M. G. Ogurtsov

The height-increment record of summer temperature extended over the last millennium in Fennoscandia

New data have allowed us to extend a previous height-increment chronology of Scots pine (Pinus sylvestris L.) at the northern Fennoscandian timberline 817 years backwards in time, from 1561 to 745. Our final transfer model accounts for 31% of the dependent instrumental (mean June—August) temperature variance between 1908 and 2007. According to the 1263 yr long summer temperature proxy, the most severe summers were experienced in 1601, 1790 and 782. Correspondingly, the summers of 1689, 885 and 1123 were the most favourable for growth. Two drastic shifts in temperature variability were also found. The twentieth century experienced a multidecadal change as the cold 1905—1914 period was immediately followed by a warm period from 1915 to 1944. An even more prominent shift occurred in the Middle Ages, as the most severe cold spell during 1135—1164 was preceded by the warmest period only a decade earlier, during 1115—1124. The Fourier spectrum of the reconstruction shows significant concentrations of variance around 33.3, 23.3 and 11 years, and between 2.6 and 3.0 years. The wavelet spectrum was able to date several centres of fluctuating periodicities between 745 and 2007. Furthermore, daily temperature records allowed us to define the major growth forcing climatic factor in more detail than in previous response analyses. The mean temperature during a 53 day season from 14 June to 6 August produced the strongest positive growth response (r 2 = 0.36).

On the link between northern fennoscandian climate and length of the quasi-eleven-year cycle in galactic cosmic-ray flux

Bidecadal fluctuations in terrestrial climate were analyzed. It was shown that this variability might arise if Earths climate reacts to galactic cosmic-ray intensity, integrated over its full quasi-11-year cycle. It was further shown that this integral effect should also lead to an effective link between climate and the duration of the quasi-11-year cycle in cosmic ray flux. That, in turn, must result in appearance of some connection between climate and the length of the solar cycle, which is currently a topic of active debate. Analyses of temperature proxies, obtained for northern Fennoscandia, confirmed the connection of the climate in this region and the length of the cycle in galactic cosmic-ray intensity. Decadal and bidecadal variability of integrated cosmic-ray flux was quantitatively estimated.

A summer temperature proxy from height increment of Scots pine since 1561 at the northern timberline in Fennoscandia

Height increments of 60 Scots pine trees were used to reconstruct mean June—August temperature variability at interannual to decadal scales from 1561 to 2004. Three standardization methods (67%, 33% flexible splines, and a fixed 22 years spline) were compared in building chronologies in order to optimize the frequency response in relation to major climatic forcing factors. The height-growth chronology built using the 33% spline standardization proved to have the most consistent and time-stable relationship with the summer temperatures. Among the monthly precipitation and temperature variables from previous June to current August, previous July shows the highest correlation with height growth. In addition, both previous June and previous August have significant positive correlations. Our final transfer model accounts for 32.5% of the dependent instrumental temperature variance between 1909 and 2004. The Fourier spectra of the height-growth chronology and mean summer temperature are very similar in appearance, both series having peaks at 2.7—3.2 years, 6.7 years and 15.7 years. Thus, the 444 years long summer temperature reconstruction is limited to high and medium frequencies. The coldest three summers in this record were experienced in years 1601, 1790 and 1903. Correspondingly, the summers of 1626, 1689 and 1598 were the warmest. The 1820s experienced the warmest 10-year mean, while the first decade of the twentieth century was the coldest. Among the 14 non-overlapping 30-year periods between 1561 and 1980, the period 1621—1650 was the warmest and the period 1591—1620 the coldest.

Solar activity and regional climate.

We performed a statistical analysis of the data on summer temperature anomalies in northern Fennoscandia (8-1995 AD) and found that a 70-130-yr cycle is present in this series during most of the time period. A comparison of the reconstructed northern Fennoscandia temperature with different indicators of solar activity (Wolf numbers, the length of solar Schwabe cycle, extended bi-decadal radiocarbon series, and data on sunspots observed by naked eye) shows that the more probable cause of the periodicity is the modulation of regional northern Fennoscandia climate by the long-term solar cycle of Gleissberg. The effect of this century-scale solar modulation of the global Northern Hemisphere temperature is weaker.

Secular variation in aerosol transparency of the atmosphere as the possible link between long-term variations in solar activity and climate

Information about variations in solar activity and climate on the time intervals from 130 years to four–five last centuries, including results of instrumental measurements (Wolf numbers, actinometry, thermometry) and indirect indicators (ice core acidity, NO3− ion concentration in polar ice, temperature tree-ring reconstructions), has been analyzed for the Northern Hemisphere and its high-latitude part. It has been obtained that the observed relation between secular variations in solar activity and near-Earth temperature resulted from the effect of the corresponding variation in aerosol transparency of the stratosphere on terrestrial climate. It has been also indicated that long-term variations in the aerosol content of the stratosphere can, in turn, be related to secular cycles in atmospheric ionization caused by variations in fluxes of ionizing cosmic particles.

