Oligocene

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Oligocene 

Oligocene
33.9 – 23.03 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Chronology
−65 — – −60 — – −55 — – −50 — – −45 — – −40 — – −35 — – −30 — – −25 — – M Z C e n o z o i c K P a l e o g e n e N Late K P a l e o c e n e E o c e n e O l i g o c e n e Miocene Danian Selandian Thanetian Ypresian Lutetian Bartonian Priabonian Rupelian Chattian         ← PETM ← First Antarctic permanent ice-sheets ← K-Pg mass extinction Subdivision of the Paleogene according to the ICS, as of 2021. Vertical axis scale: millions of years ago
Etymology
Name formality	Formal
Name ratified	1978
Usage information
Celestial body	Earth
Regional usage	Global (ICS)
Time scale(s) used	ICS Time Scale
Definition
Chronological unit	Epoch
Stratigraphic unit	Series
Time span formality	Formal
Lower boundary definition	LAD of Planktonic Foraminifers Hantkenina and Cribrohantkenina
Lower boundary GSSP	Massignano quarry section, Massignano, Ancona, Italy 43.5328°N 13.6011°E
GSSP ratified	1992
Upper boundary definition	Base of magnetic polarity chronozone C6Cn.2n. FAD of the Planktonic Foraminiferan Paragloborotalia kugleri
Upper boundary GSSP	Lemme-Carrosio Section, Carrosio, Italy 44.6589°N 8.8364°E
GSSP ratified	1996

The Oligocene ( /ˈɒlɪɡəsiːn, -ɡoʊ-/ OL-ə-gə-seen, -⁠goh-) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present (33.9±0.1 to 23.03±0.05 Ma). As with other older geologic periods, the rock beds that define the epoch are well identified but the exact dates of the start and end of the epoch are slightly uncertain. The name Oligocene was coined in 1854 by the German paleontologist Heinrich Ernst Beyrich from his studies of marine beds in Belgium and Germany. The name comes from the Ancient Greek ὀλίγος (olígos, "few") and καινός (kainós, "new"), and refers to the sparsity of extant forms of molluscs. The Oligocene is preceded by the Eocene Epoch and is followed by the Miocene Epoch. The Oligocene is the third and final epoch of the Paleogene Period.

The Oligocene is often considered an important time of transition, a link between the archaic world of the tropical Eocene and the more modern ecosystems of the Miocene. Major changes during the Oligocene included a global expansion of grasslands, and a regression of tropical broad leaf forests to the equatorial belt.

The start of the Oligocene is marked by a notable extinction event called the Grande Coupure; it featured the replacement of European fauna with Asian fauna, except for the endemic rodent and marsupial families. By contrast, the Oligocene–Miocene boundary is not set at an easily identified worldwide event but rather at regional boundaries between the warmer late Oligocene and the relatively cooler Miocene.

Boundaries and subdivisions

The lower boundary of the Oligocene (its Global Boundary Stratotype Section and Point or GSSP) is placed at the last appearance of the foraminiferan genus Hantkenina in a quarry at Massignano, Italy. However, this GSSP has been criticized as excluding the uppermost part of the type Eocene Priabonian Stage and because it is slightly earlier than important climate shifts that form natural markers for the boundary, such as the global oxygen isotope shift marking the expansion of Antarctic glaciation (the Oi1 event).

The upper boundary of the Oligocene is defined by its GSSP at Carrosio, Italy, which coincides with the first appearance of the foraminiferan Paragloborotalia kugleri and with the base of magnetic polarity chronozone C6Cn.2n.

Oligocene faunal stages from youngest to oldest are:

Chattian or late Oligocene	(27.82  –  23.03 mya)
Rupelian or early Oligocene	(33.9  –  27.82 mya)

Tectonics and paleogeography

Neotethys during the Oligocene (Rupelian, 33.9–28.4 mya)

During the Oligocene Epoch, the continents continued to drift toward their present positions. Antarctica became more isolated as deep ocean channels were established between Antarctica and Australia and South America. Australia had been very slowly rifting away from West Antarctica since the Jurassic, but the exact timing of the establishment of ocean channels between the two continents remains uncertain. However, one estimate is that a deep channel was in place between the two continents by the end of the early Oligocene. The timing of the formation of the Drake Passage between South America and Australia is also uncertain, with estimates ranging from 49 to 17 mya (early Eocene to Miocene), but oceanic circulation through the Drake Passage may also have been in place by the end of the early Oligocene. This may have been interrupted by a temporary constriction of the Drake Passage from sometime in the middle to late Oligocene (29 to 22 mya) to the middle Miocene (15 mya).

The reorganization of the oceanic tectonic plates of the northeastern Pacific, which had begun in the Paleocene, culminated with the arrival of the Murray and Mendocino Fracture Zones at the North American subduction zone in the Oligocene. This initiated strike-slip movement along the San Andreas Fault and extensional tectonics in the Basin and Range province, ended volcanism south of the Cascades, and produced clockwise rotation of many western North American terranes. The Rocky Mountains were at their peak. A new volcanic arc was established in western North America, far inland from the coast, reaching from central Mexico through the Mogollon-Datil volcanic field to the San Juan volcanic field, then through Utah and Nevada to the ancestral Northern Cascades. Huge ash deposits from these volcanoes created the White River and Arikaree Groups of the High Plains, with their excellent fossil beds.

Between 31 and 26 mya, the Ethiopia-Yemen Continental Flood Basalts were emplaced by the East African large igneous province, which also initiated rifting along the Red Sea and Gulf of Aden.

