A Medley of Potpourri

The Miocene (/ˈmaɪ.əsiːn, -oʊ-/ MY-ə-seen, -⁠oh-) is the first geological epoch of the Neogene Period and extends from about 23.03 to 5.333 million years ago (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words μείων (meíōn, "less") and καινός (kainós, "new") and means "less recent" because it has 18% fewer modern marine invertebrates than the Pliocene has. The Miocene is preceded by the Oligocene and is followed by the Pliocene.

As Earth went from the Oligocene through the Miocene and into the Pliocene, the climate slowly cooled towards a series of ice ages. The Miocene boundaries are not marked by a single distinct global event but consist rather of regionally defined boundaries between the warmer Oligocene and the cooler Pliocene Epoch.

During the Early Miocene, the Arabian Peninsula collided with Eurasia, severing the connection between the Mediterranean and Indian Ocean, and allowing a faunal interchange to occur between Eurasia and Africa, including the dispersal of proboscideans into Eurasia. During the late Miocene, the connections between the Atlantic and Mediterranean closed, causing the Mediterranean Sea to nearly completely evaporate, in an event called the Messinian salinity crisis. The Strait of Gibraltar opened and the Mediterranean refilled at the Miocene–Pliocene boundary, in an event called the Zanclean flood.

The apes first evolved, arose, and diversified during the early Miocene (Aquitanian and Burdigalian Stages), becoming widespread in the Old World. By the end of this epoch and the start of the following one, the ancestors of humans had split away from the ancestors of the chimpanzees to follow their own evolutionary path during the final Messinian Stage (7.5–5.3 Ma) of the Miocene. As in the Oligocene before it, grasslands continued to expand and forests to dwindle in extent. In the seas of the Miocene, kelp forests made their first appearance and soon became one of Earth's most productive ecosystems.

The plants and animals of the Miocene were recognizably modern. Mammals and birds were well-established. Whales, pinnipeds, and kelp spread.

The Miocene is of particular interest to geologists and palaeoclimatologists as major phases of the geology of the Himalaya occurred during the Miocene, affecting monsoonal patterns in Asia, which were interlinked with glacial periods in the northern hemisphere.

Subdivisions

Subdivisions of the Miocene

The Miocene faunal stages from youngest to oldest are typically named according to the International Commission on Stratigraphy:

Sub-epoch	Faunal stage	Time range
Late Miocene	Messinian	7.246–5.333 Ma
Late Miocene	Tortonian	11.63–7.246 Ma
Middle Miocene	Serravallian	13.82–11.63 Ma
Middle Miocene	Langhian	15.97–13.82 Ma
Early Miocene	Burdigalian	20.44–15.97 Ma
Early Miocene	Aquitanian	23.03–20.44 Ma

Regionally, other systems are used, based on characteristic land mammals; some of them overlap with the preceding Oligocene and following Pliocene Epochs:

European Land Mammal Ages

Turolian (9.0 to 5.3 Ma)
Vallesian (11.6 to 9.0 Ma)
Astaracian (16.0 to 11.6 Ma)
Orleanian (20.0 to 16.0 Ma)
Agenian (23.8 to 20.0 Ma)

North American Land Mammal Ages

Hemphillian (10.3 to 4.9 Ma)
Clarendonian (13.6 to 10.3 Ma)
Barstovian (16.3 to 13.6 Ma)
Hemingfordian (20.6 to 16.3 Ma)
Arikareean (30.6 to 20.6 Ma)

South American Land Mammal Ages

Montehermosan (6.8 to 4.0 Ma)
Huayquerian (9.0 to 6.8 Ma)
Mayoan (11.8 to 9.0 Ma)
Laventan (13.8 to 11.8 Ma)
Colloncuran (15.5 to 13.8 Ma)
Friasian (16.3 to 15.5 Ma)
Santacrucian (17.5 to 16.3 Ma)
Colhuehuapian (21.0 to 17.5 Ma)

Paleogeography

Japan during the Early Miocene

The Mediterranean during the Late Miocene

Continents continued to drift toward their present positions. Of the modern geologic features, only the land bridge between South America and North America was absent, although South America was approaching the western subduction zone in the Pacific Ocean, causing both the rise of the Andes and a southward extension of the Meso-American peninsula.

Mountain building took place in western North America, Europe, and East Asia. Both continental and marine Miocene deposits are common worldwide with marine outcrops common near modern shorelines. Well studied continental exposures occur in the North American Great Plains and in Argentina.

