title: Schutzstaffel

text: An "SS-Sonderkommando" unit led by "SS-Sturmbannführer" Herbert Lange murdered 1,201 psychiatric patients at the Tiegenhof psychiatric hospital in the Free City of Danzig, 1,100 patients in Owińska, 2,750 patients at Kościan, and 1,558 patients at Działdowo, as well as hundreds of Poles at Fort VII, where the mobile gas van and gassing bunker were developed. In 1941–42, "SS-Sonderkommando Lange" set up and managed the first extermination camp, at Chełmno, where 152,000 Jews were killed using gas vans.


title: Schutzstaffel

text: After the Battle of Stalingrad in February 1943, Himmler realized that Germany would likely lose the war, and ordered the formation of " Sonderkommando" 1005, a special task force under SS-"Standartenführer" Paul Blobel. The unit's assignment was to visit mass graves on the Eastern Front to exhume bodies and burn them in an attempt to cover up the genocide. The task remained unfinished at the end of the war, and many mass graves remain unmarked and unexcavated.


title: Schutzstaffel

text: The "Eichmann Sonderkommando" was a task force headed by Adolf Eichmann that arrived in Budapest on 19 March 1944, the same day that Axis forces invaded Hungary. Their task was to take a direct role in the deportation of Hungarian Jews to Auschwitz. The "SS-Sonderkommandos" enlisted the aid of antisemitic elements from the Hungarian gendarmerie and pro-German administrators from within the Hungarian Interior Ministry. Round-ups began on 16 April, and from 14 May, four trains of 3,000 Jews per day left Hungary and traveled to the camp at Auschwitz II-Birkenau, arriving along a newly built spur line that terminated a few hundred meters from the gas chambers. Between 10 and 25 percent of the people on each train were chosen as forced laborers; the rest were killed within hours of arrival. Under international pressure, the Hungarian government halted deportations on 6 July 1944, by which time over 437,000 of Hungary's 725,000 Jews had been murdered.


title: Schutzstaffel

text: The "Einsatzgruppen" had its origins in the ad hoc "Einsatzkommando" formed by Heydrich following the "Anschluss" in Austria in March 1938. Two units of "Einsatzgruppen" were stationed in the Sudetenland in October 1938. When military action turned out not to be necessary because of the Munich Agreement, the "Einsatzgruppen" were assigned to confiscate government papers and police documents. They secured government buildings, questioned senior civil servants, and arrested as many as 10,000 Czech communists and German citizens. The "Einsatzgruppen" also followed Wehrmacht troops and killed potential partisans. Similar groups were used in 1939 for the occupation of Czechoslovakia.


title: Schutzstaffel

text: Hitler felt that the planned extermination of the Jews was too difficult and important to be entrusted to the military. In 1941 the "Einsatzgruppen" were sent into the Soviet Union to begin large-scale genocide of Jews, Romani people, and communists. Historian Raul Hilberg estimates that between 1941 and 1945 the "Einsatzgruppen" and related agencies killed more than two million people, including 1.3 million Jews. The largest mass shooting perpetrated by the "Einsatzgruppen" was at Babi Yar outside Kiev, where 33,771 Jews were killed in a single operation on 29–30 September 1941. In the Rumbula massacre (November–December 1941), 25,000 victims from the Riga ghetto were killed. Another mass shooting early in 1942 claimed the lives of over 10,000 Jews in Kharkov.


title: Schutzstaffel

text: The last "Einsatzgruppen" were disbanded in mid-1944 (although some continued to exist on paper until 1945) due to the German retreat on both fronts and the consequent inability to continue extermination activities. Former "Einsatzgruppen" members were either assigned duties in the Waffen-SS or concentration camps. Twenty-four "Einsatzgruppen" commanders were tried for war crimes following the war.


title: Schutzstaffel

text: The SS Court Main Office ("Hauptamt SS-Gericht") was an internal legal system for conducting investigations, trials, and punishment of the SS and police. It had more than 600 lawyers on staff in the main offices in Berlin and Munich. Proceedings were conducted at 38 regional SS courts throughout Germany. It was the only authority authorized to try SS personnel, except for SS members who were on active duty in the Wehrmacht (in such cases, the SS member in question was tried by a standard military tribunal). Its creation placed the SS beyond the reach of civilian legal authority. Himmler personally intervened as he saw fit regarding convictions and punishment. The historian Karl Dietrich Bracher describes this court system as one factor in the creation of the Nazi totalitarian police state, as it removed objective legal procedures, rendering citizens defenseless against the "summary justice of the SS terror."


title: Schutzstaffel

text: Shortly after Hitler seized power in 1933, most horse riding associations were taken over by the SA and SS. Members received combat training to serve in the "Reiter-SS" (SS Cavalry Corps). The first SS cavalry regiment, designated "SS-Totenkopf Reitstandarte 1", was formed in September 1939. Commanded by then SS-"Standartenführer" Hermann Fegelein, the unit was assigned to Poland, where they took part in the extermination of Polish intelligentsia. Additional squadrons were added in May 1940, for a total of fourteen.


title: Schutzstaffel

text: The unit was split into two regiments in December 1939, with Fegelein in charge of both. By March 1941 their strength was 3,500 men. In July 1941, they were assigned to the Pripyat swamps punitive operation, tasked with rounding up and exterminating Jews and partisans. The two regiments were amalgamated into the SS Cavalry Brigade on 31 July, twelve days after the operation started. Fegelein's final report, dated 18 September 1941, states that they killed 14,178 Jews, 1,001 partisans, and 699 Red Army soldiers, with 830 prisoners taken. The historian Henning Pieper estimates the actual number of Jews killed was closer to 23,700. The SS Cavalry Brigade took serious losses in November 1941 in the Battle of Moscow, with casualties of up to 60 percent in some squadrons. Fegelein was appointed as commander of the 8th SS Cavalry Division "Florian Geyer" on 20 April 1943. This unit saw service in the Soviet Union in attacks on partisans and civilians. In addition, SS Cavalry regiments served in Croatia and Hungary.


title: Schutzstaffel

text: The SS Medical Corps were initially known as the "Sanitätsstaffel" (sanitary units). After 1931, the SS formed the headquarters office "Amt"  V as the central office for SS medical units. An SS medical academy was established in Berlin in 1938 to train Waffen-SS physicians. SS medical personnel did not often provide actual medical care; their primary responsibility was medicalized genocide. At Auschwitz, about three-quarters of new arrivals, including almost all children, women with small children, all the elderly, and all those who appeared on brief and superficial inspection by an SS doctor not to be completely fit were killed within hours of arrival. In their role as "Desinfektoren" (disinfectors), SS doctors also made selections among existing prisoners as to their fitness to work and supervised the murder of those deemed unfit. Inmates in deteriorating health were examined by SS doctors, who decided whether or not they would be able to recover in less than two weeks. Those too ill or injured to recover in that time frame were killed.


title: Schutzstaffel

text: At Auschwitz, the actual delivery of gas to the victims was always handled by the SS, on the order of the supervising SS doctor. Many of the SS doctors also conducted inhumane medical experiments on camp prisoners. The most infamous SS doctor, Josef Mengele, served as a medical officer at Auschwitz under the command of Eduard Wirths of the camp's medical corps. Mengele undertook selections even when he was not assigned to do so in the hope of finding subjects for his experiments. He was particularly interested in locating sets of twins. In contrast to most of the doctors, who viewed undertaking selections as one of their most stressful and horrible duties, Mengele undertook the task with a flamboyant air, often smiling or whistling a tune. After the war, many SS doctors were charged with war crimes for their inhumane medical experiments and for their role in gas chamber selections.


title: Schutzstaffel

text: The "Ahnenerbe" (Ancestral Heritage Organization) was founded in 1935 by Himmler and became part of the SS in 1939. It was an umbrella agency for more than fifty organizations tasked with studying German racial identity and ancient Germanic traditions and language. The agency sponsored archaeological expeditions in Germany, Scandinavia, the Middle East, Tibet, and elsewhere to search for evidence of Aryan roots, influence, and superiority. Further planned expeditions were postponed indefinitely at the start of the war.


title: Schutzstaffel

text: The "SS-Frauenkorps" was an auxiliary reporting and clerical unit, which included the "SS-Helferinnenkorps" (Women Helper Corps), made up of female volunteers. Members were assigned as administrative staff and supply personnel and served in command positions and as guards at women's concentration camps. While female concentration and extermination camp guards were civilian employees of the SS, the "SS-Helferinnen" who completed training at the "Reichsschule für SS-Helferinnen" in Oberehnheim (Alsace) were members of the Waffen-SS. Like their male equivalents in the SS, females participated in atrocities against Jews, Poles, and others.


title: Schutzstaffel

text: In 1942, Himmler set up the "Reichsschule für SS Helferinnen" (Reich school for SS helpers) in Oberehnheim to train women in communications so that they could free up men for combat roles. Himmler also intended to replace all female civilian employees in his service with "SS-Helferinnen" members, as they were selected and trained according to Nazi ideology. The school was closed on 22 November 1944 due to the Allied advance.


title: Schutzstaffel

text: The "SS-Mannschaften" (Auxiliary-SS) were not considered regular SS members, but were conscripted from other branches of the German military, the Nazi Party, the SA, and the "Volkssturm" for service in concentration camps and extermination camps.


