title: Common cold

text: The symptoms of the common cold are believed to be primarily related to the immune response to the virus. The mechanism of this immune response is virus specific. For example, the rhinovirus is typically acquired by direct contact; it binds to humans via ICAM-1 receptors and the CDHR3 receptor through unknown mechanisms to trigger the release of inflammatory mediators. These inflammatory mediators then produce the symptoms. It does not generally cause damage to the nasal epithelium. The respiratory syncytial virus (RSV), on the other hand, is contracted by direct contact and airborne droplets. It then replicates in the nose and throat before frequently spreading to the lower respiratory tract. RSV does cause epithelium damage. Human parainfluenza virus typically results in inflammation of the nose, throat, and bronchi. In young children when it affects the trachea it may produce the symptoms of croup due to the small size of their airways.


title: Common cold

text: The distinction between viral upper respiratory tract infections is loosely based on the location of symptoms, with the common cold affecting primarily the nose (rhinitis), throat (pharyngitis), and lungs (bronchitis). There can be significant overlap, and more than one area can be affected. Self-diagnosis is frequent. Isolation of the viral agent involved is rarely performed, and it is generally not possible to identify the virus type through symptoms.


title: Common cold

text: The only useful ways to reduce the spread of cold viruses are physical measures such as using correct hand washing technique and face masks; in the healthcare environment, gowns and disposable gloves are also used. Isolation or quarantine is not used as the disease is so widespread and symptoms are non-specific. There is no vaccine to protect against the common cold. Vaccination has proven difficult as there are many viruses involved and they mutate rapidly. Creation of a broadly effective vaccine is, therefore, highly improbable.


title: Common cold

text: Regular hand washing appears to be effective in reducing the transmission of cold viruses, especially among children. Whether the addition of antivirals or antibacterials to normal hand washing provides greater benefit is unknown. Wearing face masks when around people who are infected may be beneficial; however, there is insufficient evidence for maintaining a greater social distance.


title: Common cold

text: It is unclear whether zinc supplements affect the likelihood of contracting a cold. Routine vitamin C supplements do not reduce the risk or severity of the common cold, though they may reduce its duration. Gargling with water was found useful in one small trial.


title: Common cold

text: Treatments of the common cold primarily involve medications and other therapies for symptomatic relief. Getting plenty of rest, drinking fluids to maintain hydration, and gargling with warm salt water are reasonable conservative measures. Much of the benefit from symptomatic treatment is, however, attributed to the placebo effect. no medications or herbal remedies had been conclusively demonstrated to shorten the duration of infection.


title: Common cold

text: Treatments that may help with symptoms include simple pain medication and medications for fevers such as ibuprofen and acetaminophen (paracetamol). However, it is not clear whether acetaminophen helps with symptoms. It is not known if over the counter cough medications are effective for treating an acute cough. Cough medicines are not recommended for use in children due to a lack of evidence supporting effectiveness and the potential for harm. In 2009, Canada restricted the use of over-the-counter cough and cold medication in children six years and under due to concerns regarding risks and unproven benefits. The misuse of dextromethorphan (an over-the-counter cough medicine) has led to its ban in a number of countries. Intranasal corticosteroids have not been found to be useful.


title: Common cold

text: In adults short term use of nasal decongestants may have a small benefit. Antihistamines may improve symptoms in the first day or two; however, there is no longer-term benefit and they have adverse effects such as drowsiness. Other decongestants such as pseudoephedrine appear effective in adults. Combined oral analgesics, antihistaminics and decongestants are generally effective for older children and adults. Ipratropium nasal spray may reduce the symptoms of a runny nose but has little effect on stuffiness. Ipratropium may also help with cough in adults. The safety and effectiveness of nasal decongestant use in children is unclear.


title: Common cold

text: Due to lack of studies, it is not known whether increased fluid intake improves symptoms or shortens respiratory illness. As of 2017 heated and humidified air, such as via RhinoTherm, is of unclear benefit. One study has found chest vapor rub to provide some relief of nocturnal cough, congestion, and sleep difficulty.


title: Common cold

text: Some advise to avoid physical exercise if there are symptoms such as fever, widespread muscle aches or fatigue. It is regarded as safe to perform moderate exercise if the symptoms are confined to the head, including runny nose, nasal congestion, sneezing, or a minor sore throat. There is a popular belief that having a hot drink can help with cold symptoms, but evidence to support this is very limited.


title: Common cold

text: Antibiotics have no effect against viral infections, including the common cold. Due to their side effects, antibiotics cause overall harm but are still frequently prescribed. Some of the reasons that antibiotics are so commonly prescribed include people's expectations for them, physicians' desire to help, and the difficulty in excluding complications that may be amenable to antibiotics. There are no effective antiviral drugs for the common cold even though some preliminary research has shown benefits.


title: Common cold

text: Zinc supplements may shorten the duration of colds by up to 33% and reduce the severity of symptoms if supplementation begins within 24 hours of the onset of symptoms. Some zinc remedies directly applied to the inside of the nose have led to the loss of the sense of smell. A 2017 review did not recommend the use of zinc for the common cold for various reasons; whereas a 2017 and 2018 review both recommended the use of zinc, but also advocated further research on the topic.


title: Common cold

text: While there are many alternative medicines and Chinese herbal medicines supposed to treat the common cold, there is insufficient scientific evidence to support their use. As of 2015, there is weak evidence to support nasal irrigation with saline. There is no firm evidence that Echinacea products or garlic provide any meaningful benefit in treating or preventing colds.


title: Common cold

text: Vitamin C supplementation does not affect the incidence of the common cold, but may reduce its duration. There is no conclusive evidence that vitamin D supplementation is efficacious in the prevention or treatment of respiratory tract infections.


title: Common cold

text: The common cold is generally mild and self-limiting with most symptoms generally improving in a week. In children, half of cases go away in 10 days and 90% in 15 days. Severe complications, if they occur, are usually in the very old, the very young, or those who are immunosuppressed. Secondary bacterial infections may occur resulting in sinusitis, pharyngitis, or an ear infection. It is estimated that sinusitis occurs in 8% and ear infection in 30% of cases.


title: Common cold

text: The common cold is the most common human disease and affects people all over the globe. Adults typically have two to three infections annually, and children may have six to ten colds a year (and up to twelve colds a year for school children). Rates of symptomatic infections increase in the elderly due to declining immunity.


title: Common cold

text: Native Americans and Inuit are more likely to be infected with colds and develop complications such as otitis media than Caucasians. This may be explained as much by issues such as poverty and overcrowding as by ethnicity.


title: Common cold

text: While the cause of the common cold was identified in the 1950s, the disease appears to have been with humanity since its early history. Its symptoms and treatment are described in the Egyptian Ebers papyrus, the oldest existing medical text, written before the 16th century BCE. The name "cold" came into use in the 16th century, due to the similarity between its symptoms and those of exposure to cold weather.


title: Common cold

text: In the United Kingdom, the Common Cold Unit (CCU) was set up by the Medical Research Council in 1946 and it was where the rhinovirus was discovered in 1956. In the 1970s, the CCU demonstrated that treatment with interferon during the incubation phase of rhinovirus infection protects somewhat against the disease, but no practical treatment could be developed. The unit was closed in 1989, two years after it completed research of zinc gluconate lozenges in the prevention and treatment of rhinovirus colds, the only successful treatment in the history of the unit.


title: Common cold

text: Antivirals have been tested for effectiveness in the common cold; as of 2009, none had been both found effective and licensed for use. There are ongoing trials of the anti-viral drug pleconaril which shows promise against picornaviruses as well as trials of BTA-798. The oral form of pleconaril had safety issues and an aerosol form is being studied. Double-stranded RNA activated caspase oligomerizer (DRACO), a broad-spectrum antiviral therapy, has shown preliminary effectiveness in treating rhinovirus, as well as other infectious viruses.


title: Common cold

text: The economic impact of the common cold is not well understood in much of the world. In the United States, the common cold leads to 75–100 million physician visits annually at a conservative cost estimate of $7.7 billion per year. Americans spend $2.9 billion on over-the-counter drugs and another $400 million on prescription medicines for symptom relief. More than one-third of people who saw a doctor received an antibiotic prescription, which has implications for antibiotic resistance. An estimated 22–189 million school days are missed annually due to a cold. As a result, parents missed 126 million workdays to stay home to care for their children. When added to the 150 million workdays missed by employees who have a cold, the total economic impact of cold-related work loss exceeds $20 billion per year. This accounts for 40% of time lost from work in the United States.


title: Edging (sexual practice)

text: Edging, peaking, or surfing is a sexual technique whereby an orgasm is controlled. It is practiced alone or with a partner and involves the maintenance of a high level of sexual arousal for an extended period without reaching climax.


title: Edging (sexual practice)

text: When practiced by males, direct sexual stimulation without the refractory period after orgasm is observed. When the controlled orgasm is achieved, the physical sensations are greater as compared to conventional orgasm. Orgasm control is referenced as "slow masturbation" in Alex Comfort's "The New Joy of Sex" (1993) and "extended massive orgasm" in Vera and Steve Bodansky's 2000 book of the same name. It is similar to the Venus Butterfly technique used in the volume "The One Hour Orgasm" (1988) by Leah and Bob Schwartz. Detailed practices including exercises can be found in several books, such as Mantak Chia's "The Multi-Orgasmic Man".


title: Edging (sexual practice)

text: Orgasm control involves either sex partner being in control of the other partner's orgasm, or a person delaying their own orgasm during sexual activity with a partner or by masturbation. Any method of sexual stimulation can be used to experience controlled orgasm.


title: Edging (sexual practice)

text: During intercourse or other forms of sexual stimulation with a partner, one person stimulates the other(s) and reduces the level of stimulation when approaching orgasm. Erotic sexual denial occurs when the partner who is in control of the other partner's orgasm prolongs the orgasm to allow for an increased level of sexual tension.


