Extinctions and climate change

Climate change, particularly global warming, has been correlated with past extinction events. Any future short- or medium-term global warming would likely contribute to further extinctions.

Background

Although consensus about categorization is lacking, five major mass extinctions (the “Big Five”) and numerous smaller extinction events are generally recognized. Mass extinctions involve global losses of at least 30 percent of species of various types and sizes occupying various habitats. Less severe extinction events involve fewer species losses (less than 25 percent) and fail to meet one or more additional criteria defining mass extinctions. Their scope may be regional rather than global, for example. Background extinctions occur between extinction events, at relatively low rates, and affect only one or a very few species at a time. Thus, they reflect the mean duration—or “lifespan”—of species in general as reflected in the fossil record. Although background extinctions occur infrequently because they accumulate over very long periods of time, they account for far more species losses than do mass extinctions.

Extinction, Mass Extinction, and Global Warming

It is generally impossible to identify a single cause for an extinction. By way of example, imagine that global warming allows a pathogen to extend its range from the tropics into what was formerly a temperate region. Imagine further that the pathogen’s new range overlaps that of a bird that has become highly endangered by habitat loss. The pathogen causes 90 percent mortality among the birds, and the remainder dies off shortly thereafter during a series of severe storms. It would be an oversimplification to state that only habitat loss, global warming, infectious disease, or severe weather caused the extinction. Habitat loss represents the ultimate, underlying cause, although additional proximate stressors subsequently eliminated the species. Relative to single-species extinctions, extinction events typically involve far more complicated chains of causation.

Global warming is thought to have contributed to numerous extinctions—including the Permian-Triassic and Triassic-Jurassic events, as well as less severe events during the early Jurassic (around 183 million years ago) and the Paleocene-Eocene boundary (around 55 million years ago). If global warming were to occur again, it would likely contribute to future extinctions via several main mechanisms. First, warming conditions would reduce or degrade the habitats of high-latitude, high-altitude, and temperate species. For example, polar bears rely for food on the formation of winter ice in which their seal prey dens, so they would be threatened by a decrease in such ice formation. Similarly, rising sea levels resulting from melting glaciers would flood coastal areas and reduce Earth’s overall island area, threatening the disproportionately high levels of occupying these relatively small landmasses.

Although rising sea levels might increase the total volume of aquatic habitat, global warming would also place important, interrelated stresses on aquatic species. The first of these would be thermal stress. Because of water’s ability to absorb and retain heat, aquatic environments experience less temperature variation than do terrestrial environments. As a consequence, aquatic species are generally adapted to a narrower range of temperature tolerances than are terrestrial species. Thus, any change in sea temperature is liable to threaten aquatic species. Because excessive heat poses a greater physiological threat to most organisms than does excessive cold, global warming would represent an especially serious stress. Contemporary coral die-offs resulting from warming oceans provide a noteworthy example.

A second threat to aquatic species relates to the fact that as water’s temperature increases, it holds less dissolved oxygen, while warming waters tend to increase organismal metabolism and oxygen demand. Consequently, warmer water can support relatively fewer aerobic organisms than can colder water. Hypoxia (oxygen scarcity) and anoxia (absence of oxygen) in warming waters have been linked to a variety of extinctions. Although these factors would most directly affect aquatic species, they would also exert significant indirect effects on species dependent on aquatic organisms, such as fish-eating birds.

Warming oceans pose an additional threat to marine species, since warmer seawater would likely melt extensive seafloor deposits of methane hydrate. Melted methane hydrate reacts with the dissolved oxygen in seawater to form carbon dioxide, and the release of large quantities would greatly reduce oceanic oxygen levels, placing considerable stress on both marine and marine-dependent species by exacerbating the effects of the reduced oxygen levels associated with increased water temperature. Such a is thought to have occurred during the Paleocene-Eocene extinction, when high-latitude and deep-sea temperatures may have increased by as much as 7° Celsius. It has been predicted that contemporary global warming could cause similar effects. In 2019, scientists predicted that about 5 percent of species worldwide would face extinction if the global average temperatures rose 2 degrees Celsius above preindustrial levels. That same year, the United Nations (UN) estimated that an increase of 3 degrees Celsius over preindustrial levels would cause mass extinction events and lead to large portions of the Earth becoming uninhabitable. Although the UN's Paris Agreement created a global effort to limit global warming to 1.5 degrees Celsius above preindustrial levels by the end of the twenty-first century, the organization warned that significant action was needed in order to meet its goals.

Modern Versus Paleoextinctions

The fossil record’s limitations make detailed characterizations of the biodiversity losses associated with extinctions difficult. Paleontologists estimate that fossils preserve less than 4-5 percent of all species that have ever lived. The fossil record’s incompleteness results from numerous factors. First, species vary in composition and habitat. Hard tissue such as shell, tooth, and bone is much more prone to fossilization than is soft tissue such as muscle and skin. Since fossilization occurs only in environments where dead organisms can be preserved in sediment, species occupying habitats where sediments were routinely deposited are more common as fossils. By contrast, in areas such as mountain tops, where erosional processes dominated, fossils are rare or unknown. Thus, the fossil record reveals far more about shelled marine mollusks (which had hard parts and occupied depositional environments) than about small, delicately built uplands birds (which had fragile bones and occupied erosional environments).

