| 1. |
- Anchukaitis, Kevin, et al.
(författare)
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Last millennium Northern Hemisphere summer temperatures from tree rings: Part II, spatially resolved reconstructions
- 2017
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Ingår i: Quaternary Science Reviews. - : Elsevier BV. - 0277-3791 .- 1873-457X. ; 163, s. 1-22
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Tidskriftsartikel (refereegranskat)abstract
- Climate field reconstructions from networks of tree-ring proxy data can be used to characterize regionalscale climate changes, reveal spatial anomaly patterns associated with atmospheric circulation changes, radiative forcing, and large-scale modes of ocean-atmosphere variability, and provide spatiotemporal targets for climate model comparison and evaluation. Here we use a multiproxy network of tree-ring chronologies to reconstruct spatially resolved warm season (MayeAugust) mean temperatures across the extratropical Northern Hemisphere (40-90N) using Point-by-Point Regression (PPR). The resulting annual maps of temperature anomalies (750e1988 CE) reveal a consistent imprint of volcanism, with 96% of reconstructed grid points experiencing colder conditions following eruptions. Solar influences are detected at the bicentennial (de Vries) frequency, although at other time scales the influence of insolation variability is weak. Approximately 90% of reconstructed grid points show warmer temperatures during the Medieval Climate Anomaly when compared to the Little Ice Age, although the magnitude varies spatially across the hemisphere. Estimates of field reconstruction skill through time and over space can guide future temporal extension and spatial expansion of the proxy network.
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| 2. |
- Björklund, Jesper, 1979, et al.
(författare)
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Blue intensity and density from Northern Fennoscandian tree rings, exploring the potential to improve summer temperature reconstructions with earlywood information
- 2014
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Ingår i: Climate of the Past. - : Copernicus GmbH. - 1814-9324 .- 1814-9332. ; 10, s. 877-885
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Tidskriftsartikel (refereegranskat)abstract
- Here we explore two new tree-ring parameters, derived from measurements of wood density and blue intensity (BI). The new proxies show an increase in the interannual summer temperature signal compared to established proxies, and present the potential to improve long-term performance. At high latitudes, where tree growth is mainly limited by low temperatures, radiodensitometric measurements of wood density, specifically maximum latewood density (MXD), provides a temperature proxy that is superior to that of tree-ring widths. The high cost of developing MXD has led to experimentation with a less expensive method using optical flatbed scanners to produce a new proxy, herein referred to as maximum latewood blue absorption intensity (abbreviated MXBI). MXBI is shown to be very similar to MXD on annual timescales but less accurate on centennial timescales. This is due to the fact that extractives, such as resin, stain the wood differentially from tree to tree and from heartwood to sapwood. To overcome this problem, and to address similar potential problems in radiodensitometric measurements, the new parameters 1blue intensity (1BI) and 1density are designed by subtracting the ambient BI/density in the earlywood, as a background value, from the latewood measurements. As a case-study, based on Scots pine trees from Northern Sweden, we show that 1density can be used as a quality control of MXD values and that the reconstructive performance of warm-season mean temperatures is more focused towards the summer months (JJA – June, July, August), with an increase by roughly 20% when also utilising the interannual information from the earlywood. However, even though the new parameter 1BI experiences an improvement as well, there are still puzzling dissimilarities between 1density and 1BI on multicentennial timescales. As a consequence, temperature reconstructions based on 1BI will presently only be able to resolve information on decadalto- centennial timescales. The possibility of trying to calibrate BI into a measure of lignin content or density, similarly to how radiographic measurements are calibrated into density, could be a solution. If this works, only then can 1BI be used as a reliable proxy in multicentennial-scale climate reconstructions.
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| 3. |
- Björklund, Jesper, et al.
(författare)
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Fennoscandian tree-ring anatomy shows a warmer modern than medieval climate.