India continued to collide with Asia, creating dramatic new mountain ranges. The Tethys seaway continued to shrink and then disappeared as Africa collided with Eurasia in the Turkish–Arabian region between 19 and 12 Ma. The subsequent uplift of mountains in the western Mediterranean region and a global fall in sea levels combined to cause a temporary drying up of the Mediterranean Sea (known as the Messinian salinity crisis) near the end of the Miocene.

The global trend was towards increasing aridity caused primarily by global cooling reducing the ability of the atmosphere to absorb moisture. Uplift of East Africa in the late Miocene was partly responsible for the shrinking of tropical rain forests in that region, and Australia got drier as it entered a zone of low rainfall in the Late Miocene.

At the beginning of the Miocene, the northern margin of the Arabian plate collided with Eurasia, causing the closure of the Indian Ocean-Mediterranean Seaway, severing the connection between the two bodies of water, and forming a land connection between Afro-Arabia and Eurasia.

South America

During the Oligocene and Early Miocene the coast of northern Brazil, Colombia, south-central Peru, central Chile and large swathes of inland Patagonia were subject to a marine transgression. The transgressions in the west coast of South America are thought to be caused by a regional phenomenon while the steadily rising central segment of the Andes represents an exception. While there are numerous registers of Oligo-Miocene transgressions around the world it is doubtful that these correlate.

It is thought that the Oligo-Miocene transgression in Patagonia could have temporarily linked the Pacific and Atlantic Oceans, as inferred from the findings of marine invertebrate fossils of both Atlantic and Pacific affinity in La Cascada Formation. Connection would have occurred through narrow epicontinental seaways that formed channels in a dissected topography.

The Antarctic Plate started to subduct beneath South America 14 million years ago in the Miocene, forming the Chile Triple Junction. At first the Antarctic Plate subducted only in the southernmost tip of Patagonia, meaning that the Chile Triple Junction lay near the Strait of Magellan. As the southern part of Nazca Plate and the Chile Rise became consumed by subduction the more northerly regions of the Antarctic Plate begun to subduct beneath Patagonia so that the Chile Triple Junction advanced to the north over time. The asthenospheric window associated to the triple junction disturbed previous patterns of mantle convection beneath Patagonia inducing an uplift of ca. 1 km that reversed the Oligocene–Miocene transgression.

As the southern Andes rose in the Middle Miocene (14–12 million years ago) the resulting rain shadow originated the Patagonian Desert to the east.

Climate

Climates remained moderately warm, although the slow global cooling that eventually led to the Pleistocene glaciations continued.

Although a long-term cooling trend was well underway, there is evidence of a warm period during the Miocene when the global climate rivalled that of the Oligocene. The Miocene warming began 21 million years ago and continued until 14 million years ago, when global temperatures took a sharp drop—the Middle Miocene Climate Transition (MMCT). By 8 million years ago, temperatures dropped sharply once again, and the Antarctic ice sheet was already approaching its present-day size and thickness. Greenland may have begun to have large glaciers as early as 7 to 8 million years ago, although the climate for the most part remained warm enough to support forests there well into the Pliocene.

Life

Life during the Miocene Epoch was mostly supported by the two newly formed biomes, kelp forests and grasslands. Grasslands allow for more grazers, such as horses, rhinoceroses, and hippos. Ninety-five percent of modern plants existed by the end of this epoch. Modern bony fish genera were established.

Flora

The dragon blood tree is considered a remnant of the Mio-Pliocene Laurasian subtropical forests that are now almost extinct in North Africa.

The coevolution of gritty, fibrous, fire-tolerant grasses and long-legged gregarious ungulates with high-crowned teeth, led to a major expansion of grass-grazer ecosystems, with roaming herds of large, swift grazers pursued by predators across broad sweeps of open grasslands, displacing desert, woodland, and browsers.

The higher organic content and water retention of the deeper and richer grassland soils, with long-term burial of carbon in sediments, produced a carbon and water vapor sink. This, combined with higher surface albedo and lower evapotranspiration of grassland, contributed to a cooler, drier climate. C₄ grasses, which are able to assimilate carbon dioxide and water more efficiently than C₃ grasses, expanded to become ecologically significant near the end of the Miocene between 6 and 7 million years ago. The expansion of grasslands and radiations among terrestrial herbivores correlates to fluctuations in CO₂.

Cycads between 11.5 and 5 million years ago began to rediversify after previous declines in variety due to climatic changes, and thus modern cycads are not a good model for a "living fossil". Eucalyptus fossil leaves occur in the Miocene of New Zealand, where the genus is not native today, but have been introduced from Australia.

