Using proxy data from various natural sources (including deuterium content in ice cores, calcium content and thickness of sea shells and tree ring growth data) we can establish that these cycles do exist, and that they occur for several reasons, perhaps the most prominent of which is the Milkanovitch cycle. | Using proxy data from various natural sources (including deuterium content in ice cores, calcium content and thickness of sea shells and tree ring growth data) we can establish that these cycles do exist, and that they occur for several reasons, perhaps the most prominent of which is the Milkanovitch cycle. |