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1] About 6,000 years ago, temperatures on the surface of Earth about 3 degrees warmer than currently. The Arctic Ocean is ice-free, and mountain glaciers have disappeared from the mountains of Norway and the Alps in Europe, and from the Rocky Mountains of the United States and Canada. The ocean of the world is some three meters higher than currently. A lot of the present desert of the Sahara has a more humid, savannah-like climate, with giraffes and savannah fauna species.
2] 4,000 years ago to AD 900: Global cooling begins. The Arctic Ocean freezes over, mountain glaciers form once more in the Rocky Mountains, in Norway and in the Alps. The Black Sea freezes over several times, and ice forms on the Nile in Egypt. Northern Europe gets a lot wetter, and the marshes develop again in previously dry areas. The sea level drops to approximately its present level. The temperatures on the surface of the Earth are about 0.5-1 degree cooler than at present.
The causes of this period of cooling are unknown.