Galactic cosmic rays (GCR) previously affected the climate through cloud formation, that is according to research conducted by Dr. Ikuko Kitaba, an associate professor at the Research Centre for Palaeoclimatology, Ritsumeikan University, Professor Takeshi Nakagawa (head of the Research Centre for Palaeoclimatology) and their colleagues in an international research team.

In ancient times, the flux of GCR that fell to Earth increased repeatedly. There is a growing body of evidence that suggests that climatic cooling occurred during those times. Cloud formation by GCR has been claimed as the most likely cause. However, the mechanism was not fully understood due to the difficulty of accurately estimating the amount of cloud cover in the past.

Dr. Kitaba and her colleagues focused on the geomagnetic field and the climate in East Asia. The Earth’s magnetic field provides a shield against GCR. The East Asian climate reflects the temperature balance between the Eurasian landmass and the Pacific Ocean.

During the geomagnetic polarity reversals that took place at roughly about 780,000 years ago and 1,070,000 years ago, the geomagnetic field decreased to about 10% of its present level and intensity of GCR almost doubled as a consequence. Temperature and rainfall variations were reconstructed using sediment cores from Osaka Bay, Japan. The reconstructed records show more significant temperature drops on the Eurasian continent than over the Pacific, and a decrease of summer rainfall in East Asia. These observed climate changes can be accounted for if the landmasses were more strongly cooled than the oceans. The most likely mechanism behind such asymmetric cooling is the so-called ‘umbrella effect’ (increased cloud cover shading sunlight) that induces cooling of objects with smaller heat capacities. The weakening of geomagnetic field intensity can be seen as a simulation of reduced solar activity. Recently, a reduction of solar activity is raising concerns as a possible cause of future climate cooling. Our research offers an insight into the link between the sun and climate change, and invites a new perspective to the discussion of global warming which currently emphasizes only the effect of greenhouse gases.