A thought experiment is an imaginary scenario that is meant to elucidate or test an argument or theory. It is often an experiment that would be hard, impossible, or unethical to actually perform. It can also be an abstract hypothetical that is meant to test our intuitions about morality or other fundamental philosophical questions.
History
The ancient Greek δείκνυμι, deiknymi, 'thought experiment', "was the most ancient pattern of mathematical proof", and existed before Euclidean mathematics, where the emphasis was on the conceptual, rather than on the experimental part of a thought experiment.
Johann Witt-Hansen established that Hans Christian Ørsted was the first to use the equivalent German term Gedankenexperiment c. 1812.Ørsted was also the first to use the equivalent term Gedankenversuch in 1820.
By 1883, Ernst Mach used Gedankenexperiment in a different sense, to denote exclusively the imaginary conduct of a real experiment that would be subsequently performed as a real physical experiment by his students. Physical and mental experimentation could then be contrasted: Mach
asked his students to provide him with explanations whenever the results
from their subsequent, real, physical experiment differed from those of
their prior, imaginary experiment.
The English term thought experiment was coined as a calque of Gedankenexperiment, and it first appeared in the 1897 English translation of one of Mach's papers. Prior to its emergence, the activity of posing hypothetical questions
that employed subjunctive reasoning had existed for a very long time for
both scientists and philosophers. The irrealis moods
are ways to categorize it or to speak about it. This helps explain the
extremely wide and diverse range of the application of the term thought experiment once it had been introduced into English.
Galileo's thought experiment concerned the outcome (c) of attaching a small stone (a) to a larger one (b).
Galileo's
demonstration that falling objects must fall at the same rate
regardless of their masses was a significant step forward in the history
of modern science. This is widely thought to have been a straightforward physical demonstration, involving
climbing up the Leaning Tower of Pisa and dropping two heavy weights off
it, whereas in fact, it was a logical demonstration, using the thought
experiment technique. The experiment is described by Galileo in his 1638
work Two New Sciences thus:
Salviati: If then we take two bodies whose natural
speeds are different, it is clear that on uniting the two, the more
rapid one will be partly retarded by the slower, and the slower will be
somewhat hastened by the swifter. Do you not agree with me in this
opinion? Simplicio: You are unquestionably right. Salviati: But
if this is true, and if a large stone moves with a speed of, say, eight
while a smaller moves with a speed of four, then when they are united,
the system will move with a speed less than eight; but the two stones
when tied together make a stone larger than that which before moved with
a speed of eight. Hence the heavier body moves with less speed than the
lighter; an effect which is contrary to your supposition. Thus you see
how, from your assumption that the heavier body moves more rapidly than
the lighter one, I infer that the heavier body moves more slowly.
Uses
Thought experiments may be used to explore a hypothesis and the
implementation of theories around it. They are also used in education,
or for personal entertainment.
Examples of thought experiments include Schrödinger's cat, that was meant to attack the Copenhagen Interpretation
of quantum mechanics by showing that its assumptions could lead to the
seemingly absurd condition of a cat being simultaneously alive and dead,
and Maxwell's demon, which attempts to demonstrate the ability of a hypothetical finite being to violate the 2nd law of thermodynamics.
Thought experiments, which are well-structured, well-defined hypothetical questions that employ subjunctive reasoning (irrealis moods)
– "What might happen (or, what might have happened) if . . . " – have
been used to pose questions in philosophy at least since Greek
antiquity, some pre-dating Socrates. In physics and other sciences many thought experiments date from the
19th and especially the 20th Century, but examples can be found at least
as early as Galileo.
In thought experiments, we gain new information by rearranging or
reorganizing empirical data in a new way and drawing new inferences
from them, or by looking at these data from a different and unusual
perspective. In Galileo's thought experiment, for example, the
rearrangement of empirical experience consists of the original idea of
combining bodies of different weights.
Regardless of their intended goal, all thought experiments
display a patterned way of thinking that is designed to allow us to
explain, predict, and control events in a better and more productive
way.
Theoretical consequences
In terms of their theoretical consequences, thought experiments generally:
challenge (or even refute) a prevailing theory, often involving the device known as reductio ad absurdum, (as in Galileo's original argument, a proof by contradiction),
confirm a prevailing theory,
establish a new theory, or
simultaneously refute a prevailing theory and establish a new theory through a process of mutual exclusion
Practical applications
Thought experiments can produce some very important and different
outlooks on previously unknown or unaccepted theories. However, they may
make those theories themselves irrelevant, and could possibly create
new problems that are just as difficult, or possibly more difficult to
resolve.
In terms of their practical application, thought experiments are generally created to:
challenge the prevailing status quo (which includes activities such as correcting misinformation
(or misapprehension), identify flaws in the argument(s) presented, to
preserve (for the long-term) objectively established fact, and to refute
specific assertions that some particular thing is permissible,
forbidden, known, believed, possible, or necessary)
examine the extent to which past events might have occurred differently
ensure the future avoidance of past failures
Fields
Thought experiments have been used in a variety of fields, including philosophy, law, physics, and mathematics. In philosophy they have been used at least since classical antiquity, some pre-dating Socrates. In law, they were well known to Roman lawyers quoted in the Digest. In physics and other sciences, notable thought experiments date from
the 19th and, especially, the 20th century; but examples can be found at
least as early as Galileo.
Philosophy
In philosophy, a thought experiment typically presents an imagined
scenario with the intention of eliciting an intuitive or reasoned
response about the way things are in the thought experiment.
(Philosophers might also supplement their thought experiments with
theoretical reasoning designed to support the desired intuitive
response.) The scenario will typically be designed to target a
particular philosophical notion, such as morality, or the nature of the
mind or linguistic reference. The response to the imagined scenario is
supposed to tell us about the nature of that notion in any scenario,
real or imagined.
For example, a thought experiment might present a situation in
which an agent intentionally kills an innocent for the benefit of
others. Here, the relevant question is not whether the action is moral
or not, but more broadly whether a moral theory is correct that says
morality is determined solely by an action's consequences (See Consequentialism). John Searle
imagines a man in a locked room who receives written sentences in
Chinese, and returns written sentences in Chinese, according to a
sophisticated instruction manual. Here, the relevant question is not
whether or not the man understands Chinese, but more broadly, whether a functionalist theory of mind is correct.
It is generally hoped that there is universal agreement about the
intuitions that a thought experiment elicits. (Hence, in assessing
their own thought experiments, philosophers may appeal to "what we
should say," or some such locution.) A successful thought experiment
will be one in which intuitions about it are widely shared. But often,
philosophers differ in their intuitions about the scenario.
Other philosophical uses of imagined scenarios arguably are
thought experiments also. In one use of scenarios, philosophers might
imagine persons in a particular situation (maybe ourselves), and ask
what they would do.
