About five and a half millennia ago, North Africa underwent a dramatic transformation. The Sahara desert expanded and the grasslands, forests and lakes that people loved disappeared. People were forced to retreat to the mountains, oases, the Nile valley and the delta.

As a relatively large and dispersed population was squeezed into smaller and more fertile areas, it needed to innovate new ways of producing food and organizing society. Soon after, one of the first great civilizations in the world arose - ancient Egypt.

This transition from the last "African Wet Period," which lasted 15,000 to 5,500 years ago, to the current dry conditions in North Africa is the clearest example of a climate tipping point in recent geologic history. Climate tipping points are threshold values that, when exceeded, lead to a dramatic change in climate to a new stable climate. ( Martin H. Trauth , University of Potsdam , Asfawossen Asrat , Addis Ababa University , Mark Maslin , UCL, more at theconversation.com)