Cosmic rays and climate change over the past 1000 million years

TL;DR

This study challenges previous claims that cosmic ray intensity varies greatly over galactic regions and significantly influences Earth's climate, showing instead that such variations are likely much smaller, around 10-20%.

Contribution

The paper critically evaluates models of cosmic ray variation and their impact on climate, providing evidence that the intensity changes are less extreme than previously suggested.

Findings

GCR intensity variation is likely 10-20%, not 3:1 ratio.

Gamma ray data supports smaller GCR intensity variations.

GCR fluctuations are insufficient to explain mass extinctions.

Abstract

The Galactic cosmic ray (GCR) intensity has been postulated by others to vary cyclically with a peak to valley ratio of ~3:1, as the Solar System moves from the Spiral Arm to the Inter-Arm regions of the Galaxy. These intensities have been correlated with global temperatures and used to support the hypothesis of GCR induced climate change. In this paper we show that the model used to deduce such a large ratio of Arm to Interarm GCR intensity requires unlikely values of some of the GCR parameters, particularly the diffusion length in the interstellar medium, if as seems likely to be the case, the diffusion is homogeneous. Comparison is made with the existing gamma ray astronomy data and this also indicates that the ratio is not large. The variation in the intensity is probably of order 10-20% and should be no more than 30% as the Solar System moves between these two regions, unless the conventional parameters of the GCR are incorrect. In addition we show that the variation of the GCR intensity, as the trajectory of the Solar System oscillates about the Galactic Plane, is too small to account for the extinctions of species as has been postulated unless, again, conventional assumptions about the GCR parameters are not correct.