Scientists tend to use thought experiments as imaginary, "proxy" experiments prior to a real, "physical" experiment (Ernst Mach always argued that these gedankenexperiments
were "a necessary precondition for physical experiment"). In these
cases, the result of the "proxy" experiment will often be so clear that
there will be no need to conduct a physical experiment at all.
Scientists also use thought experiments when particular physical experiments are impossible to conduct (Carl Gustav Hempel labeled these sorts of experiment "theoretical experiments-in-imagination"), such as Einstein's thought experiment of chasing a light beam, leading to special relativity.
This is a unique use of a scientific thought experiment, in that it
was never carried out, but led to a successful theory, proven by other
empirical means.
Properties
Further categorization of thought experiments can be attributed to specific properties.
Some thought experiments present scenarios that are not nomologically possible. In his Twin Earth thought experiment, Hilary Putnam
asks us to imagine a scenario in which there is a substance with all of
the observable properties of water (e.g., taste, color, boiling point),
but is chemically different from water. It has been argued that this
thought experiment is not nomologically possible, although it may be
possible in some other sense, such as metaphysical possibility. It is debatable whether the nomological impossibility of a thought experiment renders intuitions about it moot.
In some cases, the hypothetical scenario might be considered metaphysically impossible, or impossible in any sense at all. David Chalmers says that we can imagine that there are zombies, or persons who are physically identical to us in every way but who lack consciousness. This is supposed to show that physicalism
is false. However, some argue that zombies are inconceivable: we can no
more imagine a zombie than we can imagine that 1+1=3. Others have
claimed that the conceivability of a scenario may not entail its
possibility.
Causal reasoning
The first characteristic pattern that thought experiments display is their orientation
in time. They are either:
Antefactual speculations: experiments that speculate about what might have happened prior to a specific, designated event, or
Postfactual speculations: experiments that speculate about what may happen subsequent to (or consequent upon) a specific, designated event.
The second characteristic pattern is their movement in time in relation to "the present
moment standpoint" of the individual performing the experiment; namely, in terms of:
Their temporal direction: are they past-oriented or future-oriented?
Their temporal sense:
(a) in the case of past-oriented thought experiments, are they
examining the consequences of temporal "movement" from the present to
the past, or from the past to the present? or,
(b) in the case of future-oriented thought experiments, are they
examining the consequences of temporal "movement" from the present to
the future, or from the future to the present?
Relation to real experiments
The relation to real experiments can be quite complex, as can be seen
again from an example going back to Albert Einstein. In 1935, with two
coworkers, he published a paper on a newly created subject called later
the EPR effect (EPR paradox). In this paper, starting from certain philosophical assumptions, on the basis of a rigorous analysis of a certain, complicated, but in
the meantime assertedly realizable model, he came to the conclusion that
quantum mechanics should be described as "incomplete". Niels Bohr asserted a refutation of Einstein's analysis immediately, and his view prevailed. After some decades, it was asserted that feasible experiments could
prove the error of the EPR paper. These experiments tested the Bell inequalities
published in 1964 in a purely theoretical paper. The above-mentioned
EPR philosophical starting assumptions were considered to be falsified
by the empirical fact (e.g. by the optical real experiments of Alain Aspect).
Thus thought experiments belong to a theoretical discipline, usually to theoretical physics, but often to theoretical philosophy.
In any case, it must be distinguished from a real experiment, which
belongs naturally to the experimental discipline and has "the final
decision on true or not true", at least in physics.
Interactivity
Thought experiments can also be interactive where the author invites
people into his thought process through providing alternative paths with
alternative outcomes within the narrative, or through interaction with a
programmed machine, like a computer program.
Thanks to the advent of the Internet, the digital space has lent
itself as a new medium for a new kind of thought experiments. The
philosophical work of Stefano Gualeni,
for example, focuses on the use of virtual worlds to materialize
thought experiments and to playfully negotiate philosophical ideas. His arguments were originally presented in his 2015 book Virtual Worlds as Philosophical Tools.
Gualeni's argument is that the history of philosophy has, until
recently, merely been the history of written thought, and digital media
can complement and enrich the limited and almost exclusively linguistic
approach to philosophical thought. He considers virtual worlds (like those interactively encountered in
videogames) to be philosophically viable and advantageous. This is
especially the case in thought experiments, when the recipients of a
certain philosophical notion or perspective are expected to objectively
test and evaluate different possible courses of action, or in cases
where they are confronted with interrogatives concerning non-actual or
non-human phenomenologies.
Relative
to now, key areas for wildlife will retain less of their biodiversity
under 2 °C (3.6 °F) of global warming, and even less under 4.5 °C
(8.1 °F).
There are several plausible pathways that could lead to plant and animalspeciesextinction from climate change. Every species has evolved to exist within a certain ecological niche, but climate change leads to changes of temperature and average weather patterns. These changes can push climatic conditions outside of the species' niche, and ultimately render it extinct. Normally, species faced with changing conditions can either adapt in place through microevolution
or move to another habitat with suitable conditions. However, the speed
of recent climate change is very fast. Due to this rapid change, for
example cold-blooded animals (a category which includes amphibians, reptiles and all invertebrates) may struggle to find a suitable habitat within 50 km of their current location at the end of this century (for a mid-range scenario of future global warming).
Climate change also increases both the frequency and intensity of extreme weather events, which can directly wipe out regional populations of species. Those species occupying coastal and low-lying island habitats can also become extinct by sea level rise. This has already happened with Bramble Cay melomys in Australia, which was the first mammal to go extinct due to human-induced sea level rise, with the Australian government officially confirming its extinction in 2019.
So far, climate change has not yet been a major contributor to the ongoing holocene extinction. In fact, nearly all of the irreversible biodiversity loss to date has been caused by other anthropogenic pressures such as habitat destruction.Yet, its effects are certain to become more prevalent in the future. As of 2021, 19% of species on the IUCN Red List of Threatened Species are already being impacted by climate change. Out of 4000 species analyzed by the IPCC Sixth Assessment Report,
half were found to have shifted their distribution to higher latitudes
or elevations in response to climate change. According to IUCN,
once a species has lost over half of its geographic range, it is
classified as "endangered", which is considered equivalent to a >20%
likelihood of extinction over the next 10–100 years. If it loses 80% or
more of its range, it is considered "critically endangered", and has a very high (over 50%) likelihood of going extinct over the next 10–100 years.
The IPCC Sixth Assessment Report
projected that in the future, 9%-14% of the species assessed would be
at a very high risk of extinction under 1.5 °C (2.7 °F) of global
warming over the preindustrial levels, and more warming means more
widespread risk, with 3 °C (5.4 °F) placing 12%-29% at very high risk,
and 5 °C (9.0 °F) 15%-48%. In particular, at 3.2 °C (5.8 °F), 15% of invertebrates (including 12% of pollinators), 11% of amphibians and 10% of flowering plants would be at a very high risk of extinction, while ~49% of insects, 44% of plants, and 26% of vertebrates would be at a high risk of extinction. In contrast, even the more modest Paris Agreement goal of limiting warming to 2 °C (3.6 °F) reduces the fraction of invertebrates, amphibians and flowering plants at a very high
risk of extinction to below 3%. However, while the more ambitious
1.5 °C (2.7 °F) goal dramatically cuts the proportion of insects,
plants, and vertebrates at high risk of extinction to 6%, 4% and
8%, the less ambitious target triples (to 18%) and doubles (8% and 16%)
the proportion of respective species at risk.
