In the 1920s and 1930s, Milutin Milankovitch worked out an account of the cause of Earth's glaciations. A. Milankovitch argued that glaciations would result when three cycles that affect characteristics of Earth's orbit combined in a certain way. B. Milankovitch's predictions have been shown to be in agreement with periods of glaciation over the past 65 million years. C. Since ice ages are much less frequent than Milankovitch's explanation predicts, some factor, such as low levels of atmospheric carbon dioxide, must also be involved in triggering glaciation. D. Milankovitch showed that changes in Earth's orbit periodically length cold seasons and shorten warm ones so that more ice forms than melts and glaciers build up. E. Dust from volcanic eruptions that blocks the Sun's warming radiation has been proposed as an explanation alternative to Milankovitch cycles for the cause of ice ages. F. Studies of gas bubbles preserved in the Greenland ice cap strongly suggest that Milankovitch cycles may also influence atmospheric carbon dioxide levels.