The Alps were rapidly rising in Europe as the African plate continued to push north into the Eurasian plate, isolating the remnants of the Tethys Sea. Sea levels were lower in the Oligocene than in the early Eocene, exposing large coastal plains in Europe and the Gulf Coast and Atlantic Coast of North America. The Obik Sea, which had separated Europe from Asia, retreated early in the Oligocene, creating a persistent land connection between the continents. There appears to have been a land bridge in the early Oligocene between North America and Europe, since the faunas of the two regions are very similar. However, towards the end of the Oligocene, there was a brief marine incursion in Europe.

The rise of the Himalayas during the Oligocene remains poorly understood. One recent hypothesis is that a separate microcontinent collided with south Asia in the early Eocene, and India itself did not collide with south Asia until the end of the Oligocene. The Tibetan Plateau may have reached nearly its present elevation by the late Oligocene.

The Andes first became a major mountain chain in the Oligocene, as subduction became more direct into the coastline.

Climate

Climate change during the last 65 million years

Climate during the Oligocene reflected a general cooling trend following the Early Eocene Climatic Optimum. This transformed the Earth's climate from a greenhouse to an icehouse climate.

Eocene-Oligocene transition and Oi1 event

The Eocene-Oligocene transition, peaking around 33.5 mya, was a major cooling event and reorganization of the biosphere. The transition is marked by the Oi1 event, in which oxygen isotope ratios decreased by 1.3‰. About 0.3–0.4‰ of this is estimated to be due to major expansion of Antarctic ice sheets. The remaining 0.9 to 1.0‰ was due to about 5 to 6 °C (9 to 10 °F) of global cooling. The transition likely took place in three closely spaced steps over the period from 33.8 to 33.5 mya. By the end of the transition, sea levels had dropped by 105 meters (344 ft), and ice sheets were 25% greater in extent than in the modern world.

The effects of the transition can be seen in the geological record at many locations around the world. Ice volumes rose as temperature and sea levels dropped. Playa lakes of the Tibetan Plateau disappeared at the transition, pointing to cooling and aridification of central Asia. Pollen and spore counts in marine sediments of the Norwegian-Greenland Sea indicate a drop in winter temperatures at high latitudes of about 5 °C (9.0 °F) just prior to the Oi1 event. Borehole dating from the Southeast Faroes drift indicates that deep-ocean circulation from the Arctic Ocean to the North Atlantic Ocean began in the early Oligocene.

The best terrestrial record of Oligocene climate comes from North America, where temperatures dropped by 7 to 11 °C (13 to 20 °F) in the earliest Oligocene. This change is seen from Alaska to the Gulf Coast. Upper Eocene paleosols reflect annual precipitation of over a meter of rain, but early Oligocene precipitation was less than half this. In central North America, the cooling was by 8.2 ± 3.1 °C over a period of 400,000 years, though there is little indication of significant increase in aridity during this interval. Ice-rafted debris in the Norwegian-Greenland Sea indicated that glaciers had appeared in Greenland by the start of the Oligocene.

Continental ice sheets in Antarctica reached sea level during the transition. Glacially rafted debris of early Oligocene age in the Weddell Sea and Kerguelen Plateau, in combination with Oi1 isotope shift, provides unambiguous evidence of a continental ice sheet on Antarctica by the early Oligocene.

The causes of the Eocene-Oligocene transition are not yet fully understood. The timing is wrong for this to be caused either by known impact events or by the volcanic activity on the Ethiopean Plateau. Two other possible drivers of climate change, not mutually exclusive, have been proposed. The first is thermal isolation of the continent of Antarctica by development of the Antarctic Circumpolar Current. Deep sea cores from south of New Zealand suggest that cold deep-sea currents were present by the early Oligocene. However, the timing of this event remains controversial. The other possibility, for which there is considerable evidence, is a drop in atmospheric carbon dioxide levels (pCO2) during the transition. The pCO2 is estimated to have dropped just before the transition, to 760 ppm at the peak of ice sheet growth, then rebounded slightly before resuming a more gradual fall. Climate modeling suggests that glaciation of Antarctica took placed only when pCO2 dropped below a critical threshold value.

Middle Oligocene climate and the Oi2 event

Oligocene climate following the Eocene-Oligocene event is poorly known. There were several pulses of glaciation in middle Oligocene, about the time of the Oi2 oxygen isotope shift. This led to the largest drop of sea level in past 100 million years, by about 75 meters (246 ft). This is reflected in a mid-Oligocene incision of continental shelves and unconformities in marine rocks around the world.

Some evidence suggests that the climate remained warm at high latitudes even as ice sheets experienced cyclical growth and retreat in response to orbital forcing and other climate drivers. Other evidence indicates significant cooling at high latitudes. Part of the difficulty may be that there were strong regional variations in the response to climate shifts. Evidence of a relatively warm Oligocene suggests an enigmatic climate state, neither hothouse nor icehouse.

Late Oligocene warming

The late Oligocene (26.5 to 24 mya) likely saw a warming trend in spite of low pCO2 levels, though this appears to vary by region. However, Antarctica remained heavily glaciated during this warming period. The late Oligocene warming is discernible in pollen counts from the Tibetan Plateau, which also show that the south Asian monsoon had already developed by the late Oligocene.

A deep 400,000-year glaciated Oligocene-Miocene boundary event is recorded at McMurdo Sound and King George Island.

Biosphere

Restoration of Nimravus (far left) and other animals from the Turtle Cove Formation

The early Eocene climate was very warm, with crocodilians and temperate plants thriving above the Arctic Circle. The cooling trend that began in the middle Eocene continued into the Oligocene, bringing the poles well below freezing for the first time in the Phanerozoic. The cooling climate, together with the opening of some land bridges and the closing of others, led to a profound reorganization of the biosphere and loss of taxonomic diversity. Land animals and marine organisms reached a Phanerozoic low in diversity by the late Oligocene, and the temperate forests and jungles of the Eocene were replaced by forest and scrubland. The closing of the Tethys Seaway destroyed its tropical biota.