Fauna

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Hominini

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Oreopithecus

Sahelanthropus

Orrorin

Ardipithecus

Australopithecus

Homo habilis

Homo erectus

Homo bodoensis

Homo sapiens

Neanderthals,Denisovans

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Cameloid footprint (Lamaichnum alfi Sarjeant and Reynolds, 1999; convex hyporelief) from the Barstow Formation (Miocene) of Rainbow Basin, California.

Both marine and continental fauna were fairly modern, although marine mammals were less numerous. Only in isolated South America and Australia did widely divergent fauna exist.

In the Early Miocene, several Oligocene groups were still diverse, including nimravids, entelodonts, and three-toed equids. Like in the previous Oligocene Epoch, oreodonts were still diverse, only to disappear in the earliest Pliocene. During the later Miocene mammals were more modern, with easily recognizable canids, bears, red pandas, procyonids, equids, beavers, deer, camelids, and whales, along with now extinct groups like borophagine canids, certain gomphotheres, three-toed horses, and hornless rhinos like Teleoceras and Aphelops. Islands began to form between South and North America in the Late Miocene, allowing ground sloths like Thinobadistes to island-hop to North America. The expansion of silica-rich C₄ grasses led to worldwide extinctions of herbivorous species without high-crowned teeth.

Unequivocally recognizable dabbling ducks, plovers, typical owls, cockatoos and crows appear during the Miocene. By the epoch's end, all or almost all modern bird groups are believed to have been present; the few post-Miocene bird fossils which cannot be placed in the evolutionary tree with full confidence are simply too badly preserved, rather than too equivocal in character. Marine birds reached their highest diversity ever in the course of this epoch.

The youngest representatives of Choristodera, an extinct order of aquatic reptiles that first appeared in the Middle Jurassic, are known from the Miocene of Europe, belonging to the genus Lazarussuchus, which had been the only known surviving genus of the group since the beginning of the Eocene.

The last known representativeof the archaic primitive mammal order Meridiolestida, which dominated South America during the Late Cretaceous, are known from the Miocene of Patagonia, represented by the mole-like Necrolestes.

The youngest known representatives of metatherians (marsupial relatives) in the Northern Hemisphere landmasses (Asia, North America and Europe) and Africa are known from the Miocene, including the North American herpetotheriid Herpetotherium, the European herpetotheriid Amphiperatherium, the peradectids Siamoperadectes and Sinoperadectes from Asia, and the possible herpetotheriid Morotodon from the late Early Miocene of Uganda.

Approximately 100 species of apes lived during this time, ranging throughout Africa, Asia and Europe and varying widely in size, diet, and anatomy. Due to scanty fossil evidence it is unclear which ape or apes contributed to the modern hominid clade, but molecular evidence indicates this ape lived between 18 and 13 million years ago. The first hominins (bipedal apes of the human lineage) appeared in Africa at the very end of the Miocene, including Sahelanthropus, Orrorin, and an early form of Ardipithecus (A. kadabba) The chimpanzee–human divergence is thought to have occurred at this time.

The expansion of grasslands in North America also led to an explosive radiation among snakes. Previously, snakes were a minor component of the North American fauna, but during the Miocene, the number of species and their prevalence increased dramatically with the first appearances of vipers and elapids in North America and the significant diversification of Colubridae (including the origin of many modern genera such as Nerodia, Lampropeltis, Pituophis and Pantherophis).

Fossils from the Calvert Formation, Zone 10, Calvert Co., MD (Miocene)

A Miocene crab (Tumidocarcinus giganteus) from the collection of the Children's Museum of Indianapolis

In the oceans, brown algae, called kelp, proliferated, supporting new species of sea life, including otters, fish and various invertebrates.

Cetaceans attained their greatest diversity during the Miocene, with over 20 recognized genera of baleen whales in comparison to only six living genera. This diversification correlates with emergence of gigantic macro-predators such as megatoothed sharks and raptorial sperm whales. Prominent examples are O. megalodon and L. melvillei. Other notable large sharks were O. chubutensis, Isurus hastalis, and Hemipristis serra.

Crocodilians also showed signs of diversification during Miocene. The largest form among them was a gigantic caiman Purussaurus which inhabited South America. Another gigantic form was a false gharial Rhamphosuchus, which inhabited modern age India. A strange form, Mourasuchus also thrived alongside Purussaurus. This species developed a specialized filter-feeding mechanism, and it likely preyed upon small fauna despite its gigantic size. The youngest members of Sebecidae, a clade of terrestrial crocodylfomes distantly related to modern crocodilians, are known from the Miocene of South America.