title: Edging (sexual practice)

text: When a partner eventually provides enough stimulation to achieve an orgasm, it may be stronger than usual due to increased tension and arousal that builds up during the extended stimulation. An example of the use of orgasm control in partnered sex can be seen in BDSM; if the partner whose orgasm is being controlled (sometimes referred to as the submissive partner) is tied up, it may better control the orgasm (the activity is sometimes called tie and tease; if orgasm is denied, it is then known as tease and denial).


title: Edging (sexual practice)

text: When practicing alone in masturbation, orgasm control can heighten sexual pleasure. Another reason for practicing orgasm control is to extend the amount of time it takes to orgasm. For a female, the practitioner can enjoy direct sexual stimulation for longer periods of time, as well as increasing frequency and intensity. For a man, the speed of masturbation may vary to navigate right to the edge of ejaculation. With orgasm control, a male can experience a more intense orgasm, as well as a larger volume of semen expelled during his ejaculation. One technique, commonly referred to as 'edging', involves masturbating up until the moment before reaching the plateau phase just before orgasm occurs, and then stopping suddenly before experiencing a climax. Another technique, commonly referred to as 'surfing', involves reaching the plateau phase and slowing down the stimulation to maintain a heightened level of sensation for an extended time. Repeating either of these techniques many times during a single masturbation session may result in a stronger, more intense orgasm.


title: Edging (sexual practice)

text: Orgasm control is more often possible with masturbation. This is because masturbation allows for individual control and has no reliance on a sexual partner to achieve orgasm. Masturbation can be seen as the starting point in the technique of orgasm control. Self-stimulation can help people learn about the limits of their body and the techniques that may help them in controlling their orgasms. Recent research suggests masturbation may decrease the incidence of prostate cancer.


title: Edging (sexual practice)

text: As it has been described by Steve Bodansky and Vera Bodansky in "Extended Massive Orgasm: How You Can Give and Receive Intense Sexual Pleasure", masturbation practiced with the aim of orgasm control should be carried on with the purpose of endured sexual gratification and not to relieve tension, as 'simple' masturbation does. In 2009, Masanobu Sato masturbated for 9 hours and 58 minutes at the 9th annual World Masturbate-a-thon.


title: Photoelectric effect

text: The photoelectric effect is the emission of electrons when electromagnetic radiation, such as light, hits a material. Electrons emitted in this manner are called photoelectrons. The phenomenon is studied in condensed matter physics, and solid state and quantum chemistry to draw inferences about the properties of atoms, molecules and solids. The effect has found use in electronic devices specialized for light detection and precisely timed electron emission.


title: Photoelectric effect

text: The experimental results disagree with classical electromagnetism, which predicts that continuous light waves transfer energy to electrons, which would then be emitted when they accumulate enough energy. An alteration in the intensity of light would theoretically change the kinetic energy of the emitted electrons, with sufficiently dim light resulting in a delayed emission. The experimental results instead show that electrons are dislodged only when the light exceeds a certain frequency—regardless of the light's intensity or duration of exposure. Because a low-frequency beam at a high intensity does not build up the energy required to produce photoelectrons, as would be the case if light's energy accumulated over time from a continuous wave, Albert Einstein proposed that a beam of light is not a wave propagating through space, but a swarm of discrete energy packets, known as photons.


title: Photoelectric effect

text: Emission of conduction electrons from typical metals requires a few electron-volt (eV) light quanta, corresponding to short-wavelength visible or ultraviolet light. In extreme cases, emissions are induced with photons approaching zero energy, like in systems with negative electron affinity and the emission from excited states, or a few hundred keV photons for core electrons in elements with a high atomic number. Study of the photoelectric effect led to important steps in understanding the quantum nature of light and electrons and influenced the formation of the concept of wave–particle duality. Other phenomena where light affects the movement of electric charges include the photoconductive effect, the photovoltaic effect, and the photoelectrochemical effect.


title: Photoelectric effect

text: The photons of a light beam have a characteristic energy, called photon energy, which is proportional to the frequency of the light. In the photoemission process, when an electron within some material absorbs the energy of a photon and acquires more energy than its binding energy, it is likely to be ejected. If the photon energy is too low, the electron is unable to escape the material. Since an increase in the intensity of low-frequency light will only increase the number of low-energy photons, this change in intensity will not create any single photon with enough energy to dislodge an electron. Moreover, the energy of the emitted electrons will not depend on the intensity of the incoming light of a given frequency, but only on the energy of the individual photons.


title: Photoelectric effect

text: While free electrons can absorb any energy when irradiated as long as this is followed by an immediate re-emission, like in the Compton effect, in quantum systems all of the energy from one photon is absorbed—if the process is allowed by quantum mechanics—or none at all. Part of the acquired energy is used to liberate the electron from its atomic binding, and the rest contributes to the electron's kinetic energy as a free particle. Because electrons in a material occupy many different quantum states with different binding energies, and because they can sustain energy losses on their way out of the material, the emitted electrons will have a range of kinetic energies. The electrons from the highest occupied states will have the highest kinetic energy. In metals, those electrons will be emitted from the Fermi level.


title: Photoelectric effect

text: When the photoelectron is emitted into a solid rather than into a vacuum, the term "internal photoemission" is often used, and emission into a vacuum is distinguished as "external photoemission".


title: Photoelectric effect

text: Even though photoemission can occur from any material, it is most readily observed from metals and other conductors. This is because the process produces a charge imbalance which, if not neutralized by current flow, results in the increasing potential barrier until the emission completely ceases. The energy barrier to photoemission is usually increased by nonconductive oxide layers on metal surfaces, so most practical experiments and devices based on the photoelectric effect use clean metal surfaces in evacuated tubes. Vacuum also helps observing the electrons since it prevents gases from impeding their flow between the electrodes.


title: Photoelectric effect

text: As sunlight, due to atmosphere's absorption, does not provide much ultraviolet light, the light rich in ultraviolet rays used to be obtained by burning magnesium or from an arc lamp. At the present time, mercury-vapor lamps, noble-gas discharge UV lamps and radio-frequency plasma sources, ultraviolet lasers, and synchrotron insertion device light sources prevail.


title: Photoelectric effect

text: The classical setup to observe the photoelectric effect includes a light source, a set of filters to monochromatize the light, a vacuum tube transparent to ultraviolet light, an emitting electrode (E) exposed to the light, and a collector (C) whose voltage "V" can be externally controlled.


title: Photoelectric effect

text: A positive external voltage is used to direct the photoemitted electrons onto the collector. If the frequency and the intensity of the incident radiation are fixed, the photoelectric current "I" increases with an increase in the positive voltage, as more and more electrons are directed onto the electrode. When no additional photoelectrons can be collected, the photoelectric current attains a saturation value. This current can only increase with the increase of the intensity of light.


title: Photoelectric effect

text: An increasing negative voltage prevents all but the highest-energy electrons from reaching the collector. When no current is observed through the tube, the negative voltage has reached the value that is high enough to slow down and stop the most energetic photoelectrons of kinetic energy "K". This value of the retarding voltage is called the "stopping potential" or "cut off" potential "V". Since the work done by the retarding potential in stopping the electron of charge "e" is "eV", the following must hold "eV" = "K"


title: Photoelectric effect

text: The current-voltage curve is sigmoidal, but its exact shape depends on the experimental geometry and the electrode material properties.


title: Photoelectric effect

text: For a given metal surface, there exists a certain minimum frequency of incident radiation below which no photoelectrons are emitted. This frequency is called the "threshold frequency". Increasing the frequency of the incident beam increases the maximum kinetic energy of the emitted photoelectrons, and the stopping voltage has to increase. The number of emitted electrons may also change because the probability that each photon results in an emitted electron is a function of photon energy.


title: Photoelectric effect

text: An increase in the intensity of the same monochromatic light (so long as the intensity is not too high), which is proportional to the number of photons impinging on the surface in a given time, increases the rate at which electrons are ejected—the photoelectric current "I—"but the kinetic energy of the photoelectrons and the stopping voltage remain the same. For a given metal and frequency of incident radiation, the rate at which photoelectrons are ejected is directly proportional to the intensity of the incident light.


title: Photoelectric effect

text: The time lag between the incidence of radiation and the emission of a photoelectron is very small, less than 10 second. Angular distribution of the photoelectrons is highly dependent on polarization (the direction of the electric field) of the incident light, as well as the emitting material's quantum properties such as atomic and molecular orbital symmetries and the electronic band structure of crystalline solids. In materials without macroscopic order, the distribution of electrons tends to peak in the direction of polarization of linearly polarized light. The experimental technique that can measure these distributions to infer the material's properties is angle-resolved photoemission spectroscopy.


title: Photoelectric effect

text: In 1905, Einstein proposed a theory of the photoelectric effect using a concept first put forward by Max Planck that light consists of tiny packets of energy known as photons or light quanta. Each packet carries energy formula_1 that is proportional to the frequency formula_2 of the corresponding electromagnetic wave. The proportionality constant formula_3 has become known as the Planck constant. The maximum kinetic energy formula_4 of the electrons that were delivered this much energy before being removed from their atomic binding is


title: Photoelectric effect

text: where formula_6 is the minimum energy required to remove an electron from the surface of the material. It is called the work function of the surface and is sometimes denoted formula_7 or formula_8. If the work function is written as


title: Photoelectric effect

text: Kinetic energy is positive, and formula_11 is required for the photoelectric effect to occur. The frequency formula_12 is the threshold frequency for the given material. Above that frequency, the maximum kinetic energy of the photoelectrons as well as the stopping voltage in the experiment formula_13 rise linearly with the frequency, and have no dependence on the number of photons and the intensity of the impinging monochromatic light. Einstein's formula, however simple, explained all the phenomenology of the photoelectric effect, and had far-reaching consequences in the development of quantum mechanics.


