Ice age usually refers to the Pleistocene epoch, the most recent occurrence of continental glaciation. Beginning several million years ago in Antarctica, it is marked by at least four major advances and retreats (excluding Antarctica). Ice ages occur during times when more snow falls during the winter than is lost by melting, evaporation, and loss of ice chunks in water during the summer. Alternating glacial and interglacial stages are best explained by a combination of Earth's orbital cycles and changes in carbon dioxide levels. These cycles operate on time scales of tens of millennia. By contrast, global warming projections involve decades, a far more imminent concern for humankind.