The pinnipeds, which appeared near the end of the Oligocene, became more aquatic. A prominent genus was Allodesmus. A ferocious walrus, Pelagiarctos may have preyed upon other species of pinnipeds including Allodesmus.

Furthermore, South American waters witnessed the arrival of Megapiranha paranensis, which were considerably larger than modern age piranhas.

New Zealand's Miocene fossil record is particularly rich. Marine deposits showcase a variety of cetaceans and penguins, illustrating the evolution of both groups into modern representatives. The early Miocene Saint Bathans Fauna is the only Cenozoic terrestrial fossil record of the landmass, showcasing a wide variety of not only bird species, including early representatives of clades such as moas, kiwis and adzebills, but also a diverse herpetofauna of sphenodontians, crocodiles and turtle as well as a rich terrestrial mammal fauna composed of various species of bats and the enigmatic Saint Bathans Mammal.

Oceans

There is evidence from oxygen isotopes at Deep Sea Drilling Program sites that ice began to build up in Antarctica about 36 Ma during the Eocene. Further marked decreases in temperature during the Middle Miocene at 15 Ma probably reflect increased ice growth in Antarctica. It can therefore be assumed that East Antarctica had some glaciers during the early to mid Miocene (23–15 Ma). Oceans cooled partly due to the formation of the Antarctic Circumpolar Current, and about 15 million years ago the ice cap in the southern hemisphere started to grow to its present form. The Greenland ice cap developed later, in the Middle Pliocene time, about 3 million years ago.

Middle Miocene disruption

The "Middle Miocene disruption" refers to a wave of extinctions of terrestrial and aquatic life forms that occurred following the Miocene Climatic Optimum (18 to 16 Ma), around 14.8 to 14.5 million years ago, during the Langhian Stage of the mid-Miocene. A major and permanent cooling step occurred between 14.8 and 14.1 Ma, associated with increased production of cold Antarctic deep waters and a major growth of the East Antarctic ice sheet. A Middle Miocene δ¹⁸O increase, that is, a relative increase in the heavier isotope of oxygen, has been noted in the Pacific, the Southern Ocean and the South Atlantic.

Impact event

A large impact event occurred either during the Miocene (23 Ma – 5.3 Ma) or the Pliocene (5.3 Ma – 2.6 Ma). The event formed the Karakul crater (52 km diameter), in Tajikistan which is estimated to have an age of less than 23 Ma or less than 5 Ma.

Special Report on Global Warming of 1.5 °C

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Special_Report_on_Global_Warming_of_1.5_%C2%B0C

The Special Report on Global Warming of 1.5 °C (SR15) was published by the Intergovernmental Panel on Climate Change (IPCC) on 8 October 2018. The report, approved in Incheon, South Korea, includes over 6,000 scientific references, and was prepared by 91 authors from 40 countries. In December 2015, the 2015 United Nations Climate Change Conference called for the report. The report was delivered at the United Nations' 48th session of the IPCC to "deliver the authoritative, scientific guide for governments" to deal with climate change.

Its key finding is that meeting a 1.5 °C (2.7 °F) target is possible but would require "deep emissions reductions" and "rapid, far-reaching and unprecedented changes in all aspects of society." Furthermore, the report finds that "limiting global warming to 1.5 °C compared with 2 °C would reduce challenging impacts on ecosystems, human health and well-being" and that a 2 °C temperature increase would exacerbate extreme weather, rising sea levels and diminishing Arctic sea ice, coral bleaching, and loss of ecosystems, among other impacts. SR15 also has modelling that shows that, for global warming to be limited to 1.5 °C, "Global net human-caused emissions of carbon dioxide (CO2) would need to fall by about 45 percent from 2010 levels by 2030, reaching 'net zero' around 2050." The reduction of emissions by 2030 and its associated changes and challenges, including rapid decarbonisation, was a key focus on much of the reporting which was repeated through the world.

Main statements

Cover of the Special Report on Global Warming of 1.5 °C

Global warming will likely rise to 1.5 °C above pre-industrial levels between 2030 and 2052 if warming continues to increase at the current rate. SR15 provides a summary of, on one hand, existing research on the impact that a warming of 1.5 °C (equivalent to 2.7 °F) would have on the planet, and on the other hand, the necessary steps to limit global warming.