title: Schutzstaffel

text: Beginning in 1940, Himmler opened up Waffen-SS recruiting to ethnic Germans that were not German citizens. In March 1941, the SS Main Office established the "Germanische Leitstelle" (Germanic Guidance Office) to establish Waffen-SS recruiting offices in Nazi-occupied Europe. The majority of the resulting foreign Waffen-SS units wore a distinctive national collar patch and preceded their SS rank titles with the prefix "Waffen" instead of SS. Volunteers from Scandinavian countries filled the ranks of two divisions, the "SS-Wiking" and "SS-Nordland". Swiss German speakers joined in substantial numbers. Belgian Flemings joined Dutchmen to form the "SS-Nederland" legion, and their Walloon compatriots joined the "SS-Wallonien". By the end of 1943 about a quarter of the SS were ethnic Germans from across Europe, and by June 1944, half the Waffen-SS were foreign nationals.


title: Schutzstaffel

text: Additional Waffen-SS units were added from the Ukrainians, Albanians from Kosovo, Serbians, Croatians, Turkic, Caucasians, Cossack, and Tatars. The Ukrainians and Tatars, who had suffered persecution under Stalin, were likely motivated primarily by opposition to the Soviet government rather than ideological agreement with the SS. The exiled Grand Mufti of Jerusalem Amin al-Husseini was made an SS-"Gruppenführer" by Himmler in May 1943. He subsequently used antisemitism and anti-Serb racism to recruit a Waffen-SS division of Bosnian Muslims, the "SS-Handschar". The year-long Soviet occupation of the Baltic states at the beginning of World War II resulted in volunteers for Latvian and Estonian Waffen-SS units. The Estonian Legion had 1,280 volunteers under training by the end of 1942. Approximately 25,000 men served in the Estonian SS division, with thousands more conscripted into Police Front battalions and border guard units. Most of the Estonians were fighting primarily to regain their independence and as many as 15,000 of them died fighting alongside the Germans. In early 1944, Himmler even contacted Pohl to suggest releasing Muslim prisoners from concentration camps to supplement his SS troops.


title: Schutzstaffel

text: The Indian Legion was a Wehrmacht unit formed in August 1942 chiefly from disaffected Indian soldiers of the British Indian Army captured in the North African Campaign. In August 1944 it was transferred to the auspices of the Waffen-SS as the "Indische Freiwilligen-Legion der Waffen-SS". There was also a French volunteer division, "SS-Charlemagne", which was formed in 1944 mainly from the remnants of the Legion of French Volunteers Against Bolshevism and French "Sturmbrigade".


title: Schutzstaffel

text: The SS established its own symbolism, rituals, customs, ranks, and uniforms to set itself apart from other organizations. Before 1929, the SS wore the same brown uniform as the SA, with the addition of a black tie and a black cap with a "Totenkopf" (death's head) skull and bones symbol, moving to an all-black uniform in 1932. In 1935, the SS combat formations adopted a service uniform in field grey for everyday wear. The SS also developed its own field uniforms, which included reversible smocks and helmet covers printed with camouflage patterns. Uniforms were manufactured in hundreds of licensed factories, with some workers being prisoners of war performing forced labor. Many were produced in concentration camps.


title: Schutzstaffel

text: Hitler and the Nazi Party understood the power of emblems and insignia to influence public opinion. The stylized lightning bolt logo of the SS was chosen in 1932. The logo is a pair of runes from a set of 18 Armanen runes created by Guido von List in 1906. It is similar to the ancient Sowilō rune, which symbolizes the sun, but was renamed as "Sig" (victory) in List's iconography. The "Totenkopf" symbolized the wearer's willingness to fight unto the death, and also served to frighten the enemy.


title: Schutzstaffel

text: After 1933 a career in the SS became increasingly attractive to Germany's social elite, who began joining the movement in great numbers, usually motivated by political opportunism. By 1938 about one-third of the SS leadership were members of the upper middle class. The trend reversed after the first Soviet counter-offensive of 1942.


title: Schutzstaffel

text: The term "Austrian SS" is often used to describe that portion of the SS membership from Austria, but it was never a recognized branch of the SS. In contrast to SS members from other countries, who were grouped into either the Germanic-SS or the Foreign Legions of the Waffen-SS, Austrian SS members were regular SS personnel. It was technically under the command of the SS in Germany but often acted independently concerning Austrian affairs. The Austrian SS was founded in 1930 and by 1934 was acting as a covert force to bring about the "Anschluss" with Germany, which occurred in March 1938. Early Austrian SS leaders were Kaltenbrunner and Arthur Seyss-Inquart. Austrian SS members served in every branch of the SS. Austrians constituted 8 percent of the Third Reich's population and 13 percent of the SS; 40 percent of the staff and 75 percent of commanders at death camps were Austrian.


title: Schutzstaffel

text: After the "Anschluss", the Austrian SS was folded into "SS-Oberabschnitt Donau". The third regiment of the "SS-Verfügungstruppe" ("Der Führer") and the fourth "Totenkopf" regiment ("Ostmark") were recruited in Austria shortly thereafter. On Heydrich's orders, mass arrests of potential enemies of the Reich began immediately after the "Anschluss". Mauthausen was the first concentration camp opened in Austria following the "Anschluss". Before the invasion of the Soviet Union, Mauthausen was the harshest of the camps in the Greater German Reich.


title: Schutzstaffel

text: The Hotel Metropole was transformed into Gestapo headquarters in Vienna in April 1938. With a staff of 900 (80 percent of whom were recruited from the Austrian police), it was the largest Gestapo office outside Berlin. An estimated 50,000 people were interrogated or tortured there. The Gestapo in Vienna was headed by Franz Josef Huber, who also served as chief of the Central Agency for Jewish Emigration in Vienna. Although its de facto leaders were Adolf Eichmann and later Alois Brunner, Huber was nevertheless responsible for the mass deportation of Austrian Jews.


title: Schutzstaffel

text: Following Nazi Germany's collapse, the SS ceased to exist. Numerous members of the SS, many of them still committed Nazis, remained at large in Germany and across Europe. On 21 May 1945, the British captured Himmler, who was in disguise and using a false passport. At an internment camp near Lüneburg, he committed suicide by biting down on a cyanide capsule. Several other leading members of the SS fled, but some were quickly captured. Kaltenbrunner, chief of the RSHA and the highest-ranking surviving SS main department chief upon Himmler's suicide, was captured and arrested in the Bavarian Alps. He was among the 22 defendants put on trial at the International Military Tribunal in 1945–46.


title: Schutzstaffel

text: Some SS members were subject to summary execution, torture, and beatings at the hands of freed prisoners, displaced persons, or Allied soldiers. American soldiers of the 157th Regiment, who entered the concentration camp at Dachau in April 1945 and saw the human deprivation and cruelty committed by the SS, shot some of the remaining SS camp guards. On 15 April 1945, British troops entered Bergen-Belsen. They placed the SS guards on starvation rations, made them work without breaks, forced them to deal with the remaining corpses, and stabbed them with bayonets or struck them with their rifle butts if they slowed their pace. Some members of the US Army Counter Intelligence Corps delivered captured SS camp guards to displaced person camps, where they knew they would be subject to summary execution.


title: Schutzstaffel

text: The Allies commenced legal proceedings against captured Nazis, establishing the International Military Tribunal at Nuremberg in 1945. The first war crimes trial of 24 prominent figures such as Hermann Göring, Albert Speer, Joachim von Ribbentrop, Alfred Rosenberg, Hans Frank, and Kaltenbrunner took place beginning in November 1945. They were accused of four counts: conspiracy, waging a war of aggression, war crimes, and crimes against humanity in violation of international law. Twelve received the death penalty, including Kaltenbrunner, who was convicted of crimes against humanity and executed on 16 October 1946. The former commandant at Auschwitz, Rudolf Höss, who testified on behalf of Kaltenbrunner and others, was tried and executed in 1947.


title: Schutzstaffel

text: Additional SS trials and convictions followed. Many defendants attempted to exculpate themselves using the excuse that they were merely following superior orders, which they had to obey unconditionally as part of their sworn oath and duty. The courts did not find this to be a legitimate defense. A trial of 40 SS officers and guards from Auschwitz took place in Kraków in November 1947. Most were found guilty, and 23 received the death penalty. In addition to those tried by the Western allies, an estimated 37,000 members of the SS were tried and convicted in Soviet courts. Sentences included hangings and long terms of hard labor. Piotr Cywiński, the director of the Auschwitz-Birkenau Museum, estimates that of the 70,000 members of the SS involved in crimes in concentration camps, only about 1,650 to 1,700 were tried after the war. The International Military Tribunal declared the SS a criminal organization in 1946.


title: Schutzstaffel

text: After the war, many former Nazis fled to South America, especially to Argentina, where they were welcomed by Juan Perón's regime. In the 1950s, former Dachau inmate Lothar Hermann discovered that Buenos Aires resident Ricardo Klement was, in fact, Adolf Eichmann, who had in 1948 obtained false identification and a landing permit for Argentina through an organization directed by Bishop Alois Hudal, an Austrian cleric with Nazi sympathies, then residing in Italy. Eichmann was captured in Buenos Aires on 11 May 1960 by Mossad, the Israeli intelligence agency. At his trial in Jerusalem in 1961, he was found guilty and sentenced to death by hanging. Eichmann was quoted as having stated, "I will jump into my grave laughing because the fact that I have the death of five million Jews [or Reich enemies, as he later claimed to have said] on my conscience gives me extraordinary satisfaction." Franz Stangl, the commandant of Treblinka, also escaped to South America with the assistance of Hudal's network. He was deported to Germany in 1967 and was sentenced to life in prison in 1970. He died in 1971.


title: Schutzstaffel

text: Mengele, worried that his capture would mean a death sentence, fled Germany on 17 April 1949. Assisted by a network of former SS members, he traveled to Genoa, where he obtained a passport under the alias "Helmut Gregor" from the International Committee of the Red Cross. He sailed to Argentina in July. Aware that he was still a wanted man, he moved to Paraguay in 1958 and Brazil in 1960. In both instances he was assisted by former Luftwaffe pilot Hans-Ulrich Rudel. Mengele suffered a stroke while swimming and drowned in 1979.


