The Solar-terrestrial climate link in the past millennia and its influence on future climate

The aims of the project

1. To reconstruct different parameters describing the solar activity over time scales of 1000 years and longer, using available proxies as direct observations, historic data, and records of production of cosmogenic isotopes and nitrate concentration in terrestrial archives and ice cores.
2. To reconstruct summer temperatures in northern Fennoscandia during the last 7000 years.
3. To reconstruct solar irradiance during the last 300-400 years.
4. To measure the concentration of stable isotope ¹³C in tree-rings from northern Finland during the last 500 years.
5. To develop new mathematical methods for statistical processing of long non-stationary time series corrupted by colored and non-additive noises and irregularly distributed gaps.
6. To perform statistical analyses on all the available data (both direct measurements and proxies, including the data obtained in framework of the project) in order to identify factors, which have influenced on the terrestrial climate in the past few thousands of years.
7. To investigate the solar-terrestrial climate link, its character, extent, and time evolution over time intervals from the last decades up to the last few millennia.
8. To investigate the mechanisms of solar-climate interrelation
9. To make a forecast for climatic changes for the future 1-2 centuries.

The influence of changes in the terrestrial climate (both global and regional) on almost all the aspects of social and economic activity of mankind is tremendous and doubtless. For example, the well-known problem of global warming has already transferred from the field of pure science into a global political sphere. Is the global warming a result only of the greenhouse effect or gives variations in solar activity also some contribution to this process? If solar contribution really exists, what is its extent? Can some other natural climatic phenomena lead to increase of the global temperature? The answer to these questions will provide us with information about the behavior of climate in the future and, hence, is of a great importance for the humanity. But it is evident that a precise and reliable forecast needs detailed information about the variability of climatic in the past and its causes. The knowledge in this domain is, However, still quite poor and has substantial gaps. Direct temperature records usually cover no more than the last 100-150 years. Series of measurements of different parameters of solar activity – the important source of climatic changes –are also short. The longest of them – the record of Wolf numbers – started around A.D. 1700. Other direct data concerning activity of the Sun and terrestrial and interplanetary phenomena are even shorter. Despite that the presence of a link between Sun’s activity and Earth’s climate has been claimed in many works, are the conclusions often based on relatively short time scales while the question about long term (centennial and multicentennial) solar modulation of climate is much less investigated.

Methods of dendrochronology, give us an opportunity to fill one of these gaps. The temperature reconstructions, based on ring width, obtained recently provide information about past climate up to 1000 years ago and more. Annual reconstruction of northern Fennoscandian July temperature, obtained at the University of Helsinki, is today one of the longest known series and covers the time interval since A.D. 50 (about 2000 years).

Stable isotope ¹³C concentration in tree-rings is another climate proxy. It reflects summer temperature, environmental conditions and precipitation regime. The concentration of ¹³C in trees from northern Finland is measured in the Dating laboratory at the University of Helsinki.

Information about the solar activity in the past can be obtained by two ways. First of them are data on concentration of cosmogenic isotopes (¹⁴C, ¹⁰Be) and nitrates (NO₃- ions) in terrestrial archives. Cosmogenic isotopes are generated in stratosphere and troposphere due to galactic cosmic rays (GCR), which are strongly modulated by Sun’s activity, included in a number of geophysical and geochemical processes and finally fixed in tree-rings (¹⁴C) and polar ices (¹⁰Be).

Nitrates are formed at high altitudes in the atmosphere, precipitate and are finally captured in polar ice. Because a large part of nitrate is generated due to energetic solar protons, the nitrate record reflects the flare activity of the Sun. Available cosmogenic isotope and nitrate records, usually cover the last few centuries but the longest of them – the decadal radiocarbon series obtained at University of Washington –starts from 12 620 B.P.