Study of spatial and temporal structure of long-term effects of solar activity and cosmic ray variations on the lower atmosphere circulation

The spatial and temporal structure of the effects of solar activity (SA) and galactic cosmic ray (GCR) flux variations on the lower atmosphere circulation has been studied based on NCEP/NCAR reanalysis archive for 1948–2006 and MSLP (Climatic Research Unit, UK) data for 1873–2000. It has been shown that the GCR effects on pressure variations are characterized by a strong latitudinal and regional dependence, which is determined by specific features of the tropospheric circulation in the studied regions. The distribution of the correlation coefficients for mean yearly values of atmospheric pressure with the GCR flux intensity is closely related to the position of the main climatological fronts. The periodic (∼60 years) changes in the correlation sign of the pressure at high and middle latitudes with Wolf numbers have been revealed. It has been suggested that the changes of the sign of SA/GCR effects on atmospheric pressure are caused by the changes of the macrocirculation epochs, which, in turn, may be related to large-scale processes on the Sun.

CLIMATIC VARIABILITY ALONG A NORTH–SOUTH TRANSECT OF FINLAND OVER THE LAST 500 YEARS: SIGNATURE OF SOLAR INFLUENCE OR INTERNAL CLIMATE OSCILLATIONS?

Abstract. Statistical analysis of a multi‐centennial dendrochronological proxy dataset of regional climate, constructed across the latitudinal gradient of 1000 km, was performed. It was shown that centennial (c. 100 year), tri‐decadal (27‐32 year), bi‐decadal (17‐23 year) and decadal (9‐13 year) periodicities governed the climate variability in Finland over the last five centuries. Despite the fact that many of the climatic periodicities bore great resemblance to periodicities of solar cycles, little evidence of actual solar influence on Finnish climate was found when the climate proxy records were subjected to linear correlation analysis with sunspot numbers. Highly non‐linear response of Northern Fennoscandian climate to solar forcing might be a cause of this result, as well as influence of terrestrial climatic processes (e.g. effect of other forcing factors and internal dynamics of regional climate). Our results show that the presence of internal climate variability at time‐scales of solar activity might distort the solar signature in climatic data and complicate its detection.

Modern progress in solar paleoastrophysics and long-range solar-activity forecasts

Decade-averaged Wolf numbers are reconstructed for the time interval 8005 B.C.–1945 A.D. using radiocarbon data derived from tree rings. Comparisons of other paleoastrophysical reconstructions of solar activity with this temporal series verify its validity and reliability. A prediction of the mean solar activity for the next forty years is made using these reconstructed Wolf numbers. It is likely that the mean solar activity during 2005–2045 will be lower than the activity of recent decades. This conclusion is compared with the long-term predictions proposed by other researchers. The prospects for paleoastrophysical predictions for the long-term variations of solar activity in the future are discussed.

On the influence of climatic factors on the ratio between the cosmogenic isotope 14C and total carbon in the atmosphere in the past

Radiocarbon 14C is a cosmogenic isotope, which is most extensively used by scientists from a wide variety of fields. Its rate of generation in the atmosphere depends on solar modulation and thus, studying 14C concentration in natural archives, one can reconstruct solar activity level in the past. The paper shows results of box-model calculations of generation of the 14C isotope in the atmosphere and its relative abundance during the time interval 1389–1800 AD, taking into account influence of changing climate. This interval includes the deep minimum of solar activity and period of significant change in atmospheric concentration of CO2 and global temperature. The performed analysis showed that concentration of 14C in the atmosphere reflects not only variations of the galactic cosmic rays intensity but as well changes of temperature and atmospheric CO2 concentration. It is shown that the decrease in CO2 concentration in the atmosphere during 1550–1600 can be connected with absorption of CO2 by the ocean surface layer. Thus, taking into account the climatic changes is an important condition for the reconstruction of solar activity in the past using data based on cosmogenic isotopes.

Solar Cyclicity during the Maunder Minimum

A multifaceted statistical study of all available data on solar activity during the Maunder minimum (1645–1715) is presented. The data include European telescope observations, Asian sunspot observations using the unaided eye, concentrations of cosmogeneous isotopes, and catalogues of polar aurorae. Joint analyses of data on the cosmogeneous isotopes 10Be and 14C are a promising source of information on solar activity in the past. The dates of relative sunspot maxima during the Maunder minimum are consistent with the idea that there were chaotic bursts of solar activity randomly distributed in time during this interval. The available evidence that the 11-year cyclicity was preserved in 1645–1715 are worthy of attention but require additional deep study and verification. No convincing evidence for a 22-year periodicity of the occurrence of sunspots during the Maunder minimum has been found.