title: Schutzstaffel

text: Thousands of Nazis, including former SS members such as Trawniki guard Jakob Reimer and Circassian collaborator Tscherim Soobzokov, fled to the United States under the guise of refugees, sometimes using forged documents. Other SS men, such as Soobzokov, SD officer Wilhelm Höttl, Eichmann aide Otto von Bolschwing, and accused war criminal Theodor Saevecke, were employed by American intelligence agencies against the Soviets. As CIA officer Harry Rositzke noted, "It was a visceral business of using any bastard so long as he was anti-Communist ... The eagerness or desire to enlist collaborators means that sure, you didn't look at their credentials too closely." Similarly, the Soviets used SS personnel after the war; Operation Theo, for instance, disseminated "subversive rumours" in Allied-occupied Germany.


title: Schutzstaffel

text: Simon Wiesenthal and others have speculated about the existence of a Nazi fugitive network code-named ODESSA (an acronym for "Organisation der ehemaligen SS-Angehörigen", Organization of former SS members) that allegedly helped war criminals find refuge in Latin America. British writer Gitta Sereny, who conducted interviews with SS men, considers the story untrue and attributes the escapes to postwar chaos and Hudal's Vatican-based network. While the existence of ODESSA remains unproven, Sereny notes that "there certainly were various kinds of Nazi aid organizations after the war — it would have been astonishing if there hadn't been."


title: Occam's razor

text: Occam's razor, Ockham's razor, or Ocham's razor (), also known as the principle of parsimony or the law of parsimony (), is the problem-solving principle that "entities should not be multiplied beyond necessity". It is generally understood in the sense that with competing theories or explanations, the simpler one, for example a model with fewer parameters, is to be preferred. The idea is frequently attributed to English Franciscan friar William of Ockham (), a scholastic philosopher and theologian, although he never used these exact words. This philosophical razor advocates that when presented with competing hypotheses about the same prediction, one should select the solution with the fewest assumptions, and that this is not meant to be a way of choosing between hypotheses that make different predictions.


title: Occam's razor

text: Similarly, in science, Occam's razor is used as an abductive heuristic in the development of theoretical models rather than as a rigorous arbiter between candidate models. In the scientific method, Occam's razor is not considered an irrefutable principle of logic or a scientific result; the preference for simplicity in the scientific method is based on the falsifiability criterion. For each accepted explanation of a phenomenon, there may be an extremely large, perhaps even incomprehensible, number of possible and more complex alternatives. Since failing explanations can always be burdened with "ad hoc" hypotheses to prevent them from being falsified, simpler theories are preferable to more complex ones because they tend to be more testable.


title: Occam's razor

text: The phrase "Occam's razor" did not appear until a few centuries after William of Ockham's death in 1347. Libert Froidmont, in his "On Christian Philosophy of the Soul", takes credit for the phrase, speaking of ""novacula occami"". Ockham did not invent this principle, but the "razor"—and its association with him—may be due to the frequency and effectiveness with which he used it. Ockham stated the principle in various ways, but the most popular version, "Entities are not to be multiplied without necessity" () was formulated by the Irish Franciscan philosopher John Punch in his 1639 commentary on the works of Duns Scotus.


title: Occam's razor

text: The origins of what has come to be known as Occam's razor are traceable to the works of earlier philosophers such as John Duns Scotus (1265–1308), Robert Grosseteste (1175–1253), Maimonides (Moses ben-Maimon, 1138–1204), and even Aristotle (384–322 BC). Aristotle writes in his "Posterior Analytics", "We may assume the superiority [other things being equal] of the demonstration which derives from fewer postulates or hypotheses." Ptolemy () stated, "We consider it a good principle to explain the phenomena by the simplest hypothesis possible."


title: Occam's razor

text: Phrases such as "It is vain to do with more what can be done with fewer" and "A plurality is not to be posited without necessity" were commonplace in 13th-century scholastic writing. Robert Grosseteste, in "Commentary on" [Aristotle's] "the Posterior Analytics Books" ("Commentarius in Posteriorum Analyticorum Libros") (c. 1217–1220), declares: "That is better and more valuable which requires fewer, other circumstances being equal... For if one thing were demonstrated from many and another thing from fewer equally known premises, clearly that is better which is from fewer because it makes us know quickly, just as a universal demonstration is better than particular because it produces knowledge from fewer premises. Similarly in natural science, in moral science, and in metaphysics the best is that which needs no premises and the better that which needs the fewer, other circumstances being equal."


title: Occam's razor

text: The "Summa Theologica" of Thomas Aquinas (1225–1274) states that "it is superfluous to suppose that what can be accounted for by a few principles has been produced by many." Aquinas uses this principle to construct an objection to God's existence, an objection that he in turn answers and refutes generally (cf. "quinque viae"), and specifically, through an argument based on causality. Hence, Aquinas acknowledges the principle that today is known as Occam's razor, but prefers causal explanations to other simple explanations (cf. also Correlation does not imply causation).


title: Occam's razor

text: William of Ockham ("circa" 1287–1347) was an English Franciscan friar and theologian, an influential medieval philosopher and a nominalist. His popular fame as a great logician rests chiefly on the maxim attributed to him and known as Occam's razor. The term "razor" refers to distinguishing between two hypotheses either by "shaving away" unnecessary assumptions or cutting apart two similar conclusions.


title: Occam's razor

text: While it has been claimed that Occam's razor is not found in any of William's writings, one can cite statements such as ("Plurality must never be posited without necessity"), which occurs in his theological work on the "Sentences of Peter Lombard" ("Quaestiones et decisiones in quattuor libros Sententiarum Petri Lombardi"; ed. Lugd., 1495, i, dist. 27, qu. 2, K).


title: Occam's razor

text: Nevertheless, the precise words sometimes attributed to William of Ockham, (Entities must not be multiplied beyond necessity), are absent in his extant works; this particular phrasing comes from John Punch, who described the principle as a "common axiom" ("axioma vulgare") of the Scholastics. William of Ockham's contribution seems to restrict the operation of this principle in matters pertaining to miracles and God's power; so, in the Eucharist, a plurality of miracles is possible, simply because it pleases God.


title: Occam's razor

text: This principle is sometimes phrased as ("Plurality should not be posited without necessity"). In his "Summa Totius Logicae", i. 12, William of Ockham cites the principle of economy, ("It is futile to do with more things that which can be done with fewer"; Thorburn, 1918, pp. 352–53; Kneale and Kneale, 1962, p. 243.)


title: Occam's razor

text: To quote Isaac Newton, "We are to admit no more causes of natural things than such as are both true and sufficient to explain their appearances. Therefore, to the same natural effects we must, as far as possible, assign the same causes."


title: Occam's razor

text: Bertrand Russell offers a particular version of Occam's razor: "Whenever possible, substitute constructions out of known entities for inferences to unknown entities."


title: Occam's razor

text: Around 1960, Ray Solomonoff founded the theory of universal inductive inference, the theory of prediction based on observationsfor example, predicting the next symbol based upon a given series of symbols. The only assumption is that the environment follows some unknown but computable probability distribution. This theory is a mathematical formalization of Occam's razor.


title: Occam's razor

text: Another technical approach to Occam's razor is ontological parsimony. Parsimony means spareness and is also referred to as the Rule of Simplicity. This is considered a strong version of Occam's razor. A variation used in medicine is called the "Zebra": a physician should reject an exotic medical diagnosis when a more commonplace explanation is more likely, derived from Theodore Woodward's dictum "When you hear hoofbeats, think of horses not zebras".


title: Occam's razor

text: Ernst Mach formulated the stronger version of Occam's razor into physics, which he called the Principle of Economy stating: "Scientists must use the simplest means of arriving at their results and exclude everything not perceived by the senses."


title: Occam's razor

text: This principle goes back at least as far as Aristotle, who wrote "Nature operates in the shortest way possible." The idea of parsimony or simplicity in deciding between theories, though not the intent of the original expression of Occam's razor, has been assimilated into common culture as the widespread layman's formulation that "the simplest explanation is usually the correct one."


title: Occam's razor

text: Prior to the 20th century, it was a commonly held belief that nature itself was simple and that simpler hypotheses about nature were thus more likely to be true. Thomas Aquinas made this argument in the 13th century, writing, "If a thing can be done adequately by means of one, it is superfluous to do it by means of several; for we observe that nature does not employ two instruments [if] one suffices."


title: Occam's razor

text: Beginning in the 20th century, epistemological justifications based on induction, logic, pragmatism, and especially probability theory have become more popular among philosophers.


title: Occam's razor

text: Occam's razor has gained strong empirical support in helping to converge on better theories (see Uses section below for some examples).


title: Occam's razor

text: In the related concept of overfitting, excessively complex models are affected by statistical noise (a problem also known as the bias-variance trade-off), whereas simpler models may capture the underlying structure better and may thus have better predictive performance. It is, however, often difficult to deduce which part of the data is noise (cf. model selection, test set, minimum description length, Bayesian inference, etc.).


title: Occam's razor

text: The razor's statement that "other things being equal, simpler explanations are generally better than more complex ones" is amenable to empirical testing. Another interpretation of the razor's statement would be that "simpler hypotheses are generally better than the complex ones". The procedure to test the former interpretation would compare the track records of simple and comparatively complex explanations. If one accepts the first interpretation, the validity of Occam's razor as a tool would then have to be rejected if the more complex explanations were more often correct than the less complex ones (while the converse would lend support to its use). If the latter interpretation is accepted, the validity of Occam's razor as a tool could possibly be accepted if the simpler hypotheses led to correct conclusions more often than not.


