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.
Highlights
Debate continues over whether changes in the galactic cosmic ray (GCR) flux may induce climate change
Numerous experiments and modelling studies have examined the production and growth of cloud condensation nuclei by the atmospheric ionisation generated by GCR4–8
The geomagnetic field intensity drops to 10–20% of the present value, which increases the GCR by 90–70%11
Summary
These cooling events cannot be explained by conventional Milankovitch theory and seem to have occurred across a widespread area in low- and mid-latitudinal regions[14,16], such as southeastern Siberia[17], Italy[18,19,20] and Israel[21] Despite all these efforts, all that has been observed so far is ‘synchronicity’ between geomagnetic and temperature changes, leaving more detailed linking mechanisms such as the GCR-induced cloud formation and the subsequent reduction of insolation (umbrella effect) still hypothetical. Using pollen analysis of a sediment core from Japan, we demonstrate differential cooling in seasonal temperatures, as well as weakening of the East Asian summer monsoon These observations in combination support the idea that the differences in response to a reduction of insolation caused by cloud formation was a central factor in the mechanism that connects GCR and climate changes
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