Causes
Projections of extreme weather under different levels of global warming
When the IPCC Fourth Assessment Report
was published in 2007, expert assessments concluded that over the last
three decades, human-induced warming had likely had a discernible
influence on many physical and biological systems, and that regional temperature trends had already affected species and ecosystems around the world. By the time of the Sixth Assessment Report,
it was found that for all species for which long-term records are
available, half have shifted their ranges poleward (and/or upward for
mountain species), while two-thirds have had their spring events occur
earlier.
Many of the species at risk are Arctic and Antarctic fauna such as polar bears In the Arctic, the waters of Hudson Bay are ice-free for three weeks longer than they were thirty years ago, affecting polar bears, which prefer to hunt on sea ice. Species that rely on cold weather conditions such as gyrfalcons, and snowy owls that prey on lemmings that use the cold winter to their advantage may be negatively affected. Climate change is also leading to a mismatch between the snow camouflage of arctic animals such as snowshoe hares with the increasingly snow-free landscape.
Then, many species of freshwater and saltwater plants and animals are dependent on glacier-fed
waters to ensure a cold water habitat that they have adapted to. Some
species of freshwater fish need cold water to survive and to reproduce,
and this is especially true with salmon and cutthroat trout. Reduced glacier runoff can lead to insufficient stream flow to allow these species to thrive. Ocean krill, a cornerstone species, prefer cold water and are the primary food source for aquatic mammals such as the blue whale. Marine invertebrates achieve peak growth at the temperatures they have adapted to, and cold-blooded animals found at high latitudes and altitudes generally grow faster to compensate for the short growing season. Warmer-than-ideal conditions result in higher metabolism and consequent reductions in body size despite increased foraging, which in turn elevates the risk of predation. Indeed, even a slight increase in temperature during development impairs growth efficiency and survival rate in rainbow trout.
Eagle River in central Alaska, home to various indigenous freshwater species
Species of fish living in cold or cool water can see a reduction in
population of up to 50% in the majority of U.S. freshwater streams,
according to most climate change models. The increase in metabolic demands due to higher water temperatures, in
combination with decreasing amounts of food will be the main
contributors to their decline. Additionally, many fish species (such as salmon) use seasonal water
levels of streams as a means of reproducing, typically breeding when
water flow is high and migrating to the ocean after spawning. Because snowfall is expected to be reduced due to climate change, water
runoff is expected to decrease which leads to lower flowing streams,
affecting the spawning of millions of salmon. To add to this, rising seas will begin to flood coastal river systems,
converting them from fresh water habitats to saline environments where
indigenous species will likely perish. In southeast Alaska, the sea
rises by 3.96 cm/year, redepositing sediment in various river channels
and bringing salt water inland. This rise in sea level not only contaminates streams and rivers with
saline water, but also the reservoirs they are connected to, where
species such as sockeye salmon
live. Although this species of Salmon can survive in both salt and
fresh water, the loss of a body of fresh water stops them from
reproducing in the spring, as the spawning process requires fresh water.
Furthermore, climate change may disrupt ecological partnerships among interacting species, via changes on behaviour and phenology, or via climate niche mismatch. The disruption of species-species associations is a potential
consequence of climate-driven movements of each individual species
towards opposite directions. Climate change may, thus, lead to another extinction, more silent and
mostly overlooked: the extinction of species' interactions. As a
consequence of the spatial decoupling of species-species associations, ecosystem services derived from biotic interactions are also at risk from climate niche mismatch. Whole ecosystem disruptions will occur earlier under more intense climate change: under the high-emissions RCP8.5
scenario, ecosystems in the tropical oceans would be the first to
experience abrupt disruption before 2030, with tropical forests and
polar environments following by 2050. In total, 15% of ecological
assemblages would have over 20% of their species abruptly disrupted if
as warming eventually reaches 4 °C (7.2 °F); in contrast, this would
happen to fewer than 2% if the warming were to stay below 2 °C (3.6 °F).
Extinctions attributed to climate change
Besides Bramble Cay melomys
(see below), few recorded species extinctions are thought to have been
caused by climate change, as opposed to the other drivers of the Holocene extinction.
For example, only 20 of 864 species extinctions are considered by the
IUCN to potentially be the result of climate change, either wholly or in
part, and the evidence linking them to climate change is typically
considered as weak or insubstantial. These species' extinctions are listed in the table below.
Causes of global extinction for 20 species whose declines were possibly linked to climate change (data from IUCN)
Higher taxon
Species
Possible link to climate change
Hypothesized causes of extinction
Snail
Graecoanatolica macedonica
Drought
Loss of aquatic habitat due to drought
Snail
Pachnodus velutinus
Drought
Habitat degradation, drought related to climate change, hybridization
Snail
Pseudamnicola desertorum
Possibly related to drought
Loss of aquatic habitat
Snail
Rhachistia aldabrae
Drought
Drought related to recent climate change
Fish*
Acanthobrama telavivensis
Drought
Loss of aquatic habitat
Fish
Tristramella magdelainae
Drought
Loss of aquatic habitat due to drought, pollution and water extraction
Frog*
Anaxyrus (Bufo) baxteri
Chytrid
Chytrid fungus
Frog
Atelopus ignescens
Chytrid
Synergistic effects of chytrid and climate change
Frog
Atelopus longirostris
Chytrid
Chytrid, climate change, pollution, and habitat loss
Habitat destruction, introduced predators and diseases, and hurricanes
Bird
Myadestes myadestinus
Storms
Habitat destruction, introduced predators and diseases, and hurricanes
Bird
Porzana palmeri
Storms
Habitat destruction and predation by introduced species, storms
Bird
Psephotus pulcherrimus
Drought
Drought and overgrazing reduced food supply, other factors include
introduced species, disease, habitat destruction, and overharvesting
Rodent
Geocapromys thoracatus
Storm
Introduced predators, storm
Acanthobrama telavivensis and Anaxyrus (Bufo) baxteri are extinct in the wild rather than globally extinct.
However, there is abundant evidence for local extinctions from contractions at the warm edges of species' ranges. Hundreds of animal species have been documented to shift their range
(usually polewards and upwards) as a signal of biotic change due to
climate warming. Warm-edge populations tend to be the most logical place to search for
causes of climate-related extinctions since these species may already be
at the limits of their climatic tolerances. This pattern of warm-edge contraction provides indications that many
local extinctions have already occurred as a result of climate change. Further, an Australian review of 519 observational studies
over 74 years found more than 100 cases where extreme weather events
reduced animal species abundance by over 25%, including 31 cases of
complete local extirpation.
