually resolved environmental variations during the Aller\uf8d interstadial is presented using 81 fossil Scots pine tree-ring series from Gaenziloo and Landikon, near Zurich, Switzerland. The absolute age of the trees ranges between 11,920 and 10,61014C BP, which was determined by wiggle-matching radiocarbon ages to the Cariaco14C data set. From the two sites we created a composite floating Aller\uf8d chronology on the basis of their 632 years of overlap (r=0.57), after individual spline detrending. Merging both data sets resulted in a Lateglacial tree-ring chronology covering 1050 years. Regional curve standardization (RCS) was applied to preserve low-frequency information. Growth behavior of the fossil trees was compared with a recent composite pine data set from the central Swiss Alps and reveals distinct differences. The new Aller\uf8d RCS chronology reveals major Lateglacial variations, such as the Gerzensee oscillation, the abrupt climate shift towards the Younger Dryas and some short-term events. Radiocarbon ages agree well with those from other sites on the Swiss Plateau. For hemispheric comparisons we used the annual layer thickness record from the NGRIP ice core and the gray-scale varve record from the Cariaco basin. Even though the amplitudes are not yet fully understood, similarities on decadal-to-centennial scales are apparent. \ua9 2007 Elsevier Ltd. All rights reserved

Low-frequency noise in delta C-13 and delta O-18 tree ring data: A case study of Pinus uncinata in the Spanish Pyrenees

Isotopic discrimination measurements in tree rings are becoming increasingly important estimators of past environmental change. Potential biases inherent to these parameters, including age trend and level offset are, however, not well understood. We here perform measurements on a new millennium-long data set of decadally resolved δ18O and δ13C discrimination from 25 high-elevation pine trees in the Spanish Pyrenees to investigate whether such low-frequency biases exist and how they alter the long-term behavior of derived time series. Alignment of the tree ring data by biological age reveals age trends over the first one to four centuries after germination. On average, isotope values change by -0.089‰ δ18O and +0.064‰ δ13C per decade over the first 100 years of tree growth. This trend persists into the forth century after germination for δ18O but diminishes to ∼0‰ over the 100-390 year period for δ13C. We also find level offsets up to 7‰ δ18O and 3‰ δ 13C between single trees. Analysis of the shape of age trends reveals negative exponential functions as reasonable choices for detrending of δ18O and (inverted) δ13C time series. The detrended isotope chronologies share low-frequency variance with traditional ring width and density measurements beyond statistical chance, suggesting that the various parameters reflect some of the same environmental forcing. Our results show that δ18O and δ13C from these Spanish pines need to be detrended to assess long-term environmental changes. To evaluate the general applicability of this conclusion, production of (1) well-replicated, (2) nonpooled, and (3) composite chronologies from other species and regions will be required. Increases in measurement speed and technology will make these tests feasible in the coming years. Copyright 2010 by the American Geophysical Union