Holocene climatic optimum

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The Holocene Climate Optimum (HCO) was a warm period in the first half of the Holocene epoch, that occurred in the interval roughly 9,500 to 5,500 years BP, [1] 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.

Contents

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]

Global effects

Temperature variations during the Holocene from a collection of different reconstructions and their average. The most recent period is on the right, but the recent warming is seen only in the inset. Holocene Temperature Variations.png
Temperature variations during the Holocene from a collection of different reconstructions and their average. The most recent period is on the right, but the recent warming is seen only in the inset.

The HCO was approximately 4.9 °C warmer than the Last 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 the Industrial Revolution, and 0.3 °C cooler than the average for 2011-2019. [6] The 2021 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 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. [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 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. [9]

Temperatures during the HCO were higher than in the present by around 6 °C in Svalbard, 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 the Laurentide Ice Sheet still chilled eastern Canada. Northeastern North America experienced peak warming 4,000 years later. Along the Arctic 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] The Greenland Ice Sheet thinned, particularly at its margins. [13]

Northwestern Europe experienced warming, but there was cooling in Southern Europe. [14] In the southwestern Iberian 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. [15] In Central Europe, the HCO was when human impact on the environment first became clearly detectable in sedimentological records, [16] 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. [17]

In the Middle East, the HCO was associated with frost-free winters and abundant Pistacia savannas. It was during this interval that the domestication of cereals and Neolithic population growth occurred in the region. [18]

The onset of the HCO in the southern Ural Mountains was simultaneous with that in Northern Europe, while its termination occurred between 6,300 and 5,100 BP. [19] Winter warming of 3 to 9 °C and summer warming of 2 to 6 °C occurred in northern central Siberia. [20]

The HCO was highly asynchronous in Central and East Asia. [21] As a result of rising sea levels and decay of ice sheets in the Northern Hemisphere, the East Asian Summer Monsoon (EASM) rain belt expanded to the northwest, penetrating deep into the Asian interior. [22] The EASM, being significantly weaker before and after the HCO, peaked in strength during this interval, [23] though the exact timing of its maximum intensity varied by region. [24] Current desert regions of Central Asia were extensively forested because of higher rainfall, and the warm temperate forest belts in China and Japan were extended northwards. [25] In the Yarlung Tsangpo valley of southern Tibet, precipitation was up to twice as high as it is today during the middle Holocene. [26] Pollen records from Lake Tai in Jiangsu, China shed light on increased summer precipitation and a warmer and wetter overall climate in the region. [27] The stability of the Middle Holocene climate in China fostered the development of agriculture and animal husbandry in the region. [28] 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. [29] Sea levels in the Sea of Japan were 2-6 metres higher than in the present, with sea surface temperatures being 1-2 °C higher. The East Korea Warm Current reached as far as Primorye and pushed cold water off of the cooler Primorsky Current to the northeast. The Tsushima Current warmed the northern shores of Hokkaido penetrated into the Sea of Okhotsk. [30] In the northern South China Sea, the HCO was associated with colder winters due to a stronger East Asian Winter Monsoon (EAWM), causing frequent coral die-offs. [31]

In the Indian Subcontinent, the Indian Summer Monsoon (ISM) heavily intensified, creating a hot and wet climate in India along with high sea levels. [32]

Relative sea level in the Spermonde Archipelago was approximately 0.5 metres higher than it is today. [33] [34] Sedimentary infill of lagoons was retarded by the sea level highstand and accelerated after the HCO, when sea levels dropped. [35]

Vegetation and water bodies in northern and central Africa in the Eemian (bottom) and Holocene (top) Journal.pone.0076514.g004.png
Vegetation and water bodies in northern and central Africa in the Eemian (bottom) and Holocene (top)

West African sediments additionally record the African humid period, an interval between 16,000 and 6,000 years ago during which Africa was much wetter than now. That was caused by a strengthening of the African monsoon by changes in summer radiation, which resulted from long-term variations in the Earth's orbit around the Sun. The "Green Sahara" 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 the wet period occurred within decades, not the previously-thought extended periods. [36] 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 the Sahara. [37] Further south, in Central Africa, the savannas that make up the coastal lowlands of the Congo River drainage basin in the present were entirely absent. [38] Southwestern Africa experienced increased humidity during the HCO. [39]

