About 15,000 BC, the Ice Age came to an end as the Earth's climate warmed up. Evidence from the Greenland ice cores suggests that average temperatures rose by as much as fifteen degrees Celsius in a short span of time. This warming seems to have coincided with rapid increases in human populations as the global warming led to expanding animal populations and much greater availability of wild plants and foods. This process was put into rapid reverse at about 14,000 BC, by a period of cooling known as the Younger Dryas, but after 9600 BC, global temperatures rose again, by seven degrees Celsius in less than a decade, and have since stayed high. Archaeologist Brian Fagan calls it the Long Summer.