Relationship between impact-crater size and severity of related extinction episodes

How large must an extraterrestrial impact be to cause a peak episode of increased extinctions of life? Impact energies ≥ 3 × 107 Mt TNT (associated with terrestrial impact craters with final diameters ≥ 100 km) seem to be required to generate significant widespread climatic effects from sub-micron dust and soot in the atmosphere, leading to a distinct extinction episode (≥ 15% extinction of marine genera). Impacts creating craters smaller than...