Since fossilization is a rare outcome, species that were rare in life are correspondingly rare in the fossil record. As a consequence, while complete or nearly complete fossils of some marine invertebrates are common, most dinosaurs are known from one or a very few specimens, many of them fragmentary. Because marine fossils are much more common than terrestrial fossils, they are frequently used in computing extinction rates.

The final key factor limiting the fossil record’s completeness is erosion, which eventually reduces fossil-bearing rock to sediment, destroying any fossils it contains. The longer fossil-bearing rock is in existence, the more opportunity there is for erosion to act on it. As a result, the fossil record of more recent extinctions is far more complete than that of older extinctions. Consequently, the details of the Cretaceous-Tertiary extinction are better understood than those of the much earlier Ordovician-Silurian extinction.

Because the fossil record is so often imprecise at the species level, paleontologists often assess extinctions on the basis of the persistence or disappearance of higher taxa, such as genera, families, or orders. The extinction of even one order, family, or genus would likely involve the loss of numerous species, although the vagaries of fossilization preclude precise estimates of species losses.

Although contemporary biologists face logistical difficulties in locating and cataloging existing biodiversity, they clearly have significant advantages over paleontologists attempting to characterize the biodiversity of vanished ecosystems. It would be highly unlikely for relatively small, rare, and delicately boned species analogous to the black-footed ferret to be abundant or well-preserved in the fossil record. However, some of the same traits that make such creatures rare in the fossil record place them in danger of extinction in the modern world, where an ability to study their distant ancestors through fossil records could help scientists better understand their ecological and evolutionary roles.

Possibility of A Mass Extinction

Despite the difficulties paleontologists face in providing detailed characterizations of vanished life, they can render informative sketches of lost biological communities. In comparing estimates of present-day extinction rates, whose main cause is habitat loss, to those computed from the fossil record, ecologists have concluded that recent human history, including the present, probably qualifies as a period of mass extinction.

Mass extinctions are generally thought to have resulted from nonbiological causes, such as climate change, sea-level shift, intense volcanism, or meteor impact. Current extinctions, linked as they are to the activities of a single species, humans, appear unprecedented: Collectively, they represent the first mass extinction with a biotic cause. The rate of extinction might also be unusually high, at least fifty to one hundred times greater than background extinction rates. Although often referred to as “events,” mass extinctions are generally thought to have been protracted by human standards, occurring over thousands or, in many cases, millions of years. By contrast, the spike in extinction rates associated with human activities may be unprecedentedly high, occurring as it has over only several thousand years.

A study published in Nature Geoscience in 2023 indicated that extreme heat caused by climate change will likely lead to the next mass extinction, which will eradicate all mammals, including humans, in 250 million years. The scientists in the study cautioned that this may occur sooner unless people stop burning fossil fuels. Specifically, activities like farming, poaching, mining, overfishing, and deforestation have impacted the environment so much that about one million plant and animal species have become at risk for extinction. By 2023, there were over 650 US species that had been lost to extinction, with twenty-two animals and one plant moving from the US Fish and Wildlife Service's (USFWS) endangered species list to the extinction list in 2021 alone. By 2024, there were 1,300 endangered or threatened species within the United States, and according to the IUCN Red List, 46,300 animal and plant species throughout the world were threatened with extinction.

Context

The fossil record provides invaluable insights into extinction, but it may prove insufficient for use in predicting future extinction risks. For instance, it has been noted that few North American extinctions are associated with climate change during the roughly two-million-year duration of the Pleistocene. During periods of Pleistocene cooling and warming, species simply shifted their ranges. The assumption that species would do the same globally under near-term warming conditions ignores a complicating factor: Humans now occupy and modify a far larger percentage of the Earth’s habitats. While Pleistocene species could readily shift their ranges in response to climate change, they did not face the challenge of doing so across such large areas of potentially inhospitable human-modified habitats. Vulnerability to human barriers would vary from species to species. While many birds could simply fly to preferred habitats and shift their migratory patterns in response to changing conditions, less-mobile species—particularly those with specialized habitat requirements—would be at greater risk. Just as important, anthropogenic stressors could interact both with global warming and with other abiotic stressors such as drought or volcanism to increase extinction risk. Land degradation and the decline of wild bee populations put annual crop production at risk; and coral reef and mangrove loss along coasts cause an increased risk of flooding that could affect up to three million people. Experts predict that radical changes to strengthen and create environmental protection laws to protect both endangered species and to expand protected nature preserves across the globe are needed in order to slow the decline of biodiversity.

Key Concepts

• anthropogenic: resulting from human actions
• extinction: species loss; the death of the last member of a particular species
• extinction event: an unusually large number of extinctions occurring in a relatively short period of time
• methane hydrate: also known as methane clathrate; an extremely abundant, solid, ice-like form of methane that occurs at or beneath the ocean floor in especially deep or cold waters

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