Variations of galactic cosmic rays and the Earth's climate

Abstract

The galactic environment of Earth appears to have varied only subtly over mankind's time scale. Have these variations had any perceptible effect on Earth's climate? Surprisingly, they may; palaeoclimatic evidence suggests that the climate may be influenced by solar/cosmic ray forcing on all time scales from decades to billions of years. However, despite an intense scientific interest spanning two centuries, the mechanism underlying solar-climate variability has remained a mystery. Recent satellite data, however, have provided an intriguing new clue: low cloud cover may be influenced by galactic cosmic rays. Since cosmic rays are modulated by the solar wind, this would link Earth's climate to solar magnetic activity, which is known to be highly variable. On longer time scales, it would also imply that the climate responds to changes in the geomagnetic field and the galactic environment of Earth, both of which affect the cosmic ray flux. The least understood aspect of this sequence of processes is the microphysical mechanism by which cosmic rays may affect clouds. Physical mechanisms have been proposed and modelled, but definitive experiments are lacking. However, the presence of ioninduced nucleation of new particles in the atmosphere is supported by recent observations, and new experiments are underway to investigate the nature and significance of cosmic ray-cloud-climate interactions. © 2006 Springer. All Rights Reserved.