Impact of Galactic Cosmic Rays on Earth's Climate During an Encounter With the Local Lynx of Cold Clouds 2–3 Myr Ago

Abstract As the protective cocoon of solar wind enveloping the solar system, the heliosphere shields Earth from low‐energy Galactic Cosmic Rays (GCRs) in the interstellar medium. Recent work that mapped the trajectory of the Sun found that the solar system may have passed through a massive interstellar cold cloud–the Local Lynx of Cold Clouds–with density 3,000 two to three million years ago. Such crossing would have compressed the heliosphere and directly exposed Earth to the interstellar flux of GCRs, which will react with and molecules to produce ion pairs in the atmosphere. Using a 2D atmospheric chemistry and dynamics model, we simulate how the increased low‐energy GCR flux affects Earth's climate. We study two cases: the removal of heliospheric shielding, and the concurrent effect of exposure to interstellar GCRs and a geomagnetic pole reversal, such as the Gauss‐Matuyama pole reversal which occurred 2.58 Mya. Without heliospheric shielding, we find a 5%–15% increase in polar stratospheric when compared to pre‐industrial solar minimum conditions. This produces a 1%–2% decrease in polar stratospheric ozone which decreases stratospheric temperatures by 0.1–0.2 K. In the case that a geomagnetic pole reversal would occur during the cold cloud crossing, we see almost no shielding of cosmic rays at the equator and mid‐latitudes; this produces over 50% enhancement, 14% polar stratospheric ozone depletion, and up to 1 K decrease in stratospheric temperature. Changes in stratospheric temperatures could have localized effects on surface temperatures, and ozone depletion could increase UV exposure on Earth's surface.