Holocene climatic optimum

The Holocene Climate Optimum (HCO) was a warm period that occurred in the interval roughly 9,500 to 5,500 years ago BP, with a thermal maximum around 8000 years BP. It has also been known by many other names, such as Altithermal, Climatic Optimum, Holocene Megathermal, Holocene Optimum, Holocene Thermal Maximum, Hypsithermal, and Mid-Holocene Warm Period. The warm period was followed by a gradual decline, of about 0.1 to 0.3 °C per millennium, until about two centuries ago. However, on a sub-millennial scale, there were regional warm periods superimposed on this decline. For other temperature fluctuations, see temperature record. For other past climate fluctuation, see paleoclimatology. For the pollen zone and Blytt–Sernander period, associated with the climate optimum, see Atlantic (period). A study in 2020 estimated that the average global temperature during the warmest 200 year period of the HCO, around 6,500 years ago, was around 0.7 °C warmer than the mean for nineteenth century AD, immediately before the Industrial Revolution, and 0.3 °C cooler than the average for 2011-2019. The 2021 IPCC report expressed medium confidence that temperatures in the last decade are higher than they were in the Mid-Holocene Warm Period. Temperatures in the Northern Hemisphere are simulated to be warmer than present average during the summers, but the tropics and parts of the Southern Hemisphere were colder than average. The HCO consisted of increases of up to 4 °C near the North Pole (in one study, winter warming of 3 to 9 °C and summer of 2 to 6 °C in northern central Siberia). Northwestern Europe experienced warming, but there was cooling in Southern Europe. The average temperature change appears to have declined rapidly with latitude and so essentially no change in mean temperature is reported at low and middle latitudes. Tropical reefs tend to show temperature increases of less than 1 °C. The tropical ocean surface at the Great Barrier Reef about 5350 years ago was 1 °C warmer and enriched in 18O by 0.5 per mil relative to modern seawater.