title: Geological support for the Umbrella Effect as a link between geomagnetic field and climate.
abstract: The weakening of the geomagnetic field causes an increase in galactic cosmic ray (GCR) flux. Some researchers argue that enhanced GCR flux might lead to a climatic cooling by increasing low cloud formation, which enhances albedo (umbrella effect). Recent studies have reported geological evidence for a link between weakened geomagnetic field and climatic cooling. However, more work is needed on the mechanism of this link, including whether the umbrella effect is playing a central role. In this research, we present new geological evidence that GCR flux change had a greater impact on continental climate than on oceanic climate. According to pollen data from Osaka Bay, Japan, the decrease in temperature of the Siberian air mass was greater than that of the Pacific air mass during geomagnetic reversals in marine isotope stages (MIS) 19 and 31. Consequently, the summer land-ocean temperature gradient was smaller, and the summer monsoon was weaker. Greater terrestrial cooling indicates that a reduction of insolation is playing a key role in the link between the weakening of the geomagnetic field and climatic cooling. The most likely candidate for the mechanism seems to be the increased albedo of the umbrella effect.
authors.affiliation: Research Centre for Palaeoclimatology, Ritsumeikan University, Kusatsu 525-8577, Japan.|Research Center for Inland Seas, Kobe University, Kobe 657-8501, Japan.
authors.forename: Ikuko
authors.initials: I
authors.lastname: Kitaba
authors.affiliation: Research Center for Inland Seas, Kobe University, Kobe 657-8501, Japan.
authors.forename: Masayuki
authors.initials: M
authors.lastname: Hyodo
authors.affiliation: Research Centre for Palaeoclimatology, Ritsumeikan University, Kusatsu 525-8577, Japan.
authors.forename: Takeshi
authors.initials: T
authors.lastname: Nakagawa
authors.affiliation: Museum of Nature and Human Activities, Hyogo, Sanda 669-1546, Japan.
authors.forename: Shigehiro
authors.initials: S
authors.lastname: Katoh
authors.affiliation: Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA.
authors.forename: David L
authors.initials: DL
authors.lastname: Dettman
authors.affiliation: Institute of Natural and Environmental Sciences, University of Hyogo, Sanda 669-1546, Japan.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Sato