title: Photoelectric effect

text: Electrons that are bound in atoms, molecules and solids each occupy distinct states of well-defined binding energies. When light quanta deliver more than this amount of energy to an individual electron, the electron may be emitted into free space with excess (kinetic) energy that is formula_1 higher than the electron's binding energy. The distribution of kinetic energies thus reflects the distribution of the binding energies of the electrons in the atomic, molecular or crystalline system: an electron emitted from the state at binding energy formula_15 is found at kinetic energy formula_16. This distribution is one of the main characteristics of the quantum system, and can be used for further studies in quantum chemistry and quantum physics.


title: Photoelectric effect

text: The electronic properties of ordered, crystalline solids are determined by the distribution of the electronic states with respect to energy and momentum—the electronic band structure of the solid. Theoretical models of photoemission from solids show that this distribution is, for the most part, preserved in the photoelectric effect. The phenomenological "three-step model" for ultraviolet and soft X-ray excitation decomposes the effect into these steps:


title: Photoelectric effect

text: There are cases where the three-step model fails to explain peculiarities of the photoelectron intensity distributions. The more elaborate "one-step model" treats the effect as a coherent process of photoexcitation into the final state of a finite crystal for which the wave function is free-electron-like outside of the crystal, but has a decaying envelope inside.


title: Photoelectric effect

text: In 1839, Alexandre Edmond Becquerel discovered the photovoltaic effect while studying the effect of light on electrolytic cells. Though not equivalent to the photoelectric effect, his work on photovoltaics was instrumental in showing a strong relationship between light and electronic properties of materials. In 1873, Willoughby Smith discovered photoconductivity in selenium while testing the metal for its high resistance properties in conjunction with his work involving submarine telegraph cables.


title: Photoelectric effect

text: Johann Elster (1854–1920) and Hans Geitel (1855–1923), students in Heidelberg, investigated the effects produced by light on electrified bodies and developed the first practical photoelectric cells that could be used to measure the intensity of light. They arranged metals with respect to their power of discharging negative electricity: rubidium, potassium, alloy of potassium and sodium, sodium, lithium, magnesium, thallium and zinc; for copper, platinum, lead, iron, cadmium, carbon, and mercury the effects with ordinary light were too small to be measurable. The order of the metals for this effect was the same as in Volta's series for contact-electricity, the most electropositive metals giving the largest photo-electric effect.


title: Photoelectric effect

text: In 1887, Heinrich Hertz observed the photoelectric effect and reported on the production and reception of electromagnetic waves. The receiver in his apparatus consisted of a coil with a spark gap, where a spark would be seen upon detection of electromagnetic waves. He placed the apparatus in a darkened box to see the spark better. However, he noticed that the maximum spark length was reduced when inside the box. A glass panel placed between the source of electromagnetic waves and the receiver absorbed ultraviolet radiation that assisted the electrons in jumping across the gap. When removed, the spark length would increase. He observed no decrease in spark length when he replaced the glass with quartz, as quartz does not absorb UV radiation.


title: Photoelectric effect

text: The discoveries by Hertz led to a series of investigations by Hallwachs, Hoor, Righi and Stoletov on the effect of light, and especially of ultraviolet light, on charged bodies. Hallwachs connected a zinc plate to an electroscope. He allowed ultraviolet light to fall on a freshly cleaned zinc plate and observed that the zinc plate became uncharged if initially negatively charged, positively charged if initially uncharged, and more positively charged if initially positively charged. From these observations he concluded that some negatively charged particles were emitted by the zinc plate when exposed to ultraviolet light.


title: Photoelectric effect

text: With regard to the "Hertz effect", the researchers from the start showed the complexity of the phenomenon of photoelectric fatigue—the progressive diminution of the effect observed upon fresh metallic surfaces. According to Hallwachs, ozone played an important part in the phenomenon, and the emission was influenced by oxidation, humidity, and the degree of polishing of the surface. It was at the time unclear whether fatigue is absent in a vacuum.


title: Photoelectric effect

text: In the period from 1888 until 1891, a detailed analysis of the photoeffect was performed by Aleksandr Stoletov with results reported in six publications. Stoletov invented a new experimental setup which was more suitable for a quantitative analysis of the photoeffect. He discovered a direct proportionality between the intensity of light and the induced photoelectric current (the first law of photoeffect or Stoletov's law). He measured the dependence of the intensity of the photo electric current on the gas pressure, where he found the existence of an optimal gas pressure corresponding to a maximum photocurrent; this property was used for the creation of solar cells.


title: Photoelectric effect

text: Many substances besides metals discharge negative electricity under the action of ultraviolet light. G. C. Schmidt and O. Knoblauch compiled a list of these substances.


title: Photoelectric effect

text: In 1897, J. J. Thomson investigated ultraviolet light in Crookes tubes. Thomson deduced that the ejected particles, which he called corpuscles, were of the same nature as cathode rays. These particles later became known as the electrons. Thomson enclosed a metal plate (a cathode) in a vacuum tube, and exposed it to high-frequency radiation. It was thought that the oscillating electromagnetic fields caused the atoms' field to resonate and, after reaching a certain amplitude, caused subatomic corpuscles to be emitted, and current to be detected. The amount of this current varied with the intensity and color of the radiation. Larger radiation intensity or frequency would produce more current.


title: Photoelectric effect

text: During the years 1886–1902, Wilhelm Hallwachs and Philipp Lenard investigated the phenomenon of photoelectric emission in detail. Lenard observed that a current flows through an evacuated glass tube enclosing two electrodes when ultraviolet radiation falls on one of them. As soon as ultraviolet radiation is stopped, the current also stops. This initiated the concept of photoelectric emission. The discovery of the ionization of gases by ultraviolet light was made by Philipp Lenard in 1900. As the effect was produced across several centimeters of air and yielded a greater number of positive ions than negative, it was natural to interpret the phenomenon, as J. J. Thomson did, as a "Hertz effect" upon the particles present in the gas.


title: Photoelectric effect

text: In 1902, Lenard observed that the energy of individual emitted electrons increased with the frequency (which is related to the color) of the light. This appeared to be at odds with Maxwell's wave theory of light, which predicted that the electron energy would be proportional to the intensity of the radiation.


title: Photoelectric effect

text: Lenard observed the variation in electron energy with light frequency using a powerful electric arc lamp which enabled him to investigate large changes in intensity, and that had sufficient power to enable him to investigate the variation of the electrode's potential with light frequency. He found the electron energy by relating it to the maximum stopping potential (voltage) in a phototube. He found that the maximum electron kinetic energy is determined by the frequency of the light. For example, an increase in frequency results in an increase in the maximum kinetic energy calculated for an electron upon liberation – ultraviolet radiation would require a higher applied stopping potential to stop current in a phototube than blue light. However, Lenard's results were qualitative rather than quantitative because of the difficulty in performing the experiments: the experiments needed to be done on freshly cut metal so that the pure metal was observed, but it oxidized in a matter of minutes even in the partial vacuums he used. The current emitted by the surface was determined by the light's intensity, or brightness: doubling the intensity of the light doubled the number of electrons emitted from the surface.


title: Photoelectric effect

text: The researches of Langevin and those of Eugene Bloch have shown that the greater part of the Lenard effect is certainly due to the "Hertz effect". The Lenard effect upon the gas itself nevertheless does exist. Refound by J. J. Thomson and then more decisively by Frederic Palmer, Jr., the gas photoemission was studied and showed very different characteristics than those at first attributed to it by Lenard.


title: Photoelectric effect

text: In 1900, while studying black-body radiation, the German physicist Max Planck suggested in his "On the Law of Distribution of Energy in the Normal Spectrum" paper that the energy carried by electromagnetic waves could only be released in "packets" of energy. In 1905, Albert Einstein published a paper advancing the hypothesis that light energy is carried in discrete quantized packets to explain experimental data from the photoelectric effect. Einstein theorized that the energy in each quantum of light was equal to the frequency of light multiplied by a constant, later called the Planck constant. A photon above a threshold frequency has the required energy to eject a single electron, creating the observed effect. This was a key step in the development of quantum mechanics. In 1914, Robert A. Millikan's highly accurate measurements of the Planck constant from the photoelectric effect supported Einstein's model, even though a corpuscular theory of light was for Millikan, at the time, "quite unthinkable". Einstein was awarded the 1921 Nobel Prize in Physics for "his discovery of the law of the photoelectric effect", and Millikan was awarded the Nobel Prize in 1923 for "his work on the elementary charge of electricity and on the photoelectric effect". In quantum perturbation theory of atoms and solids acted upon by electromagnetic radiation, the photoelectric effect is still commonly analyzed in terms of waves; the two approaches are equivalent because photon or wave absorption can only happen between quantized energy levels whose energy difference is that of the energy of photon.


title: Photoelectric effect

text: Albert Einstein's mathematical description of how the photoelectric effect was caused by absorption of quanta of light was in one of his Annus Mirabilis papers, named "On a Heuristic Viewpoint Concerning the Production and Transformation of Light". The paper proposed a simple description of "light quanta", or photons, and showed how they explained such phenomena as the photoelectric effect. His simple explanation in terms of absorption of discrete quanta of light agreed with experimental results. It explained why the energy of photoelectrons was dependent only on the "frequency" of the incident light and not on its "intensity": at low-intensity, the high-frequency source could supply a few high energy photons, whereas at high-intensity, the low-frequency source would supply no photons of sufficient individual energy to dislodge any electrons. This was an enormous theoretical leap, but the concept was strongly resisted at first because it contradicted the wave theory of light that followed naturally from James Clerk Maxwell's equations of electromagnetism, and more generally, the assumption of infinite divisibility of energy in physical systems. Even after experiments showed that Einstein's equations for the photoelectric effect were accurate, resistance to the idea of photons continued.


