2026ApJ...996...43B

Space weather could have a profound impact on Earth's climate. Compared to the present-day Sun, the young Sun would have been more magnetically active and should have experienced more frequent extreme space weather events, in the form of coronal mass ejections and solar energetic particles, which steadily bombarded Earth's upper atmosphere. These particles enhanced atmospheric chemistry, potentially resulting in large amounts of kinetically produced greenhouse gases, such as CO, H₂, N₂O, and HCN. In this work, to explore whether the Sun-Earth interaction could address the faint young Sun paradox (FYSP), we use a chain of three models: (i) an Earth-like planet surface temperature model, (ii) a radiative-convective model (EOS), and (iii) a thermochemical and photochemical kinetic model. We also apply this modeling pipeline to the present-day Earth atmosphere to assess the potential impact of a prolonged period of intense solar activity. Our main results can be summarized as follows. First, we find that, for an Archean Earth-like atmosphere of 90% N₂, 10% CO_2, and trace amounts of either CH₄ or H₂, the surface temperature increase due to space weather-produced greenhouse species is no larger than 0.3 K, which makes this solution to the FYSP unviable. Second, we find that the contribution of nitrogen species (N₂O and HCN) to this temperature increase is negligible and that the main greenhouse contributors are instead CO and H₂. Third, on present-day conditions, the cumulative effects of a prolonged period of intense solar activity would decrease the surface temperature by ∼3 K.