Flora

The Oi1 event of the Eocene-Oligocene transition covered the continent of Antarctica with ice sheets, leaving Nothofagus and mosses and ferns clinging to life around the periphery of Antarctica in tundra conditions.

Angiosperms continued their expansion throughout the world as tropical and sub-tropical forests were replaced by temperate deciduous forests. Open plains and deserts became more common and grasses expanded from their water-bank habitat in the Eocene moving out into open tracts. The decline in pCO2 favored C4 photosynthesis, which is found only in angiosperms and is particularly characteristic of grasses. However, even at the end of the period, grass was not quite common enough for modern savannas.

In North America, much of the dense forest was replaced by patchy scrubland with riparian forests. Subtropical species dominated with cashews and lychee trees present, and temperate woody plants such as roses, beeches, and pines were common. The legumes spread, while sedges and ferns continued their ascent.

Fauna

Life restoration of Daeodon

 

Paraceratherium restored next to Hyaenodon

Most extant mammal families had appeared by the end of the Oligocene. These included primitive three-toed horses, rhinoceroses, camels, deer, and peccaries. Carnivores such as dogs, nimravids (ancestor of cats), bears, weasels, and raccoons began to replace the creodonts that had dominated the Paleocene in the Old World. Rodents and rabbits underwent tremendous diversification due to the increase in suitable habitats for ground-dwelling seed eaters, as habitats for squirrel-like nut- and fruit-eaters diminished. The primates, once present in Eurasia, were reduced in range to Africa and South America. Many groups, such as equids, entelodonts, rhinos, merycoidodonts, and camelids, became more able to run during this time, adapting to the plains that were spreading as the Eocene rainforests receded. Brontotheres died out in the Earliest Oligocene, and creodonts died out outside Africa and the Middle East at the end of the period. Multituberculates, an ancient lineage of primitive mammals that originated back in the Jurassic, also became extinct in the Oligocene, aside from the gondwanatheres.

The Eocene-Oligocene transition in Europe and Asia has been characterized as the Grande Coupure. The lowering of sea levels closed the Turgai Strait across the Obik Sea, which had previously separated Asia from Europe. This allowed Asian mammals, such as rhinoceroses and ruminants, to enter Europe and drive endemic species to extinction. Lesser faunal turnovers occurred simultaneously with the Oi2 event and towards the end of the Oligocene. There was significant diversification of mammals in Eurasia, including the giant indricotheres, that grew up to 6 meters (20 ft) at the shoulder and weighed up to 20 tons. Paraceratherium was one of the largest land mammals ever to walk the Earth. However, the indricotheres were an exception to a general tendency for Oligocene mammals to be much smaller than their Eocene counterparts. The earliest deer, giraffes, pigs, and cattle appeared in the mid-Oligocene in Eurasia. The first felid, Proailurus, originated in Asia during the late Oligocene and spread to Europe.

There was only limited migration between Asia and North America. The cooling of central North America at the Eocene-Oligocene transition resulted in a large turnover of gastropods, amphibians, and reptiles. Mammals were much less affected. Crocodilians and pond turtles replaced by dry land tortoises. Molluscs shifted to more drought-tolerant forms. The White River Fauna of central North America inhabited a semiarid prairie home and included entelodonts like Archaeotherium, camelids (such as Poebrotherium), running rhinoceratoids, three-toed equids (such as Mesohippus), nimravids, protoceratids, and early canids like Hesperocyon. Merycoidodonts, an endemic American group, were very diverse during this time.

Australia and South America became geographically isolated and developed their own distinctive endemic fauna. These included the New World and Old World monkeys. The South American continent was home to animals such as pyrotheres and astrapotheres, as well as litopterns and notoungulates. Sebecosuchians, terror birds, and carnivorous metatheres, like the borhyaenids remained the dominant predators.

Africa was also relative isolated and retained its endemic fauna. These included mastodonts, hyraxes, arsinoitheres, and other archaic forms. Egypt in the Oligocene was an environment of lush forested deltas.

At sea, 97% of marine snail species, 89% of clams, and 50% of echinoderms of the Gulf Coast did not survive past the earliest Oligocene. New species evolved, but the overall diversity diminished. Cold-water mollusks migrated around the Pacific Rim from Alaska and Siberia. The marine animals of Oligocene oceans resembled today's fauna, such as the bivalves. Calcareous cirratulids appeared in the Oligocene. The fossil record of marine mammals is a little spotty during this time, and not as well known as the Eocene or Miocene, but some fossils have been found. The baleen whales and toothed whales had just appeared, and their ancestors, the archaeocete cetaceans began to decrease in diversity due to their lack of echolocation, which was very useful as the water became colder and cloudier. Other factors to their decline could include climate changes and competition with today's modern cetaceans and the requiem sharks, which also appeared in this epoch. Early desmostylians, like Behemotops, are known from the Oligocene. Pinnipeds appeared near the end of the epoch from an otter-like ancestor.

• 

  Poebrotherium

• 

  Merycoidodon

• 

  Hoplophoneus

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  Mesohippus

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  Paraceratherium

• 

  Paleoparadoxia

• 

  Protoceras

• 

  Archaeotherium

• 

  Janjucetus

Oceans

The Oligocene sees the beginnings of modern ocean circulation, with tectonic shifts causing the opening and closing of ocean gateways. Cooling of the oceans had already commenced by the Eocene/Oligocene boundary, and they continued to cool as the Oligocene progressed. The formation of permanent Antarctic ice sheets during the early Oligocene and possible glacial activity in the Arctic may have influenced this oceanic cooling, though the extent of this influence is still a matter of some significant dispute.

The effects of oceanic gateways on circulation

The opening and closing of ocean gateways: the opening of the Drake Passage; the opening of the Tasmanian Gateway and the closing of the Tethys seaway; along with the final formation of the Greenland–Iceland–Faroes Ridge; played vital parts in reshaping oceanic currents during the Oligocene. As the continents shifted to a more modern configuration, so too did ocean circulation.

