排序方式： 共有18条查询结果，搜索用时 46 毫秒
1.
2.
Geomagnetism and Aeronomy - Reconstruction of summer temperature at subarctic Fennoscandia (68°–70° N, 20°–30° E), based on tree-ring data, was compared with three... 相似文献
3.
Samuli Helama Jianmin Jiang Johanna Korhonen Jari Holopainen Mauri Timonen 《地理学报(英文版)》2013,23(6):1069-1079
Cryophenological records （i.e. observational series of freeze and breakup dates of ice） are of great importance when assessing the environmental variations in cold regions. Here we employed the extraordinarily long observational records of river ice breakup dates and air temperatures in northern Fennoscandia to examine their interrelations since 1802. Historical observations, along with modern data, comprise the informational setting for this analysis carried out using t-test. Temperature history of April-May season was used as cli- matic counterpart for the breakup timings. Both records （temperature and breakup） showed seven sub-periods during which their local means were distinctly different relative to preced- ing and subsequent sub-periods. The starting and ending years of these sub-periods oc- curred in temporal agreement. The main findings of this study are summarized as follows： （1） the synchrony between the temperature and river ice breakup records ruled out the possibility that the changes would have occurred due to quality of the historical series （i.e. inhomoge- neity problems often linked to historical time-series）; （2） the studied records agreed to show lower spring temperatures and later river ice breakups during the 19th century, in comparison to the 20th century conditions, evidencing the prevalence of cooler spring temperatures in the study region, in agreement with the concept of the Little Ice Age （1570-1900） climate in North-West Europe; （3） the most recent sub-period demonstrate the highest spring tem- peratures with concomitantly earliest river ice breakups, showing the relative warmth of the current springtime climate in the study region in the context of the past two centuries; （4） the effects of anthropogenic changes in the river environment （e.g. construction and demolition of dams） during the 20th century should be considered for non-climatic variations in the breakup records; （5） this study emphasizes the importance of multi-centurial （i.e. historical） cryo- phenological information for highly interesting viewpoints of climate and environmental his- tory. 相似文献
4.
Samuli Helama Heikki Sepp? Anne E. Bjune H. John B. Birks 《Journal of Paleolimnology》2012,48(1):275-286
A new palaeoclimatic reconstruction of mid-summer (July) temperatures for the last 7.5?ka in northern Fennoscandia is presented. It is based on two botanical proxies: spectra of fossil pollen and tree rings of Scots pine logs recovered from lacustrine sediments in the Arctic tree-line region. A newly developed method of proxy fusion is used to integrate the proxy-specific reconstructions of past summer temperature variability based on the pollen-stratigraphic and dendroclimatic data. The rationale behind the method is that the two proxies are likely to be connected to climate variability in a timescale-dependent fashion and, accordingly, the new reconstruction makes use of the low- and high-frequencies from pollen-stratigraphic and tree-ring data, respectively. The most prominent features of the new reconstruction are: (1) the long-term decline of temperatures by 2.0?°C over the past 7.5?ka, (2) the mid-Holocene warmth culminating between 5 and 4?ka as a deviation from the cooling trend, (3) the Little Ice Age cool phase between 0.7 and 0.1?ka, and (4) the subsequent warming during the past century. These periods are superimposed on year-to-year variations in climate as dated to calendar-year accuracy by dendrochronology. Within the modern period, the years 1934 and 1937 are among the warmest, and the years 1903 and 1910 are among the coldest summers in the context of the past 7.5?ka. On average, the reconstructed Holocene climate was approximately 0.85?°C warmer than the twentieth century. 相似文献
5.
Detection of climate signal in dendrochronological data analysis: a comparison of tree-ring standardization methods 总被引：7，自引：0，他引：7
S. Helama M. Lindholm M. Timonen M. Eronen 《Theoretical and Applied Climatology》2004,79(3-4):239-254
Summary Tree-ring standardization methods were compared. Traditional methods along with the recently introduced approaches of regional curve standardization (RCS) and power-transformation (PT) were included. The difficulty in removing non-climatic variation (noise) while simultaneously preserving the low-frequency variability in the tree-ring series was emphasized. The potential risk of obtaining inflated index values was analysed by comparing methods to extract tree-ring indices from the standardization curve. The material for the tree-ring series, previously used in several palaeoclimate predictions, came from living and dead wood of high-latitude Scots pine in northernmost Europe. This material provided a useful example of a long composite tree-ring chronology with the typical strengths and weaknesses of such data, particularly in the context of standardization. PT stabilized the heteroscedastic variation in the original tree-ring series more efficiently than any other standardization practice expected to preserve the low-frequency variability. RCS showed great potential in preserving variability in tree-ring series at centennial time scales; however, this method requires a homogeneous sample for reliable signal estimation. It is not recommended to derive indices by subtraction without first stabilizing the variance in the case of series of forest-limit tree-ring data. Index calculation by division did not seem to produce inflated chronology values for the past one and a half centuries of the chronology (where mean sample cambial age is high). On the other hand, potential bias of high RCS chronology values was observed during the period of anomalously low mean sample cambial age. An alternative technique for chronology construction was proposed based on series age decomposition, where indices in the young vigorously behaving part of each series are extracted from the curve by division and in the mature part by subtraction. Because of their specific nature, the dendrochronological data here should not be generalized to all tree-ring records. The examples presented should be used as guidelines for detecting potential sources of bias and as illustrations of the usefulness of tree-ring records as palaeoclimate indicators. 相似文献
6.
