text: Past hydrological changes on our planet have had enormous effects on the environment. Over most of geologic time, the long-term mean sea level has been higher than today. Only at the Permian-Triassic boundary ~250 million years ago was the long-term mean sea level lower than today. Long term changes in the mean sea level are the result of changes in the oceanic crust, with a downward trend expected to continue in the very long term. Twenty thousand years ago, the sea level was  lower than at present with vast amounts of water being locked up as ice; in the northern hemisphere, there was twenty times more ice than the present volume.
  The climate of the Sahara, for example, has undergone enormous variations between wet and dry over the last few hundred thousand years, believed to be caused by long-term changes in the North African climate cycle that alters the path of the North African Monsoon. Paleohydrological studies of sediments in the Fazzan Basin in Libya show that humid conditions once prevailed there that were capable of creating a lake with a surface area of around . Before the abrupt desertification of North Africa about five thousand years ago, the Sahara was the home of Neolithic men and supported verdant vegetation and diverse wildlife.