Northwestern Patagonia, in a region known as the Arid Diagonal, was significantly drier during the Early and Middle Holocene, with the region becoming more humid during the Late Holocene following the end of the HCO. [40]

In the far Southern Hemisphere (New Zealand and Antarctica), the warmest period during the Holocene appears to have been roughly 10,500 to 8,000 years ago, immediately after the end of the last ice age. [41] [42] The Amery Ice Shelf retreated approximately 80 kilometres landward during this warm interval. [43] 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. [44] [45]

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. [46]

Comparison of ice cores

A comparison of the delta profiles at Byrd Station, West Antarctica (2164 m ice core recovered, 1968), and Camp Century, Northwest Greenland, shows the post-glacial climatic optimum. [47] 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. [47]

The Hans Tausen Ice Cap, in Peary Land (northern Greenland), 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. [47]

From the delta-profile, the Renland ice cap in the Scoresby 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. [47] The Renland ice core from East Greenland apparently covers a full glacial cycle from the Holocene into the previous Eemian interglacial. The Renland ice core is 325 m long. [48]

Although the depths are different, the GRIP and NGRIP cores also contain the climatic optimum at very similar times. [47]

Milankovitch cycles

Milankovitch cycles. Orbital variation.svg
Milankovitch cycles.

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 last glacial 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 calculated Milankovitch Forcing would have provided 0.2% more solar 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, the Intertropical 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 to ice–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 ]

Other changes

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 Japan and desert-like conditions in the Midwestern United States. Areas around the Amazon show temperature increases and drier conditions. [49]

See also

Related Research Articles

The Holocene is the current geological epoch, beginning approximately 11,700 years ago. It follows the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene together form the Quaternary period. The Holocene is an interglacial period within the ongoing glacial cycles of the Quaternary, and is equivalent to Marine Isotope Stage 1.

The Younger Dryas, which occurred circa 12,900 to 11,700 years BP, was a return to glacial conditions which temporarily reversed the gradual climatic warming after the Last Glacial Maximum, which lasted from circa 27,000 to 20,000 years BP. The Younger Dryas was the last stage of the Pleistocene epoch that spanned from 2,580,000 to 11,700 years BP and it preceded the current, warmer Holocene epoch. The Younger Dryas was the most severe and longest lasting of several interruptions to the warming of the Earth's climate, and it was preceded by the Late Glacial Interstadial, an interval of relative warmth that lasted from 14,670 to 12,900 BP.

<span class="mw-page-title-main">Last Glacial Period</span> Period of major glaciations of the Northern Hemisphere (115,000–12,000 years ago)

The Last Glacial Period (LGP), also known colloquially as the Last Ice Age or simply Ice Age, occurred from the end of the Last Interglacial to the end of the Younger Dryas, encompassing the period c. 115,000 – c. 11,700 years ago.

<span class="mw-page-title-main">Last Interglacial</span> Interglacial period which began 130,000 years ago

The Last Interglacial, also known as the Eemian among other names was the interglacial period which began about 130,000 years ago at the end of the Penultimate Glacial Period and ended about 115,000 years ago at the beginning of the Last Glacial Period. It corresponds to Marine Isotope Stage 5e. It was the second-to-latest interglacial period of the current Ice Age, the most recent being the Holocene which extends to the present day. During the Last Interglacial, the proportion of CO2 in the atmosphere was about 280 parts per million. The Last Interglacial was one of the warmest periods of the last 800,000 years, with temperatures comparable to and at times warmer than the contemporary Holocene interglacial, with the maximum sea level being up to 6 to 9 metres higher than at present, with global ice volume likely also being smaller than the Holocene interglacial.

<span class="mw-page-title-main">Last Glacial Maximum</span> Most recent time during the Last Glacial Period that ice sheets were at their greatest extent

The Last Glacial Maximum (LGM), also referred to as the Last Glacial Coldest Period, was the most recent time during the Last Glacial Period where ice sheets were at their greatest extent 26,000 and 20,000 years ago. Ice sheets covered much of Northern North America, Northern Europe, and Asia and profoundly affected Earth's climate by causing a major expansion of deserts, along with a large drop in sea levels.