Changes in tree density are estimated for the northern forest‐limit region of Finnish Lapland over the past seven and a half millennia. This is done using dendrochronologically dated Scots pine megafossils and their tree‐ring series. Direct and indirect estimates of past tree density are derived from chronology sample size (CSS) and growth trend modelling (GTM) respectively. The latter is a new method, where the past levels of growth competition (the influence of nearby trees) are extracted from the behaviour of growth trends in cross‐dated tree‐ring series, trends that are expected to be driven by tree‐density control. Two records constructed from the same original data set but by different means (independently) correlate significantly. Both records show a tree‐density maximum around 3000–1750 BC , indicating relative warmth during that time. Another positive tree‐density anomaly occurred in accordance with the ‘Medieval Warm Period’, preceding the thinning at the forest‐limit due to the ‘Little Ice Age’. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
7.
Helama S Schöne BR Kirchhefer AJ Nielsen JK Rodland DL Janssen R 《Marine environmental research》2007,63(3):185-199
Annual growth increments were examined from shells of the ocean quahog (Arctica islandica L.) from northwest Norway and from tree-ring samples of the Scots pine (Pinus sylvestris L.) from nearby coastal areas. The reconstructed annual growth increments were used to compare growth variability in marine and terrestrial ecosystems. Spatiotemporal comparison of the growth records showed statistically significant correlation during the 19th century A.D., indicative of ecosystem-independent response to pre-anthropogenic climate variations. Geographical correlation between marine and terrestrial records was only observed at the local scale. Years with particularly low winter or high summer North Atlantic Oscillation (NAO) indices showed the best synchronization of marine and terrestrial growth. Despite strong correlation during historical time, our palaeoecological evidence suggests that marine and terrestrial ecosystems may show dissimilar growth reaction to recently observed positive winter-NAO phases. 相似文献
8.
9.
Unprecedented low twentieth century winter sea ice extent in the Western Nordic Seas since A.D. 1200
M. Macias Fauria A. Grinsted S. Helama J. Moore M. Timonen T. Martma E. Isaksson M. Eronen 《Climate Dynamics》2010,34(6):781-795
We reconstructed decadal to centennial variability of maximum sea ice extent in the Western Nordic Seas for A.D. 1200–1997
using a combination of a regional tree-ring chronology from the timberline area in Fennoscandia and δ18O from the Lomonosovfonna ice core in Svalbard. The reconstruction successfully explained 59% of the variance in sea ice extent
based on the calibration period 1864–1997. The significance of the reconstruction statistics (reduction of error, coefficient
of efficiency) is computed for the first time against a realistic noise background. The twentieth century sustained the lowest
sea ice extent values since A.D. 1200: low sea ice extent also occurred before (mid-seventeenth and mid-eighteenth centuries,
early fifteenth and late thirteenth centuries), but these periods were in no case as persistent as in the twentieth century.
Largest sea ice extent values occurred from the seventeenth to the nineteenth centuries, during the Little Ice Age (LIA),
with relatively smaller sea ice-covered area during the sixteenth century. Moderate sea ice extent occurred during thirteenth–fifteenth
centuries. Reconstructed sea ice extent variability is dominated by decadal oscillations, frequently associated with decadal
components of the North Atlantic Oscillation/Arctic Oscillation (NAO/AO), and multi-decadal lower frequency oscillations operating
at ~50–120 year. Sea ice extent and NAO showed a non-stationary relationship during the observational period. The present
low sea ice extent is unique over the last 800 years, and results from a decline started in late-nineteenth century after
the LIA. 相似文献
10.
Jari Holopainen Samuli Helama Juha M. Kajander Johanna Korhonen Jouko Launiainen Heikki Nevanlinna Anni Reissell Veli-Pekka Salonen 《Climatic change》2009,92(1-2):213-233
Spring temperatures were reconstructed by multiproxy database for south-west Finland since 1750. Proxy records used here were ice break-up in the Aurajoki River, the Baltic Sea ice extent, the plant phenological index and the annual varve thickness in the Pyhäjärvi Lake. Records were integrated into one palaeoclimate model using time-scale dependent calibration techniques. Reconstruction was verified with statistics showing a high degree of validation between the reconstructed and observed temperatures in Turku, south-west Finland. Reconstruction demonstrates that the springs have become warmer and reveals a warming trend since 1850s. Except for the period from 1750 to around 1850, the springs have been characterized as having a larger low-frequency variability, as well as by having a smaller range of annual temperature variations. Analyses of decadal variations revealed that the coldest springtimes occurred in the 1840s and 1850s and the first decade of the 19th century. Reconstruction was compared with the available meteorological series of central England, Stockholm, St. Petersburg, Uppsala and the spring-temperature reconstruction from western Norway. The effect of global solar, volcanic, greenhouse gases and aerosol forcings were examined together with the North Atlantic Oscillation (NAO) indices at local scale over the reconstructed period. Reconstructed spring-temperature changes have been related to changes in the atmospheric circulation, as indicated by the NAO (February–June). 相似文献