In general, the following additional constraints can be posed on dynamo models aiming to describe the long-term (during the past 11,000 years) evolution of solar magnetic activity.
The sun ultimately defines the climate on Earth supplying it with energy via radiation received by the terrestrial system, but the role of solar variability in climate variations is far from being clear. Solar variability can affect the Earth’s environment and climate in different ways (see, e.g., a review by Haigh, 2007). Variability of total solar irradiance (TSI) measured during recent decades is known to be too small to explain observed climate variations (e.g., Foukal et al., 2006; Fröhlich, 2006). On the other hand, there are other ways solar variability may affect the climate, e.g., an unknown long-term trend in TSI (Solanki and Krivova, 2004; Wang et al., 2005) or a terrestrial amplifier of spectral irradiance variations (Shindell et al., 1999). Alternatively, an indirect mechanism also driven by solar activity, such as ionization of the atmosphere by CR (Usoskin and Kovaltsov, 2006) or the global terrestrial current system (Tinsley and Zhou, 2006) can modify atmospheric properties, in particular cloud cover (Ney, 1959; Svensmark, 1998). Even a small change in cloud cover modifies the transparency/absorption/reflectance of the atmosphere and affects the amount of absorbed solar radiation, even without changes in the solar irradiance. Since the CR flux at Earth is modulated not only by solar activity, but also by the slowly changing geomagnetic field, the two CR modulation mechanisms are independent and act on different timescales, thus giving one the opportunity to study the CR effect on Earth separately from solar irradiance (de Jager and Usoskin, 2006; Usoskin et al., 2005b).
Accordingly, improved knowledge of the solar driver’s variability may help in disentangling various effects in the very complicated system that is the terrestrial climate (e.g., de Jager, 2005; Versteegh, 2005). It is of particular importance to know the driving forces in the pre-industrial era, when all climate changes were natural. Knowledge of the natural variability can lead to an improved understanding of anthropogenic effects upon the Earth’s climate.
Studies of the long-term solar-terrestrial terrestrial are mostly phenomenological, lacking a clear quantitative physical mechanism. Even phenomenological and empirical studies suffer from large uncertainties, related to the quantitative interpretation of proxy data, temporal and spatial resolution (Versteegh, 2005). Therefore, more precise knowledge of past solar activity, especially since it is accompanied by continuous efforts of the paleo-climatic community on improving climatic data sets, is crucial for improved understanding of the natural (including solar) variability of the terrestrial environment.
The proxy method of solar-activity reconstruction, based on cosmogenic isotopes, was developed from the radiocarbon dating method, when it was recognized that the production rate of 14C is not constant and may vary in time due to solar variability and geomagnetic field changes. Neglect of these effects can lead to inaccurate radiocarbon (or more generally, cosmogenic nuclide) dating, which is a key for, e.g., archeology and Quaternary geology. Thus, knowledge of past solar activity and geomagnetic changes allows for the improvement of the quality of calibration curves, such as the IntCal (Stuiver et al., 1998; Reimer et al., 2004) for radiocarbon, eventually leading to more precise dating.
Long-term variations in the geomagnetic field are often evaluated using cosmogenic isotope data. Knowledge of source variability due to solar modulation is important for better results.
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