title: Photoelectric effect

text: Einstein's work predicted that the energy of individual ejected electrons increases linearly with the frequency of the light. Perhaps surprisingly, the precise relationship had not at that time been tested. By 1905 it was known that the energy of photoelectrons increases with increasing "frequency" of incident light and is independent of the "intensity" of the light. However, the manner of the increase was not experimentally determined until 1914 when Millikan showed that Einstein's prediction was correct.


title: Photoelectric effect

text: The photoelectric effect helped to propel the then-emerging concept of wave–particle duality in the nature of light. Light simultaneously possesses the characteristics of both waves and particles, each being manifested according to the circumstances. The effect was impossible to understand in terms of the classical wave description of light, as the energy of the emitted electrons did not depend on the intensity of the incident radiation. Classical theory predicted that the electrons would 'gather up' energy over a period of time, and then be emitted.


title: Photoelectric effect

text: These are extremely light-sensitive vacuum tubes with a coated photocathode inside the envelope. The photo cathode contains combinations of materials such as cesium, rubidium, and antimony specially selected to provide a low work function, so when illuminated even by very low levels of light, the photocathode readily releases electrons. By means of a series of electrodes (dynodes) at ever-higher potentials, these electrons are accelerated and substantially increased in number through secondary emission to provide a readily detectable output current. Photomultipliers are still commonly used wherever low levels of light must be detected.


title: Photoelectric effect

text: Video camera tubes in the early days of television used the photoelectric effect, for example, Philo Farnsworth's "Image dissector" used a screen charged by the photoelectric effect to transform an optical image into a scanned electronic signal.


title: Photoelectric effect

text: Because the kinetic energy of the emitted electrons is exactly the energy of the incident photon minus the energy of the electron's binding within an atom, molecule or solid, the binding energy can be determined by shining a monochromatic X-ray or UV light of a known energy and measuring the kinetic energies of the photoelectrons. The distribution of electron energies is valuable for studying quantum properties of these systems. It can also be used to determine the elemental composition of the samples. For solids, the kinetic energy and emission angle distribution of the photoelectrons is measured for the complete determination of the electronic band structure in terms of the allowed binding energies and momenta of the electrons. Modern instruments for angle-resolved photoemission spectroscopy are capable of measuring these quantities with a precision better than 1 meV and 0.1°.


title: Photoelectric effect

text: Photoelectron spectroscopy measurements are usually performed in a high-vacuum environment, because the electrons would be scattered by gas molecules if they were present. However, some companies are now selling products that allow photoemission in air. The light source can be a laser, a discharge tube, or a synchrotron radiation source.


title: Photoelectric effect

text: The concentric hemispherical analyzer is a typical electron energy analyzer. It uses an electric field between two hemispheres to change (disperse) the trajectories of incident electrons depending on their kinetic energies.


title: Photoelectric effect

text: Photons hitting a thin film of alkali metal or semiconductor material such as gallium arsenide in an image intensifier tube cause the ejection of photoelectrons due to the photoelectric effect. These are accelerated by an electrostatic field where they strike a phosphor coated screen, converting the electrons back into photons. Intensification of the signal is achieved either through acceleration of the electrons or by increasing the number of electrons through secondary emissions, such as with a micro-channel plate. Sometimes a combination of both methods is used. Additional kinetic energy is required to move an electron out of the conduction band and into the vacuum level. This is known as the electron affinity of the photocathode and is another barrier to photoemission other than the forbidden band, explained by the band gap model. Some materials such as gallium arsenide have an effective electron affinity that is below the level of the conduction band. In these materials, electrons that move to the conduction band all have sufficient energy to be emitted from the material, so the film that absorbs photons can be quite thick. These materials are known as negative electron affinity materials.


title: Photoelectric effect

text: The photoelectric effect will cause spacecraft exposed to sunlight to develop a positive charge. This can be a major problem, as other parts of the spacecraft are in shadow which will result in the spacecraft developing a negative charge from nearby plasmas. The imbalance can discharge through delicate electrical components. The static charge created by the photoelectric effect is self-limiting, because a higher charged object doesn't give up its electrons as easily as a lower charged object does.


title: Photoelectric effect

text: Light from the Sun hitting lunar dust causes it to become positively charged from the photoelectric effect. The charged dust then repels itself and lifts off the surface of the Moon by electrostatic levitation. This manifests itself almost like an "atmosphere of dust", visible as a thin haze and blurring of distant features, and visible as a dim glow after the sun has set. This was first photographed by the Surveyor program probes in the 1960s, and most recently the Chang'e 3 rover observed dust deposition on lunar rocks as high as about 28 cm. It is thought that the smallest particles are repelled kilometers from the surface and that the particles move in "fountains" as they charge and discharge.


title: Photoelectric effect

text: When photon energies are as high as the electron rest energy of , yet another process, the Compton scattering, may take place. Above twice this energy, at pair production is also more likely. Compton scattering and pair production are examples of two other competing mechanisms.


title: Photoelectric effect

text: Even if the photoelectric effect is the favoured reaction for a particular interaction of a single photon with a bound electron, the result is also subject to quantum statistics and is not guaranteed. The probability of the photoelectric effect occurring is measured by the cross section of the interaction, σ. This has been found to be a function of the atomic number of the target atom and photon energy. In a crude approximation, for photon energies above the highest atomic binding energy, the cross section is given by:


title: Photoelectric effect

text: Here "Z" is the atomic number and "n" is a number which varies between 4 and 5. The photoelectric effect rapidly decreases in significance in the gamma-ray region of the spectrum, with increasing photon energy. It is also more likely from elements with high atomic number. Consequently, high-"Z" materials make good gamma-ray shields, which is the principal reason why lead ("Z" = 82) is preferred and most widely used.


title: Christa McAuliffe

text: Sharon Christa McAuliffe ( Corrigan; September 2, 1948 – January 28, 1986) was an American teacher and astronaut from Concord, New Hampshire, who was killed on the Space Shuttle "Challenger" on mission STS-51-L where she was serving as a payload specialist.


title: Christa McAuliffe

text: She received her bachelor's degree in education and history from Framingham State College in 1970 and her master's degree in education, supervision and administration from Bowie State University in 1978. She took a teaching position as a social studies teacher at Concord High School in New Hampshire in 1983.


title: Christa McAuliffe

text: In 1985, McAuliffe was selected from more than 11,000 applicants to the NASA Teacher in Space Project and was scheduled to become the first teacher to fly in space. As a member of mission STS-51-L, she was planning to conduct experiments and teach two lessons from "Challenger". On January 28, 1986, the shuttle broke apart 1 minute 13 seconds after launch, killing all onboard. After her death, several schools were named in her honor, and she was posthumously awarded the Congressional Space Medal of Honor in 2004.


title: Christa McAuliffe

text: Sharon Christa Corrigan was born on September 2, 1948, in Boston as the oldest of the five children of accountant Edward Christopher Corrigan (1922–1990), who was of Irish descent; and Grace Mary Corrigan (1924–2018; née George), a substitute teacher, whose father was of Lebanese Maronite descent. McAuliffe was a great niece of Lebanese-American historian Philip Khuri Hitti. She was known by her middle name from an early age, although in later years she signed her name "S. Christa Corrigan", and eventually "S. Christa McAuliffe".


title: Christa McAuliffe

text: The year she was born, her father was completing his sophomore year at Boston College. Not long after, he took a job as an assistant comptroller in a Boston department store, and they moved to Framingham, Massachusetts, where she attended and graduated from Marian High School in 1966. She went on to earn a bachelor's degree in 1970 from Framingham State College, now Framingham State University. As a youth, she was inspired by Project Mercury and the Apollo Moon landing program. The day after John Glenn orbited the Earth in "Friendship 7", she told a friend at Marian High, "Do you realize that someday people will be going to the Moon? Maybe even taking a bus, and I want to do that!" She wrote years later on her NASA application form: "I watched the Space Age being born, and I would like to participate."


title: Christa McAuliffe

text: In 1970, she married her longtime boyfriend whom she had known since high school, Steven J. McAuliffe, a 1970 graduate of the Virginia Military Institute, and they moved closer to Washington, D.C., so that he could attend the Georgetown University Law Center. They had two children, Scott and Caroline, who were nine and six, respectively, when she died.


title: Christa McAuliffe

text: She obtained her first teaching position in 1970, as an American history teacher at Benjamin Foulois Junior High School in Morningside, Maryland. From 1971 to 1978, she taught history and civics at Thomas Johnson Middle School in Lanham, Maryland. In addition to teaching, she completed a Master of Arts in education supervision and administration from Bowie State University in Maryland. In 1978, she moved to Concord, New Hampshire, when Steven accepted a job as an assistant to the New Hampshire Attorney General. McAuliffe taught 7th and 8th grade American history and English in Concord, New Hampshire, and 9th grade English in Bow, New Hampshire, before taking a teaching post at Concord High School in 1983.


title: Christa McAuliffe

text: She was a social studies teacher, and taught several courses including American history, law, and economics, in addition to a self-designed course: "The American Woman". Taking field trips and bringing in speakers were an important part of her teaching techniques. According to "The New York Times", she "emphasized the impact of ordinary people on history, saying they were as important to the historical record as kings, politicians or generals."


title: Christa McAuliffe

text: In 1984, President Ronald Reagan announced the Teacher in Space Project, and Christa learned about NASA's efforts to find their first civilian, an educator, to fly into space. NASA wanted to find an "ordinary person," a gifted teacher who could communicate with students while in orbit. McAuliffe became one of more than 11,000 applicants.