title: Occam's razor

text: Even if some increases in complexity are sometimes necessary, there still remains a justified general bias toward the simpler of two competing explanations. To understand why, consider that for each accepted explanation of a phenomenon, there is always an infinite number of possible, more complex, and ultimately incorrect, alternatives. This is so because one can always burden a failing explanation with an ad hoc hypothesis. Ad hoc hypotheses are justifications that prevent theories from being falsified.


title: Occam's razor

text: For example, if a man, accused of breaking a vase, makes supernatural claims that leprechauns were responsible for the breakage, a simple explanation might be that the man did it, but ongoing ad hoc justifications (e.g. "... and that's not me breaking it on the film; they tampered with that, too") could successfully prevent complete disproof. This endless supply of elaborate competing explanations, called saving hypotheses, cannot be technically ruled out – except by using Occam's razor.


title: Occam's razor

text: Any more complex theory might still possibly be true. A study of the predictive validity of Occam's razor found 32 published papers that included 97 comparisons of economic forecasts from simple and complex forecasting methods. None of the papers provided a balance of evidence that complexity of method improved forecast accuracy. In the 25 papers with quantitative comparisons, complexity increased forecast errors by an average of 27 percent.


title: Occam's razor

text: One justification of Occam's razor is a direct result of basic probability theory. By definition, all assumptions introduce possibilities for error; if an assumption does not improve the accuracy of a theory, its only effect is to increase the probability that the overall theory is wrong.


title: Occam's razor

text: There have also been other attempts to derive Occam's razor from probability theory, including notable attempts made by Harold Jeffreys and E. T. Jaynes. The probabilistic (Bayesian) basis for Occam's razor is elaborated by David J. C. MacKay in chapter 28 of his book "Information Theory, Inference, and Learning Algorithms", where he emphasizes that a prior bias in favor of simpler models is not required.


title: Occam's razor

text: William H. Jefferys and James O. Berger (1991) generalize and quantify the original formulation's "assumptions" concept as the degree to which a proposition is unnecessarily accommodating to possible observable data. They state, "A hypothesis with fewer adjustable parameters will automatically have an enhanced posterior probability, due to the fact that the predictions it makes are sharp." The use of "sharp" here is not only a tongue-in-cheek reference to the idea of a razor, but also indicates that such predictions are more accurate than competing predictions. The model they propose balances the precision of a theory's predictions against their sharpness, preferring theories that sharply make correct predictions over theories that accommodate a wide range of other possible results. This, again, reflects the mathematical relationship between key concepts in Bayesian inference (namely marginal probability, conditional probability, and posterior probability).


title: Occam's razor

text: The bias–variance tradeoff is a framework that incorporates the Occam's razor principle in its balance between overfitting (associated with lower bias but higher variance) and underfitting (associated with lower variance but higher bias).


title: Occam's razor

text: Karl Popper argues that a preference for simple theories need not appeal to practical or aesthetic considerations. Our preference for simplicity may be justified by its falsifiability criterion: we prefer simpler theories to more complex ones "because their empirical content is greater; and because they are better testable". The idea here is that a simple theory applies to more cases than a more complex one, and is thus more easily falsifiable. This is again comparing a simple theory to a more complex theory where both explain the data equally well.


title: Occam's razor

text: The philosopher of science Elliott Sober once argued along the same lines as Popper, tying simplicity with "informativeness": The simplest theory is the more informative, in the sense that it requires less information to a question. He has since rejected this account of simplicity, purportedly because it fails to provide an epistemic justification for simplicity. He now believes that simplicity considerations (and considerations of parsimony in particular) do not count unless they reflect something more fundamental. Philosophers, he suggests, may have made the error of hypostatizing simplicity (i.e., endowed it with a "sui generis" existence), when it has meaning only when embedded in a specific context (Sober 1992). If we fail to justify simplicity considerations on the basis of the context in which we use them, we may have no non-circular justification: "Just as the question 'why be rational?' may have no non-circular answer, the same may be true of the question 'why should simplicity be considered in evaluating the plausibility of hypotheses?'"


title: Occam's razor

text: According to Swinburne, since our choice of theory cannot be determined by data (see Underdetermination and Duhem–Quine thesis), we must rely on some criterion to determine which theory to use. Since it is absurd to have no logical method for settling on one hypothesis amongst an infinite number of equally data-compliant hypotheses, we should choose the simplest theory: "Either science is irrational [in the way it judges theories and predictions probable] or the principle of simplicity is a fundamental synthetic a priori truth.".


title: Occam's razor

text: In science, Occam's razor is used as a heuristic to guide scientists in developing theoretical models rather than as an arbiter between published models. In physics, parsimony was an important heuristic in Albert Einstein's formulation of special relativity, in the development and application of the principle of least action by Pierre Louis Maupertuis and Leonhard Euler, and in the development of quantum mechanics by Max Planck, Werner Heisenberg and Louis de Broglie.


title: Occam's razor

text: In chemistry, Occam's razor is often an important heuristic when developing a model of a reaction mechanism. Although it is useful as a heuristic in developing models of reaction mechanisms, it has been shown to fail as a criterion for selecting among some selected published models. In this context, Einstein himself expressed caution when he formulated Einstein's Constraint: "It can scarcely be denied that the supreme goal of all theory is to make the irreducible basic elements as simple and as few as possible without having to surrender the adequate representation of a single datum of experience". An often-quoted version of this constraint (which cannot be verified as posited by Einstein himself) says "Everything should be kept as simple as possible, but not simpler."


title: Occam's razor

text: In the scientific method, parsimony is an epistemological, metaphysical or heuristic preference, not an irrefutable principle of logic or a scientific result. As a logical principle, Occam's razor would demand that scientists accept the simplest possible theoretical explanation for existing data. However, science has shown repeatedly that future data often support more complex theories than do existing data. Science prefers the simplest explanation that is consistent with the data available at a given time, but the simplest explanation may be ruled out as new data become available. That is, science is open to the possibility that future experiments might support more complex theories than demanded by current data and is more interested in designing experiments to discriminate between competing theories than favoring one theory over another based merely on philosophical principles.


title: Occam's razor

text: When scientists use the idea of parsimony, it has meaning only in a very specific context of inquiry. Several background assumptions are required for parsimony to connect with plausibility in a particular research problem. The reasonableness of parsimony in one research context may have nothing to do with its reasonableness in another. It is a mistake to think that there is a single global principle that spans diverse subject matter.


title: Occam's razor

text: It has been suggested that Occam's razor is a widely accepted example of extraevidential consideration, even though it is entirely a metaphysical assumption. Most of the time, however, Occam's razor is a conservative tool, cutting out "crazy, complicated constructions" and assuring "that hypotheses are grounded in the science of the day", thus yielding "normal" science: models of explanation and prediction. There are, however, notable exceptions where Occam's razor turns a conservative scientist into a reluctant revolutionary. For example, Max Planck interpolated between the Wien and Jeans radiation laws and used Occam's razor logic to formulate the quantum hypothesis, even resisting that hypothesis as it became more obvious that it was correct.


title: Occam's razor

text: Appeals to simplicity were used to argue against the phenomena of meteorites, ball lightning, continental drift, and reverse transcriptase. One can argue for atomic building blocks for matter, because it provides a simpler explanation for the observed reversibility of both and chemical reactions as simple separation and rearrangements of atomic building blocks. At the time, however, the atomic theory was considered more complex because it implied the existence of invisible particles that had not been directly detected. Ernst Mach and the logical positivists rejected John Dalton's atomic theory until the reality of atoms was more evident in Brownian motion, as shown by Albert Einstein.


title: Occam's razor

text: In the same way, postulating the aether is more complex than transmission of light through a vacuum. At the time, however, all known waves propagated through a physical medium, and it seemed simpler to postulate the existence of a medium than to theorize about wave propagation without a medium. Likewise, Isaac Newton's idea of light particles seemed simpler than Christiaan Huygens's idea of waves, so many favored it. In this case, as it turned out, neither the wave—nor the particle—explanation alone suffices, as light behaves like waves and like particles.


title: Occam's razor

text: Three axioms presupposed by the scientific method are realism (the existence of objective reality), the existence of natural laws, and the constancy of natural law. Rather than depend on provability of these axioms, science depends on the fact that they have not been objectively falsified. Occam's razor and parsimony support, but do not prove, these axioms of science. The general principle of science is that theories (or models) of natural law must be consistent with repeatable experimental observations. This ultimate arbiter (selection criterion) rests upon the axioms mentioned above.


title: Occam's razor

text: If multiple models of natural law make exactly the same testable predictions, they are equivalent and there is no need for parsimony to choose a preferred one. For example, Newtonian, Hamiltonian and Lagrangian classical mechanics are equivalent. Physicists have no interest in using Occam's razor to say the other two are wrong. Likewise, there is no demand for simplicity principles to arbitrate between wave and matrix formulations of quantum mechanics. Science often does not demand arbitration or selection criteria between models that make the same testable predictions.


title: Occam's razor

text: Biologists or philosophers of biology use Occam's razor in either of two contexts both in evolutionary biology: the units of selection controversy and systematics. George C. Williams in his book "Adaptation and Natural Selection" (1966) argues that the best way to explain altruism among animals is based on low-level (i.e., individual) selection as opposed to high-level group selection. Altruism is defined by some evolutionary biologists (e.g., R. Alexander, 1987; W. D. Hamilton, 1964) as behavior that is beneficial to others (or to the group) at a cost to the individual, and many posit individual selection as the mechanism that explains altruism solely in terms of the behaviors of individual organisms acting in their own self-interest (or in the interest of their genes, via kin selection). Williams was arguing against the perspective of others who propose selection at the level of the group as an evolutionary mechanism that selects for altruistic traits (e.g., D. S. Wilson & E. O. Wilson, 2007). The basis for Williams' contention is that of the two, individual selection is the more parsimonious theory. In doing so he is invoking a variant of Occam's razor known as Morgan's Canon: "In no case is an animal activity to be interpreted in terms of higher psychological processes, if it can be fairly interpreted in terms of processes which stand lower in the scale of psychological evolution and development." (Morgan 1903).