Historic data are another source of knowledge about the Sun’s activity in the past. The most complete catalogue of the ancient oriental sunspot observations made by naked eye, (B.C. 165 – A.D. 1684) is the longest historic proxy of solar activity. Works made in the Central Astronomical Observatory (Pulkovo) proved that the catalogue contains quite reliable and valuable information. The performed researches have shown, that new original statistical approaches makes it possible to extract from descriptive historical data such information as frequency and amplitude components of the long-term solar variability, period of solar rotation and probably north-southern asymmetry of activity over a two-millennial time scale. It should be noted that not only Sun but also a variety of terrestrial and extraterrestrial phenomena (volcanic activity, influence of Moon and large planets, biospheric changes, human activity) could affect climatic changes at different time scales. To separate the possible solar contribution from that of other factors is another problem that needs complex statistical (spectral, correlation, cluster) analyses of all the direct and indirect indicators of climate and all phenomena, which may cause its changes.

First results of statistical analysis, made for some of the data mentioned above, has given us evidence that the century-type time variation, present in many climatic series during 1-2 millennia, very likely was caused by acting of centennial cycle of solar activity (Gleissberg cycle). The analyses show that this solar-climate link was more evident in northern Fennoscandia. It allows us to hope that the next annual northern Fennoscandian temperature reconstruction, which is under preparation at the University of Helsinki and which will cover the last 7000 years, can provide valuable information not only about regional climatic variability in common but also about the history of the connection between Sun’s activity and the terrestrial climate during seven millennia.

Despite the fact that some progress already has been made, is the main work still to be executed. A lot of data obtained recently (temperature dendro-reconstructions, data on cosmogenic isotopes, volcanic activity and other phenomena carrying valuable information from the point of view of this project) still need thorough statistical analysis. Comparison of tree-ring temperature proxies with other climatic indicators is desirable for a more reliable reconstruction of climatic variability in the past. The series of measurement of ¹³C in tree-rings from northern Finland, which have been started in the Dating laboratory of the University of Helsinki and which will cover the last centuries, should help us to realize this purpose. Including ¹³C measurements of Baltic tree samples should help us to trace regional temperature differences and, therefore, to analyze the strength of local climatic effects over multicentennial time scale. A serious problem is connected with the fact that all data sets involved into analyses are very non-stationary and many of them contain a strong noise component. Moreover, noises, contained in climatic signals, often are “colored” and non-additive. Hence, the analysis of time structure of the available datasets and revealing possible interrelation between them is difficult. It should also be noted that many historical and paleo-data have irregularly distributed gaps, which complicates substantially the analysis. A solution of these difficulties demands considerable improvement of modern statistical methods (including wavelet analyses, chaotic dynamics and fractal geometry) and, probably, elaboration of new methods. Our knowledge about the solar activity in the past, especially about long-term changes of solar irradiance – likely an important factor affecting terrestrial climate - also should be enhanced. Analysis of ancient aurora borealis catalogues is anticipated to supply us with more detailed information about solar activity than decadal radiocarbon series and ancient naked eye observations. The measurements of the cosmogenic isotope ¹⁴C in tree-rings from northern Finland and Baltic will help us to restore the intensity of GCR during the last few centuries and to clear up a question about possible regional variability of concentration of radiocarbon in the atmosphere. Reliable reconstruction of solar irradiance needs further progress both in our knowledge about long-term changes of solar magnetic dynamo and about time evolution of the helio-latitudinal structure of solar activity. Investigation of the mechanisms of solar influence upon Earth’s weather and climate, which still is unknown, demands also progress in our knowledge about solar modulation of interplanetary magnetic field and galactic cosmic rays intensity at the Earth’s orbit, and about influence of solar and galactic cosmic rays on the upper atmosphere.

Obviously the solution of these problems can be attained only by combined efforts of specialists, working in dendroclimatology, paleoastrophysics, mathematical statistics, geophysics, magnetohydrodynamics, solar physics and cosmic plasma physics.

In spite of a lot of difficulties, the studies of the mechanisms and effects of solar activity on terrestrial climate, and their evolution during the last few millennia, are very important and valuable. The answers to these questions allow us to increase reliability and correctness of global and regional climatic prognosis for future centuries that is of tremendous importance for all the humanity.