The sunspot cycle is a cyclical rise and fall of the number of magnetic storms on the Sun and is associated with increased and decreased solar radiation received by the Earth. b. The nitrogen-isotope ratios of ancient glacial ice indicate the atmospheric temperature of the snowfall that created the ice. c. Bubbles of air trapped in old ice show the oxygen concentration of the ancient atmosphere, which is an indicator of its temperature. d. Icehouse periods occur when the Earth's atmosphere has significantly high concentrations of CO2 and water vapor.\