The Last Glacial Maximum (LGM) was the last period in the Earth's climate history during the last glacial period when ice sheets were at their greatest extension. Growth of the ice sheets reached their maximum positions 24,500 BCE. Deglaciation commenced in the Northern Hemisphere gradually between approximately 18,000 to 17,000 BCE, and in Antarctica approximately 12,500 BCE which is consistent with evidence that this was the primary source for an abrupt rise in the sea level 12,500 BCE. During the LGM, vast ice sheets covered much of North America, northern Europe and Asia. These ice sheets profoundly affected Earth's climate, causing drought, desertification and a dramatic drop in sea levels. It was followed by the Late Glacial Maximum.