title: Occam's razor

text: However, more recent biological analyses, such as Richard Dawkins' "The Selfish Gene", have contended that Morgan's Canon is not the simplest and most basic explanation. Dawkins argues the way evolution works is that the genes propagated in most copies end up determining the development of that particular species, i.e., natural selection turns out to select specific genes, and this is really the fundamental underlying principle that automatically gives individual and group selection as emergent features of evolution.


title: Occam's razor

text: Zoology provides an example. Muskoxen, when threatened by wolves, form a circle with the males on the outside and the females and young on the inside. This is an example of a behavior by the males that seems to be altruistic. The behavior is disadvantageous to them individually but beneficial to the group as a whole and was thus seen by some to support the group selection theory. Another interpretation is kin selection: if the males are protecting their offspring, they are protecting copies of their own alleles. Engaging in this behavior would be favored by individual selection if the cost to the male musk ox is less than half of the benefit received by his calf – which could easily be the case if wolves have an easier time killing calves than adult males. It could also be the case that male musk oxen would be individually less likely to be killed by wolves if they stood in a circle with their horns pointing out, regardless of whether they were protecting the females and offspring. That would be an example of regular natural selection – a phenomenon called "the selfish herd".


title: Occam's razor

text: Systematics is the branch of biology that attempts to establish patterns of relationship among biological taxa, today generally thought to reflect evolutionary history. It is also concerned with their classification. There are three primary camps in systematics: cladists, pheneticists, and evolutionary taxonomists. Cladists hold that classification should be based on synapomorphies (shared, derived character states), pheneticists contend that overall similarity (synapomorphies and complementary symplesiomorphies) is the determining criterion, while evolutionary taxonomists say that both genealogy and similarity count in classification (in a manner determined by the evolutionary taxonomist).


title: Occam's razor

text: It is among the cladists that Occam's razor is applied, through the method of "cladistic parsimony". Cladistic parsimony (or maximum parsimony) is a method of phylogenetic inference that yields phylogenetic trees (more specifically, cladograms). Cladograms are branching, diagrams used to represent hypotheses of relative degree of relationship, based on synapomorphies. Cladistic parsimony is used to select as the preferred hypothesis of relationships the cladogram that requires the fewest implied character state transformations (or smallest weight, if characters are differentially weighted). Critics of the cladistic approach often observe that for some types of data, parsimony could produce the wrong results, regardless of how much data is collected (this is called statistical inconsistency, or long branch attraction). However, this criticism is also potentially true for any type of phylogenetic inference, unless the model used to estimate the tree reflects the way that evolution actually happened. Because this information is not empirically accessible, the criticism of statistical inconsistency against parsimony holds no force. For a book-length treatment of cladistic parsimony, see Elliott Sober's "Reconstructing the Past: Parsimony, Evolution, and Inference" (1988). For a discussion of both uses of Occam's razor in biology, see Sober's article "Let's Razor Ockham's Razor" (1990).


title: Occam's razor

text: Other methods for inferring evolutionary relationships use parsimony in a more general way. Likelihood methods for phylogeny use parsimony as they do for all likelihood tests, with hypotheses requiring fewer differing parameters (i.e., numbers or different rates of character change or different frequencies of character state transitions) being treated as null hypotheses relative to hypotheses requiring more differing parameters. Thus, complex hypotheses must predict data much better than do simple hypotheses before researchers reject the simple hypotheses. Recent advances employ information theory, a close cousin of likelihood, which uses Occam's razor in the same way. Of course, the choice of the "shortest tree" relative to a not-so-short tree under any optimality criterion (smallest distance, fewest steps, or maximum likelihood) is always based on parsimony


title: Occam's razor

text: Francis Crick has commented on potential limitations of Occam's razor in biology. He advances the argument that because biological systems are the products of (an ongoing) natural selection, the mechanisms are not necessarily optimal in an obvious sense. He cautions: "While Ockham's razor is a useful tool in the physical sciences, it can be a very dangerous implement in biology. It is thus very rash to use simplicity and elegance as a guide in biological research." This is an ontological critique of parsimony.


title: Occam's razor

text: In biogeography, parsimony is used to infer ancient vicariant events or migrations of species or populations by observing the geographic distribution and relationships of existing organisms. Given the phylogenetic tree, ancestral population subdivisions are inferred to be those that require the minimum amount of change.


title: Occam's razor

text: In the philosophy of religion, Occam's razor is sometimes applied to the existence of God. William of Ockham himself was a Christian. He believed in God, and in the authority of Scripture; he writes that "nothing ought to be posited without a reason given, unless it is self-evident (literally, known through itself) or known by experience or proved by the authority of Sacred Scripture." Ockham believed that an explanation has no sufficient basis in reality when it does not harmonize with reason, experience, or the Bible. However, unlike many theologians of his time, Ockham did not believe God could be logically proven with arguments. To Ockham, science was a matter of discovery, but theology was a matter of revelation and faith. He states: "only faith gives us access to theological truths. The ways of God are not open to reason, for God has freely chosen to create a world and establish a way of salvation within it apart from any necessary laws that human logic or rationality can uncover."


title: Occam's razor

text: St. Thomas Aquinas, in the "Summa Theologica", uses a formulation of Occam's razor to construct an objection to the idea that God exists, which he refutes directly with a counterargument:


title: Occam's razor

text: Further, it is superfluous to suppose that what can be accounted for by a few principles has been produced by many. But it seems that everything we see in the world can be accounted for by other principles, supposing God did not exist. For all natural things can be reduced to one principle which is nature; and all voluntary things can be reduced to one principle which is human reason, or will. Therefore there is no need to suppose God's existence.


title: Occam's razor

text: In turn, Aquinas answers this with the "quinque viae", and addresses the particular objection above with the following answer:


title: Occam's razor

text: Since nature works for a determinate end under the direction of a higher agent, whatever is done by nature must needs be traced back to God, as to its first cause. So also whatever is done voluntarily must also be traced back to some higher cause other than human reason or will, since these can change or fail; for all things that are changeable and capable of defect must be traced back to an immovable and self-necessary first principle, as was shown in the body of the Article.


title: Occam's razor

text: Rather than argue for the necessity of a god, some theists base their belief upon grounds independent of, or prior to, reason, making Occam's razor irrelevant. This was the stance of Søren Kierkegaard, who viewed belief in God as a leap of faith that sometimes directly opposed reason. This is also the doctrine of Gordon Clark's presuppositional apologetics, with the exception that Clark never thought the leap of faith was contrary to reason (see also Fideism).


title: Occam's razor

text: Various arguments in favor of God establish God as a useful or even necessary assumption. Contrastingly some anti-theists hold firmly to the belief that assuming the existence of God introduces unnecessary complexity (Schmitt 2005, e.g., the Ultimate Boeing 747 gambit).


title: Occam's razor

text: Another application of the principle is to be found in the work of George Berkeley (1685–1753). Berkeley was an idealist who believed that all of reality could be explained in terms of the mind alone. He invoked Occam's razor against materialism, stating that matter was not required by his metaphysics and was thus eliminable. One potential problem with this belief is that it's possible, given Berkeley's position, to find solipsism itself more in line with the razor than a God-mediated world beyond a single thinker.


title: Occam's razor

text: Occam's razor may also be recognized in the apocryphal story about an exchange between Pierre-Simon Laplace and Napoleon. It is said that in praising Laplace for one of his recent publications, the emperor asked how it was that the name of God, which featured so frequently in the writings of Lagrange, appeared nowhere in Laplace's. At that, he is said to have replied, "It's because I had no need of that hypothesis." Though some points of this story illustrate Laplace's atheism, more careful consideration suggests that he may instead have intended merely to illustrate the power of methodological naturalism, or even simply that the fewer logical premises one assumes, the stronger is one's conclusion.


title: Occam's razor

text: In his article "Sensations and Brain Processes" (1959), J. J. C. Smart invoked Occam's razor with the aim to justify his preference of the mind-brain identity theory over spirit-body dualism. Dualists state that there are two kinds of substances in the universe: physical (including the body) and spiritual, which is non-physical. In contrast, identity theorists state that everything is physical, including consciousness, and that there is nothing nonphysical. Though it is impossible to appreciate the spiritual when limiting oneself to the physical, Smart maintained that identity theory explains all phenomena by assuming only a physical reality. Subsequently, Smart has been severely criticized for his use (or misuse) of Occam's razor and ultimately retracted his advocacy of it in this context. Paul Churchland (1984) states that by itself Occam's razor is inconclusive regarding duality. In a similar way, Dale Jacquette (1994) stated that Occam's razor has been used in attempts to justify eliminativism and reductionism in the philosophy of mind. Eliminativism is the thesis that the ontology of folk psychology including such entities as "pain", "joy", "desire", "fear", etc., are eliminable in favor of an ontology of a completed neuroscience.


title: Occam's razor

text: In penal theory and the philosophy of punishment, parsimony refers specifically to taking care in the distribution of punishment in order to avoid excessive punishment. In the utilitarian approach to the philosophy of punishment, Jeremy Bentham's "parsimony principle" states that any punishment greater than is required to achieve its end is unjust. The concept is related but not identical to the legal concept of proportionality. Parsimony is a key consideration of the modern restorative justice, and is a component of utilitarian approaches to punishment, as well as the prison abolition movement. Bentham believed that true parsimony would require punishment to be individualised to take account of the sensibility of the individual—an individual more sensitive to punishment should be given a proportionately lesser one, since otherwise needless pain would be inflicted. Later utilitarian writers have tended to abandon this idea, in large part due to the impracticality of determining each alleged criminal's relative sensitivity to specific punishments.