The Drake Passage

Eocene-Oligocene circum-Antarctic oceanic changes

The Drake Passage is located between South America and Antarctica. Once the Tasmanian Gateway between Australia and Antarctica opened, all that kept Antarctica from being completely isolated by the Southern Ocean was its connection to South America. As the South American continent moved north, the Drake Passage opened and enabled the formation of the Antarctic Circumpolar Current (ACC), which would have kept the cold waters of Antarctica circulating around that continent and strengthened the formation of Antarctic Bottom Water (ABW). With the cold water concentrated around Antarctica, sea surface temperatures and, consequently, continental temperatures would have dropped. The onset of Antarctic glaciation occurred during the early Oligocene, and the effect of the Drake Passage opening on this glaciation has been the subject of much research. However, some controversy still exists as to the exact timing of the passage opening, whether it occurred at the start of the Oligocene or nearer the end. Even so, many theories agree that at the Eocene/Oligocene (E/O) boundary, a yet shallow flow existed between South America and Antarctica, permitting the start of an Antarctic Circumpolar Current.

Stemming from the issue of when the opening of the Drake Passage took place, is the dispute over how great of an influence the opening of the Drake Passage had on the global climate. While early researchers concluded that the advent of the ACC was highly important, perhaps even the trigger, for Antarctic glaciation and subsequent global cooling, other studies have suggested that the δ¹⁸O signature is too strong for glaciation to be the main trigger for cooling. Through study of Pacific Ocean sediments, other researchers have shown that the transition from warm Eocene ocean temperatures to cool Oligocene ocean temperatures took only 300,000 years, which strongly implies that feedbacks and factors other than the ACC were integral to the rapid cooling.

The late Oligocene opening of the Drake Passage

The latest hypothesized time for the opening of the Drake Passage is during the early Miocene. Despite the shallow flow between South America and Antarctica, there was not enough of a deep water opening to allow for significant flow to create a true Antarctic Circumpolar Current. If the opening occurred as late as hypothesized, then the Antarctic Circumpolar Current could not have had much of an effect on early Oligocene cooling, as it would not have existed.

The early Oligocene opening of the Drake Passage

The earliest hypothesized time for the opening of the Drake Passage is around 30 Ma. One of the possible issues with this timing was the continental debris cluttering up the seaway between the two plates in question. This debris, along with what is known as the Shackleton Fracture Zone, has been shown in a recent study to be fairly young, only about 8 million years old. The study concludes that the Drake Passage would be free to allow significant deep water flow by around 31 Ma. This would have facilitated an earlier onset of the Antarctic Circumpolar Current.

Currently, an opening of the Drake Passage during the early Oligocene is favored.

The opening of the Tasman Gateway

The other major oceanic gateway opening during this time was the Tasman, or Tasmanian, depending on the paper, gateway between Australia and Antarctica. The time frame for this opening is less disputed than the Drake Passage and is largely considered to have occurred around 34 Ma. As the gateway widened, the Antarctic Circumpolar Current strengthened.

The Tethys Seaway closing

The Tethys Seaway was not a gateway, but rather a sea in its own right. Its closing during the Oligocene had significant impact on both ocean circulation and climate. The collisions of the African plate with the European plate and of the Indian subcontinent with the Asian plate, cut off the Tethys Seaway that had provided a low-latitude ocean circulation. The closure of Tethys built some new mountains (the Zagros range) and drew down more carbon dioxide from the atmosphere, contributing to global cooling.

Greenland–Iceland–Faroes

The gradual separation of the clump of continental crust and the deepening of the tectonic ridge in the North Atlantic that would become Greenland, Iceland, and the Faroe Islands helped to increase the deep water flow in that area. More information about the evolution of North Atlantic Deep Water will be given a few sections down.

Ocean cooling

Evidence for ocean-wide cooling during the Oligocene exists mostly in isotopic proxies. Patterns of extinction and patterns of species migration can also be studied to gain insight into ocean conditions. For a while, it was thought that the glaciation of Antarctica may have significantly contributed to the cooling of the ocean, however, recent evidence tends to deny this.

Deep water

Reconstruction of Aglaocetus moreni

Isotopic evidence suggests that during the early Oligocene, the main source of deep water was the North Pacific and the Southern Ocean. As the Greenland-Iceland-Faroe Ridge sank and thereby connected the Norwegian–Greenland sea with the Atlantic Ocean, the deep water of the North Atlantic began to come into play as well. Computer models suggest that once this occurred, a more modern in appearance thermo-haline circulation started.

North Atlantic deep water

Evidence for the early Oligocene onset of chilled North Atlantic deep water lies in the beginnings of sediment drift deposition in the North Atlantic, such as the Feni and Southeast Faroe drifts.

South Ocean deep water

The chilling of the South Ocean deep water began in earnest once the Tasmanian Gateway and the Drake Passage opened fully. Regardless of the time at which the opening of the Drake Passage occurred, the effect on the cooling of the Southern Ocean would have been the same.

Impact events

Recorded extraterrestrial impacts:

• Haughton impact crater, Nunavut, Canada (23 Ma, crater 24 km (15 mi) diameter) (now considered questionable as an Oligocene event; later analyses have concluded the crater dates to 39 Ma, placing the event in the Eocene.)

Supervolcanic explosions

• La Garita Caldera (28–26 million years ago)
• Wah Wah Springs Caldera (30 million years ago)

IPCC Sixth Assessment Report

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/IPCC_Sixth_Assessment_Report 

 

Intergovernmental Panel on Climate Change

The Sixth Assessment Report (AR6) of the United Nations (UN) Intergovernmental Panel on Climate Change (IPCC) is the sixth in a series of reports which assess scientific, technical, and socio-economic information concerning climate change. Three Working Groups (WGI, II, and III) have been working on the following topics: The Physical Science Basis (WGI); Impacts, Adaptation and Vulnerability (WGII); Mitigation of Climate Change (WGIII). Of these, the first study was published in 2021, the second report February 2022, and the third in April 2022. The final synthesis report is due to be finished by late 2022.