Even assuming full implementation of conditional and unconditional Nationally Determined Contributions submitted by nations in the Paris Agreement, net emissions would increase compared to 2010, leading to a warming of about 3 °C by 2100, and more afterwards. In contrast, limiting warming below or close to 1.5 °C would require to decrease net emissions by around 45% by 2030 and reach net zero by 2050 (i.e. keeping total cumulative emissions within a carbon budget). Even just for limiting global warming to below 2 °C, CO2 emissions should decline by 25% by 2030 and by 100% by 2075.

Pathways (i.e. scenarios and portfolios of mitigation options) that would allow such reduction by 2050 describe a rapid transition towards producing electricity through lower-emission methods, and increasing use of electricity instead of other fuels in sectors such as transportation. On average, the pathways describing the proportion of primary energy produced by renewables as increasing to 60%, while the proportion produced by coal drops to 5% and oil to 13%. Most pathways describe a larger role for nuclear energy and carbon capture and storage, and less usage of natural gas. They also assume that other measures are simultaneously undertaken: e.g. non-CO2 emissions (such as methane, black carbon, nitrous oxide) are to be similarly reduced, energy demand is unchanged, reduced by even 30% or offsetted by an unprecedented scale of carbon dioxide removal methods yet to be developed, while new policies and research allows to improve efficiency in agriculture and industry.

Pathways limiting global warming to 1.5 °C with no or limited overshoot would require rapid and far-reaching transitions in energy, land, urban and infrastructure (including transport and buildings), and industrial systems. These systems transitions are unprecedented in terms of scale, but not necessarily in terms of speed, and imply deep emissions reductions in all sectors, a wide portfolio of mitigation options and a significant upscaling of investments in those options. The rates of system changes [...] have occurred in the past within specific sectors, technologies and spatial contexts, but there is no documented historic precedent for their scale.
— IPCC, SR15 Summary for policymakers, p. 17

Impact of 1.5 °C or 2 °C warming

According to the report, with global warming of 1.5 °C there would be increased risks to "health, livelihoods, food security, water supply, human security, and economic growth." Impact vectors include reduction in crop yields and nutritional quality. Livestock are also affected with rising temperatures through "changes in feed quality, spread of diseases, and water resource availability." "Risks from some vector-borne diseases, such as malaria and dengue fever, are projected to increase."

"Limiting global warming to 1.5°C, compared with 2°C, could reduce the number of people both exposed to climate-related risks and susceptible to poverty by up to several hundred million by 2050." Climate-related risks associated with increasing global warming depend on geographic location, "levels of development and vulnerability", and the speed and reach of climate mitigation and climate adaptation practices. For example, "urban heat islands amplify the impacts of heatwaves in cities." In general, "countries in the tropics and Southern Hemisphere subtropics are projected to experience the largest impacts on economic growth."

Weather, sea level and ice

Many regions and seasons experience warming greater than the global annual average, e.g. "2–3 times higher in the Arctic. Warming is generally higher over land than over the ocean," and it correlates with temperature extremes (which are projected to warm up to twice more on land than the global mean surface temperature) as well as precipitation extremes (both heavy rain and droughts). The assessed levels of risk generally increased compared to the previous IPCC report.

The "global mean sea level is projected rise (relative to 1986–2005) by 0.26 to 0.77 m by 2100 for 1.5 °C global warming" and about 0.1 m more for 2 °C. A difference of 0.1 m may correspond to 10 million more or fewer people exposed to related risks. "Sea level rise will continue beyond 2100 even if global warming is limited to 1.5 °C. Around 1.5 °C to 2 °C of global warming," irreversible instabilities could be triggered in Antarctica and "Greenland ice sheet, resulting in multi-metre rise in sea level." "An ice-free Arctic summer is projected once per century" (per decade) for 1.5 °C (respectively 2 °C). "Limiting global warming to 1.5 °C rather than 2 °C is projected to prevent the thawing over centuries of a permafrost area in the range of 1.5 to 2.5 million km²."

Ecosystems

"A decrease in global annual catch for marine fisheries of about 1.5 or 3 million tonnes for 1.5 °C or 2 °C of global warming" is projected by one global fishery model cited in the report. Coral reefs are projected to decline by a further 70–90% at 1.5 °C, and even more than 99% at 2 °C. "Of 105,000 species studied, 18% of insects, 16% of plants and 8% of vertebrates fare projected to lose over half of their climatically determined geographic range for global warming of 2 °C."

