The Holocene Climate Optimum (HCO) was a warm period during the first half of the Holocene epoch, occurring roughly between 9,500 and 5,500 years before the present. The warmest part of this period happened around 8,000 years before the present. This time has also been called by other names, such as Altithermal, Climatic Optimum, Holocene Megathermal, Holocene Optimum, Holocene Thermal Maximum, Holocene global thermal maximum, Hypsithermal, and Mid-Holocene Warm Period.
After the warm period, temperatures gradually decreased by about 0.1 to 0.3 degrees Celsius every thousand years until about 200 years ago. However, during shorter time periods, some regions experienced warm spells that occurred alongside this overall cooling trend.
Global effects
The Holocene Climatic Optimum (HCO) was approximately 4.9 °C warmer than the Last Glacial Maximum. 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 about 0.7 °C warmer than the average for the nineteenth century AD, just before the Industrial Revolution, and 0.3 °C cooler than the average for 2011–2019. The 2021 IPCC report stated that temperatures in the last decade are likely higher than they were during the Mid-Holocene Warm Period. Simulations show that temperatures in the Northern Hemisphere were warmer than present averages during summers, but the tropics and parts of the Southern Hemisphere were colder than average. The average temperature change decreased rapidly with latitude, and little to no change in mean temperature was reported at low and middle latitudes. Tropical reefs generally showed temperature increases of less than 1 °C. The tropical ocean surface near the Great Barrier Reef about 5,350 years ago was 1 °C warmer and had oxygen levels 0.5 per mil higher than modern seawater.
Temperatures during the HCO were about 6 °C higher than present temperatures in Svalbard, near the North Pole. Of 140 sites across the western Arctic, 120 showed evidence of warmer conditions than today. At 16 sites with quantitative estimates, local temperatures were on average 1.6±0.8 °C higher during the HCO than now. Northwestern North America reached peak warmth first, from 11,000 to 9,000 years ago, but the Laurentide Ice Sheet still made eastern Canada colder. Northeastern North America experienced peak warming 4,000 years later. Along the Arctic Coastal Plain in Alaska, summer temperatures were 2–3 °C warmer than today. Research suggests that the Arctic had less sea ice than now. The Greenland Ice Sheet thinned, especially at its edges. In addition, Arctic Alaska became wetter.
Northwestern Europe experienced warming, but Southern Europe cooled. In the southwestern Iberian Peninsula, forest cover reached its peak between 9,760 and 7,360 years ago due to high moisture and warm temperatures during the HCO. In Central Europe, the HCO was the first time human impact on the environment became clearly visible in sediment records. From 9,000 to 7,500 years ago, human impact was minimal and the environment was stable. From 7,500 to 6,300 years ago, human influence was only seen in pollen records. After 6,300 years ago, humans had a significant impact on the environment.
In the Middle East, the HCO was marked by frost-free winters and abundant Pistacia savannas. This period also saw the domestication of cereals and Neolithic population growth in the region. The HCO began in the southern Ural Mountains at the same time as in Northern Europe, but ended between 6,300 and 5,100 years ago. In northern central Siberia, winter temperatures were 3 to 9 °C warmer, and summer temperatures were 2 to 6 °C warmer than today.
The HCO was not synchronized in Central and East Asia, but it occurred at the same time in the Loess Plateau, the Inner Mongolian Plateau, and Xinjiang. Rising sea levels and melting ice sheets in the Northern Hemisphere caused the East Asian Summer Monsoon (EASM) rain belt to expand westward into the Asian interior. The EASM was strongest during the HCO, though its peak varied by region. Stronger westerly winds sometimes caused dry periods in China during the HCO. Current desert regions in Central Asia were once heavily forested due to higher rainfall, and warm temperate forests in China and Japan extended northward. In southern Tibet’s Yarlung Tsangpo valley, precipitation was twice as high as today during the middle Holocene. In the Huai River basin, the HCO began 9,100 to 8,000 years ago. Pollen records from Lake Tai in Jiangsu, China, show increased summer rainfall and a warmer, wetter climate in the region. The stable climate of the Middle Holocene in China supported the development of agriculture and animal husbandry. In the Korean Peninsula, the HCO occurred from 8,900 to 4,400 years ago, with its peak between 7,600 and 4,800 years ago. Sea levels in the Sea of Japan were 2–6 meters higher than today, and sea surface temperatures were 1–2 °C warmer. The East Korea Warm Current extended as far as Primorye, pushing colder water from the Primorsky Current northeastward. The Tsushima Current warmed the northern shores of Hokkaido and reached the Sea of Okhotsk. In the northern South China Sea, the HCO was linked to colder winters due to a stronger East Asian Winter Monsoon, causing frequent coral die-offs.