title: Christa McAuliffe

text: NASA hoped that sending a teacher into space would increase public interest in the Space Shuttle program, and also demonstrate the reliability of space flight at a time when the agency was under continuous pressure to find financial support. President Reagan said it would also remind Americans of the important role that teachers and education serve in their country.


title: Christa McAuliffe

text: The Council of Chief State School Officers, a non-profit organization of public officials in education, was chosen by NASA to coordinate the selection process. Out of the initial applicant pool, 114 semi-finalists were nominated by state, territorial, and agency review panels. McAuliffe was one of two teachers nominated by the state of New Hampshire. The semi-finalists gathered in Washington, D.C., from June 22–27, 1985, for a conference on space education and to meet with the Review Panel that would select the 10 finalists.


title: Christa McAuliffe

text: On July 1, 1985, she was announced as one of the 10 finalists, and on July 7 she traveled to Johnson Space Center for a week of thorough medical examinations and briefings about space flight. The finalists were interviewed by an evaluation committee composed of senior NASA officials, and the committee made recommendations to NASA Administrator James M. Beggs for the primary and backup candidates for the Teacher in Space Project. On July 19, 1985, Vice President George H. W. Bush announced that she had been selected for the position. Another teacher, Barbara Morgan, served as her backup. According to Mark Travis of the "Concord Monitor", it was her manner that set her apart from the other candidates. NASA official Alan Ladwig said "she had an infectious enthusiasm", and NASA psychiatrist Terrence McGuire told "New Woman" magazine that "she was the most broad-based, best-balanced person of the 10."


title: Christa McAuliffe

text: Later that year, McAuliffe and Morgan each took a year-long leave of absence from teaching in order to train for a Space Shuttle mission in early 1986. NASA paid both their salaries. While not a member of the NASA Astronaut Corps, McAuliffe was to be part of the STS-51-L crew, and would conduct experiments and teach lessons from space. Her planned duties included basic science experiments in the fields of chromatography, hydroponics, magnetism, and Newton's laws. She was also planning to conduct two 15-minute classes from space, including a tour of the spacecraft, called "The Ultimate Field Trip", and a lesson about the benefits of space travel, called "Where We've Been, Where We're Going, Why". The lessons were to be broadcast to millions of schoolchildren via closed-circuit TV. To record her thoughts, McAuliffe intended to keep a personal journal like a "woman on the Conestoga wagons pioneering the West."


title: Christa McAuliffe

text: After being chosen to be the first teacher in space, she was a guest on several television programs, including "Good Morning America"; the "CBS Morning News"; the "Today Show"; and "The Tonight Show Starring Johnny Carson", where, when asked about the mission, she stated, "If you're offered a seat on a rocket ship, don't ask what seat. Just get on." She had an immediate rapport with the media, and the Teacher in Space Project received popular attention as a result.


title: Christa McAuliffe

text: On January 28, 1986, McAuliffe boarded "Challenger" with the other six crew members of STS-51-L. Seventy-three seconds into its flight at an altitude of , the shuttle broke apart, resulting in the deaths of all seven crew members.


title: Christa McAuliffe

text: According to NASA, it was in part because of the excitement over her presence on the shuttle that the accident had such a significant effect on the nation. Many schoolchildren were viewing the launch live, and media coverage of the accident was extensive.


title: Christa McAuliffe

text: Barbara Morgan, her backup, became a professional astronaut in January 1998, and flew on Space Shuttle mission STS-118, to the International Space Station, on August 8, 2007, aboard "Endeavour", the orbiter that replaced "Challenger".


title: Christa McAuliffe

text: McAuliffe was buried at Blossom Hill Cemetery in her hometown, Concord. She has since been honored at many events, including the Daytona 500 NASCAR race in 1986. The McAuliffe-Shepard Discovery Center in Concord, the Christa Corrigan McAuliffe Center for Education and Teaching Excellence at Framingham State University, the Christa McAuliffe Intermediate School in Brooklyn, NY and the S. Christa McAuliffe Elementary School in Lowell, Massachusetts, were named in her memory, as are the asteroid 3352 McAuliffe, the crater McAuliffe on the Moon, and a crater on the planet Venus, which was named McAuliffe by the Soviet Union. Approximately 40 schools around the world have been named after her, including the Christa McAuliffe Space Education Center in Pleasant Grove, Utah.


title: Christa McAuliffe

text: Scholarships and other events have also been established in her memory. The Christa McAuliffe Technology Conference has been held in Nashua, New Hampshire, every year since 1986, and is devoted to the use of technology in all aspects of education. The Nebraska McAuliffe Prize honors a Nebraska teacher each year for courage and excellence in education. Grants in her name, honoring innovative teachers, are provided by the American Association of State Colleges and Universities and the National Council for the Social Studies.


title: Christa McAuliffe

text: She was portrayed by Karen Allen in the 1990 TV movie "Challenger". In October 2018, it was announced that Michelle Williams was cast to portray McAuliffe in "The Challenger", another retelling of the Space Shuttle Challenger disaster. In 2019, McAuliffe was portrayed by Erika Waldorf in the independent film "The Challenger Disaster". The spaceship on the 1996–1997 children's science-fiction series "Space Cases", about a group of students lost in space, was called "Christa". In 2006, a documentary film about her and Morgan called "Christa McAuliffe: Reach for the Stars" aired on CNN in the "CNN Presents" format. The film, produced by Renee Sotile and Mary Jo Godges, commemorated the 20th anniversary of her death. It was narrated by Susan Sarandon, and included an original song by Carly Simon.


title: Christa McAuliffe

text: Her parents worked with Framingham State College to establish the McAuliffe Center. Her husband Steven J. McAuliffe remarried and in 1992 became a federal judge, serving with the United States District Court for the District of New Hampshire in Concord. Her son, Scott, completed graduate studies in marine biology, and her daughter, Caroline, went on to pursue the same career as her mother: teaching. On July 23, 2004, she and all the other 13 astronauts lost in both the "Challenger" and "Columbia" disasters were posthumously awarded the Congressional Space Medal of Honor by President George W. Bush.


title: Christa McAuliffe

text: On January 28, 2016, several teachers who competed alongside McAuliffe for a seat on the "Challenger" traveled to Cape Canaveral, Florida, for a 30th anniversary remembrance service, along with her widower, Steven and son, Scott. After remarking that 30 years had passed, Steven said ""Challenger" will always be an event that occurred just recently. Our thoughts and memories of Christa will always be fresh and comforting." In 2017, McAuliffe was inducted into the International Air & Space Hall of Fame at the San Diego Air & Space Museum.


title: Christa McAuliffe

text: In 2019, Congress passed the Christa McAuliffe Commemorative Coin Act which was signed into law by President Donald Trump on October 9, 2019. The bill allows the Department of the Treasury to "issue not more than 350,000 $1 silver coins in commemoration of Christa McAuliffe." The coins were minted in 2021.


title: Holocene extinction

text: The Holocene extinction, or Anthropocene extinction, is the ongoing extinction event during the Holocene epoch. The extinctions span numerous families of bacteria, fungi, plants, and animals, including mammals, birds, reptiles, amphibians, fish, invertebrates, and affecting not just terrestrial species but also large sectors of marine life. With widespread degradation of biodiversity hotspots, such as coral reefs and rainforests, as well as other areas, the vast majority of these extinctions are thought to be undocumented, as the species are undiscovered at the time of their extinction, which goes unrecorded. The current rate of extinction of species is estimated at 100 to 1,000 times higher than natural background extinction rates, and is increasing.


title: Holocene extinction

text: During the past 100–200 years, biodiversity loss and species extinction have accelerated, to the point that most conservation biologists now believe that humankind has either entered a period of mass extinction, or is on the cusp of doing so. As such, the event has also been referred to as the sixth mass extinction or sixth extinction.


title: Holocene extinction

text: The Holocene extinction includes the disappearance of large land animals known as megafauna, starting at the end of the last glacial period. Megafauna outside of the African mainland, which did not evolve alongside humans, proved highly sensitive to the introduction of human predation, and many died out shortly after early humans began spreading and hunting across the Earth. Many African species have also gone extinct in the Holocene, along with species in North America, South America, and Australia, but – with some exceptions – the megafauna of the Eurasian mainland was largely unaffected until a few hundred years ago. These extinctions, occurring near the Pleistocene–Holocene boundary, are sometimes referred to as the Quaternary extinction event.


title: Holocene extinction

text: The most popular theory is that human overhunting of species added to existing stress conditions as the Holocene extinction coincides with human colonization of many new areas around the world. Although there is debate regarding how much human predation and habitat loss affected their decline, certain population declines have been directly correlated with the onset of human activity, such as the extinction events of New Zealand and Hawaii. Aside from humans, climate change may have been a driving factor in the megafaunal extinctions, especially at the end of the Pleistocene.


title: Holocene extinction

text: In the twentieth century, human numbers quadrupled, and the size of the global economy increased twenty-five-fold. This Great Acceleration or Anthropocene Epoch has also accelerated species extinction. Ecologically, humanity is now an unprecedented "global superpredator", which consistently preys on the adults of other apex predators, takes over other species' essential habitats and displaces them, and has worldwide effects on food webs. There have been extinctions of species on every land mass and in every ocean: there are many famous examples within Africa, Asia, Europe, Australia, North and South America, and on smaller islands.


title: Holocene extinction

text: Overall, the Holocene extinction can be linked to the human impact on the environment. The Holocene extinction continues into the 21st century, with human population growth, increasing per capita consumption, and meat production, among others, being the primary drivers of mass extinction. Deforestation, overfishing, ocean acidification, the destruction of wetlands, and the decline in amphibian populations, among others, are a few broader examples of global biodiversity loss.


