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TheHolocene Climate Optimum (HCO) was awarm period in the first half of theHoloceneepoch, that occurred in the interval roughly 9,500 to 5,500 yearsBP,^[1] with a thermal maximum around 8000 years BP. It has also been known by many other names, such asAltithermal,Climatic Optimum,Holocene Megathermal,Holocene Optimum,Holocene Thermal Maximum,Holocene global thermal maximum,Hypsithermal, andMid-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.^[2][3][4]

• For other temperature fluctuations, seetemperature record.
• For other past climate fluctuation, seepaleoclimatology.
• For thepollen zone andBlytt–Sernander period, associated with the climate optimum, seeAtlantic (period).

The HCO was approximately 4.9 °C warmer than theLast Glacial Maximum.^[5] 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 theIndustrial Revolution, and 0.3 °C cooler than the average for 2011-2019.^[6] The2021 IPCC report expressed medium confidence that temperatures in the last decade are higher than they were in the Mid-Holocene Warm Period.^[7] Temperatures in theNorthern Hemisphere are simulated to be warmer than present average during the summers, but thetropics and parts of theSouthern Hemisphere were colder than average.^[8] 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 theGreat Barrier Reef about 5350 years ago was 1 °C warmer and enriched in¹⁸O by 0.5 per mil relative to modern seawater.^[9]

Temperatures during the HCO were higher than in the present by around 6 °C inSvalbard, near the North Pole.^[10]

Of 140 sites across the western Arctic, there is clear evidence for conditions that were warmer than now at 120 sites. At 16 sites for which quantitative estimates have been obtained, local temperatures were on average 1.6±0.8 °C higher during the optimum than now. Northwestern North America reached peak warmth first, from 11,000 to 9,000 years ago, but theLaurentide Ice Sheet still chilled eastern Canada. Northeastern North America experienced peak warming 4,000 years later. Along theArctic Coastal Plain in Alaska, there are indications of summer temperatures 2–3 °C warmer than now.^[11] Research indicates that the Arctic had less sea ice than now.^[12] TheGreenland Ice Sheet thinned, particularly at its margins.^[13] In addition to being warmer, Arctic Alaska also became wetter.^[14]

Northwestern Europe experienced warming, but there was cooling inSouthern Europe.^[15] In the southwesternIberian Peninsula, forest cover reached its peak between 9,760 and 7,360 years BP as a result of high moisture availability and warm temperatures during the HCO.^[16] InCentral Europe, the HCO was when human impact on the environment first became clearly detectable in sedimentological records,^[17] with the portion of the HCO from 9,000 to 7,500 BP being associated with minimal human impact and environmental stability, the portion from 7,500 to 6,300 BP with human impact only observed in pollen records, and the portion after 6,300 BP with substantial human influence on the environment.^[18]

In theMiddle East, the HCO was associated with frost-free winters and abundantPistaciasavannas. It was during this interval that the domestication ofcereals and Neolithic population growth occurred in the region.^[19]

The onset of the HCO in the southernUral Mountains was simultaneous with that inNorthern Europe, while its termination occurred between 6,300 and 5,100 BP.^[20] Winter warming of 3 to 9 °C and summer warming of 2 to 6 °C occurred in northern centralSiberia.^[21]

The HCO was highly asynchronous in Central and East Asia,^[22] though it at least occurred contemporaneously in the Loess Plateau, the Inner Mongolian Plateau, and Xinjiang.^[23] As a result of rising sea levels and decay ofice sheets in the Northern Hemisphere, the East Asian Summer Monsoon (EASM) rain belt expanded to the northwest, penetrating deep into the Asian interior.^[24] The EASM, being significantly weaker before and after the HCO, peaked in strength during this interval,^[25] though the exact timing of its maximum intensity varied by region;^[26] intensified westerlies occasionally caused dry spells in China during the HCO.^[27] Current desert regions ofCentral Asia were extensively forested because of higher rainfall, and the warm temperate forest belts in China and Japan were extended northwards.^[28] In theYarlung Tsangpo valley of southern Tibet, precipitation was up to twice as high as it is today during the middle Holocene.^[29] In the Huai River basin, the HCO began 9,100 to 8,000 BP.^[30] Pollen records fromLake Tai inJiangsu,China shed light on increased summer precipitation and a warmer and wetter overall climate in the region.^[31] The stability of the Middle Holocene climate in China fostered the development of agriculture and animal husbandry in the region.^[32] In the Korean Peninsula, arboreal pollen records the HCO as occurring from 8,900 to 4,400 BP, with its core period being 7,600 to 4,800 BP.^[33] Sea levels in theSea of Japan were 2-6 metres higher than in the present, withsea surface temperatures being 1-2 °C higher. TheEast Korea Warm Current reached as far asPrimorye and pushed cold water off of the cooler Primorsky Current to the northeast. TheTsushima Current warmed the northern shores ofHokkaido penetrated into theSea of Okhotsk.^[34] In the northernSouth China Sea, the HCO was associated with colder winters due to a stronger East Asian Winter Monsoon (EAWM), causing frequent coral die-offs.^[35]

In theIndian Subcontinent, the Indian Summer Monsoon (ISM) heavily intensified, creating a hot and wet climate in India along with high sea levels.^[36]

