Global warming.
Has anyone here ever wondered what lies underneath a glacier?
Since 1974, Thompson has made the trek to the Quelccaya ice cap at least 27 times, drilling cores through to bedrock, taking samples and periodically monitoring its slow but accelerating retreat. Ancient plant beds have been newly uncovered as the ice retreats. The first were discovered in 2002, more are uncovered each year, and carbon dating indicates that most have been buried for at least 5,000 years. They indicate that the current retreat of the ice exceeds any other retreat in at least the last 50 centuries.
So what caused the warm period a mere 5,000 years ago? Where now there's ice, not that long ago was lush fertile land. These plants were growing after the first pyramids were built.