The Holocene Climate Optimum was a warm period during roughly the interval 7,000 to 5,000 years B.P.. This event has also been known by many other names, including: Hypisthermal, Altithermal, Climatic Optimum, Holocene Optimum, Holocene Thermal Maximum, and Holocene Megathermal.

This warm period was followed by a gradual decline until about 2,000 years ago.

• For other temperature fluctuations see: Temperature record
• For other past climate fluctuation see: Paleoclimatology

Global effects

The Holocene Climate Optimum warm event consisted of increases of up to 4 °C near the North Pole (in one study, winter warming of 3-9°C and summer of 2-6°C in northern central Siberia). Northwestern Europe experienced warming, while there was cooling in the south.^{10.1098/rstb.2003.1434}