Swiss Virtual Institute for Solar Science
Irradiance
Solar Irradiance Variability and Earth's climate
One of the burning issues facing mankind today is that of global warming. While it is now widely understood that this problem is man-made, caused by the increasing greenhouse effect as we are modifying the global atmospheric chemistry by the burning of fossil fuels, it is urgent to improve on our capabilities to forecast how the climate will evolve in the future. When doing this we need to understand how the climate system responds to perturbations of different kinds. One such perturbation is the variable energy input from the Sun, which fluctuates due to the Sun's magnetic activity. In the visible spectral range the solar irradiance fluctuations are only in the promille range, but they affect the troposphere directly and dominate energetically. In comparison, the variations in the solar UV may reach 100% or more, but these variations contain much less energy and do not reach ground level. Still they cause large temperature variations in the stratospheric ozone layer, and this might indirectly affect the climate through coupling between stratosphere and troposphere. To clarify the various physical mechanisms a so-called Poly project of ETH Zurich with the title "Variability of the Sun and Global Climate" was set up, involving a close collaboration between the four institutes PMOD/WRC in Davos, EAWAG in Dübendorf (research group of Jürg Beer), and the Institute of Astronomy and the Institute for Atmospheric and Climate Science of ETH Zurich.
Figure: The PMOD composite Total Solar Irradiance (TSI) as daily values plotted in different colors for the different originating experiments. Moreover, the difference between the minimum values is also indicated.
Within the framework of this collaboration the Institute of Astronomy has focused on the physical origins of the solar irradiance variability and been able to show that this variability can be fully described in terms of changes in the distribution of surface magnetic fields on the Sun, i.e., it is possible to translate maps of solar magnetic fields into irradiance. A deep understanding of the internal structure of sunspots, in particular cool and strongly magnetized sunspot umbrae, is important to correctly predict variations of the solar irradiance and their effect on the evolution of the terrestrial climate. With the developed tools it becomes possible to reconstruct the solar irradiance in the more extended past from other indices of solar activity.
Research at PMOD/WRC has focused on the reconstruction of the solar irradiance variability in the deep UV part of the spectrum. This involves using extensive non-LTE radiative transfer codes to compute synthetic UV spectra that are representative of various types of active features on the Sun (sunspots, faculae, network), to model the observed solar UV spectrum and its fluctuations.
The level of solar activity in the past can be determined over time scales of thousands of years by using cosmogenic radionuclides like 10Be and 14C that are produced by galactic cosmic rays in the terrestrial atmosphere. The Sun's magnetic activity modulates the propagation conditions in the heliosphere (the solar wind region), such that the abundance of 14C in tree rings and 10Be in ice cores contain imprints from which the level of solar activity can be extracted. The group at EAWAG has been a world leader in using such proxies for deducing the longer term variability of the Sun, and this work is now coordinated as part of the ETH Poly project.