In the Indian Subcontinent, the Indian Summer Monsoon (ISM) intensified, creating a hot and wet climate in India and raising sea levels. Relative sea level in the Spermonde Archipelago was about 0.5 meters higher than today. Sediment buildup in lagoons slowed during the HCO but accelerated afterward as sea levels dropped.
West African sediments show evidence of the African Humid Period, a time between 16,000 and 6,000 years ago when Africa was much wetter than today. This was caused by a stronger African monsoon due to changes in summer sunlight, which resulted from long-term changes in Earth’s orbit around the Sun. The "Green Sahara" had many lakes and supported crocodile and hippopotamus populations. Marine sediments suggest that the transition into and out of the wet period occurred within decades, not over long periods. Some scientists think humans may have altered North African vegetation after 8,000 years ago by introducing domesticated animals, contributing to the rapid shift to arid conditions in the Sahara. Further south, in Central Africa, the coastal lowlands of the Congo River drainage basin were entirely covered by savannas, not forests, during the HCO. Southwestern Africa had increased humidity during the HCO.
In northwestern Patagonia, a region called the Arid Diagonal, the area was much drier during the Early and Middle Holocene but became more humid in the Late
Comparison of ice cores
A comparison of the graphs showing changes in ice layers at Byrd Station, West Antarctica (where an ice core 2164 meters long was collected in 1968), and Camp Century, Northwest Greenland, shows evidence of a warm period after the last ice age. Matching points in the data suggest that this warm period, called the post-glacial climatic optimum, likely happened at the same time in both locations. Similar patterns are also seen in the Dye 3 ice core from 1979 and the Camp Century ice core from 1963 for this time period.
The Hans Tausen Ice Cap in Peary Land, northern Greenland, was drilled to a depth of 325 meters in 1977 using a new deep drill. The ice core showed clear layers formed by melting all the way to the bedrock. This suggests that the Hans Tausen Ice Cap did not contain ice from the last ice age and melted completely during the post-glacial climatic optimum. The ice cap was later rebuilt when the climate cooled about 4000 years ago.
The graphs from the Renland ice cap in Scoresby Sound show that this ice cap has always been separate from the larger inland ice. However, the same patterns found in the Camp Century 1963 ice core also appear in the Renland 1985 ice core. The Renland ice core from East Greenland seems to cover a full cycle of ice ages, including the time of the Holocene and the previous Eemian interglacial period. The Renland ice core is 325 meters long.
Although the depths of the GRIP and NGRIP ice cores are different, both cores show evidence of the warm period (post-glacial climatic optimum) occurring at very similar times.
Milankovitch cycles
The climate event was likely caused by regular changes in Earth's orbit, known as Milankovitch cycles, and was a follow-up to changes that ended the last ice age.
Around 9,000 years ago, the Northern Hemisphere experienced the most heating. At that time, Earth's tilt was 24°, and Earth was closest to the Sun during its summer. This increased solar energy by 0.2% (+40 W/m²), leading to more heating. Scientists also observed a shift in the global band of thunderstorms, known as the Intertropical Convergence Zone, moving southward.
However, changes in Earth's orbit predicted the strongest climate changes to happen thousands of years earlier than what was seen in the Northern Hemisphere. This delay might be due to ongoing climate changes as Earth recovered from the last ice age and the effect of ice reflecting sunlight. Climate changes occurred at different times and lasted different lengths in various places. In some areas, changes started as early as 11,000 years ago and lasted until 4,000 years ago. As mentioned earlier, the warmest time in the far south happened before warming in the north.
Other changes
Major changes in temperature have not been observed in most areas near the equator. However, other climate changes have been recorded, such as much wetter weather in Africa, Australia, and Japan, and conditions similar to deserts in the Midwestern United States. Regions around the Amazon River have experienced higher temperatures and drier weather.