How might climate be influenced by cosmic rays?
In 1913, Victor Hess measured the background level of atmospheric ionization while ascending with a balloon. By doing so, he discovered that Earth is continuously bathed in ionizing radiation. These cosmic rays primarily consist of protons and heavier nuclei with energies between their rest mass and a trillion times larger. In 1934, Walter Baade and Fritz Zwicky suggested that cosmic rays originate from supernovae, the explosive death of massive stars. However, only in 2013 was it directly proved, using gamma-ray observations with the FERMI satellite, that cosmic rays are indeed accelerated by supernova remnants. Thus, the amount of ionization in the lower atmosphere is almost entirely governed by supernova explosions that took place in the solar system’s galactic neighborhood in the past twenty million years or so.
Besides being messengers from ancient explosions, cosmic rays are extremely interesting because they link together so many different phenomena. They tell us about the galactic geography, about the history of meteorites or of solar activity, they can potentially tell us about the existence of dark matter, and apparently they can even affect climate here on Earth. They can explain many of the past climate variations, which in turn can be used to study the Milky Way.
The idea that cosmic rays may affect climate through modulation of the cosmic ray ionization in the atmosphere goes back to Edward Ney in 1959. It was known that solar wind modulates the flux of cosmic rays reaching Earth—a high solar activity deflects more of the cosmic rays reaching the inner solar system, and with it reduces the atmospheric ionization. Ney raised the idea that this ionization could have some climatic effect. This would immediately link solar activity with climate variations, and explain things like the little ice age during the Maunder minimum, when sunspots were a rare occurrence on the solar surface.