Evidence for solar forcing: Some selected aspects

It is believed that the global warming since the mid-20th century is primarily the result of the combustion of fossil fuel. The fact that the climate also changed in the past during periods of rather constant atmospheric greenhouse gas concentrations points to additional factors such as solar and volcanic forcing. The Sun is by far the most important source of energy for Earth and direct satellite based observations during the past 30 years show that the solar constant (total solar irradiance TSI) changes in phase with the solar magnetic activity. The past 30 years are characterized by a high, rather constant mean level of activity, however, during the last 2 years the minima in TSI, IMF (interplanetary magnetic field), NM (neutron monitor count rate), and (solar modulation function) have clearly deviated from the earlier minima, suggesting that TSI is now decreasing in response to a lower level of solar magnetic activity. Unfortunately our knowledge of past solar activity is very limited, the longest record available being the sunspot record going back to 1610. The record can be extended from centuries to millennia by using the cosmogenic radionuclides which are primarily produced by the galactic cosmic rays. Their intensity is modulated by the open solar magnetic and the geomagnetic field. Removing the geomagnetic effects results in the solar modulation function which can be reconstructed for the past 10,000 years, as can the strength of the interplanetary magnetic field. The comparison of with selected climate records provides strong evidence that solar forcing was important in the past and will possibly play a role in the future. Confirmation of the synchronous declines in TSI and IMF will allow the reconstructed IMF to be used to estimate TSI for the past 10,000 years.