- 2023
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Ingår i: Nature. - 1476-4687 .- 0028-0836. ; 620:7972, s. 97-103
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Tidskriftsartikel (refereegranskat)abstract
- Earth system models and various climate proxy sources indicate global warming is unprecedented during at least the Common Era1. However, tree-ring proxies often estimate temperatures during the Medieval Climate Anomaly (950-1250CE) that are similar to, or exceed, those recorded for the past century2,3, in contrast tosimulation experiments at regional scales4. This not only calls into question the reliability of models and proxies but also contributes to uncertainty in future climate projections5. Here we show that the current climate of the Fennoscandian Peninsula is substantially warmer than that of the medieval period. This highlights the dominant role of anthropogenic forcing in climate warming even at the regional scale, thereby reconciling inconsistencies between reconstructions and model simulations. We used an annually resolved 1,170-year-long tree-ring record that relies exclusively on tracheid anatomical measurements from Pinus sylvestris trees, providing high-fidelity measurements of instrumental temperature variability during the warm season. We therefore call for the construction of more such millennia-long records to further improve our understanding and reduce uncertainties around historical and future climate change at inter-regional and eventually global scales.
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| 4. |
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| 5. |
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| 6. |
- Büntgen, Ulf, et al.
(författare)
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Cooling and societal change during the Late Antique Little Ice Age from 536 to around 660 AD
- 2016
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Ingår i: Nature Geoscience. - 1752-0894 .- 1752-0908. ; 9:3, s. 231-236
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Tidskriftsartikel (refereegranskat)abstract
- Climatic changes during the first half of the Common Era have been suggested to play a role in societal reorganizations in Europe and Asia. In particular, the sixth century coincides with rising and falling civilizations, pandemics, human migration and political turmoil. Our understanding of the magnitude and spatial extent as well as the possible causes and concurrences of climate change during this period is, however, still limited. Here we use tree-ring chronologies from the Russian Altai and European Alps to reconstruct summer temperatures over the past two millennia. We find an unprecedented, long-lasting and spatially synchronized cooling following a cluster of large volcanic eruptions in 536, 540 and 547 AD, which was probably sustained by ocean and sea-ice feedbacks, as well as a solar minimum. We thus identify the interval from 536 to about 660 AD as the Late Antique Little Ice Age. Spanning most of the Northern Hemisphere, we suggest that this cold phase be considered as an additional environmental factor contributing to the establishment of the Justinian plague, transformation of the eastern Roman Empire and collapse of the Sasanian Empire, movements out of the Asian steppe and Arabian Peninsula, spread of Slavic-speaking peoples and political upheavals in China.
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| 7. |
- Büntgen, Ulf, et al.
(författare)
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Global tree-ring response and inferred climate variation following the mid-thirteenth century Samalas eruption
- 2022
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Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 59:1-2, s. 531-546
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Tidskriftsartikel (refereegranskat)abstract
- The largest explosive volcanic eruption of the Common Era in terms of estimated sulphur yield to the stratosphere was identified in glaciochemical records 40 years ago, and dates to the mid-thirteenth century. Despite eventual attribution to the Samalas (Rinjani) volcano in Indonesia, the eruption date remains uncertain, and the climate response only partially understood. Seeking a more global perspective on summer surface temperature and hydroclimate change following the eruption, we present an analysis of 249 tree-ring chronologies spanning the thirteenth century and representing all continents except Antarctica. Of the 170 predominantly temperature sensitive high-frequency chronologies, the earliest hints of boreal summer cooling are the growth depressions found at sites in the western US and Canada in 1257 CE. If this response is a result of Samalas, it would be consistent with an eruption window of circa May-July 1257 CE. More widespread summer cooling across the mid-latitudes of North America and Eurasia is pronounced in 1258, while records from Scandinavia and Siberia reveal peak cooling in 1259. In contrast to the marked post-Samalas temperature response at high-elevation sites in the Northern Hemisphere, no strong hydroclimatic anomalies emerge from the 79 precipitation-sensitive chronologies. Although our findings remain spatially biased towards the western US and central Europe, and growth-climate response patterns are not always dominated by a single meteorological factor, this study offers a global proxy framework for the evaluation of paleoclimate model simulations.
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| 8. |
- Büntgen, Ulf, et al.