Approximately "4% or 13% of the global terrestrial land area is projected to undergo a transformation of ecosystems from one type to another" at 1 °C or 2 °C, respectively. "High-latitude tundra and boreal forests are particularly at risk of climate change-induced degradation and loss, with woody shrubs already encroaching into the tundra and will proceed with further warming."

Limiting the temperature increase

Human activities (anthropogenic greenhouse gas emissions) have already contributed 0.8–1.2 °C (1.4–2.2 °F) of warming. Nevertheless, the gases which have been emitted so far are unlikely to cause global temperature to rise to 1.5 °C alone, meaning a global temperature rise to 1.5 °C above pre-industrial levels is avoidable, assuming net zero emissions are reached soon.

Carbon budget

Limiting global warming to 1.5 °C requires staying within a total carbon budget, i.e. limiting total cumulative emissions of CO₂. In other words, if net anthropogenic CO₂ emissions are kept above zero, a global warming of 1.5 °C and more will eventually be reached.

The value of the total net anthropogenic CO₂ budget since the pre-industrial era is not assessed in the report. Estimates of 400–800 GtCO₂ (gigatonnes of CO₂) for the remaining budget are given (580 GtCO₂ and 420 GtCO₂ for a 50% and 66% probability of limiting warming to 1.5 °C, using global mean surface air temperature (GSAT); or 770 and 570 GtCO₂, for 50% and 66% probabilities, using global mean surface temperature (GMST)). This is about 300 GtCO₂ more compared to a previous IPCC report, due to updated understanding and further advances in methods.

Emissions around the time of the report were depleting this budget at 42±3 GtCO₂ per year. Anthropogenic emissions from the pre-industrial period to the end of 2017 are estimated to have reduced the budget for 1.5 °C by approximately 2200±320 GtCO₂.

The estimates for the budget come with significant uncertainties, associated with: climate response to CO₂ and non-CO₂ emissions (these contribute about ±400 GtCO₂ in uncertainty), the level of historic warming (±250 GtCO₂), potential additional carbon release from future permafrost thawing and methane release from wetlands (reducing the budget by up to 100 GtCO₂ over the century), and the level of future non-CO₂ mitigation (±400 GtCO₂).

Necessary emission reductions

Current nationally stated mitigation ambitions, as submitted under the Paris Agreement, would lead to global greenhouse gas emissions of 52–58 GtCO₂eq per year, by 2030. "Pathways reflecting these ambitions would not limit global warming to 1.5 °C, even if supplemented by very challenging increases in the scale and ambition of emissions reductions after 2030." Instead, they are "broadly consistent" with a warming of about 3 °C by 2100, and more afterwards.

Limit global warming to 1.5 °C with no or limited overshoot would require reducing emissions to below 35 GtCO₂eq per year in 2030, regardless of the modelling pathway chosen. Most fall within 25–30 GtCO₂eq per year, a 40–50% reduction from 2010 levels.

The report says that for limiting warming to below 1.5 C "global net human-caused emissions of CO₂ would need to fall by about 45% from 2010 levels by 2030, reaching net zero around 2050." Even just for limiting global warming to below 2 °C, CO₂ emissions should decline by 25% by 2030 and by 100% by 2070.

Non-CO₂ emissions should decline in more or less similar ways. This involves deep reductions in emissions of methane and black carbon: at least 35% of both by 2050, relative to 2010, to limit warming near 1.5 °C. Such measures could be undertaken in the energy sector and by reducing nitrous oxide and methane from agriculture, methane from the waste sector, and some other sources of black carbon and hydrofluorocarbons.

On timescales longer than tens of years, it may still be necessary to sustain net negative CO₂ emissions and/or further reduce non-CO₂ radiative forcing (*), in order to prevent further warming (due to Earth system feedbacks), reverse ocean acidification, and minimise sea level rise.

(*) Non-CO₂ emissions included in this Report are all anthropogenic emissions other than CO₂ that result in radiative forcing. These include short-lived climate forcers, such as methane, some fluorinated gases, ozone precursors, aerosols or aerosol precursors, such as black carbon and sulphur dioxide, respectively, as well as long-lived greenhouse gases, such as nitrous oxide or some fluorinated gases. The radiative forcing associated with non-CO₂ emissions and changes in surface albedo is referred to as non-CO₂ radiative forcing.

Pathways to 1.5 °C

Various pathways are considered, describing scenarios for mitigation of global warming, including portfolios for energy supply and negative emission technologies (like afforestation or carbon dioxide removal).

