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Tree Rings as Sensitive Proxies of Past Climate Change
Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
2006 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In the boreal forests of the Northern Hemisphere, time series of tree-ring width (TRW) and maximum density in the latewood (MXD) are highly correlated to local instrumental summer-temperature data and are thus widely used as proxies in high-resolution climate reconstructions. Hence, much of our present knowledge about climatic variability in the last millennium is based on tree-rings. However, many tree-ring records have a lack of data in the most recent decades, which severely hampers our ability to place the recent temperature increase in a longer-timescale perspective of natural variability.

The main objective of this thesis is to update and extend the Torneträsk TRW and MXD records in northern Sweden. Local instrumental climate-data is used to calibrate the new tree-ring records. The results show that TRW is mainly forced by temperature in the early growing season (June/July) while MXD has a wider response window (June – August) and has a higher correlation to temperature. Two reconstructions of summer temperature are made for (i) the last 7,400 years based on TRW, and (ii) the last 1,500 years based on a combination of TRW and MXD. The reconstructions show natural variability on timescales from years to several centuries. The 20th century does not stand out as a notably warm period in the long timescale perspective. A medieval period from AD 900 – 1100 is markedly warmer than the 20th century.

The environmental impact from a large explosive volcanic eruption in 1628/1627 BC is analysed in the tree rings of 14C-dated bog pines in south-central Sweden and in absolutely-dated subfossil pines from Torneträsk. The results show evidence of an impact in the southern site at approximately this time but no detectable impact in the North.

Subfossil trees of Fitzroya cupressoides in southern Chile were 14C-dated to approx. 50,000 years BP and amalgamated into a 1,229-year TRW chronology. This tree-ring record is the oldest in the world. The variability in this Last-glacial chronology is similar to the variability in present-day living trees of the same species. These results suggest that the growth–forcing mechanisms 50,000 years ago were similar to those at present.

Place, publisher, year, edition, pages
Stockholm: Institutionen för naturgeografi och kvartärgeologi , 2006. , 75 p.
Series
Dissertations from the Department of Physical Geography and Quaternary Geology, ISSN 1653-7211 ; 1
Keyword [en]
Tree ring, width, density, climate, temperature, change, variability, sensitivity, Torneträsk, Tornetrask, Sweden, Fennoscandia, explosive, volcanic, eruption, Santorini, Thera, subfossil, Fitzroya, Chile
National Category
Physical Geography
Identifiers
URN: urn:nbn:se:su:diva-1034ISBN: 91-7155-251-0 (print)OAI: oai:DiVA.org:su-1034DiVA: diva2:189273
Public defence
2006-06-02, De Geersalen, Geovetenskapens hus, Svante Arrhenius väg 8 A, Stockholm, 13:00
Opponent
Supervisors
Available from: 2006-05-08 Created: 2006-05-08Bibliographically approved
List of papers
1. A 7400-year tree-ring chronology in northern Swedish Lapland: Natural climatic variability expressed on annual to millennial timescales
Open this publication in new window or tab >>A 7400-year tree-ring chronology in northern Swedish Lapland: Natural climatic variability expressed on annual to millennial timescales
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2002 (English)In: The Holocene, ISSN 0959-6836, E-ISSN 1477-0911, Vol. 12, no 6, 657-665 p.Article in journal (Refereed) Published
Abstract [en]

Tree-ring widths from 880 living, dry dead, and subfossil northern Swedish pines (Pinus sylvestris L.) have been assembled into a continuous and precisely dated chronology (the Tornetrask chronology) covering the period 5407 BC to AD 1997. Biological trends in the data were removed with autoregressive standardization (ARS) to emphasize year-to-year variability, and with regional curve standardization (RCS) to emphasize variability on timescales from decades to centuries. The strong association with summer mean temperature (June-August) has enabled the production of a temperature reconstruction for the last 7400 years, providing information on natural summer-temperature variability on timescales from years to centuries. Numerous cold episodes, comparable in severity and duration to the severe summers of the seventeenth century, are shown throughout the last seven millennia. Particularly severe conditions suggested between 600 and 1 BC correspond to a known period of glacier expansion, The relatively warm conditions of the late twentieth century do not exceed those reconstructed for several earlier time intervals, although replication is relatively poor and confidence in the reconstructions is correspondingly reduced in the pre-Christian period, particularly around 3000, 1600 and 330 BC. Despite the use of the RCS approach in chronology construction, the 7400-year chronology does not express the full range of millennial-timescale temperature change in northern Sweden.