(författare)
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Global wood anatomical perspective on the onset of the Late Antique Little Ice Age (LALIA) in the mid-6th century CE
- 2022
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Ingår i: Science Bulletin. - : Elsevier BV. - 2095-9273. ; 67:22, s. 2336-2344
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Tidskriftsartikel (refereegranskat)abstract
- Linked to major volcanic eruptions around 536 and 540 CE, the onset of the Late Antique Little Ice Age has been described as the coldest period of the past two millennia. The exact timing and spatial extent of this exceptional cold phase are, however, still under debate because of the limited resolution and geographical distribution of the available proxy archives. Here, we use 106 wood anatomical thin sections from 23 forest sites and 20 tree species in both hemispheres to search for cell-level fingerprints of ephemeral summer cooling between 530 and 550 CE. After cross-dating and double-staining, we identified 89 Blue Rings (lack of cell wall lignification), nine Frost Rings (cell deformation and collapse), and 93 Light Rings (reduced cell wall thickening) in the Northern Hemisphere. Our network reveals evidence for the strongest temperature depression between mid-July and early-August 536 CE across North America and Eurasia, whereas more localised cold spells occurred in the summers of 532, 540–43, and 548 CE. The lack of anatomical signatures in the austral trees suggests limited incursion of stratospheric volcanic aerosol into the Southern Hemisphere extra-tropics, that any forcing was mitigated by atmosphere-ocean dynamical responses and/or concentrated outside the growing season, or a combination of factors. Our findings demonstrate the advantage of wood anatomical investigations over traditional dendrochronological measurements, provide a benchmark for Earth system models, support cross-disciplinary studies into the entanglements of climate and history, and question the relevance of global climate averages.
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| 9. |
- Büntgen, Ulf, et al.
(författare)
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Limited capacity of tree growth to mitigate the global greenhouse effect under predicted warming
- 2019
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 10
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Tidskriftsartikel (refereegranskat)abstract
- It is generally accepted that animal heartbeat and lifespan are often inversely correlated, however, the relationship between productivity and longevity has not yet been described for trees growing under industrial and pre-industrial climates. Using 1768 annually resolved and absolutely dated ring width measurement series from living and dead conifers that grew in undisturbed, high-elevation sites in the Spanish Pyrenees and the Russian Altai over the past 2000 years, we test the hypothesis of grow fast-die young. We find maximum tree ages are significantly correlated with slow juvenile growth rates. We conclude, the interdependence between higher stem productivity, faster tree turnover, and shorter carbon residence time, reduces the capacity of forest ecosystems to store carbon under a climate warming-induced stimulation of tree growth at policy-relevant timescales.
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| 10. |
- Büntgen, Ulf, et al.
(författare)
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Multi-proxy dating of Iceland's major pre-settlement Katla eruption to 822-823 CE
- 2017
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Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 45:9, s. 783-786
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Tidskriftsartikel (refereegranskat)abstract
- Investigations of the impacts of past volcanic eruptions on climate, environment, and society require accurate chronologies. However, eruptions that are not recorded in historical documents can seldom be dated exactly. Here we use annually resolved radiocarbon (C-14) measurements to isolate the 775 CE cosmogenic C-14 peak in a subfossil birch tree that was buried by a glacial outburst flood in southern Iceland. We employ this absolute time marker to date a subglacial eruption of Katla volcano at late 822 CE to early 823 CE. We argue for correlation between the 822-823 CE eruption and a conspicuous sulfur anomaly evident in Greenland ice cores, which follows in the wake of an even larger volcanic signal (ca. 818-820 CE) as yet not attributed to a known eruption. An abrupt summer cooling in 824 CE, evident in tree-ring reconstructions for Fennoscandia and the Northern Hemisphere, suggests a climatic response to the Katla eruption. Written historical sources from Europe and China corroborate our proposed tree ring-radiocarbon-ice core linkage but also point to combined effects of eruptions occurring during this period. Our study describes the oldest precisely dated, high-latitude eruption and reveals the impact of an extended phase of volcanic forcing in the early 9th century. It also provides insight into the existence of prehistoric woodland cover and the nature of volcanism several decades before Iceland's permanent settlement began.
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