Changes in Galactic Cosmic Ray (GCR) flux have been proposed by some as the main factor controlling Earth's climate. This hypothesis, which invokes enhanced formation of low clouds due to ionization of atmospheric aerosol by GCR flux as a control mechanism, implies that climate sensitivity to atmospheric CO 2 levels is overestimated. Here we propose to test this conjecture by comparing a deep-time 3.6 million year–long (~30–26.4 million years ago) record of global climate changes with a proxy record of geomagnetic paleointensity fluctuations. At the time scale adopted for this study, the geomagnetic field intensity is the major controller of GCR reaching the Earth. We compare the records of paleointensity, as a proxy for GCR flux fluctuations, and a record of global climate showing that they are substantially independent. We conclude that, the putative role of GCR flux as a cause for medium to long term (103–5 yr) changes in Earth's climate is not supported by evidence.