title: Holocene extinction

text: Mass extinctions are characterized by the loss of at least 75% of species within a geologically short period of time. The Holocene extinction is also known as the "sixth extinction", as it is possibly the sixth mass extinction event, after the Ordovician–Silurian extinction events, the Late Devonian extinction, the Permian–Triassic extinction event, the Triassic–Jurassic extinction event, and the Cretaceous–Paleogene extinction event.


title: Holocene extinction

text: There is no general agreement on where the Holocene, or anthropogenic, extinction begins, and the Quaternary extinction event, which includes climate change resulting in the end of the last ice age, ends, or if they should be considered separate events at all. The Holocene extinction is mainly caused by human activities. Some have suggested that anthropogenic extinctions may have begun as early as when the first modern humans spread out of Africa between 200,000 and 100,000 years ago; this is supported by rapid megafaunal extinction following recent human colonisation in Australia, New Zealand and Madagascar. In many cases, it is suggested that even minimal hunting pressure was enough to wipe out large fauna, particularly on geographically isolated islands. Only during the most recent parts of the extinction have plants also suffered large losses.


title: Holocene extinction

text: The contemporary rate of extinction of species is estimated at 100 to 1,000 times higher than the background extinction rate, the historically typical rate of extinction (in terms of the natural evolution of the planet); also, the current rate of extinction is 10 to 100 times higher than in any of the previous mass extinctions in the history of Earth. One scientist estimates the current extinction rate may be 10,000 times the background extinction rate, although most scientists predict a much lower extinction rate than this outlying estimate. Theoretical ecologist Stuart Pimm stated that the extinction rate for plants is 100 times higher than normal.


title: Holocene extinction

text: Some contend that contemporary extinction has yet to reach the level of the previous five mass extinctions, and that this comparison downplays how severe the first five mass extinctions were. John Briggs argues that there is inadequate data to determine the real rate of extinctions, and shows that estimates of current species extinctions varies enormously, ranging from 1.5 species to 40,000 species going extinct due to human activities each year. Both papers from Barnosky et al. (2011) and Hull et al. (2015) point out that the real rate of extinction during previous mass extinctions is unknown, both as only some organisms leave fossil remains, and as the temporal resolution of the fossil layer is larger than the time frame of the extinction events. However, all these authors agree that there is a modern biodiversity crisis with population declines affecting numerous species, and that a future anthropogenic mass extinction event is a big risk. The 2011 study by Barnosky et al. confirms that "current extinction rates are higher than would be expected from the fossil record" and adds that anthropogenic ecological stressors, including climate change, habitat fragmentation, pollution, overfishing, overhunting, invasive species and expanding human biomass will intensify and accelerate extinction rates in the future without significant mitigation efforts.


title: Holocene extinction

text: In "The Future of Life" (2002), Edward Osborne Wilson of Harvard calculated that, if the current rate of human disruption of the biosphere continues, one-half of Earth's higher lifeforms will be extinct by 2100. A 1998 poll conducted by the American Museum of Natural History found that 70% of biologists acknowledge an ongoing anthropogenic extinction event.


title: Holocene extinction

text: In a pair of studies published in 2015, extrapolation from observed extinction of Hawaiian snails led to the conclusion that 7% of all species on Earth may have been lost already. A 2021 study published in the journal "Frontiers in Forests and Global Change" found that only around 3% of the planet's terrestrial surface is ecologically and faunally intact, meaning areas with healthy populations of native animal species and little to no human footprint.


title: Holocene extinction

text: The 2019 "Global Assessment Report on Biodiversity and Ecosystem Services", published by the United Nations' Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES), posits that roughly one million species of plants and animals face extinction within decades as the result of human actions. Organized human existence is jeopardized by increasingly rapid destruction of the systems that support life on Earth, according to the report, the result of one of the most comprehensive studies of the health of the planet ever conducted. Moreover, the 2021 "Economics of Biodiversity" review, published by the UK government, asserts that "biodiversity is declining faster than at any time in human history." According to a 2022 study published in "Frontiers in Ecology and the Environment", a survey of more than 3,000 experts says that the extent of the mass extinction might be greater than previously thought, and estimates that roughly 30% of species "have been globally threatened or driven extinct since the year 1500." In a 2022 report, IPBES listed unsustainable fishing, hunting and logging as being some of the primary drivers of the global extinction crisis.


title: Holocene extinction

text: There is widespread consensus among scientists that human activity is accelerating the extinction of many animal species through the destruction of habitats, the consumption of animals as resources, and the elimination of species that humans view as threats or competitors. That humans have become the primary driver of modern extinctions is undeniable, rising extinction trends impacting numerous animal groups including mammals, birds, reptiles, and amphibians have prompted scientists to declare a biodiversity crisis.


title: Holocene extinction

text: Characterisation of recent extinction as a mass extinction has been debated among scientists. Stuart Pimm, for example, asserts that the sixth mass extinction "is something that hasn't happened yet – we are on the edge of it." Several studies posit that the earth has entered a sixth mass extinction event, including a 2015 paper by Barnosky et al. and a November 2017 statement titled "World Scientists’ Warning to Humanity: A Second Notice", led by eight authors and signed by 15,364 scientists from 184 countries which asserted that, among other things, "we have unleashed a mass extinction event, the sixth in roughly 540 million years, wherein many current life forms could be extirpated or at least committed to extinction by the end of this century." The World Wide Fund for Nature's 2020 "Living Planet Report" says that wildlife populations have declined by 68% since 1970 as a result of overconsumption, population growth and intensive farming, which is further evidence that humans have unleashed a sixth mass extinction event; however, this finding has been disputed by one 2020 study, which posits that this major decline was primarily driven by a few extreme outlier populations, and that when these outliers are removed, the trend shifts to that of a decline between the 1980s and 2000s, but a roughly positive trend after 2000. A 2021 report in "Frontiers in Conservation Science" which cites both of the aforementioned studies, says "population sizes of vertebrate species that have been monitored across years have declined by an average of 68% over the last five decades, with certain population clusters in extreme decline, thus presaging the imminent extinction of their species," and asserts "that we are already on the path of a sixth major extinction is now scientifically undeniable." A January 2022 review article published in "Biological Reviews" builds upon previous studies documenting biodiversity decline to assert that a sixth mass extinction event caused by anthropogenic activity is currently underway. A December 2022 study published in "Science Advances" states that "the planet has entered the sixth mass extinction" and warns that current anthropogenic trends, particularly regarding climate and land-use changes, could result in the loss of more than a tenth of plant and animal species by the end of the century.


title: Holocene extinction

text: According to the UNDP's 2020 Human Development Report, "The Next Frontier: Human Development and the Anthropocene":


title: Holocene extinction

text: The 2022 "Living Planet Report" found that vertebrate wildlife populations have plummeted by an average of almost 70% since 1970, with agriculture and fishing being the primary drivers of this decline.


title: Holocene extinction

text: Some scientists, including Rodolfo Dirzo and Paul R. Ehrlich, contend that the sixth mass extinction is largely unknown to most people globally, and is also misunderstood by many in the scientific community. They say it is not the disappearance of species, which gets the most attention, that is at the heart of the crisis, but "the existential threat of myriad population extinctions."


title: Holocene extinction

text: The abundance of species extinctions considered anthropogenic, or due to human activity, has sometimes (especially when referring to hypothesized future events) been collectively called the "Anthropocene extinction". "Anthropocene" is a term introduced in 2000. Some now postulate that a new geological epoch has begun, with the most abrupt and widespread extinction of species since the Cretaceous–Paleogene extinction event 66 million years ago.


title: Holocene extinction

text: The term "anthropocene" is being used more frequently by scientists, and some commentators may refer to the current and projected future extinctions as part of a longer Holocene extinction. The Holocene–Anthropocene boundary is contested, with some commentators asserting significant human influence on climate for much of what is normally regarded as the Holocene Epoch. Other commentators place the Holocene–Anthropocene boundary at the industrial revolution and also say that "[f]ormal adoption of this term in the near future will largely depend on its utility, particularly to earth scientists working on late Holocene successions."


title: Holocene extinction

text: It has been suggested that human activity has made the period starting from the mid-20th century different enough from the rest of the Holocene to consider it a new geological epoch, known as the Anthropocene, a term which was considered for inclusion in the timeline of Earth's history by the International Commission on Stratigraphy in 2016. In order to constitute the Holocene as an extinction event, scientists must determine exactly when anthropogenic greenhouse gas emissions began to measurably alter natural atmospheric levels on a global scale, and when these alterations caused changes to global climate. Using chemical proxies from Antarctic ice cores, researchers have estimated the fluctuations of carbon dioxide (CO) and methane (CH) gases in the Earth's atmosphere during the late Pleistocene and Holocene epochs. Estimates of the fluctuations of these two gases in the atmosphere, using chemical proxies from Antarctic ice cores, generally indicate that the peak of the Anthropocene occurred within the previous two centuries: typically beginning with the Industrial Revolution, when the highest greenhouse gas levels were recorded.


title: Holocene extinction

text: A 2015 article in "Science" suggested that humans are unique in ecology as an unprecedented "global superpredator", regularly preying on large numbers of fully grown terrestrial and marine apex predators, and with a great deal of influence over food webs and climatic systems worldwide. Although significant debate exists as to how much human predation and indirect effects contributed to prehistoric extinctions, certain population crashes have been directly correlated with human arrival. Human activity has been the main cause of mammalian extinctions since the Late Pleistocene. A 2018 study published in "PNAS" found that since the dawn of human civilization, the biomass of wild mammals has decreased by 83%. The biomass decrease is 80% for marine mammals, 50% for plants and 15% for fish. Currently, livestock make up 60% of the biomass of all mammals on earth, followed by humans (36%) and wild mammals (4%). As for birds, 70% are domesticated, such as poultry, whereas only 30% are wild.