60% of the studies followed the ecosystem for over a year, and
populations did not recover to pre-disturbance levels in 38% of the
cases.
Extinction risk estimates for all species
Initial estimate
The first major attempt to estimate the impact of climate change on
generalized species' extinction risks was published in the journal Nature
in 2004. It suggested that between 15% and 37% of 1103 endemic or
near-endemic known plant and animal species around the world would be
"committed to extinction" by 2050, as their habitat will no longer be
able to support their survival range by then. However, there was limited knowledge at the time about the species'
average ability to disperse or otherwise adapt in response to climate
change, and about the minimum average area needed for their
persistence, which limited the reliability of their estimate in the eyes
of the scientific community. In response, another 2004 paper found that different, yet still
plausible assumptions about those factors could result in as few as 5.6%
or as many as 78.6% of those 1103 species being committed to
extinction, although this was disputed by the original authors.
Major reports, reviews and surveys
Between 2005 and 2011, 74 studies analyzing the impact of climate
change on various species' extinction risk were published. A 2011 review
of those studies found that on average, they projected the loss of
11.2% of species by 2100. However, the average of predictions based on
the extrapolation of observed responses was 14.7%, while the model-based
estimates were at 6.7%. Further, when using IUCN criteria, 7.6% of species would become threatened based on model predictions, yet 31.7% based on extrapolated observations. The following year, this mismatch between models and observations was
primarily attributed to the models failing to properly account for
different rates of species relocation and for the emerging competition
among species, thus causing them to underestimate extinction risk.
In 2019, Global Assessment Report on Biodiversity and Ecosystem Services from Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services
(IPBES) estimated that there are 8 million animal and plant species,
including 5.5 million insect species. It found that one million species,
including 40 percent of amphibians, almost a third of reef-building corals, more than a third of marine mammals, and 10 percent of all insects are threatened with extinction due to five main stressors. The land use change and sea use change was considered the most important stressor, followed by direct exploitation of organisms (i.e. overfishing). Climate change ranked third, followed by pollution and invasive species.
The report concluded that global warming of 2 °C (3.6 °F) over the
preindustrial levels would threaten an estimated 5% of all the Earth's
species with extinction even in the absence of the other four factors,
while if the warming reached 4.3 °C (7.7 °F), 16% of the Earth's species
would be threatened with extinction. Finally, even the lower warming
levels of 1.5–2 °C (2.7–3.6 °F) would "profoundly" reduce geographical
ranges of the majority of the world's species, thus making them more
vulnerable than they would have been otherwise.
February 2022 IPCC Sixth Assessment Report included median and maximum
estimates of the percentage of species at high risk of extinction for
every level of warming, with the maximum estimates increasing much more
than the medians. For instance, for 1.5 °C (2.7 °F), the median was 9%
and the maximum 14%, for 2 °C (3.6 °F) the median was 10% and the
maximum 18%, for 3 °C (5.4 °F) the median was 12% and the maximum 29%,
for 4 °C (7.2 °F) the median was 13% and the maximum 39%, and for 5 °C
(9.0 °F) the median was 15% but the maximum 48%) at 5 °C.
In July 2022, a survey of 3331 biodiversity experts estimated
that since the year 1500, around 30% (between 16% and 50%) of all
species have been threatened with extinction – including the species
which had already gone extinct. With regards to climate change, the
experts estimated that 2 °C (3.6 °F) threatens or drives to extinction
about 25% of the species, although their estimates ranged from 15% to
40%. When asked about 5 °C (9.0 °F) warming, they believed it would
threaten or drive into extinction 50% of the species, with the range
between 32 and 70%.
A review of estimates from 82 studies, which have collectively
projected the distribution of over 400,000 species, was published in
2024. The results suggested that between 13.9% and 27.6% of all species
would be likely to go extinct by 2070 under the "moderate" emission
scenario RCP4.5 and between 22.7 and 31.6% under the high-emission RCP8.5.
Also in 2024, a synthesis of 5 million projections from 485 studies was published. The results suggested that a warming of 1.5 °C (2.7 °F) would threaten
the extinction of 1.8% of all species by 2100, while stopping the
warming at 2024's level of 1.3 °C (2.3 °F) would still cause extinctions
of 1.6% over the same timeframe. Then, greenhouse gas emissions
remaining on the "current trajectory" of year 2024 would be consistent
with extinctions of around 5% of species by the end of the century,
while very high warming of 4.3 °C (7.7 °F) or 5.4 °C (9.7 °F) would
likely result in extinctions of 15% and 30% of all species.
Fossil-based estimates
The
comparison between great historical mass extinctions, current extent of
extinctions, and the possible extent of future extinctions driven by a
plausible scenario of climate change, with and without nuclear war; PETM: Paleocene–Eocene thermal maximum case; EM: mass extinction case
2021 research found that the "Big Five" mass extinctions
were associated with a warming of around 5.2 °C (9.4 °F). The paper
estimated that this level of warming over the preindustrial occurring
today would also result in a mass extinction event of the same magnitude
(~75% of marine animals wiped out). The following year, this was disputed by the Tohoku University Earth science scholar Kunio Kaiho. Based on his reanalysis of sedimentary rockrecord, he estimated that the loss of over 60% of marine species and over 35% of marine genera was correlated to a >7 °C (13 °F) global cooling and a 7–9 °C (13–16 °F) global warming, while for the terrestrial tetrapods, the same losses would be seen under ~7 °C (13 °F) of global cooling or warming.
Kaiho's follow-up paper estimated that under what he considered
the most likely scenario of climate change, with 3 °C (5.4 °F) of
warming by 2100 and 3.8 °C (6.8 °F) by 2500 (based on the average of Representative Concentration Pathways
4.5 and 6.0), would result in 8% marine species extinctions, 16–20%
terrestrial animal species extinctions, and a combined average of 12–14%
animal species extinctions. This was defined by the paper as a minor mass extinction, comparable to the end-Guadalupian and Jurassic–Cretaceous
boundary events. It also cautioned that warming needed to be kept below
2.5 °C (4.5 °F) to prevent an extinction of >10% of animal species.
Finally, it estimated that a minornuclear war (defined as a nuclear exchange between India and Pakistan or an event of equivalent magnitude) would cause extinctions of 10–20% of species on its own, while a major nuclear war (defined as a nuclear exchange between United States and Russia) would cause the extinctions of 40-50% species.
Worldwide extinction risk estimates for specific categories
A 2018 Science Magazine
paper estimated that at 1.5 °C (2.7 °F), 2 °C (3.6 °F) and 3.2 °C
(5.8 °F), over half of climatically determined geographic range would be
lost by 4%, 8% and 26% of vertebrate species. This estimate was later directly cited in the IPCC Sixth Assessment Report. According to the IUCN Red List
criteria, such a range loss is sufficient to classify as species as
"endangered", and it is considered equivalent to >20% likelihood of
extinction over the 10–100 years.