Relative sea level in theSpermonde Archipelago was approximately 0.5 metres higher than it is today.^[37][38] Sedimentary infill of lagoons was retarded by the sea level highstand and accelerated after the HCO, when sea levels dropped.^[39]

West African sediments additionally record theAfrican humid period, an interval between 16,000 and 6,000 years ago during whichAfrica was much wetter than now. That was caused by a strengthening of theAfrican monsoon by changes in summer radiation, which resulted from long-term variations in theEarth's orbit around theSun. The "Green Sahara" was dotted with numerouslakes, containing typical African lakecrocodile andhippopotamus fauna. A curious discovery from the marine sediments is that the transitions into and out of the wet period occurred within decades, not the previously-thought extended periods.^[40] It is hypothesized that humans played a role in altering the vegetation structure of North Africa at some point after 8,000 years ago by introducing domesticated animals, which contributed to the rapid transition to the arid conditions that are now found in many locations in theSahara.^[41] Further south, inCentral Africa, thesavannas that make up the coastal lowlands of theCongo River drainage basin in the present were entirely absent.^[42] Southwestern Africa experienced increased humidity during the HCO.^[43]

NorthwesternPatagonia, in a region known as theArid Diagonal, was significantly drier during theEarly andMiddle Holocene, with the region becoming more humid during the Late Holocene following the end of the HCO.^[44]

In the far Southern Hemisphere (New Zealand and Antarctica), the warmest period during theHolocene appears to have been roughly 10,500 to 8,000 years ago, immediately after the end of thelast ice age.^[45][46] TheAmery Ice Shelf retreated approximately 80 kilometres landward during this warm interval.^[47] By 6,000 years ago, which is normally associated with the Holocene Climatic Optimum in the Northern Hemisphere, those regions had reached temperatures similar to today, and they did not participate in the temperature changes of the north. However, some authors have used the term "Holocene Climatic Optimum" to describe the earlier southern warm period as well; typically, the term "Early Holocene Climatic Optimum" is used for the Southern Hemisphere warm interval.^[48][49]

In New Zealand, the HCO was associated with a 2 °C temperature gradient across the subtropical front (STF), a sharp contrast with the 6 °C observed today. Westerly winds in New Zealand were reduced.^[50]

A comparison of the delta profiles atByrd Station, West Antarctica (2164 m ice core recovered, 1968), andCamp Century, Northwest Greenland, shows the post-glacial climatic optimum.^[51] Points of correlation indicate that in both locations, the HCO (post-glacial climatic optimum) probably occurred at the same time. A similar comparison is evident between the Dye 3 1979 and the Camp Century 1963 cores regarding this period.^[51]

TheHans Tausen Ice Cap, inPeary Land (northernGreenland), was drilled in 1977, with a new deep drill to 325 m. The ice core contained distinct melt layers all the way to the bedrock. That indicates that Hans Tausen Iskappe contains no ice from the last glaciation and so the world's northernmost ice cap melted away during the post-glacial climatic optimum and was rebuilt when the climate cooled some 4000 years ago.^[51]

From the delta-profile, theRenland ice cap in theScoresby Sound has always been separated from the inland ice, but all of the delta-leaps revealed in the Camp Century 1963 core recurred in the Renland 1985 ice core.^[51] The Renland ice core from East Greenland apparently covers a full glacial cycle from the Holocene into the previousEemian interglacial. The Renland ice core is 325 m long.^[52]

Although the depths are different, the GRIP and NGRIP cores also contain the climatic optimum at very similar times.^[51]

The 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 lastglacial period.^{[citation needed]}

The effect would have had the maximum heating of the Northern Hemisphere 9,000 years ago, when the axial tilt was 24° and the nearest approach to the Sun (perihelion) was during the Northern Hemisphere's summer. The calculatedMilankovitch Forcing would have provided 0.2% moresolar radiation (+40 W/m²) to the Northern Hemisphere in summer, which tended to cause more heating. There seems to have been the predicted southward shift in the global band of thunderstorms, theIntertropical Convergence Zone.^{[citation needed]}

However,orbital forcing would predict maximum climate response several thousand years earlier than those observed in the Northern Hemisphere. The delay may be a result of the continuing changes in climate, as the Earth emerged from the last glacial period and related toice–albedo feedback. Different sites often show climate changes at somewhat different times and lasting for different durations. At some locations, climate changes may have begun as early as 11,000 years ago or have persisted until 4,000 years ago. As noted above, the warmest interval in the far south significantly preceded warming in the north.^{[citation needed]}

Significant temperature changes do not appear to have occurred at most low-latitude sites, but other climate changes have been reported, such as significantly wetter conditions in Africa, Australia and Japanand desert-like conditions in theMidwestern United States. Areas around theAmazon show temperature increases and drier conditions.^[53]

• 8.2-kiloyear event – Rapid global cooling about 8,200 years ago
• Hockey stick graph (global temperature) – Graph in climate science
• Little Ice Age – Climatic cooling after the Medieval Warm Period (16th–19th centuries)
• Medieval Warm Period – Period of warm climate in North Atlantic region lasting from about 950 CE to about 1250
• Next glacial maximum – Series of alternating glacial and interglacial periodsPages displaying short descriptions of redirect targets
• Timeline of environmental history
• Younger Dryas – Time period c. 12,900–11,700 years ago with Northern Hemisphere glacial cooling and SH warming

• History of climate variability and change
• Holocene

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