title: Occam's razor

text: Marcus Hutter's universal artificial intelligence builds upon Solomonoff's mathematical formalization of the razor to calculate the expected value of an action.


title: Occam's razor

text: There are various papers in scholarly journals deriving formal versions of Occam's razor from probability theory, applying it in statistical inference, and using it to come up with criteria for penalizing complexity in statistical inference. Papers have suggested a connection between Occam's razor and Kolmogorov complexity.


title: Occam's razor

text: One of the problems with the original formulation of the razor is that it only applies to models with the same explanatory power (i.e., it only tells us to prefer the simplest of equally good models). A more general form of the razor can be derived from Bayesian model comparison, which is based on Bayes factors and can be used to compare models that don't fit the observations equally well. These methods can sometimes optimally balance the complexity and power of a model. Generally, the exact Occam factor is intractable, but approximations such as Akaike information criterion, Bayesian information criterion, Variational Bayesian methods, false discovery rate, and Laplace's method are used. Many artificial intelligence researchers are now employing such techniques, for instance through work on Occam Learning or more generally on the Free energy principle.


title: Occam's razor

text: Statistical versions of Occam's razor have a more rigorous formulation than what philosophical discussions produce. In particular, they must have a specific definition of the term "simplicity", and that definition can vary. For example, in the Kolmogorov–Chaitin minimum description length approach, the subject must pick a Turing machine whose operations describe the basic operations "believed" to represent "simplicity" by the subject. However, one could always choose a Turing machine with a simple operation that happened to construct one's entire theory and would hence score highly under the razor. This has led to two opposing camps: one that believes Occam's razor is objective, and one that believes it is subjective.


title: Occam's razor

text: The minimum instruction set of a universal Turing machine requires approximately the same length description across different formulations, and is small compared to the Kolmogorov complexity of most practical theories. Marcus Hutter has used this consistency to define a "natural" Turing machine of small size as the proper basis for excluding arbitrarily complex instruction sets in the formulation of razors. Describing the program for the universal program as the "hypothesis", and the representation of the evidence as program data, it has been formally proven under Zermelo–Fraenkel set theory that "the sum of the log universal probability of the model plus the log of the probability of the data given the model should be minimized." Interpreting this as minimising the total length of a two-part message encoding model followed by data given model gives us the minimum message length (MML) principle.


title: Occam's razor

text: One possible conclusion from mixing the concepts of Kolmogorov complexity and Occam's razor is that an ideal data compressor would also be a scientific explanation/formulation generator. Some attempts have been made to re-derive known laws from considerations of simplicity or compressibility.


title: Occam's razor

text: According to Jürgen Schmidhuber, the appropriate mathematical theory of Occam's razor already exists, namely, Solomonoff's theory of optimal inductive inference and its extensions. See discussions in David L. Dowe's "Foreword re C. S. Wallace" for the subtle distinctions between the algorithmic probability work of Solomonoff and the MML work of Chris Wallace, and see Dowe's "MML, hybrid Bayesian network graphical models, statistical consistency, invariance and uniqueness" both for such discussions and for (in section 4) discussions of MML and Occam's razor. For a specific example of MML as Occam's razor in the problem of decision tree induction, see Dowe and Needham's "Message Length as an Effective Ockham's Razor in Decision Tree Induction".


title: Occam's razor

text: In software development, the rule of least power argues the correct programming language to use is the one that is simplest while also solving the targeted software problem. In that form the rule is often credited to Tim Berners-Lee since it appeared in his design guidelines for the original Hypertext Transfer Protocol. Complexity in this context is measured either by placing a language into the Chomsky hierarchy or by listing idiomatic features of the language and comparing according to some agreed to scale of difficulties between idioms. Many languages once thought to be of lower complexity have evolved or later been discovered to be more complex than originally intended; so, in practice this rule is applied to the relative ease of a programmer to obtain the power of the language, rather than the precise theoretical limits of the language.


title: Occam's razor

text: Occam's razor is not an embargo against the positing of any kind of entity, or a recommendation of the simplest theory come what may. Occam's razor is used to adjudicate between theories that have already passed "theoretical scrutiny" tests and are equally well-supported by evidence. Furthermore, it may be used to prioritize empirical testing between two equally plausible but unequally testable hypotheses; thereby minimizing costs and wastes while increasing chances of falsification of the simpler-to-test hypothesis.


title: Occam's razor

text: Another contentious aspect of the razor is that a theory can become more complex in terms of its structure (or syntax), while its ontology (or semantics) becomes simpler, or vice versa. Quine, in a discussion on definition, referred to these two perspectives as "economy of practical expression" and "economy in grammar and vocabulary", respectively.


title: Occam's razor

text: Galileo Galilei lampooned the "misuse" of Occam's razor in his "Dialogue". The principle is represented in the dialogue by Simplicio. The telling point that Galileo presented ironically was that if one really wanted to start from a small number of entities, one could always consider the letters of the alphabet as the fundamental entities, since one could construct the whole of human knowledge out of them.


title: Occam's razor

text: Occam's razor has met some opposition from people who have considered it too extreme or rash. Walter Chatton () was a contemporary of William of Ockham who took exception to Occam's razor and Ockham's use of it. In response he devised his own "anti-razor": "If three things are not enough to verify an affirmative proposition about things, a fourth must be added, and so on." Although there have been a number of philosophers who have formulated similar anti-razors since Chatton's time, no one anti-razor has perpetuated in as much notability as Chatton's anti-razor, although this could be the case of the Late Renaissance Italian motto of unknown attribution ("Even if it is not true, it is well conceived") when referred to a particularly artful explanation.


title: Occam's razor

text: Anti-razors have also been created by Gottfried Wilhelm Leibniz (1646–1716), Immanuel Kant (1724–1804), and Karl Menger (1902–1985). Leibniz's version took the form of a principle of plenitude, as Arthur Lovejoy has called it: the idea being that God created the most varied and populous of possible worlds. Kant felt a need to moderate the effects of Occam's razor and thus created his own counter-razor: "The variety of beings should not rashly be diminished."


title: Occam's razor

text: Karl Menger found mathematicians to be too parsimonious with regard to variables, so he formulated his Law Against Miserliness, which took one of two forms: "Entities must not be reduced to the point of inadequacy" and "It is vain to do with fewer what requires more." A less serious but even more extremist anti-razor is 'Pataphysics, the "science of imaginary solutions" developed by Alfred Jarry (1873–1907). Perhaps the ultimate in anti-reductionism, "'Pataphysics seeks no less than to view each event in the universe as completely unique, subject to no laws but its own." Variations on this theme were subsequently explored by the Argentine writer Jorge Luis Borges in his story/mock-essay "Tlön, Uqbar, Orbis Tertius". Physicist R. V. Jones contrived Crabtree's Bludgeon, which states that "[n]o set of mutually inconsistent observations can exist for which some human intellect cannot conceive a coherent explanation, however complicated."


title: Climate change

text: In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to Earth's climate. The current rise in global average temperature is more rapid than previous changes, and is primarily caused by humans burning fossil fuels. Fossil fuel use, deforestation, and some agricultural and industrial practices increase greenhouse gases, notably carbon dioxide and methane. Greenhouse gases absorb some of the heat that the Earth radiates after it warms from sunlight. Larger amounts of these gases trap more heat in Earth's lower atmosphere, causing global warming.


title: Climate change

text: Due to climate change, deserts are expanding, while heat waves and wildfires are becoming more common. Increased warming in the Arctic has contributed to melting permafrost, glacial retreat and sea ice loss. Higher temperatures are also causing more intense storms, droughts, and other weather extremes. Rapid environmental change in mountains, coral reefs, and the Arctic is forcing many species to relocate or become extinct. Even if efforts to minimise future warming are successful, some effects will continue for centuries. These include ocean heating, ocean acidification and sea level rise.


title: Climate change

text: Climate change threatens people with food and water scarcity, increased flooding, extreme heat, more disease, and economic loss. Human migration and conflict can also be a result. The World Health Organization (WHO) calls climate change the greatest threat to global health in the 21st century. Communities may adapt to climate change through efforts like coastline protection or expanding access to air conditioning, but some impacts are unavoidable. Poorer countries are responsible for a small share of global emissions, yet they have the least ability to adapt and are most vulnerable to climate change.


title: Climate change

text: Many climate change impacts are already felt at the current level of warming. Additional warming will increase these impacts and may trigger tipping points, such as the melting of the Greenland ice sheet. Under the 2015 Paris Agreement, nations collectively agreed to keep warming "well under 2 °C". However, with pledges made under the Agreement, global warming would still reach about by the end of the century. Limiting warming to 1.5 °C will require halving emissions by 2030 and achieving net-zero emissions by 2050.


title: Climate change

text: Reducing emissions requires generating electricity from low-carbon sources rather than burning fossil fuels. This change includes phasing out coal and natural gas fired power plants, vastly increasing use of wind, solar, and other types of renewable energy, and reducing energy use. Electricity generated from non-carbon-emitting sources will need to replace fossil fuels for powering transportation, heating buildings, and operating industrial facilities. Carbon can also be removed from the atmosphere, for instance by increasing forest cover and by farming with methods that capture carbon in soil.


title: Climate change

text: Before the 1980s, when it was unclear whether the warming effect of increased greenhouse gases were stronger than the cooling effect of airborne particulates in air pollution, scientists used the term "inadvertent climate modification" to refer to human impacts on the climate.