In 2022, a Science Advances paper estimated that local extinctions of 6% of vertebrates alone would occur by 2050 under the "intermediate" SSP2-4.5
scenario, and 10.8% under the pathway of continually increasing
emissions SSP5-8.5. By 2100, those would increase to ~13% and ~27%,
respectively. These estimates included local extinctions from all
causes, not just climate change: however, it was estimated to account
for the majority (~62%) of extinctions, followed by secondary
extinctions or coextinctions (~20%), with land use change and invasive species combined accounting for less than 20%.
In 2023, a study estimated the proportion of vertebrates which
would be exposed to extreme heat beyond what they were known to have
experienced historically in at least half their distribution by the end
of the century. Under the highest-emission pathway SSP5–8.5 (a warming
of 4.4 °C (7.9 °F) by 2100, according to the paper), this would include
~41% of all land vertebrates (31.1% mammals, 25.8% birds, 55.5%
amphibians and 51% reptiles). On the other hand, SSP1–2.6 (1.8 °C
(3.2 °F) by 2100) would only see 6.1% of vertebrate species exposed to
unprecedented heat in at least of their area, while SSP2–4.5 (2.7 °C
(4.9 °F) by 2100) and SSP3–7.0 (3.6 °C (6.5 °F) by 2100) would see 15.1%
and 28.8%, respectively.
Another 2023 paper suggested that under SSP5-8.5, around 55.3% of terrestrial vertebrate species would experience some local habitat loss by 2100 due to unprecedented aridity alone, while 16.7% would lose over half of their original habitat to aridity. Around 7.18% of those species will find all of their original habitat too dry to survive in by 2100, presumably going extinct unless migration or some form of adaptation
to a dryer environment can occur. Under SSP2-4.5, 41.22% of the
terrestrial vertebrates will lose some habitat to aridity, 8.6% will
lose over half, and 4.7% will lose all of it, and under SSP1-2.6, these
figures go down to 25.2%, 4.6% and 3%, respectively.
In 2024, a major review paper projected likely extinctions of 19%
to 34% vertebrate species by the year 2070 under RCP4.5 and 36% to 44%
under RCP8.5.
Present
and future exposure of frog species around the world to unprecedented
heat, under a more intense climate change scenario SSP3-7.0. Green,
yellow and red circles show whether one, two or all three key thresholds
(annual mean temperature, coldest month temperature or temperature
variability) are exceeded by 2100.
A 2013 study estimated that 670–933 amphibian species (11–15%) are
both highly vulnerable to climate change while already being on the IUCNRed List
of threatened species. A further 698–1,807 (11–29%) amphibian species
are not currently threatened, but could become threatened in the future
due to their high vulnerability to climate change.
The IPCC Sixth Assessment Report concluded that while at 2 °C (3.6 °F), fewer than 3% of most amphibian species would be at a very high risk of extinction, salamanders are more than twice as vulnerable, with nearly 7% of species highly threatened. At 3.2 °C (5.8 °F), 11% of amphibians and 24% of salamanders would be at a very high risk of extinction.
A 2023 paper concluded that under the high-warming SSP5–8.5
scenario, 64.2% of amphibians would lose at least some habitat by 2100
purely due to an increase in aridity, with 33.3% losing over half of it,
and 16.2% finding their entire current habitat too dry for them to
survive in. These figures go down to 47.5%, 18.6% and 10.3% under the
"intermediate" SSP2-4.5 scenario and to 31.7%, 11.2% and 7.4% under the
high-mitigation SSP1-2.6.
A 2022 study estimated that while right now, 14.8% of the global range of all anurans (frogs) is in an extinction risk area, this will increase to 30.7% by 2100 under Shared Socioeconomic Pathway
SSP1-2.6 (low emission pathway), 49.9% under SSP2-4.5, 59.4% under
SSP3-7.0 and 64.4% under the highest-emitting SSP5-8.5. Extreme-sized
anuran species are disproportionately affected: while currently only
0.3% of these species have >70% of their range in a risk area, this
number will increase to 3.9% under SSP1-2.6, 14.2% under SSP2-4.5, 21.5%
under SSP3-7 and 26% under SSP5-8.5.
In 2012, it was estimated that on average, every degree of warming
results in between 100 and 500 land bird extinctions. For a warming of
3.5 °C (6.3 °F) by 2100, the same research estimated between 600 and 900
land bird extinctions, with 89% occurring in the tropical environments. A 2013 study estimated that 608–851 bird species (6–9%) are highly vulnerable to climate change while being on the IUCNRed List
of threatened species, and 1,715–4,039 (17–41%) bird species are not
currently threatened but could become threatened due to climate change
in the future.
A 2023 paper concluded that under the high-warming SSP5–8.5
scenario, 51.8% of birds would lose at least some habitat by 2100 as the
conditions become more arid, but only 5.3% would lose over half of
their habitat due to an increase in dryness alone, while 1.3% could be
expected to lose their entire habitat. These figures go down to 38.7%,
2% and 1% under the "intermediate" SSP2-4.5 scenario and to 22.8%, 0.7%
and 0.5% under the high-mitigation SSP1-2.6.
The projected changes in freshwater fish distribution in Minnesotan lakes under high future warming
A 2022 paper found that 45% of all marine species at risk of
extinction are affected by climate change, but it's currently less
damaging to their survival than overfishing, transportation, urban development and water pollution.
However, if the emissions were to rise unchecked, then by the end of
the century climate change would become as important as all of them
combined. Continued high emissions until 2300 would then risk a mass
extinction equivalent to Permian-Triassic extinction event,
or "The Great Dying". On the other hand, staying at low emissions would
reduce future climate-driven extinctions in the oceans by over 70%.
A 2021 study which analyzed around 11,500 freshwater
fish species concluded that 1-4% of those species would be likely to
lose over half of their current geographic range at 1.5 °C (2.7 °F) and
1-9% at 2 °C (3.6 °F). A warming of 3.2 °C (5.8 °F) would threaten 8-36%
of freshwater fish species with such range loss and 4.5 °C (8.1 °F)
would threaten 24-63%. The different percentages represent different
assumptions about how well freshwater fishes could disperse to new areas
and thus offset past range losses, with the highest percentages
assuming no dispersal is possible. According to the IUCN Red List
criteria, such a range loss is sufficient to classify as species as
"endangered", and it is considered equivalent to >20% likelihood of
extinction over the 10–100 years.
A 2023 paper concluded that under the high-warming SSP5–8.5 scenario,
50.3% of mammals would lose at least some habitat by 2100 as the
conditions become more arid. Out of those, 9.5% would lose over half of
their habitat due to an increase in dryness alone, while 3.2% could be
expected to lose their entire habitat ad the result. These figures go
down to 38.27%, 4.96% and 2.22% under the "intermediate" SSP2-4.5
scenario, and to 22.65%, 2.03% and 1.15% under the high-mitigation
SSP1-2.6.