The first of the three working groups published its report on 9 August 2021, Climate Change 2021: The Physical Science Basis. A total of 234 scientists from 66 countries contributed to this first working group (WGI) report. The authors built on more than 14,000 scientific papers to produce a 3,949-page report, which was then approved by 195 governments. The Summary for Policymakers (SPM) document was drafted by scientists and agreed to line-by-line by the 195 governments in the IPCC during the five days leading up to 6 August 2021.

According to the WGI report, it is only possible to avoid warming of 1.5 °C (2.7 °F) or 2.0 °C (3.6 °F) if massive and immediate cuts in greenhouse gas emissions are made. In a front-page story, The Guardian described the report as "its starkest warning yet" of "major inevitable and irreversible climate changes", a theme echoed by many newspapers as well as political leaders and activists around the world.

Production

History

After the IPCC had been founded in 1988, the First Assessment Report (AR1) was published in 1990 and received an update in 1992. In intervals of about six years, new editions of IPCC Assessment Report followed: AR2 in 1995, AR3 in 2001, AR4 in 2007, and AR5 in 2014.

In April 2016, at the 43rd session which took place in Nairobi, Kenya, the topics for three Special Reports (SR) and one methodology report on Greenhouse Gases (GHG) inventories in the AR6 assessment cycle were decided. These reports were completed in the interim phase since the finalisation of the Fifth Assessment Report and the publication of results from the Sixth Assessment Report.

Sequence of release dates:

• Special Report on Global Warming of 1.5 °C (SR15) in October 2018
  • 2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories in May 2019
• Special Report on Climate Change and Land (SRCCL) in August 2019
• Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC) in September 2019

Structure

The sixth assessment report is made up of the reports of three working groups (WG I, II, and III) and a synthesis report which concludes the assessment in late 2022.

• The Physical Science Basis of Climate Change in August 2021 (WGI contribution)
• Impacts, Adaptation and Vulnerability in February 2022 (WGII contribution)
• Mitigation of Climate Change in April 2022 (WGIII contribution)
• Synthesis Report in October 2022

Leaks

During the preparation of the three main AR6 reports, a small group of scientists leaked some information on the results of Working Group III (Mitigation of Climate Change) through the organization Scientist Rebellion. As governments can change the summaries for policymakers (SPM) for IPCC reports, the scientists were afraid that politicians might dilute this information in the summary. According to the leaked information, humanity should cut GHG emissions by 50% by 2030 and completely by 2050 in order to limit warming to 1.5 °C (2.7 °F). These efforts require strong changes in lifestyle and economy.

Geopolitics

Geopolitics has been included in climate models for the first time, in the form of five Shared Socioeconomic Pathways: SSP1 "Taking the Green Road", SSP2 "Middle of the Road", SSP3 "A Rocky Road", SSP4 "A Road Divided", and SSP5 "Taking the Highway", which have been published in 2016.

Those pathways assume that international cooperation and worldwide increase in GDP will facilitate adaptation to climate change. The geopolitical pathways served as one of the sources for the formation of the Shared Socioeconomic Pathways in the report among with other sources. One of the assumptions is that enough GDP and technology derived from fossil fuels development will permit to adapt even to 5.0 °C (9.0 °F) temperature rise. However, the report that is based on consensus science and was written by hundreds of scientists did not confirm the assumption about adaptation to 5 °C. Some experts assume, that while the odds for a worst-case scenario (5 °C) and the best base-case (1.5 °C) today seem lower, the most plausible outcome is around 3.0 °C (5.4 °F).

The report explicitly says: "Each pathway is an internally consistent, plausible and integrated description of a socio-economic future, but these socio-economic futures do not account for the effects of climate change, and no new climate policies are assumed. ... By design, the evolution of drivers and emissions within the SSP scenarios do not take into account the effects of climate change."

Like other major international scientific processes, the IPCC has been accused of not sufficiently including scholars from the Global South. For example, the biases were highlighted that prevent African scholars from participating, such as publication requirements and being an expert reviewer before joining the panel of contributors. Similarly, the physical sciences report only had 28% women in its team of authors.

The Physical Science Basis (Working Group 1 report)

Variation of annual observed global average temperature (1850–2019) relative to the 1850–1900 average (blue line), as reported in the Summary for Policymakers (SPM)

A total of 234 scientists from 66 countries contributed to the first of three working group reports. Working group 1 (WGI) published Climate Change 2021: The Physical Science Basis. The report's authors built on more than 14,000 scientific papers to produce a 3,949-page report, which was then approved by 195 governments. The Summary for Policymakers (SPM) document was drafted by scientists and agreed to line-by-line by the 195 governments in the IPCC during the five days leading up to 6 August 2021. It was published on Monday, 9 August 2021.

According to the report, it is only possible to avoid warming of 1.5 °C or 2 °C if massive and immediate cuts in greenhouse gas emissions are made. In a front-page story, The Guardian described the report as "its starkest warning yet" of "major inevitable and irreversible climate changes", a theme echoed by many newspapers around the world.

The Technical Summary (TS) provides a level of detail between the Summary for Policymakers (SPM) and the full report. In addition, an interactive atlas was made "for a flexible spatial and temporal analysis of both data-driven climate change information and assessment findings in the report." Following IPCC protocol adopted in May 2011, errata were and are being compiled. Thirty-four questions were published as part of the FAQs section, with each one related to a chapter of the report. Regional fact sheets were made available for geographic regions of the globe, drawing on facts from the reports. The data for the SPM are being held and visible at the UK Centre for Environmental Data Analysis website. Computer slides were made available as part of the "Outreach Materials" , for press conferences and basic presentations.