title: Climate change

text: In the 1980s, the terms "global warming" and "climate change" became more common. Though the two terms are sometimes used interchangeably, scientifically, "global warming" refers only to increased surface warming, while "climate change" describes the totality of changes to Earth's climate system. "Global warming"—used as early as 1975—became the more popular term after NASA climate scientist James Hansen used it in his 1988 testimony in the U.S. Senate. Since the 2000s, "climate change" has increased in usage. "Climate change" can also refer more broadly to both human-caused changes or natural changes throughout Earth's history.


title: Climate change

text: Various scientists, politicians and media now use the terms "climate crisis" or "climate emergency" to talk about climate change, and "global heating" instead of "global warming".


title: Climate change

text: Multiple independent instrumental datasets show that the climate system is warming. The 2011–2020 decade warmed to an average 1.09 °C [0.95–1.20 °C] compared to the pre-industrial baseline (1850–1900). Surface temperatures are rising by about 0.2 °C per decade, with 2020 reaching a temperature of 1.2 °C above the pre-industrial era. Since 1950, the number of cold days and nights has decreased, and the number of warm days and nights has increased.


title: Climate change

text: There was little net warming between the 18th century and the mid-19th century. Climate information for that period comes from climate proxies, such as trees and ice cores. Thermometer records began to provide global coverage around 1850. Historical patterns of warming and cooling, like the Medieval Climate Anomaly and the Little Ice Age, did not occur at the same time across different regions. Temperatures may have reached as high as those of the late-20th century in a limited set of regions. There have been prehistorical episodes of global warming, such as the Paleocene–Eocene Thermal Maximum. However, the modern observed rise in temperature and concentrations has been so rapid that even abrupt geophysical events in Earth's history do not approach current rates.


title: Climate change

text: Evidence of warming from air temperature measurements are reinforced with a wide range of other observations. For example, changes to the natural water cycle have been predicted and observed, such as an increase in the frequency and intensity of heavy precipitation, melting of snow and land ice, and increased atmospheric humidity. Flora and fauna are also behaving in a manner consistent with warming; for instance, plants are flowering earlier in spring. Another key indicator is the cooling of the upper atmosphere, which demonstrates that greenhouse gases are trapping heat near the Earth's surface and preventing it from radiating into space.


title: Climate change

text: Regions of the world warm at differing rates. The pattern is independent of where greenhouse gases are emitted, because the gases persist long enough to diffuse across the planet. Since the pre-industrial period, the average surface temperature over land regions has increased almost twice as fast as the global-average surface temperature. This is because of the larger heat capacity of oceans, and because oceans lose more heat by evaporation. The thermal energy in the global climate system has grown with only brief pauses since at least 1970, and over 90% of this extra energy has been stored in the ocean. The rest has heated the atmosphere, melted ice, and warmed the continents.


title: Climate change

text: The Northern Hemisphere and the North Pole have warmed much faster than the South Pole and Southern Hemisphere. The Northern Hemisphere not only has much more land, but also more seasonal snow cover and sea ice. As these surfaces flip from reflecting a lot of light to being dark after the ice has melted, they start absorbing more heat. Local black carbon deposits on snow and ice also contribute to Arctic warming. Arctic temperatures are increasing at over twice the rate of the rest of the world. Melting of glaciers and ice sheets in the Arctic disrupts ocean circulation, including a weakened Gulf Stream, further changing the climate.


title: Climate change

text: The climate system experiences various cycles on its own which can last for years (such as the El Niño–Southern Oscillation (ENSO)), decades or even centuries. Other changes are caused by an imbalance of energy that is "external" to the climate system, but not always external to the Earth. Examples of external forcings include changes in the concentrations of greenhouse gases, solar luminosity, volcanic eruptions, and variations in the Earth's orbit around the Sun.


title: Climate change

text: To determine the human contribution to climate change, known internal climate variability and natural external forcings need to be ruled out. A key approach is to determine unique "fingerprints" for all potential causes, then compare these fingerprints with observed patterns of climate change. For example, solar forcing can be ruled out as a major cause. Its fingerprint would be warming in the entire atmosphere. Yet, only the lower atmosphere has warmed, consistent with greenhouse gas forcing. Attribution of recent climate change shows that the main driver is elevated greenhouse gases, with aerosols having a dampening effect.


title: Climate change

text: Greenhouse gases are transparent to sunlight, and thus allow it to pass through the atmosphere to heat the Earth's surface. The Earth radiates it as heat, and greenhouse gases absorb a portion of it. This absorption slows the rate at which heat escapes into space, trapping heat near the Earth's surface and warming it over time. Before the Industrial Revolution, naturally-occurring amounts of greenhouse gases caused the air near the surface to be about 33 °C warmer than it would have been in their absence. While water vapour (~50%) and clouds (~25%) are the biggest contributors to the greenhouse effect, they increase as a function of temperature and are therefore feedbacks. On the other hand, concentrations of gases such as (~20%), tropospheric ozone, CFCs and nitrous oxide are not temperature-dependent, and are therefore external forcings.


title: Climate change

text: Human activity since the Industrial Revolution, mainly extracting and burning fossil fuels (coal, oil, and natural gas), has increased the amount of greenhouse gases in the atmosphere, resulting in a radiative imbalance. In 2019, the concentrations of and methane had increased by about 48% and 160%, respectively, since 1750. These levels are higher than they have been at any time during the last 2 million years. Concentrations of methane are far higher than they were over the last 800,000 years.


title: Climate change

text: Global anthropogenic greenhouse gas emissions in 2019 were equivalent to 59 billion tonnes of . Of these emissions, 75% was , 18% was methane, 4% was nitrous oxide, and 2% was fluorinated gases. emissions primarily come from burning fossil fuels to provide energy for transport, manufacturing, heating, and electricity. Additional emissions come from deforestation and industrial processes, which include the released by the chemical reactions for making cement, steel, aluminum, and fertiliser. Methane emissions come from livestock, manure, rice cultivation, landfills, wastewater, and coal mining, as well as oil and gas extraction. Nitrous oxide emissions largely come from the microbial decomposition of fertiliser.


title: Climate change

text: Despite the contribution of deforestation to greenhouse gas emissions, the Earth's land surface, particularly its forests, remain a significant carbon sink for . Land-surface sink processes, such as carbon fixation in the soil and photosynthesis, remove about 29% of annual global emissions. The ocean also serves as a significant carbon sink via a two-step process. First, dissolves in the surface water. Afterwards, the ocean's overturning circulation distributes it deep into the ocean's interior, where it accumulates over time as part of the carbon cycle. Over the last two decades, the world's oceans have absorbed 20 to 30% of emitted .


title: Climate change

text: Air pollution, in the form of aerosols, affects the climate on a large scale. Aerosols scatter and absorb solar radiation. From 1961 to 1990, a gradual reduction in the amount of sunlight reaching the Earth's surface was observed. This phenomenon is popularly known as "global dimming", and is attributed to aerosols produced by dust, pollution and combustion of biofuels and fossil fuels. Globally, aerosols have been declining since 1990 due to pollution controls, meaning that they no longer mask greenhouse gas warming as much.


title: Climate change

text: Aerosols also have indirect effects on the Earth's radiation budget. Sulfate aerosols act as cloud condensation nuclei and lead to clouds that have more and smaller cloud droplets. These clouds reflect solar radiation more efficiently than clouds with fewer and larger droplets. They also reduce the growth of raindrops, which makes clouds more reflective to incoming sunlight. Indirect effects of aerosols are the largest uncertainty in radiative forcing.


title: Climate change

text: While aerosols typically limit global warming by reflecting sunlight, black carbon in soot that falls on snow or ice can contribute to global warming. Not only does this increase the absorption of sunlight, it also increases melting and sea-level rise. Limiting new black carbon deposits in the Arctic could reduce global warming by 0.2 °C by 2050.


title: Climate change

text: Humans change the Earth's surface mainly to create more agricultural land. Today, agriculture takes up 34% of Earth's land area, while 26% is forests, and 30% is uninhabitable (glaciers, deserts, etc.). The amount of forested land continues to decrease, which is the main land use change that causes global warming. Deforestation releases contained in trees when they are destroyed, plus it prevents those trees from absorbing more in the future. The main causes of deforestation are: permanent land-use change from forest to agricultural land producing products such as beef and palm oil (27%), logging to produce forestry/forest products (26%), short term shifting cultivation (24%), and wildfires (23%).


title: Climate change

text: The type of vegetation in a region affects the local temperature. It impacts how much of the sunlight gets reflected back into space (albedo), and how much heat is lost by evaporation. For instance, the change from a dark forest to grassland makes the surface lighter, causing it to reflect more sunlight. Deforestation can also affect temperatures by modifying the release of chemical compounds that influence clouds, and by changing wind patterns. In tropic and temperate areas the net effect is to produce significant warming, while at latitudes closer to the poles a gain of albedo (as forest is replaced by snow cover) leads to a cooling effect. Globally, these effects are estimated to have led to a slight cooling, dominated by an increase in surface albedo.


title: Climate change

text: As the Sun is the Earth's primary energy source, changes in incoming sunlight directly affect the climate system. Solar irradiance has been measured directly by satellites, and indirect measurements are available from the early 1600s onwards. There has been no upward trend in the amount of the Sun's energy reaching the Earth.


title: Climate change

text: Explosive volcanic eruptions represent the largest natural forcing over the industrial era. When the eruption is sufficiently strong (with sulfur dioxide reaching the stratosphere), sunlight can be partially blocked for a couple of years. The temperature signal lasts about twice as long. In the industrial era, volcanic activity has had negligible impacts on global temperature trends. Present-day volcanic CO emissions are equivalent to less than 1% of current anthropogenic CO emissions.