<span class="mw-page-title-main">Mammoth steppe</span> Prehistoric biome

During the Last Glacial Maximum, the mammoth steppe, also known as steppe-tundra, was once the Earth's most extensive biome. It stretched east-to-west, from the Iberian Peninsula in the west of Europe, across Eurasia to North America, through Beringia and Canada; from north-to-south, the steppe reached from the arctic islands southward to China. The mammoth steppe was cold and dry, and relatively featureless, though topography and geography varied considerably throughout. Some areas featured rivers which, through erosion, naturally created gorges, gulleys, or small glens. The continual glacial recession and advancement over millennia contributed more to the formation of larger valleys and different geographical features. Overall, however, the steppe is known to be flat and expansive grassland. The vegetation was dominated by palatable, high-productivity grasses, herbs and willow shrubs.

<span class="mw-page-title-main">Interglacial</span> Geological interval of warmer temperature that separates glacial periods within an ice age

An interglacial period is a geological interval of warmer global average temperature lasting thousands of years that separates consecutive glacial periods within an ice age. The current Holocene interglacial began at the end of the Pleistocene, about 11,700 years ago.

The Older Dryas was a stadial (cold) period between the Bølling and Allerød interstadials, about 14,000 years Before Present, towards the end of the Pleistocene. Its date range is not well defined, with estimates varying by 400 years, but its duration is agreed to have been around two centuries.

The Holocene glacial retreat is a geographical phenomenon that involved the global retreat of glaciers (deglaciation) that previously had advanced during the Last Glacial Maximum. Ice sheet retreat initiated ca. 19,000 years ago and accelerated after ca. 15,000 years ago. The Holocene, starting with abrupt warming 11,700 years ago, resulted in rapid melting of the remaining ice sheets of North America and Europe.

<span class="mw-page-title-main">4.2-kiloyear event</span> Severe climatic event starting around 2200 BC

The 4.2-kiloyear BP aridification event, also known as the 4.2 ka event, was one of the most severe climatic events of the Holocene epoch. It defines the beginning of the current Meghalayan age in the Holocene epoch.

<span class="mw-page-title-main">Neoglaciation</span>

The neoglaciation describes the documented cooling trend in the Earth's climate during the Holocene, following the retreat of the Wisconsin glaciation, the most recent glacial period. Neoglaciation has followed the hypsithermal or Holocene Climatic Optimum, the warmest point in the Earth's climate during the current interglacial stage, excluding the global warming-induced temperature increase starting in the 20th century. The neoglaciation has no well-marked universal beginning: local conditions and ecological inertia affected the onset of detectably cooler conditions.

<span class="mw-page-title-main">8.2-kiloyear event</span> Rapid global cooling around 8,200 years ago

In climatology, the 8.2-kiloyear event was a sudden decrease in global temperatures that occurred approximately 8,200 years before the present, or c. 6,200 BC, and which lasted for the next two to four centuries. It defines the start of the Northgrippian age in the Holocene epoch. The cooling was significantly less pronounced than during the Younger Dryas cold period that preceded the beginning of the Holocene. During the event, atmospheric methane concentration decreased by 80 ppb, an emission reduction of 15%, by cooling and drying at a hemispheric scale.

<span class="mw-page-title-main">Subboreal</span> Climatic period of the Holocene

The Subboreal is a climatic period, immediately before the present one, of the Holocene. It lasted from 3710 to 450 BCE.

Deglaciation is the transition from full glacial conditions during ice ages, to warm interglacials, characterized by global warming and sea level rise due to change in continental ice volume. Thus, it refers to the retreat of a glacier, an ice sheet or frozen surface layer, and the resulting exposure of the Earth's surface. The decline of the cryosphere due to ablation can occur on any scale from global to localized to a particular glacier. After the Last Glacial Maximum, the last deglaciation begun, which lasted until the early Holocene. Around much of Earth, deglaciation during the last 100 years has been accelerating as a result of climate change, partly brought on by anthropogenic changes to greenhouse gases.