Examples of actions consistent with the 1.5 °C pathway include "shifting to low- or zero-emission power generation, such as renewables; changing food systems, such as diet changes away from land-intensive animal products; electrifying transport and developing 'green infrastructure', such as building green roofs, or improving energy efficiency by smart urban planning, which will change the layout of many cities." As another example, an increase of forestation by 10,000,000 square kilometres (3,900,000 sq mi) by 2050 relative to 2010 would be required.

The pathways also assume an increase in annual investments in low-carbon energy technologies and energy efficiency by roughly a factor of four to ten by 2050 compared to 2015.

Model pathways with no or limited overshoot of 1.5 °C
P1	P2	P3	P4
A scenario with low energy demand (LED)	S1, based on SSP1	S2, based on SSP2	S5, based on SSP5
Grubler et al., 2018	Shared Socio-Economic Pathway 1 (SSP1: Sustainable development)	Shared Socio-Economic Pathway 2 (SSP2: Middle of the road)	Shared Socio-Economic Pathway 5 (SSP5: Fossil-fuelled development)

Carbon dioxide removal

The emission pathways that reach 1.5 °C contained in the report assume the use of carbon dioxide removal (CDR) to offset for remaining emissions. Pathways that overshoot the goal rely on CDR to remove carbon dioxide at a rate that exceeds remaining emissions in order to return to 1.5 °C. However, understanding is still limited about the effectiveness of net negative emissions to reduce temperatures after an overshoot. Reversing an overshoot of 0.2 °C might not be achievable given considerable implementation challenges. The report highlights a CDR technology called bioenergy with carbon capture and storage (BECCS). The report notes that apart from afforestation/reforestation and ecosystem restoration, "the feasibility of massive-scale deployment of many CDR technologies remains an open question", with areas of uncertainty regarding technology upscaling, governance, ethical issues, policy and carbon cycle. The report notes that CDR technology is in its infancy and the feasibility is an open question. Estimates from recent literature are cited, giving a potential of up to 5 GtCO2 per year for BECCS and up to 3.6 GtCO2 per year for afforestation.

Solar radiation management

The report describes several proposals for solar radiation management (SRM). It concludes that SRMs have potential to limit warming, but "face large uncertainties and knowledge gaps as well as substantial risks, [...] and constraints"; "the impacts of SRM (both biophysical and societal), costs, technical feasibility, governance and ethical issues associated need to be carefully considered." An analysis of the geoengineering proposals published in Nature Communication confirmed findings of the SR15, stating that "all are in early stages of development, involve substantial uncertainties and risks, and raise ethical and governance dilemmas. Based on present knowledge, climate geoengineering techniques cannot be relied on to significantly contribute to meeting the Paris Agreement temperature goals".

Process

There are three IPCC working groups: Working Group I (WG I), co-chaired by Valerie Masson-Delmotte and Panmao Zhai, covers the physical science of climate change. Working Group II (WG II), co-chaired by Hans-Otto Pörtner and Debra Roberts, examines "impacts, adaptation and vulnerability". The "mitigation of climate change" is dealt with by Working Group III (WG III), co-chaired by Priyardarshi Shukla and Jim Skea. The "Task Force on National Greenhouse Gas Inventories" "develops methodologies for measuring emissions and removals". There are also Technical Support Units that guide "the production of IPCC assessment reports and other products".

Contributors

Researchers from 40 countries, representing 91 authors and editors contributed to the report, which includes over 6,000 scientific references.

Reactions

Researchers

In his 1 October 2018 opening statement at the 48th Session held in Incheon, Korea, Hoesung Lee, who has been Chair of the IPCC since 6 October 2015, described this IPCC meeting as "one of the most important" in its history. Debra Roberts, IPCC contributor called it the "largest clarion bell from the science community". Roberts hopes "it mobilises people and dents the mood of complacency."

In a CBC interview, Paul Romer was asked if the Nobel Prize in economic sciences that he and William Nordhaus received shortly before the SR15 was released, was timed as a message. Romer said that he was optimistic that measures will be taken in time to avert climate catastrophe. Romer compared the angst and lack of political will in imposing a carbon tax to the initial angst surrounding the chlorofluorocarbon (CFC) ban and the positive impact it had on restoring the depleted ozone layer. In giving the Nobel to Nordhaus and Romer, the Royal Swedish Academy of Sciences cited Nordhaus as saying "the most efficient remedy for problems caused by greenhouse gases is a global scheme of universally imposed carbon taxes".