Findings

The Working Group 1 (WGI) report, Climate Change 2021: The Physical Science Basis comprises thirteen chapters and is focused on the foundational consensus of the climate science behind the causes and effects of human greenhouse gas emissions. Compared with previous assessments, the report included much more detail on the regional effects of climate change, although more research is needed on climate change in eastern and central North America. Sea-level rise by 2100 is likely to be from half to one metre, but two to five metres is not ruled out, as ice sheet instability processes are still poorly understood.

The report quantifies climate sensitivity as between 2.5 °C (4.5 °F) and 4.0 °C (7.2 °F) for each doubling of carbon dioxide in the atmosphere, while the best estimate is 3 °C. In all the represented Shared Socioeconomic Pathways the temperature reaches the 1.5 °C warming limit, at least for some period of time in the middle of the 21st century. However, Joeri Rogelj, director of the Grantham Institute and a lead IPCC author, said that it is possible to completely avoid warming of 1.5 °C, but to achieve that the world would need to cut emissions by 50% by the year 2030 and by 100% by the year 2050. If the world does not begin to drastically cut emissions by the time of the next report of the IPCC, then it will no longer be possible to prevent 1.5 °C of warming. SSP1-1.9 is a new pathway with a rather low radiative forcing of 1.9 W/m² in 2100 to model how people could keep warming below the 1.5 °C threshold. But, even in this scenario, the global temperature peaks at 1.6 °C in the years 2041–2060 and declines after.

Shared Socioeconomic Pathways in the IPCC Sixth Assessment Report 

SSP	Scenario	Estimated warming (2041–2060)	Estimated warming (2081–2100)	Very likely range in °C (2081–2100)
SSP1-1.9	very low GHG emissions: CO2 emissions cut to net zero around 2050	1.6 °C	1.4 °C	1.0 – 1.8
SSP1-2.6	low GHG emissions: CO2 emissions cut to net zero around 2075	1.7 °C	1.8 °C	1.3 – 2.4
SSP2-4.5	intermediate GHG emissions: CO2 emissions around current levels until 2050, then falling but not reaching net zero by 2100	2.0 °C	2.7 °C	2.1 – 3.5
SSP3-7.0	high GHG emissions: CO2 emissions double by 2100	2.1 °C	3.6 °C	2.8 – 4.6
SSP5-8.5	very high GHG emissions: CO2 emissions triple by 2075	2.4 °C	4.4 °C	3.3 – 5.7

The IPCC Sixth report did not estimate the likelihoods of the scenarios but a 2020 commentary described SSP5-8.5 as highly unlikely, SSP3-7.0 as unlikely, and SSP2-4.5 as likely.

However, a report citing the above commentary shows that RCP8.5 is the best match to the cumulative emissions from 2005 to 2020. 

According to AR6 coauthors, the probable temperature rise is in the middle of the scenario spectrum that reaches from 1.5 °C to 5 °C, at about 3 °C at the end of the century. It is likely that 1.5 °C will be reached before 2040. The threats from compound impacts are rated higher than in previous IPCC reports. The famous hockey stick graph has been extended.

Extreme weather is expected to increase in line with temperature, and compound effects (such as heat and drought together) may impact more on society. The report includes a major change from previous IPCC in the ability of scientists to attribute specific extreme weather events.

The global carbon budget to keep below 1.5 °C is estimated at 500 billion more tonnes of greenhouse gas, which would need the whole world to be net zero before 2050. Staying within this budget, if counting from the beginning of the year 2020, gives a 50% chance to stay below 1.5 °C. For having a 67% chance, the budget is 400 billion tonnes and for an 83% chance it is 300 billion tonnes. The report says that rapidly reducing methane emissions is very important, to make short-term gains to buy time for carbon dioxide emission cuts to take effect.

Any future warming will increase the occurrence of extreme weather events. Even in a 1.5 °C temperature rise there will be "an increasing occurrence of some extreme events unprecedented in the observational record". The likelihood of more rare events increases more.

The frequency, and the intensity of such events will considerably increase with warming, as described in the following table:

Increase in frequency and intensity of extreme events with global warming

Name of event	Climate in 1850–1900	1 °C warming	1.5 °C warming	2 °C warming	4 °C warming
1 in 10 years heatwave	Normal	2.8 times more often, 1.2 °C hotter	4.1 times more often, 1.9 °C hotter	5.6 times more often, 2.6 °C hotter	9.4 times more often, 5.1 °C hotter
1 in 50 years heatwave	Normal	4.8 times more often, 1.2 °C hotter	8.6 times more often, 2.0 °C hotter	13.9 times more often, 2.7 °C hotter	39.2 times more often, 5.3 °C hotter
1 in 10 years heavy precipitation event	Normal	1.3 times more often, 6.7% wetter	1.5 times more often, 10.5% wetter	1.7 times more often, 14.0% wetter	2.7 times more often, 30.2% wetter
1 in 10 years drought	Normal	1.7 times more often, 0.3 sd drier	2.0 times more often, 0.5 sd drier	2.4 times more often, 0.6 sd drier	4.1 times more often, 1.0 sd drier

Increase in frequency of extreme events with global warming in the Sixth Assessment Report's Summary for Policymakers

Reception

In science

The publication of the report was during the Northern Hemisphere summer, where there was much extreme weather, such as a Western North America heat wave, flooding in Europe, extreme rainfall in India and China, and wildfires in several countries. Some scientists are describing these extreme weather events as clear gaps in the models used for writing the report, with the lived experience proving more severe than the consensus science.

In politics

After publication of the Working Group 1 report, EU Vice President Frans Timmermans said that it is not too late to prevent runaway climate change. UK Prime Minister Boris Johnson said that the next decade will be pivotal to the future of the planet.

