Past Solar Activity Variations and Sun-Climate links
Satellite measurements over the last decades indicate that the solar irradiance varies only by about 1 ‰ over a typical 11-year cycle – an amount that is considered to be too small to influence the global mean temperature by more than about ±0.1 – 0.2 ⁰C. On the other hand, there is growing evidence that, while the global mean temperature might change only to a small degree, the atmospheric circulation (i.e. regional climate) may be more sensitive to changes in solar forcing. Reconstructions of past climate can aid a better understanding of the links between solar forcing and climate variations. They allow investigations of periods that were not yet affected by humans as well as the impact of longer-term solar activity variations, as for example the so-called Grand Solar Minima. A precondition for these studies is knowledge about solar activity in the past – i.e. before the satellite era.
For approximately the last 400 years sunspot observations enable an assessment of past solar activity changes. For earlier periods so-called cosmogenic radionuclides such as 10Be and 14C are the most reliable proxies for solar activity. A simple representation of their production and geochemical pathways is given in figure 1. The production rates of cosmogenic radionuclides depend on the flux of galactic cosmic rays (charged particle radiation from space) into the atmosphere where they trigger a cascade of nuclear reactions occasionally resulting in the production of 10Be and 14C. The flux of galactic cosmic rays in turn is modulated by the variable strength of the Sun’s and Earth’s magnetic fields. Hence, records of past atmospheric 14C and 10Be concentrations in e.g. tree rings and ice cores, respectively, contain information about past solar activity levels.
However, paleorecords of atmospheric 14C and 10Be concentrations can also be affected by changes in the local or global carbon cycle (14C) and atmospheric mixing and deposition (10Be) – processes that can ‘contaminate’ the solar activity information in these records. In this project we aim at extending and improving past solar activity reconstructions back into the last ice age. We employ 10Be and 14C records from various archives, in an effort to extract robust information of past solar activity variations. We subsequently use these reconstructions in tandem with climate model experiments to test hypotheses of past solar influences on climate under different climate regimes and in various environmental settings. The derived conclusions will aid the understanding of natural climate variability and its forcing mechanisms.