Reptiles
A 2023 paper concluded that under the high-warming SSP5–8.5 scenario,
56.4% of reptiles would lose at least some habitat by 2100 as the
conditions become more arid. Out of those, 24% would lose over half of
their habitat due to an increase in dryness alone, while 10.94% could be
expected to lose their entire habitat as the result. These figures go
down to 41.7%, 12.5% and 7.2% under the "intermediate" SSP2-4.5
scenario, and to 24.6%, 6.6% and 4.4% under the high-mitigation
SSP1-2.6.
In a 2010 study led by Barry Sinervo, researchers surveyed 200 sites in Mexico which showed 24 local extinctions (also known as extirpations), of Sceloporuslizards
since 1975. Using a model developed from these observed extinctions the
researchers surveyed other extinctions around the world and found that
the model predicted those observed extirpations, thus attributing the
extirpations around the world to climate warming. These models predict
that extinctions of the lizard species around the world will reach 20%
by 2080, but up to 40% extinctions in tropical ecosystems where the
lizards are closer to their ecophysiological limits than lizards in the
temperate zone.
Invertebrates
The IPCC Sixth Assessment Report estimates that while at 2 °C (3.6 °F), fewer than 3% of invertebrates would be at a very high risk of extinction, 15% would be at a very high risk at 3.2 °C (5.8 °F). This includes 12% of pollinator species.
Almost no other ecosystem is as vulnerable to climate change as coral reefs.
Updated 2022 estimates show that even at a global average increase of
1.5 °C (2.7 °F) over pre-industrial temperatures, only 0.2% of the
world's coral reefs would still be able to withstand marine heatwaves, as opposed to 84% being able to do so now, with the figure dropping to 0% by 2 °C (3.6 °F) and beyond. However, it was found in 2021 that each square meter of coral reef area
contains about 30 individual corals, and their total number is
estimated at half a trillion - equivalent to all the trees in the
Amazon, or all the birds in the world. As such, most individual coral
reef species are predicted to avoid extinction even as coral reefs would
cease to function as the ecosystems we know. A 2013 study found that 47–73 coral species (6–9%) are vulnerable to
climate change while already threatened with extinction according to the
IUCN Red List,
and 74–174 (9–22%) coral species were not vulnerable to extinction at
the time of publication, but could be threatened under continued climate
change, making them a future conservation priority. The authors of the recent coral number estimates suggest that those
older projections were too high, although this has been disputed.
Insects account for the vast majority of invertebrate species. A 2018 Science Magazine
paper estimated that at 1.5 °C (2.7 °F), 2 °C (3.6 °F) and 3.2 °C
(5.8 °F), over half of climatically determined geographic range would be
lost by 6%, 18% and ~49% of insect species, with this loss
corresponding to >20% likelihood of extinction over the next 10–100
years according to the IUCN criteria.
A 2020 long-term study of more than 60 bee species published in the journal Science found that climate change causes drastic declines in the population and diversity of bumblebees across the two continents studied, independent of land use
change and at rates "consistent with a mass extinction." When 1901-1974
"baseline" period was compared with the 2000 to 2014 recent period,
then North America's bumblebee populations were found to have fallen by 46%, while Europe's population fell by 14%. The strongest effects were seen in the southern regions, where rapid increases in frequency of extreme warm years had exceeded the species' historical temperature ranges.
In 2024, a major review paper projected likely extinctions of 14% to 27% insects under RCP4.5 by the year 2070, and 23% to 31% under RCP8.5.
Data from 2018 found that at 1.5 °C (2.7 °F), 2 °C (3.6 °F) and
3.2 °C (5.8 °F) of global warming, over half of climatically determined
geographic range would be lost by 8%, 16%, and 44% of plant species.
This corresponds to more than 20% likelihood of extinction over the next
10–100 years under the IUCN criteria.
The 2022 IPCC Sixth Assessment Report estimates that while at 2 °C (3.6 °F) of global warming, fewer than 3% of flowering plants would be at a very high risk of extinction, this increases to 10% at 3.2 °C (5.8 °F).
A 2020 meta-analysis found that while 39% of vascular plant species were likely threatened with extinction, only 4.1% of this figure could be attributed to climate change, with land use
change activities predominating. However, the researchers suggested
that this may be more representative of the slower pace of research on
effects of climate change on plants. For fungi, it estimated that 9.4% are threatened due to climate change, while 62% are threatened by other forms of habitat loss.
2024 review paper projected likely extinctions of 8% to 16% plant
species as well as 8%–27% fungi species under RCP4.5 by 2070. Under
RCP8.5 23% to 31% of both plant and fungi species would be lost.
Predicted and observed extinctions in specific geographic areas
A 2018 study from the University of East Anglia
team analyzed the impacts of 2 °C (3.6 °F) and 4.5 °C (8.1 °F) of
warming on 80,000 plant and animal species in 35 of the world's biodiversity
hotspots. It found that these areas could lose up to 25% and 50% of
their species, respectively: they may or may not be able to survive
outside of them.
Africa
A Southern Yellow-billed Hornbill female
In 2019, it was estimated that the current great ape range in Africa will decline massively under both the severe RCP8.5
scenario and the more moderate RCP4.5. The apes could potentially
disperse to new habitats, but those would lie almost completely outside
of their current protected areas, meaning that conservation planning needs to be "urgently" updated to account for this.
In 2019, it was also estimated that multiple bird species endemic to southern Africa's Kalahari Desert (Southern Pied Babblers, Southern Yellow-billed Hornbills and Southern Fiscals)
would either be all-but-lost from it or reduced to its eastern fringes
by the end of the century, depending on the emission scenario. While the
temperatures are not projected to become so high as to kill the birds
outright, they would still be high enough to prevent them from
sustaining sufficient body mass and energy for breeding. By 2022, breeding success of the Southern Yellow-billed Hornbills was
already observed to collapse in the hottest, southern parts of the
desert. It was predicted that those particular subpopulations would
disappear by 2027. Similarly, it was found that two Ethiopian bird species, White-tailed Swallow and Ethiopian Bush-crow,
would lose 68-84% and >90% of their range by 2070. As their existing
geographical range is already very limited, this means that it would
likely end up too small to support a viable population even under the
scenario of limited climate change, rendering these species extinct in the wild.
According to 2018 research, Madagascar would lose 60% of its species under 4.5 °C (8.1 °F), while Fynbos in Western Cape region of South Africa would lose a third of its species. Miombo Woodlands of South Africa would lose around 90% of their amphibians and about 86% of their birds if the warming were to reach 4.5 °C (8.1 °F).
A 2013 paper looked at 12 900 islands in the Pacific Ocean and Southeast Asia which host over 3000 vertebrates, and how they would be affected by sea level rise
of 1, 3 and 6 meters (with the last two levels not anticipated until
after this century). Depending on the extent of sea level rise, 15–62%
of islands studied would be completely underwater, and 19–24% will lose
50–99% of their area. This was correlated with the total habitat loss
for 37 species under 1 meter of sea level rise, and for 118 species
under 3 meters. A subsequent paper found that under RCP8.5,
the scenario of continually increasing greenhouse gas emissions,
numerous vulnerable and endangered vertebrate species living on the low-lying islands
in the Pacific Ocean would be threatened by high waves at the end of
the century, with the risk substantially reduced under the more moderate
RCP4.5 scenario.