title: Holocene extinction

text: Extinction of animals, plants, and other organisms caused by human actions may go as far back as the late Pleistocene, over 12,000 years ago. There is a correlation between megafaunal extinction and the arrival of humans. Over the past 125,000 years, the average body size of wildlife has fallen by 14% as human actions eradicated megafauna on all continents with the exception of Africa.


title: Holocene extinction

text: Human civilization was founded on and grew from agriculture. The more land used for farming, the greater the population a civilization could sustain, and subsequent popularization of farming led to habitat conversion.


title: Holocene extinction

text: Habitat destruction by humans, thus replacing the original local ecosystems, is a major driver of extinction. The sustained conversion of biodiversity rich forests and wetlands into poorer fields and pastures (of lesser carrying capacity for wild species), over the last 10,000 years, has considerably reduced the Earth's carrying capacity for wild birds, among other organisms, in both population size and species count.


title: Holocene extinction

text: Other, related human causes of the extinction event include deforestation, hunting, pollution, the introduction in various regions of non-native species, and the widespread transmission of infectious diseases spread through livestock and crops.


title: Holocene extinction

text: Recent investigations into the practice of landscape burning during the Neolithic Revolution has a major implication for the current debate about the timing of the Anthropocene and the role that humans may have played in the production of greenhouse gases prior to the Industrial Revolution. Studies of early hunter-gatherers raise questions about the current use of population size or density as a proxy for the amount of land clearance and anthropogenic burning that took place in pre-industrial times. Scientists have questioned the correlation between population size and early territorial alterations. Ruddiman and Ellis' research paper in 2009 makes the case that early farmers involved in systems of agriculture used more land per capita than growers later in the Holocene, who intensified their labor to produce more food per unit of area (thus, per laborer); arguing that agricultural involvement in rice production implemented thousands of years ago by relatively small populations have created significant environmental impacts through large-scale means of deforestation.


title: Holocene extinction

text: While a number of human-derived factors are recognized as contributing to rising atmospheric concentrations of CH (methane) and CO (carbon dioxide), deforestation and territorial clearance practices associated with agricultural development may be contributing most to these concentrations globally. Scientists that are employing a variance of archaeological and paleoecological data argue that the processes contributing to substantial human modification of the environment spanned many thousands of years on a global scale and thus, not originating as late as the Industrial Revolution. Gaining popularity on his uncommon hypothesis, palaeoclimatologist William Ruddiman in 2003, has argued that in the early Holocene 11,000 years ago, atmospheric carbon dioxide and methane levels fluctuated in a pattern which was different from the Pleistocene epoch before it. He argued that the patterns of the significant decline of CO levels during the last ice age of the Pleistocene inversely correlates to the Holocene where there have been dramatic increases of CO around 8000 years ago and CH levels 3000 years after that. The correlation between the decrease of CO in the Pleistocene and the increase of it during the Holocene implies that the causation of this spark of greenhouse gases into the atmosphere was the growth of human agriculture during the Holocene such as the anthropogenic expansion of (human) land use and irrigation.


title: Holocene extinction

text: One of the main theories for the extinction's cause is historic climate change. The climate change theory has suggested that a change in climate near the end of the late Pleistocene stressed the megafauna to the point of extinction. Some scientists favor abrupt climate change as the catalyst for the extinction of the mega-fauna at the end of the Pleistocene, but there are many who believe increased hunting from early modern humans also played a part, with others even suggesting that the two interacted. However, the annual mean temperature of the current interglacial period for the last 10,000 years is no higher than that of previous interglacial periods, yet some of the same megafauna survived similar temperature increases. In the Americas, a controversial explanation for the shift in climate is presented under the Younger Dryas impact hypothesis, which states that the impact of comets cooled global temperatures.


title: Holocene extinction

text: A 2020 study published in "Science Advances" found that human population size and/or specific human activities, not climate change, caused rapidly rising global mammal extinction rates during the past 126,000 years. Around 96% of all mammalian extinctions over this time period are attributable to human impacts. According to Tobias Andermann, lead author of the study, "these extinctions did not happen continuously and at constant pace. Instead, bursts of extinctions are detected across different continents at times when humans first reached them. More recently, the magnitude of human driven extinctions has picked up the pace again, this time on a global scale."


title: Holocene extinction

text: Megafauna play a significant role in the lateral transport of mineral nutrients in an ecosystem, tending to translocate them from areas of high to those of lower abundance. They do so by their movement between the time they consume the nutrient and the time they release it through elimination (or, to a much lesser extent, through decomposition after death). In South America's Amazon Basin, it is estimated that such lateral diffusion was reduced over 98% following the megafaunal extinctions that occurred roughly 12,500 years ago. Given that phosphorus availability is thought to limit productivity in much of the region, the decrease in its transport from the western part of the basin and from floodplains (both of which derive their supply from the uplift of the Andes) to other areas is thought to have significantly impacted the region's ecology, and the effects may not yet have reached their limits. The extinction of the mammoths allowed grasslands they had maintained through grazing habits to become birch forests. The new forest and the resulting forest fires may have induced climate change. Such disappearances might be the result of the proliferation of modern humans; some recent studies favor this theory.


title: Holocene extinction

text: Large populations of megaherbivores have the potential to contribute greatly to the atmospheric concentration of methane, which is an important greenhouse gas. Modern ruminant herbivores produce methane as a byproduct of foregut fermentation in digestion, and release it through belching or flatulence. Today, around 20% of annual methane emissions come from livestock methane release. In the Mesozoic, it has been estimated that sauropods could have emitted 520 million tons of methane to the atmosphere annually, contributing to the warmer climate of the time (up to 10 °C warmer than at present). This large emission follows from the enormous estimated biomass of sauropods, and because methane production of individual herbivores is believed to be almost proportional to their mass.


title: Holocene extinction

text: Recent studies have indicated that the extinction of megafaunal herbivores may have caused a reduction in atmospheric methane. This hypothesis is relatively new. One study examined the methane emissions from the bison that occupied the Great Plains of North America before contact with European settlers. The study estimated that the removal of the bison caused a decrease of as much as 2.2 million tons per year. Another study examined the change in the methane concentration in the atmosphere at the end of the Pleistocene epoch after the extinction of megafauna in the Americas. After early humans migrated to the Americas about 13,000 BP, their hunting and other associated ecological impacts led to the extinction of many megafaunal species there. Calculations suggest that this extinction decreased methane production by about 9.6 million tons per year. This suggests that the absence of megafaunal methane emissions may have contributed to the abrupt climatic cooling at the onset of the Younger Dryas. The decrease in atmospheric methane that occurred at that time, as recorded in ice cores, was 2–4 times more rapid than any other decrease in the last half million years, suggesting that an unusual mechanism was at work.


title: Holocene extinction

text: The hyperdisease hypothesis, proposed by Ross MacPhee in 1997, states that the megafaunal die-off was due to an indirect transmission of diseases by newly arriving humans. According to MacPhee, aboriginals or animals travelling with them, such as domestic dogs or livestock, introduced one or more highly virulent diseases into new environments whose native population had no immunity to them, eventually leading to their extinction. K-selection animals, such as the now-extinct megafauna, are especially vulnerable to diseases, as opposed to r-selection animals who have a shorter gestation period and a higher population size. Humans are thought to be the sole cause as other earlier migrations of animals into North America from Eurasia did not cause extinctions.


title: Holocene extinction

text: There are many problems with this theory, as this disease would have to meet several criteria: it has to be able to sustain itself in an environment with no hosts; it has to have a high infection rate; and be extremely lethal, with a mortality rate of 50–75%. Disease has to be very virulent to kill off all the individuals in a species, and even such a virulent disease as West Nile fever is unlikely to have caused extinction. However, diseases have been the cause for some extinctions. The introduction of avian malaria and avipoxvirus, for example, have had a negative impact on the endemic birds of Hawaii.


title: Holocene extinction

text: Contemporary human overpopulation and continued population growth, along with per-capita consumption growth, prominently in the past two centuries, are regarded as the underlying causes of extinction.


title: Holocene extinction

text: The loss of animal species from ecological communities, defaunation, is primarily driven by human activity. This has resulted in empty forests, ecological communities depleted of large vertebrates. This is not to be confused with extinction, as it includes both the disappearance of species and declines in abundance. Defaunation effects were first implied at the Symposium of Plant-Animal Interactions at the University of Campinas, Brazil in 1988 in the context of Neotropical forests. Since then, the term has gained broader usage in conservation biology as a global phenomenon.


title: Holocene extinction

text: Some scholars assert that the emergence of capitalism as the dominant economic system has accelerated ecological exploitation and destruction, and has also exacerbated mass species extinction. CUNY professor David Harvey, for example, posits that the neoliberal era "happens to be the era of the fastest mass extinction of species in the Earth's recent history". Major lobbying organizations representing corporations in the agriculture, fisheries, forestry and paper, mining, and oil and gas industries, including the United States Chamber of Commerce, have been pushing back against legislation that could address the extinction crisis. A 2022 report by the climate think tank InfluenceMap stated that "although industry associations, especially in the US, appear reluctant to discuss the biodiversity crisis, they are clearly engaged on a wide range of policies with significant impacts on biodiversity loss."


title: Holocene extinction

text: Big cat populations have severely declined over the last half-century and could face extinction in the following decades. According to 2011 IUCN estimates: lions are down to 25,000, from 450,000; leopards are down to 50,000, from 750,000; cheetahs are down to 12,000, from 45,000; tigers are down to 3,000 in the wild, from 50,000. A December 2016 study by the Zoological Society of London, Panthera Corporation and Wildlife Conservation Society showed that cheetahs are far closer to extinction than previously thought, with only 7,100 remaining in the wild, existing within only 9% of their historic range. Human pressures are to blame for the cheetah population crash, including prey loss due to overhunting by people, retaliatory killing from farmers, habitat loss and the illegal wildlife trade.