<span class="mw-page-title-main">Early Holocene sea level rise</span> Sea level rise between 12,000 and 7,000 years ago

The early Holocene sea level rise (EHSLR) was a significant jump in sea level by about 60 m (197 ft) during the early Holocene, between about 12,000 and 7,000 years ago, spanning the Eurasian Mesolithic. The rapid rise in sea level and associated climate change, notably the 8.2 ka cooling event , and the loss of coastal land favoured by early farmers, may have contributed to the spread of the Neolithic Revolution to Europe in its Neolithic period.

<span class="mw-page-title-main">Late Cenozoic Ice Age</span> Ice age of the last 34 million years, in particular in Antarctica

The Late Cenozoic Ice Age, or Antarctic Glaciation, began 34 million years ago at the Eocene-Oligocene Boundary and is ongoing. It is Earth's current ice age or icehouse period. Its beginning is marked by the formation of the Antarctic ice sheets.

<span class="mw-page-title-main">Climate and vegetation interactions in the Arctic</span>

Changing climate conditions are amplified in polar regions and northern high-latitude areas are projected to warm at twice the rate of the global average. These modifications result in ecosystem interactions and feedbacks that can augment or mitigate climatic changes. These interactions may have been important through the large climate fluctuations since the glacial period. Therefore it is useful to review the past dynamics of vegetation and climate to place recent observed changes in the Arctic into context. This article focuses on northern Alaska where there has been much research on this theme.

<span class="mw-page-title-main">Penultimate Glacial Period</span> Glacial age that occurred before the Last Glacial Period

The Penultimate Glacial Period (PGP) is the glacial period that occurred before the Last Glacial Period. The penultimate glacial period is officially unnamed just like the Last Glacial Period. The PGP lasted from ~194,000 years ago, to ~135,000 years ago, and was succeed by the Last Interglacial. The PGP also occurred during Marine Isotope Stage 6 (MIS6). At the glacial ages’ height, it is known to be the most extensive expansion of glaciers in the last 400,000 years over Eurasia, and could be the second or third coolest glacial period over the last 1,000,000 years, as shown by ice cores. Due to this, the global sea level dropped to between 92 and 150 metres below modern-day global mean sea level. The penultimate glacial period expanded ice sheets and shifted temperature zones worldwide, which had a variety of effects on the world's environment, and the organisms that lived in it. At its height, the penultimate glacial period was a more severe glaciation than the Last Glacial Maximum. The PGP covers the last period of the Saalian glaciation in Europe, called the Wolstonian Stage in Britain, and is equivalent to the Illinoian in North America.

<span class="mw-page-title-main">African humid period</span> Holocene climate period during which northern Africa was wetter than today

The African humid period is a climate period in Africa during the late Pleistocene and Holocene geologic epochs, when northern Africa was wetter than today. The covering of much of the Sahara desert by grasses, trees and lakes was caused by changes in the Earth's axial tilt; changes in vegetation and dust in the Sahara which strengthened the African monsoon; and increased greenhouse gases. During the preceding Last Glacial Maximum, the Sahara contained extensive dune fields and was mostly uninhabited. It was much larger than today, and its lakes and rivers such as Lake Victoria and the White Nile were either dry or at low levels. The humid period began about 14,600–14,500 years ago at the end of Heinrich event 1, simultaneously to the Bølling–Allerød warming. Rivers and lakes such as Lake Chad formed or expanded, glaciers grew on Mount Kilimanjaro and the Sahara retreated. Two major dry fluctuations occurred; during the Younger Dryas and the short 8.2 kiloyear event. The African humid period ended 6,000–5,000 years ago during the Piora Oscillation cold period. While some evidence points to an end 5,500 years ago, in the Sahel, Arabia and East Africa, the end of the period appears to have taken place in several steps, such as the 4.2-kiloyear event.

<span class="mw-page-title-main">Medieval Warm Period</span> Time of warm climate in the North Atlantic region lasting from c. 950 to c. 1250

The Medieval Warm Period (MWP), also known as the Medieval Climate Optimum or the Medieval Climatic Anomaly, was a time of warm climate in the North Atlantic region that lasted from c. 950 to c. 1250. Climate proxy records show peak warmth occurred at different times for different regions, which indicate that the MWP was not a globally uniform event. Some refer to the MWP as the Medieval Climatic Anomaly to emphasize that climatic effects other than temperature were also important.

References

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