Howard J. Herzog, a senior research engineer at the Massachusetts Institute of Technology, said that carbon capture and storage technologies, except reforestation, are problematic because of their impact on the environment, health and high cost. In the article there is a link to another article that refers to a study published in the scientific journal "Nature Energy". The study says that we can limit warming to 1.5 degrees without carbon capture and storage, by technological innovation and changing lifestyle.

A 2021 study found that degrowth scenarios, where economic output either "declines" or declines in terms of contemporary economic metrics such as current GDP, have been neglected in considerations of 1.5 °C scenarios in the report, finding that investigated degrowth scenarios "minimize many key risks for feasibility and sustainability compared to technology-driven pathways" with a core problem of such being feasibility in the context of contemporary decision-making of politics and globalized rebound- and relocation-effects.

Politics

Australia

Prime Minister Scott Morrison emphasised that the report was not specifically for Australia but for the whole world. Energy Minister Angus Taylor said the Government would "not be distracted" by the IPCC report saying "A debate about climate change and generation technologies in 2050 won't bring down current power prices for Australian households and small businesses." Environment Minister Melissa Price said that scientists are "drawing a very long bow" to say coal should be phased out by 2050 and supported new coal-fired power stations pledging not to legislate the Paris targets. Australia is not on track to meet the commitments under Paris agreement according to modelling conducted by ClimateWorks Australia.

Canada

Canadian Environment Minister Catherine McKenna acknowledged that the SR15 report would say Canada is not "on track" for 1.5 °C.^[77] Canada will not be implementing new plans but it will continue to move forward on a "national price on carbon, eliminating coal-fired power plants, making homes and businesses more energy-efficient, and investing in clean technologies and renewable energy". In response to a question on the sense of urgency of the SR15 report during a 9 October interview on CBC News's Power and Politics Andrew Scheer, the Leader of the Opposition, promised that they are putting forward a "comprehensive plan to reduce CO2 without imposing a carbon tax" which Scheer said "raised costs without actually reducing emissions."

European Union

According to The New York Times, the European Union indicated it might add more ambitious reform goals centered around reducing emissions. On 9 October, the Council of the European Union presented their response to SR15 and their position for the Katowice Climate Change Conference of the Parties (COP 24) held in Poland in December 2018. Their environment ministers noted recent progress in legislation to reduce greenhouse gas emissions.

India

The Centre for Science and Environment said the repercussions for developing countries such as India, would be "catastrophic" at 2 °C warming and that the impact even at 1.5 °C described in SR15 is much greater than anticipated. Crop yields would decline and poverty would increase.

New Zealand

The Minister for Climate Change James Shaw said that the Report "has laid out a strong case for countries to make every effort to limit temperature rise to 1.5° Celsius above pre-industrial levels. ... The good news is that the IPCC's report is broadly in line with this Government's direction on climate change and it's highly relevant to the work we are doing with the Zero Carbon Bill."

United States

President Donald Trump said that he had received the report, but wanted to learn more about those who "drew it" before offering conclusions. In an interview with ABC's "This Week" the director of the National Economic Council, Larry Kudlow, stated, "personally, I think the UN study is way too difficult," and that the authors "overestimate" the likelihood for environmental disasters. Since the publication Trump stated in an interview on 60 Minutes that he didn't know that climate change is manmade and that "it'll change back again", the scientists who say it's worse than ever have "a very big political agenda" and that "we have scientists that disagree with [manmade climate change]."

COP24

The governments of four countries (the gas/oil-producers USA, Russia, Saudi Arabia and Kuwait) blocked a proposal to welcome the Intergovernmental Panel on Climate Change's (IPCC) Special Report on Global Warming of 1.5 °C at the 2018 United Nations Climate Change Conference (COP24).

Other

The "Special Report on Global Warming of 1.5 °C" (SR15) is cited by Greta Thunberg in her speeches "Wherever I Go I Seem to Be Surrounded by Fairy Tales" (United States Congress, Washington DC, 18 September 2019) and "We Are the Change and Change Is Coming" (Week For Future, Climate Strike, Montreal, 27 September 2019), both published in the second edition of No One Is Too Small to Make a Difference.

At the 2019 World Economic Forum, the head of the International Monetary Fund, Kristalina Georgieva, said that: "The big eye opener [into climate change and its effects] was when last year I read [the SR15] IPCC report. I tell you, I could not sleep that night. [...] What have we done?".

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