title: Holocene extinction

text: The term pollinator decline refers to the reduction in abundance of insect and other animal pollinators in many ecosystems worldwide beginning at the end of the twentieth century, and continuing into the present day. Pollinators, which are necessary for 75% of food crops, are declining globally in both abundance and diversity. A 2017 study led by Radboud University's Hans de Kroon indicated that the biomass of insect life in Germany had declined by three-quarters in the previous 25 years. Participating researcher Dave Goulson of Sussex University stated that their study suggested that humans are making large parts of the planet uninhabitable for wildlife. Goulson characterized the situation as an approaching "ecological Armageddon", adding that "if we lose the insects then everything is going to collapse." A 2019 study found that over 40% of insect species are threatened with extinction. The most significant drivers in the decline of insect populations are associated with intensive farming practices, along with pesticide use and climate change. The world's insect population decreases by around 1 to 2 per cent per year.


title: Holocene extinction

text: Various species are predicted to become extinct in the near future, among them the rhinoceros, nonhuman primates, pangolins, and giraffes. Hunting alone threatens bird and mammalian populations around the world. The direct killing of megafauna for meat and body parts is the primary driver of their destruction, with 70% of the 362 megafauna species in decline as of 2019. Mammals in particular have suffered such severe losses as the result of human activity that it could take several million years for them to recover. Contemporary assessments have discovered that roughly 41% of amphibians, 25% of mammals, 21% of reptiles and 14% of birds are threatened with extinction, which could disrupt ecosystems on a global scale and eliminate billions of years of phylogenetic diversity. 189 countries, which are signatory to the Convention on Biological Diversity (Rio Accord), have committed to preparing a Biodiversity Action Plan, a first step at identifying specific endangered species and habitats, country by country.


title: Holocene extinction

text: Recent extinctions are more directly attributable to human influences, whereas prehistoric extinctions can be attributed to other factors, such as global climate change. The International Union for Conservation of Nature (IUCN) characterises 'recent' extinction as those that have occurred past the cut-off point of 1500, and at least 875 plant and animal species have gone extinct since that time and 2009. Some species, such as the Père David's deer and the Hawaiian crow, are extinct in the wild, and survive solely in captive populations. Other populations are only locally extinct (extirpated), still existent elsewhere, but reduced in distribution, as with the extinction of gray whales in the Atlantic, and of the leatherback sea turtle in Malaysia.


title: Holocene extinction

text: Humans are rapidly driving the largest vertebrate animals towards extinction, and in the process interrupting a 66-million-year-old feature of ecosystems, the relationship between diet and body mass, which researchers suggest could have unpredictable consequences. A 2019 study published in "Nature Communications" found that rapid biodiversity loss is impacting larger mammals and birds to a much greater extent than smaller ones, with the body mass of such animals expected to shrink by 25% over the next century. Another 2019 study published in "Biology Letters" found that extinction rates are perhaps much higher than previously estimated, in particular for bird species.


title: Holocene extinction

text: The 2019 "Global Assessment Report on Biodiversity and Ecosystem Services" lists the primary causes of contemporary extinctions in descending order: (1) changes in land and sea use (primarily agriculture and overfishing respectively); (2) direct exploitation of organisms such as hunting; (3) anthropogenic climate change; (4) pollution and (5) invasive alien species spread by human trade. This report, along with the 2020 "Living Planet Report" by the WWF, both project that climate change will be the leading cause in the next several decades.


title: Holocene extinction

text: A June 2020 study published in "PNAS" posits that the contemporary extinction crisis "may be the most serious environmental threat to the persistence of civilization, because it is irreversible" and that its acceleration "is certain because of the still fast growth in human numbers and consumption rates." The study found that more than 500 vertebrate species are poised to be lost in the next two decades.


title: Holocene extinction

text: Humans both create and destroy crop cultivar and domesticated animal varieties. Advances in transportation and industrial farming has led to monoculture and the extinction of many cultivars. The use of certain plants and animals for food has also resulted in their extinction, including silphium and the passenger pigeon. It was estimated in 2012 that 13 percent of Earth's ice-free land surface is used as row-crop agricultural sites, 26 percent used as pastures, and 4 percent urban-industrial areas.


title: Holocene extinction

text: In March 2019, "Nature Climate Change" published a study by ecologists from Yale University, who found that over the next half century, human land use will reduce the habitats of 1,700 species by up to 50%, pushing them closer to extinction. That same month "PLOS Biology" published a similar study drawing on work at the University of Queensland, which found that "more than 1,200 species globally face threats to their survival in more than 90% of their habitat and will almost certainly face extinction without conservation intervention".


title: Holocene extinction

text: Since 1970, the populations of migratory freshwater fish have declined by 76%, according to research published by the Zoological Society of London in July 2020. Overall, around one in three freshwater fish species are threatened with extinction due to human-driven habitat degradation and overfishing.


title: Holocene extinction

text: Some scientists and academics assert that industrial agriculture and the growing demand for meat is contributing to significant global biodiversity loss as this is a significant driver of deforestation and habitat destruction; species-rich habitats, such as the Amazon region and Indonesia being converted to agriculture. A 2017 study by the World Wildlife Fund (WWF) found that 60% of biodiversity loss can be attributed to the vast scale of feed crop cultivation required to rear tens of billions of farm animals. Moreover, a 2006 report by the Food and Agriculture Organization (FAO) of the United Nations, "Livestock's Long Shadow", also found that the livestock sector is a "leading player" in biodiversity loss. More recently, in 2019, the IPBES "Global Assessment Report on Biodiversity and Ecosystem Services" attributed much of this ecological destruction to agriculture and fishing, with the meat and dairy industries having a very significant impact. Since the 1970s food production has soared in order to feed a growing human population and bolster economic growth, but at a huge price to the environment and other species. The report says some 25% of the earth's ice-free land is used for cattle grazing. A 2020 study published in "Nature Communications" warned that human impacts from housing, industrial agriculture and in particular meat consumption are wiping out a combined 50 billion years of earth's evolutionary history (defined as phylogenetic diversity) and driving to extinction some of the "most unique animals on the planet," among them the Aye-aye lemur, the Chinese crocodile lizard and the pangolin. Said lead author Rikki Gumbs:


title: Holocene extinction

text: Climate change is expected to be a major driver of extinctions from the 21st century. Rising levels of carbon dioxide are resulting in influx of this gas into the ocean, increasing its acidity. Marine organisms which possess calcium carbonate shells or exoskeletons experience physiological pressure as the carbonate reacts with acid. For example, this is already resulting in coral bleaching on various coral reefs worldwide, which provide valuable habitat and maintain a high biodiversity. Marine gastropods, bivalves and other invertebrates are also affected, as are the organisms that feed on them. Some studies have suggested that it is not climate change that is driving the current extinction crisis, but the demands of contemporary human civilization on nature.


title: Holocene extinction

text: Overhunting can reduce the local population of game animals by more than half, as well as reducing population density, and may lead to extinction for some species. Populations located nearer to villages are significantly more at risk of depletion. Several conservationist organizations, among them IFAW and HSUS, assert that trophy hunters, particularly from the United States, are playing a significant role in the decline of giraffes, which they refer to as a "silent extinction".


title: Holocene extinction

text: The surge in the mass killings by poachers involved in the illegal ivory trade along with habitat loss is threatening African elephant populations. In 1979, their populations stood at 1.7 million; at present there are fewer than 400,000 remaining. Prior to European colonization, scientists believe Africa was home to roughly 20 million elephants. According to the Great Elephant Census, 30% of African elephants (or 144,000 individuals) disappeared over a seven-year period, 2007 to 2014. African elephants could become extinct by 2035 if poaching rates continue.


title: Holocene extinction

text: Fishing has had a devastating effect on marine organism populations for several centuries even before the explosion of destructive and highly effective fishing practices like trawling. Humans are unique among predators in that they regularly prey on other adult apex predators, particularly in marine environments; bluefin tuna, blue whales, North Atlantic right whales and over fifty species of sharks and rays are vulnerable to predation pressure from human fishing, in particular commercial fishing. A 2016 study published in "Science" concludes that humans tend to hunt larger species, and this could disrupt ocean ecosystems for millions of years. A 2020 study published in "Science Advances" found that around 18% of marine megafauna, including iconic species such as the Great white shark, are at risk of extinction from human pressures over the next century. In a worst-case scenario, 40% could go extinct over the same time period. According to a 2021 study published in "Nature", 71% of oceanic shark and ray populations have been destroyed by overfishing (the primary driver of ocean defaunation) from 1970 to 2018, and are nearing the "point of no return" as 24 of the 31 species are now threatened with extinction, with several being classified as critically endangered.


title: Holocene extinction

text: The decline of amphibian populations has also been identified as an indicator of environmental degradation. As well as habitat loss, introduced predators and pollution, Chytridiomycosis, a fungal infection accidentally spread by human travel, globalization and the wildlife trade, has caused severe population drops of over 500 amphibian species, and perhaps 90 extinctions, including (among many others) the extinction of the golden toad in Costa Rica, the Gastric-brooding frog in Australia, the Rabb's fringe-limbed treefrog and the extinction of the Panamanian golden frog in the wild. Chytrid fungus has spread across Australia, New Zealand, Central America and Africa, including countries with high amphibian diversity such as cloud forests in Honduras and Madagascar. "Batrachochytrium salamandrivorans" is a similar infection currently threatening salamanders. Amphibians are now the most endangered vertebrate group, having existed for more than 300 million years through three other mass extinctions.