The Svensmark hypothesis posits that cosmic rays reaching the Earth's atmosphere create cloud condensation nuclei by knocking electrons out of gas molecules in the atmosphere.
- "According to the hypothesis, then, less cosmic radiation would mean less cloud formation and, ultimately, warmer temperatures – precisely what was observed during the 20th century."
- "The Svensmark hypothesis is that when the solar wind is weak, more cosmic rays penetrate to Earth. That creates more charged particles in the atmosphere, which in turn induces more clouds to form, cooling the climate."
- Coolings and warmings of around 2 deg. C have occurred repeatedly over the past 10,000 years, as the Sun's activity and the cosmic ray influx have varied.