On long timescales the intensity of cosmic rays varies more emphatically because the influx from the Galaxy changes. During the past 500 million years, the Earth has passed through four ‘hothouse’ episodes, free of ice and with high sea levels, and four ‘icehouse’ episodes like the one we live in now, with ice-sheets, glaciers and relatively low sea levels.
Nir Shaviv of the Hebrew University in Jerusalem, together with Ján Veizer of the Ruhr University and the University of Ottawa, links these changes to the journey of the Sun and the Earth through the Milky Way Galaxy. They blame the icehouse episodes on encounters with bright spiral arms, where cosmic rays are most intense (diagram 3).
3. The red curve shows changes in tropical sea-surface temperatures over the past 500 million years. In the blue curve, drawn upside down to match, cosmic-rays intensities have varied according to our position relative to the Galaxy’s spiral arms. After Shaviv and Veizer
More frequent chilling events, every 34 million years or so, occur whenever the solar system passes through with the mid-plane of the Galaxy.
Comment:~ What does not appear to be adequately explained is the divergence between tropical sea-surface temperatures and cosmic ray intensities in the latest cycle.