In 2008, the white lemuroid possum was reported to be the first known mammal species to be driven extinct by climate change. However, these reports were based on a misunderstanding. One population of these possums in the mountain forests of North Queensland
is severely threatened by climate change as the animals cannot survive
extended temperatures over 30 °C (86 °F). However, another population
100 kilometres south remains in good health. On the other hand, the Bramble Cay melomys, which lived on a Great Barrier Reef island, was reported as the first mammal to go extinct due to human-induced sea level rise, with the Australian government officially confirming its extinction in 2019. Another Australian species, the greater stick-nest rat (Leporillus conditor) may be next.
According to 2018 research, southwestern Australia would lose around 90% of amphibians if the warming were to reach 4.5 °C (8.1 °F).
2022 research predicted that in Bangladesh,
between 2% and 34% of the native butterfly species could lose their
entire habitat under scenarios SSP1-2.6 and SSP5-8.5, respectively.
Europe
Viola Calcarata or mountain violet, which is projected to go extinct in the Swiss Alps around 2050
Alpine and mountain plant species are known to be some of the most
vulnerable to climate change. In 2010, a study looking at 2,632 species
located in and around European mountain ranges
found that depending on the climate scenario, 36–55% of alpine species,
31–51% of subalpine species and 19–46% of montane species would lose
more than 80% of their suitable habitat by 2070–2100. In 2012, it was estimated that for the 150 plant species in the European Alps,
their range would, on average, decline by 44%-50% by the end of the
century - moreover, lags in their shifts would mean that around 40% of
their remaining range would soon become unsuitable as well, often
leading to an extinction debt. In 2022, it was found that those earlier studies simulated abrupt,
"stepwise" climate shifts, while more realistic gradual warming would
see a rebound in alpine plant diversity after mid-century under the
"intermediate" and most intense global warming scenarios RCP4.5
and RCP8.5. However, for RCP8.5, that rebound would be deceptive,
followed by the same collapse in biodiversity at the end of the century
as simulated in the earlier papers. This is because on average, every degree of warming reduces total species population growth by 7%, and the rebound was driven by colonization of niches left behind by most vulnerable species like Androsace chamaejasme and Viola calcarata going extinct by mid-century or earlier.
The
vulnerability of different European lizard populations to extinctions
caused by climate change. Populations in group A are already at risk; B
and C will be threatened under 2 °C (3.6 °F). Groups D and E will become
threatened under 3 °C (5.4 °F) and 4 °C (7.2 °F), and Group F is
unlikely to be threatened.
A 2015 study looked at the persistence of common lizard populations in Europe
under future climate change. It found that under 2 °C (3.6 °F), 11% of
the lizard population would be threatened with local extinction around
2050 and 14% by 2100. At 3 °C (5.4 °F) by 2100, 21% of the population
are threatened, and at 4 °C (7.2 °F), 30% of the populations are.
A 2018 estimate suggests that two prominent species of seagrasses in the Mediterranean Sea would be substantially affected under the worst-case greenhouse gas emission scenario, with Posidonia oceanica losing 75% of its habitat by 2050 and potentially becoming functionally extinct by 2100, while Cymodocea nodosa would lose ~46% of its habitat and then stabilize due to expansion into previously unsuitable areas.
A 2018 study examined the impact of climate change on Troglohyphantes cave spiders in the Alps and found that even the low-emission scenario RCP2.6
would reduce their habitat by ~45% by 2050, while the high emission
scenario would reduce it by ~55% by 2050 and ~70% by 2070. The authors
suggested that this may be sufficient to drive the most restricted
species to extinction.
In 2022, it was found that the warming which occurred over the past 40 years in Germany's Bavaria region pushed out cold-adapted grasshoppers, butterfly and dragonfly
species, while allowing warm-adapted species from those taxa to become
more widespread. Altogether, 27% of dragonfly and 41% of butterfly and
grasshopper species occupied less area, while 52% of dragonflies became
more widespread, along with 27% of grasshoppers (41%, 20 species) and
20% of butterflies, with the rest showing no trend in area change. The
study only measured geographic spread and not total abundance. While the
paper looked at both climate and land use change, it suggested the latter was only a significant negative factor for specialist butterfly species.
Central and South America
Green sea turtle grazing grass
2016 research found that sex ratios for sea turtles in the Caribbean
are being affected because of climate change. Environmental data were
collected from the annual rainfall and tide temperatures over the course
of 200 years and showed an increase in air temperature (mean of 31.0
degree Celsius). These data were used to relate the decline of the sex
ratios of sea turtles in the North East Caribbean and climate change.
The species of sea turtles include Dermochelys coriacea, Chelonia myads, and Eretmochelys imbricata.
Extinction is a risk for these species as the sex ratio is being
afflicted causing a higher female to male ratio. Projections estimate
the declining rate of male Chelonia myads as 2.4% hatchlings being male by 2030 and 0.4% by 2090.
It's been estimated that by 2050, climate change alone could reduce species richness of trees in the Amazon rainforest by 31–37%, while deforestation
alone could be responsible for 19–36%, and the combined effect might
reach 58%. The paper's worst-case scenario for both stressors had only
53% of the original rainforest area surviving as a continuous ecosystem
by 2050, with the rest reduced to a severely fragmented block. Another study estimated that the rainforest would lose 69% of its plant species under the warming of 4.5 °C (8.1 °F).
North America
Increase in extinction risk for US bird species under two different levels of warming
One of the earliest studies to link insect extinctions to recent climate change was published in 2002, when observations of two populations of Bay checkerspot butterfly found that they were threatened by changes in precipitation.
In 2015, it was projected that native forest birds in Hawaii would be threatened with extinction due to the spread of avian malaria under the high-warming RCP8.5 scenario or a similar scenario from earlier modelling, but would persist under the "intermediate" RCP4.5.
A 2017 analysis found that the mountain goat populations of coastal Alaska would go extinct sometime between 2015 and 2085 in half of the considered scenarios of climate change. Another analysis found that the Miombo Woodlands of South Africa are predicted to lose about 80% of their mammal species if the warming reached 4.5 °C (8.1 °F).
For the 604 bird species in mainland North America, 2020 research concluded that under 1.5 °C (2.7 °F) warming, 207 would be moderately vulnerable to extinction and 47 would be highly
vulnerable. At 2 °C (3.6 °F), this changes to 198 moderately
vulnerable and 91 highly vulnerable. At 3 °C (5.4 °F), there are more
highly vulnerable species (205) than moderately vulnerable species
(140). Relative to 3 °C (5.4 °F), stabilizing the warming at 1.5 °C
(2.7 °F) represents a reduction in extinction risk for 76% of those
species, and 38% stop being vulnerable.
