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The Holocene Climate Optimum was a warm period during roughly the interval 9,000 to 5,000 years B.P.. This event has also been known by many other names, including: Hypsithermal, 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
• For the pollen zone and Blytt-Sernander period associated with the climate optimum, see: Atlantic (period)

Temperature variations during the Holocene from a collection of different reconstructions and their average. Most recent period is on left.

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 to 9 °C and summer of 2 to 6 °C in northern central Siberia)^[1]. Northwestern Europe experienced warming, while there was cooling in the south.^[2] The average temperature change appears to have declined rapidly with latitude so that essentially no change in mean temperature is reported at low and mid latitudes. Tropical reefs tend to show temperature increases of less than 1 °C. In terms of the global average, temperatures were probably less than 0.5 to 2 °C warmer than the mid-20th century (depending on estimates of latitude dependence and seasonality in response patterns).

At 140 sites across the western Arctic, there is clear evidence for warmer-than-present conditions at 120 sites. At 16 sites where quantitative estimates have been obtained, local HTM temperatures were on average 1.6±0.8 °C higher than present. Northwestern North America had peak warmth first, from 11,000 to 9,000 years ago, while the Laurentide ice sheet still chilled the continent. Northeastern North America experienced peak warming 4,000 years later.^[3]

West African sediments additionally record the "African Humid Period", an interval between 16,000 and 6,000 years ago when Africa was much wetter due to a strengthening of the African monsoon by changes in summer radiation resulting from long-term variations in the Earth's orbit around the sun. During this period, the Saharan desert was dotted with numerous lakes containing typical African lake crocodile and hippopotamus fauna. A curious discovery from the marine sediments is that the transitions into and out of this wet period occurred within decades, not millennia as previously thought.^[4]

In the far southern hemisphere (e.g. New Zealand and Antarctica), the warmest period during the Holocene appears to have been roughly 8,000 to 10,500 years ago, immediately following the end of last ice age^[5][6]. By 6,000 years ago, the time normally associated with the Holocene Climatic Optimum in the Northern Hemisphere, these regions had reached temperatures similar to those existing in the modern era, and did not participate in the temperature changes of the North. However, some authors have used the term "Holocene Climatic Optimum" to describe this earlier southern warm period as well.

Main article: Milankovitch cycles

This climatic event was probably a result of predictable changes in the Earth's orbit (Milankovitch cycles) and a continuation of changes that caused the end of the last glacial period.

The effect would have had maximum Northern Hemisphere heating 9,000 years ago when axial tilt was 24° and nearest approach to the Sun (perihelion) was during boreal summer. The calculated Milankovitch forcing would have provided 8% more solar radiation (+40W/m²) to the Northern Hemisphere in summer, tending to cause greater heating at that time. There does seem to have been the predicted southward shift in the global band of thunderstorms called the Intertropical convergence zone.

However, orbital forcing would predict maximum climate response several 1,000 years earlier than those observed in the Northern Hemisphere. This delay may be a result of the continuing changes in climate as the Earth emerged from the last glacial period and related to ice feedbacks. It should also be noted that different sites often show climate changes at somewhat different times and lasting for different durations. At some locations, climate changes associated with this event may have begun as early as 11,000 years ago, or presisted until 4,000 years before present. As noted above, the warmest interval in the far south significantly preceded warming in the North.

While there do not appear to have been significant temperature changes at most low latitude sites, other climate changes have been reported. These include significantly wetter conditions in Africa, Australia and Japan, and desert-like conditions in the midwestern United States. Areas around the Amazon in South America show temperature increases and drier conditions.^[7]