Rick Spinrad, administrator of the US's National Oceanic and Atmospheric Administration, stated that his agency "will use the new insights from this IPCC report to inform the work it does with communities to prepare for, respond to, and adapt to climate change".

NGO

Swedish climate activist Greta Thunberg said that the report "confirms what we already know from thousands [of] previous studies and reports—that we are in an emergency".

In media

In a front-page story, dedicated to the report The Guardian described it as "starkest warning yet" of "major inevitable and irreversible climate changes". This message was echoed by many media channels after the release of the report.

According to CTXT, the publication that first posted the leaked materials from the report: "it showed that the global economy must be shifted rapidly away from a reliance on conventional GDP growth, but that the report underplays this."

From the United Nations

The Secretary-General of the UN, António Guterres, called the report a "code red for humanity".

Climate Change 2022: Impacts, Adaptation & Vulnerability (Working Group 2 report)

The second part of the report, a contribution of working group II (WGII), was published on 28 February 2022. Entitled Climate Change 2022: Impacts, Adaptation & Vulnerability, the full report is 3675 pages, plus a 37-page summary for policymakers. It contains information on the impacts of climate change on nature and human activity. Topics examined included biodiversity loss, migration, risks to urban and rural activities, human health, food security, water scarcity, and energy. It also assesses ways to address these risks and highlights how climate resilient development can be part of a larger shift towards sustainability.

The report found that climate impacts are at the high end of previous estimates, with all parts of the world being affected. At least 3.3 billion people, about 40% of the world population, now fall into the most serious category of "highly vulnerable", with the worst effects in the developing world. If emissions continue on their current path, Africa will lose 30% of its maize cultivation territory and 50% of its land cultivated for beans. One billion people face flooding due to sea level rise. Climate change, together with other factors, also increases the risk of infectious diseases outbreaks like the COVID-19 pandemic. The report also cites evidence that China will pay the highest financial cost if the temperature continue to rise. The impacts will include food insecurity, water scarcity, flooding, especially in coastal areas where most of the population lives due to higher than average sea level rise, and more powerful cyclones. At some point part of the country may face wet-bulb temperatures higher than humans and other mammals can tolerate more than six hours. Overall, the report identified 127 different negative impacts of climate change, some of them irreversible.

People can protect themselves to some degree from the effects of climate change, which is known as adaptation. Overall, progress on adaptation has been made in all sectors and regions, although this progress is unevenly distributed and many initiatives prioritise immediate risks over longer-term transformational changes. Still, there are feasible and effective adaptation options available and many adaption actions have benefits beyond reducing climate risks, including positive effects on the Sustainable Development Goals. For example, the majority of current adaptations address water-related risks; adaptations like improved water management, water storage and irrigation reduce vulnerability and can also provide economic and ecological benefits. Similarly, adaptation actions like agroforestry, farm- and landscape diversification and urban agriculture can increase food availability, while at the same time improving sustainability.

WGII further highlighted the need for conservation in order to maintain biodiversity, and mitigate the effects of climate change. The report reads, "Recent analyses, drawing on a range of lines of evidence, suggest that maintaining the resilience of biodiversity and ecosystem services at a global scale depends on effective and equitable conservation of approximately 30% to 50% of Earth's land, freshwater and ocean areas, including currently near-natural ecosystems." The report was critical of technological approaches to carbon dioxide removal, instead indicating that urbanisation could help drive adoption of mitigation strategies such as public transport and renewable energy. The report also warns there are high risks associated with strategies such as solar radiation management; planting forests in unnatural locations; or "poorly implemented bioenergy, with or without carbon capture and storage".

The report puts considerable emphasis on adaptation limits. It states that some human and natural systems already reached "soft adaptation limits" including human systems in Australia, Small Islands, America, Africa and Europe and some natural systems reach even the "hard adaptation limits" like part of corals, wetland, rainforests, ecosystems in polar and mountain regions. If the temperature rise will reach 1.5 °C (2.7 °F) additional ecosystems and human systems will reach hard adaptation limits, including regions depending on glaciers and snow water and small islands. At 2 °C (3.6 °F) temperature rise, soft limits will be reached by many staple crops in many areas while at 3 °C (5.4 °F) hard limits will be reached by parts of Europe.

In line with the emphasis on adaptation limits, the report also highlights loss and damage, meaning negative consequences of climate change that cannot be avoided through adaptation. The report states that such losses and damages are already widespread: droughts, floods and heatwaves are becoming more frequent, and a mass extinction is already underway. Taking near-term actions to limit warming to below 1.5°C would substantially reduce future losses and damages, but cannot eliminate them all. Previously, rich countries have resisted taking responsibility for these lossess.

The report states that even a temporary overshoot of the 1.5 degree limit will lead to negative effects on humans and ecosystems. According to the report: "Depending on the magnitude and duration of overshoot, some impacts will cause release of additional greenhouse gases (medium confidence) and some will be irreversible, even if global warming is reduced (high confidence)". Climate resilient development will be more difficult if the global temperature will rise by 1.5 degrees above pre-industrial levels, while if it will rise by more than 2 degrees it will become impossible "in some regions and sub-regions". Although the report's outlook is bleak, its conclusion argues that there is still time to limit warming to 1.5 °C (2.7 °F) by drastic cuts to greenhouse gas emission, but such action must be taken immediately. Moreover, climate resilient development can have both adaptation and mitigation benefits, but it requires international cooperation and collaborations with local communities and organisations.

Reactions

Responding to the report, António Guterres, Secretary-General of the United Nations, called it "an atlas of human suffering and a damning indictment of failed climate leadership" and "the facts are undeniable ... the world's biggest polluters are guilty of arson of our only home." The United States special presidential envoy for climate, John Kerry, said "We have seen the increase in climate-fuelled extreme events, and the damage that is left behind – lives lost and livelihoods ruined. The question at this point is not whether we can altogether avoid the crisis – it is whether we can avoid the worst consequences."