In 2023, a study looked at freshwater fish in 900 lakes of the American state of Minnesota.
It found that if their water temperature increases by 4 °C (7.2 °F) in
July (said to occur under approximately the same amount of global
warming), then cold-water fish species like cisco would disappear from 167 lakes, which represents 61% of their habitat in Minnesota. Cool-wateryellow perch would see its numbers decline by about 7% across all of Minnesota's lakes, while warm-water bluegill would increase by around 10%.
Polar regions
A polar bear
It has been projected in 2015 that many fish species will migrate
towards the North and South poles as a result of climate change. Under
the highest emission scenario RCP8.5, 2 new species would enter (invade) per 0.5° of latitude in the Arctic Ocean and 1.5 in the Southern Ocean. It would also result in an average of 6.5 local extinctions per 0.5° of latitude outside of the poles.
In 2020, a study in Nature Climate Change estimated the effects of Arctic sea ice decline on polar bear populations (which rely on the sea ice to hunt seals) under two climate change scenarios. Under high greenhouse gas
emissions, at most a few high-Arctic populations will remain by 2100:
under more moderate scenario, the species will survive this century, but
several major subpopulations will still be wiped out.
Climate change is particularly threatening to penguins. As early as in 2008, it was estimated that every time Southern Ocean temperatures increase by 0.26 °C (0.47 °F), this reduces king penguin populations by 9%. Under the worst-case warming trajectory, king penguins will permanently
lose at least two out of their current eight breeding sites, and 70% of
the species (1.1 million pairs) will have to relocate to avoid
disappearance. Emperor penguin
populations may be at a similar risk, with 80% of populations being at
risk of extinction by 2100 with no mitigation. With Paris Agreement
temperature goals in place, however, that number may decline to 31%
under the 2 °C (3.6 °F) goal or 19% under the 1.5 °C (2.7 °F) goal.
A 27-year study of the largest colony of Magellanic penguins
in the world, published in 2014, found that extreme weather caused by
climate change kills 7% of penguin chicks in an average year, accounting
for up to 50% of all chick deaths in some years. Since 1987, the number of breeding pairs in the colony has reduced by 24%. Chinstrap penguins are also known to be in decline, mainly due to corresponding declines of Antarctic krill. And it was estimated that while Adélie penguins
will retain some of its habitat past 2099, one-third of colonies along
the West Antarctic Peninsula (WAP) will be in decline by 2060. Those
colonies are believed to represent about 20% of the entire species.
Impacts of species degradation on livelihoods
The livelihoods of nature dependent communities depend on abundance and availability of certain species. Climate change conditions such as increase in atmospheric temperature
and carbon dioxide concentration directly affect availability of biomass
energy, food, fiber and other ecosystem services. Degradation of species supplying such products directly affect the livelihoods of people relying on them more so in Africa. The situation is likely to be exacerbated by changes in rainfall variability which is likely to give dominance to invasive species especially those that are spread across large latitudinal gradients. The effects that climate change has on both plant and animal species
within certain ecosystems has the ability to directly affect the human
inhabitants who rely on natural resources. Frequently, the extinction of
plant and animal species create a cyclic relationship of species
endangerment in ecosystems which are directly affected by climate
change.
Species adaptation
Museum specimens of Collared flycatcher (top) and Eurasian blackbird
(bottom) juveniles compared with modern-day birds. Nesting feathers are
replaced with adult plumage earlier, and females now complete the shift
earlier than males, while in the past it was the opposite.
Many species are already responding to climate change by moving into different areas. For instance, Antarctic hair grass is colonizing areas of Antarctica where previously their survival range was limited. Similarly, 5-20% of the United States land area is likely to end up with a different biome at the end of the century, as vegetation undergoes range shifts. However, such shifts can only go so far to protect species: globally, only 5% of ectotherm species' present locations are within 50 km of a location which would remain fully suitable and not impose evolutionary fitness
costs on them by 2100, even under "mid-range" warming scenarios.
Completely random dispersal may have an 87% chance of sending the
species to a less suitable location. Species in the tropics have the
least extensive dispersal options, while species in the temperate
mountains face the greatest risks of moving to a wrong location. Similarly, an artificial selection
experiment demonstrated that evolution of tolerance to warming can
occur in fish, but the rate of evolution appears limited to 0.04 °C
(0.072 °F) per generation, which is too slow to protect the vulnerable
species from impacts of climate change.
Rising temperatures are beginning to have a noticeable impact on birds, and butterflies nearly 160 species from 10 different zones have shifted their ranges northward by 200 km in Europe and North
America. The migration range of larger animals has been substantially
constrained by human development. In Britain, spring butterflies are appearing an average of 6 days earlier than two decades ago.
Climate change has already altered the appearance of some birds by facilitating changes to their feathers. A comparison of museum specimens of juvenile passerines
from 1800s with juveniles of the same species today had shown that
these birds now complete the switch from their nesting feathers to adult
feathers earlier in their lifecycle, and that females now do this
earlier than males. Further, blue tits are defined by blue and yellow feathers, but a study in MediterraneanFrance had shown that those contrasting colors became less bright and intense in just the period between 2005 and 2019.
A young red deer in the wild in Scotland
Climate change has affected the gene pool of the red deer population on Rùm, one of the Inner Hebrides islands, Scotland.
Warmer temperatures resulted in deer giving birth on average three days
earlier for each decade of the study. The gene which selects for
earlier birth has increased in the population because those with the
gene have more calves over their lifetime.
Prevention
In addition to reducing future warming to the lowest possible levels,
preserving the current and likely near-future habitat of endangered
species in protected areas in efforts like 30x30 is a crucial aspect of helping species survive. A more radical approach is the assisted migration of species endangered by climate change to new habitats, whether passively (through measures like the creation of wildlife corridors
to allow them to move to a new area unimpeded), or their active
transport to new areas. This is approach is more controversial, since
some of the rescued species may end up invasive in their new locations. I.e. while it would be relatively easy to move polar bears, which are currently threatened by Arctic sea ice decline, to Antarctica, the damage to Antarctica's ecosystem is considered too great to allow this. Finally, species which are extinct in the wild may be kept alive in artificial surroundings until a suitable natural habitat may be restored. In cases where captive breeding fails, embryo cryopreservation has been proposed as an option of last resort.
Apiculture initiatives to prevent human-wildlife conflict in Zimbabwe
Women in rural communities in Hurungwe rural district Zimbabwe have resorted to placing beehives at the border of fields and villages (bio fencing) to protect themselves and their crops from elephants.
Assisted migration
Assisted migration is the act of moving plants or animals to a different habitat.
It has been proposed as a way to rescue species which may not be able
to disperse easily, have long generation times or have small
populations. This strategy has already been implemented to save multiple tree species in North America. For instance, the Torreya Guardians have coordinated an assisted migration program to save the Torreya taxifolia from extinction.
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