Keyword
dendroclimatology; tree rings; climate; summer temperature; Pinus sylvestris; northern Scandinavia; Holocene
National Category
Natural Sciences
Identifiers
urn:nbn:se:su:diva-22689 (URN)10.1191/0959683602hl578rp (DOI)
Note
Part of urn:nbn:se:su:diva-1034Available from: 2006-05-08 Created: 2006-05-08 Last updated: 2017-12-13Bibliographically approved
2. Torneträsk tree-ring width and density AD 500 – 2004: A test of climatic sensitivity and a new 1500-year reconstruction of north Fennoscandian summers
Open this publication in new window or tab >>Torneträsk tree-ring width and density AD 500 – 2004: A test of climatic sensitivity and a new 1500-year reconstruction of north Fennoscandian summers
2008 (English)In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 31, 843-857 p.Article in journal (Refereed) Published
Abstract [en]

This paper presents updated tree-ring width(TRW) and maximum density (MXD) from Tornetra¨sk innorthern Sweden, now covering the period AD 500–2004. Byincluding data from relatively young trees for the most recentperiod, a previously noted decline in recent MXD is eliminated.Non-climatological growth trends in the data areremoved using Regional Curve Standardization (RCS), thusproducingTRWandMXDchronologies with preserved lowfrequencyvariability. The chronologies are calibrated usinglocal and regional instrumental climate records. A bootstrappedresponse function analysis using regional climatedata shows that tree growth is forced by April–August temperaturesand that the regression weights for MXD are muchstronger than for TRW. The robustness of the reconstructionequation is verified by independent temperature data andshows that 63–64% of the instrumental inter-annual variationis captured by the tree-ring data. This is a significantimprovement compared to previously published reconstructionsbased on tree-ring data from Tornetra¨sk. Adivergence phenomenon around AD 1800, expressed as anincrease in TRW that is not paralleled by temperature andMXD, is most likely an effect of major changes in the densityof the pine population at this northern tree-line site. The biasintroduced by this TRW phenomenon is assessed by producinga summer temperature reconstruction based onMXDexclusively. The new data show generally higher temperatureestimates than previous reconstructions based onTornetra¨sk tree-ring data. The late-twentieth century, however,is not exceptionally warm in the new record: Ondecadal-to-centennial timescales, periods around AD 750,1000, 1400, and 1750 were equally warm, or warmer. The200-year long warm period centered on AD 1000 was significantlywarmer than the late-twentieth century (p\0.05)and is supported by other local and regional paleoclimatedata. The new tree-ring evidence from Tornetra¨sk suggeststhat this ‘‘Medieval WarmPeriod’’ in northern Fennoscandiawas much warmer than previously recognized.

National Category
Climate Research
Research subject
Physical Geography
Identifiers
urn:nbn:se:su:diva-22690 (URN)10.1007/s00382-007-0358-2 (DOI)000260370300007 ()
Note
Part of urn:nbn:se:su:diva-1034Available from: 2011-02-03 Created: 2006-05-08 Last updated: 2017-12-13Bibliographically approved
3. Swedish tree rings provide new evidence in support of a major, widespread environmental disruption in 1628 BC
Open this publication in new window or tab >>Swedish tree rings provide new evidence in support of a major, widespread environmental disruption in 1628 BC
2000 (English)In: Geophysical Research Letters, Vol. 27, no 18, 2957-2960 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:su:diva-22691 (URN)
Note
Part of urn:nbn:se:su:diva-1034Available from: 2006-05-08 Created: 2006-05-08 Last updated: 2011-01-24Bibliographically approved
4. Climate variability 50,000 years ago in midlatitude Chile as reconstructed from tree rings
Open this publication in new window or tab >>Climate variability 50,000 years ago in midlatitude Chile as reconstructed from tree rings
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2001 (English)In: Nature, Vol. 410, 567-570 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:su:diva-22692 (URN)
Note
Part of urn:nbn:se:su:diva-1034Available from: 2006-05-08 Created: 2006-05-08 Last updated: 2011-01-24Bibliographically approved

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