Environmentalist Inger Andersen commented: "Nature can be our saviour ... but only if we save it first."

The report was published during the first week of the 2022 Russian invasion of Ukraine. In the context of the conflict, the Ukrainian delegation connected the Russian aggression to the global dependency on oil, and a Russian official, Oleg Anisimov, apologized for the conflict despite the possible repercussions. The Ukrainian delegation also called for news reporting on the war not to overshadow the WGII report.

See also: Environmental impact of the 2022 Russian invasion of Ukraine

Climate Change 2022: Mitigation of Climate Change (Working Group 3 report)

The report was presented on 4 April 2022. The structure of the report was adopted in 2017. Some observers are worried that the conclusions might be watered down, considering the way the reports are adopted. According to The Observer, some countries "have sought to make changes that would weaken the final warnings".

WGIII found that "net anthropogenic GHG emissions have increased since 2010 across all major sectors globally. An increasing share of emissions can be attributed to urban areas. Emissions reductions in CO2 from fossil fuels and industrial processes, due to improvements in energy intensity of GDP and carbon intensity of energy, have been less than emissions increases from rising global activity levels in industry, energy supply, transport, agriculture and buildings."

Areas of focus

Matters covered by the report include:

• Trends and drivers of greenhouse gas emissions;
• Emission reduction pathways that match up with long-term net-zero goals;
• Shorter-term pathways for emission reductions and their compatibility with “national development objectives” for job creation, competitiveness, poverty, sustainable development, and more;
• Social aspects of greenhouse gas emission reductions, including objectives to meet human needs under the UN Sustainable Development Goals;
• Energy systems;
• Agriculture, forestry, and other land uses;
• Cities and other human settlements;
• Buildings;
• Transportation;
• Industry;
• Costs and opportunities across different economic sectors;
• National and sub-national policies and institutions;
• International cooperation;
• Investment and finance;
• Innovation, technology, and technological transfer;
• Connections between sustainable development and the response to climate change.

Important findings

The report uses some new approaches like to include different social aspects, the participation of youth, indigenous people, cities, businesses in the solution. It states that "International cooperation is a critical enabler for achieving ambitious climate change mitigation goals." International cooperation has positive and measurable effect on climate mitigation. It provides critical support for many mitigation measures. Participation in international agreements leads to adoption of climate policies. For preventing global temperature from rising more than 2 degrees above the preindustrial level, international cooperation needs to be much stronger than now as many developing countries need support from other countries higher than present for strong climate action.

According to the report demand side mitigation measures can reduce GHG emissions by 40–70% by the year 2050 compared to scenarios in which countries will fulfill its national pledges given before 2020. For being implemented successfully those measures should be linked "with improving basic wellbeing for all".

The report says that for achieving net zero reduction it is necessary to use carbon dioxide removal. The report compares different methods of carbon dioxide removal (CDR) including agroforestry, reforestation, blue carbon management, restoration of peatland and others.

Cities have big potential for reducing greenhouse gas emissions. Without any action cities supposed to emit 65 GtCO₂-eq by the year 2050. With full scale mitigation action the emissions will be near zero, in the worst case they will be only 3 GtCO₂-eq. City planning, supporting mixed use of space, transit, walking, cycling, sharing vehicles can reduce urban emissions by 23–26%. Urban forests, lakes and other blue and green infrastructure can reduce emissions as directly and indirectly (trough reduce in energy demand for cooling for example).

Buildings received significant attention. Buildings emitted 21% of global GHG emissions in the year 2019. 80–90% of their emissions can be cut while helping to achieve other Sustainable Development Goals. The report introduces a new scheme for reducing GHG emissions in buildings: SER = Sufficiency, Efficiency, Renewable. Sufficiency measures do not need very complex technology, energy supply, maintenance or replacement during the life of the building. Those include, natural ventilation, green roofs, white walls, mixed use of spaces, collective use of devices etc. There are multiple links between emissions from buildings and emissions from other sectors including those discussed in chapters 6, 7, 8, 10 and 11. Reducing GHG emissions from buildings is linked to sharing economy and circular economy.

The report found that there is no evidence that sustainable development requires fossil fuels. Climate journalist Amy Westervelt reacting to the report, described this finding as one of the most radical, debunking a common refrain by energy poverty advocates, that development requires use of fossil fuels.

The IPCC found that decent living standards need less energy than was thought before. According to the report for reaching well being for all, the needed energy consumption is "between 20 and 50 GJ cap-1 yr-1 depending on the context." More equitable income distribution can lower emissions. Mitigation pathways based on low demand and high efficiency can achieve decent living standards and well being for all. Pathways based on reducing consumption, involving sustainable development have less negative outcomes than pathways based on high consumption and narrow mitigation. Table TS.29 shows that mitigation measures in the urban, buildings, AFOLU, transport sectors have considerably more synergies and less trade offs with sustainable development goals that measures in the energy sector while the second (less trade offs) is true also for the industry sector. According to table TS30 narrow mitigation can increase habitat loss by 600%, while avoiding habitat degradation by around 95%. Mitigation with sustainable development did not harm forest cover and biodiversity.

International cooperation gives the possibility to achieve climate change mitigation without compromising sustainable development goals.

The report mentions some improvement in global climate action. For example, the rate of deforestation slowed after 2010 and the total forest cover increased in the latest years due to reforestation in Europe, Asia and North America.

Responses

The Secretary-General of the United Nations, António Guterres, said the report described "litany of broken climate promises [by policy makers]" and in his remarks called for more action, saying "Climate activists are sometimes depicted as dangerous radicals. But, the truly dangerous radicals are the countries that are increasing the production of fossil fuels."