title: Climate change

text: Physical climate models are unable to reproduce the rapid warming observed in recent decades when taking into account only variations in solar output and volcanic activity. Further evidence for greenhouse gases causing global warming comes from measurements that show a warming of the lower atmosphere (the troposphere), coupled with a cooling of the upper atmosphere (the stratosphere). If solar variations were responsible for the observed warming, the troposphere and stratosphere would both warm.


title: Climate change

text: The response of the climate system to an initial forcing is modified by feedbacks: increased by "self-reinforcing" or "positive" feedbacks and reduced by "balancing" or "negative" feedbacks. The main reinforcing feedbacks are the water-vapour feedback, the ice–albedo feedback, and the net effect of clouds. The primary balancing mechanism is radiative cooling, as Earth's surface gives off more heat to space in response to rising temperature. In addition to temperature feedbacks, there are feedbacks in the carbon cycle, such as the fertilizing effect of on plant growth. Uncertainty over feedbacks is the major reason why different climate models project different magnitudes of warming for a given amount of emissions.


title: Climate change

text: As air warms, it can hold more moisture. Water vapour, as a potent greenhouse gas, holds heat in the atmosphere. If cloud cover increases, more sunlight will be reflected back into space, cooling the planet. If clouds become higher and thinner, they act as an insulator, reflecting heat from below back downwards and warming the planet. The effect of clouds is the largest source of feedback uncertainty.


title: Climate change

text: Another major feedback is the reduction of snow cover and sea ice in the Arctic, which reduces the reflectivity of the Earth's surface.


title: Climate change

text: More of the Sun's energy is now absorbed in these regions, contributing to amplification of Arctic temperature changes. Arctic amplification is also melting permafrost, which releases methane and into the atmosphere. Climate change can also cause methane releases from wetlands, marine systems, and freshwater systems. Overall, climate feedbacks are expected to become increasingly positive.


title: Climate change

text: Around half of human-caused emissions have been absorbed by land plants and by the oceans. On land, elevated and an extended growing season have stimulated plant growth. Climate change increases droughts and heat waves that inhibit plant growth, which makes it uncertain whether this carbon sink will continue to grow in the future. Soils contain large quantities of carbon and may release some when they heat up. As more and heat are absorbed by the ocean, it acidifies, its circulation changes and phytoplankton takes up less carbon, decreasing the rate at which the ocean absorbs atmospheric carbon. Overall, at higher concentrations the Earth will absorb a reduced fraction of our emissions.


title: Climate change

text: A climate model is a representation of the physical, chemical, and biological processes that affect the climate system. Models also include natural processes like changes in the Earth's orbit, historical changes in the Sun's activity, and volcanic forcing. Models are used to calculate the degree of warming future emissions will cause when accounting for the strength of climate feedbacks, or reproduce and predict the circulation of the oceans, the annual cycle of the seasons, and the flows of carbon between the land surface and the atmosphere.


title: Climate change

text: The physical realism of models is tested by examining their ability to simulate contemporary or past climates. Past models have underestimated the rate of Arctic shrinkage and underestimated the rate of precipitation increase. Sea level rise since 1990 was underestimated in older models, but more recent models agree well with observations. The 2017 United States-published National Climate Assessment notes that "climate models may still be underestimating or missing relevant feedback processes".


title: Climate change

text: A subset of climate models add societal factors to a simple physical climate model. These models simulate how population, economic growth, and energy use affectand interact withthe physical climate. With this information, these models can produce scenarios of future greenhouse gas emissions. This is then used as input for physical climate models and carbon cycle models to predict how atmospheric concentrations of greenhouse gases might change in the future. Depending on the socioeconomic scenario and the mitigation scenario, models produce atmospheric CO concentrations that range widely between 380 and 1400 ppm.


title: Climate change

text: The IPCC Sixth Assessment Report projects that global warming is very likely to reach 1.0 °C to 1.8 °C by the late 21st century under the . In an global warming would reach 2.1 °C to 3.5 °C, and 3.3 °C to 5.7 °C under the . These projections are based on climate models in combination with observations.


title: Climate change

text: The remaining carbon budget is determined by modelling the carbon cycle and the climate sensitivity to greenhouse gases. According to the IPCC, global warming can be kept below 1.5 °C with a two-thirds chance if emissions after 2018 do not exceed 420 or 570 gigatonnes of . This corresponds to 10 to 13 years of current emissions. There are high uncertainties about the budget. For instance, it may be 100 gigatonnes of smaller due to methane release from permafrost and wetlands. However, it is clear that fossil fuel resources are too abundant for shortages to be relied on to limit carbon emissions in the 21st century.


title: Climate change

text: The environmental effects of climate change are broad and far-reaching, affecting oceans, ice, and weather. Changes may occur gradually or rapidly. Evidence for these effects comes from studying climate change in the past, from modelling, and from modern observations. Since the 1950s, droughts and heat waves have appeared simultaneously with increasing frequency. Extremely wet or dry events within the monsoon period have increased in India and East Asia. The rainfall rate and intensity of hurricanes and typhoons is likely increasing, and the geographic range likely expanding poleward in response to climate warming. Frequency of tropical cyclones has not increased as a result of climate change.


title: Climate change

text: Global sea level is rising as a consequence of glacial melt, melt of the ice sheets in Greenland and Antarctica, and thermal expansion. Between 1993 and 2020, the rise increased over time, averaging 3.3 ± 0.3 mm per year. Over the 21st century, the IPCC projects that in a very high emissions scenario the sea level could rise by 61–110 cm. Increased ocean warmth is undermining and threatening to unplug Antarctic glacier outlets, risking a large melt of the ice sheet and the possibility of a 2-meter sea level rise by 2100 under high emissions.


title: Climate change

text: Climate change has led to decades of shrinking and thinning of the Arctic sea ice. While ice-free summers are expected to be rare at 1.5 °C degrees of warming, they are set to occur once every three to ten years at a warming level of 2 °C. Higher atmospheric concentrations have led to changes in ocean chemistry. An increase in dissolved is causing oceans to acidify. In addition, oxygen levels are decreasing as oxygen is less soluble in warmer water. Dead zones in the ocean, regions with very little oxygen, are expanding too.


title: Climate change

text: Greater degrees of global warming increase the risk of passing through ‘tipping points’—thresholds beyond which certain impacts can no longer be avoided even if temperatures are reduced. An example is the collapse of West Antarctic and Greenland ice sheets, where a temperature rise of 1.5 to 2 °C may commit the ice sheets to melt, although the time scale of melt is uncertain and depends on future warming. Some large-scale changes could occur over a short time period, such as a shutdown of certain ocean currents like the Atlantic Meridional Overturning Circulation (AMOC). Tipping points can also include irreversible damage to ecosystems like the Amazon rainforest and coral reefs.


title: Climate change

text: The long-term effects of climate change on oceans include further ice melt, ocean warming, sea level rise, and ocean acidification. On the timescale of centuries to millennia, the magnitude of climate change will be determined primarily by anthropogenic emissions. This is due to 's long atmospheric lifetime. Oceanic uptake is slow enough that ocean acidification will continue for hundreds to thousands of years. These emissions are estimated to have prolonged the current interglacial period by at least 100,000 years. Sea level rise will continue over many centuries, with an estimated rise of after 2000 years.


title: Climate change

text: Recent warming has driven many terrestrial and freshwater species poleward and towards higher altitudes. Higher atmospheric levels and an extended growing season have resulted in global greening. However, heatwaves and drought have reduced ecosystem productivity in some regions. The future balance of these opposing effects is unclear. Climate change has contributed to the expansion of drier climate zones, such as the expansion of deserts in the subtropics. The size and speed of global warming is making abrupt changes in ecosystems more likely. Overall, it is expected that climate change will result in the extinction of many species.


title: Climate change

text: The oceans have heated more slowly than the land, but plants and animals in the ocean have migrated towards the colder poles faster than species on land. Just as on land, heat waves in the ocean occur more frequently due to climate change, harming a wide range of organisms such as corals, kelp, and seabirds. Ocean acidification makes it harder for marine calcifying organisms such as mussels, barnacles and corals to produce shells and skeletons; and heatwaves have bleached coral reefs. Harmful algal blooms enhanced by climate change and eutrophication lower oxygen levels, disrupt food webs and cause great loss of marine life. Coastal ecosystems are under particular stress. Almost half of global wetlands have disappeared due to climate change and other human impacts.


title: Climate change

text: The effects of climate change are impacting humans everywhere in the world. Impacts can now be observed on all continents and ocean regions, with low-latitude, less developed areas facing the greatest risk. Continued warming has potentially “severe, pervasive and irreversible impacts” for people and ecosystems. The risks are unevenly distributed, but are generally greater for disadvantaged people in developing and developed countries.


title: Climate change

text: The WHO has classified climate change as the greatest threat to global health in the 21st century. Extreme weather leads to injury and loss of life, and crop failures to undernutrition. Various infectious diseases are more easily transmitted in a warmer climate, such as dengue fever and malaria. Young children are the most vulnerable to food shortages. Both children and older people are vulnerable to extreme heat. The World Health Organization (WHO) has estimated that between 2030 and 2050, climate change would cause around 250,000 additional deaths per year. They assessed deaths from heat exposure in elderly people, increases in diarrhea, malaria, dengue, coastal flooding, and childhood undernutrition. Over 500,000 more adult deaths are projected yearly by 2050 due to reductions in food availability and quality. By 2100, 50% to 75% of the global population may face climate conditions that are life-threatening due to combined effects of extreme heat and humidity.


title: Climate change

text: Climate change is affecting food security. It has caused reduction in global yields of maize, wheat, and soybeans between 1981 and 2010. Future warming could further reduce global yields of major crops. Crop production will probably be negatively affected in low-latitude countries, while effects at northern latitudes may be positive or negative. Up to an additional 183 million people worldwide, particularly those with lower incomes, are at risk of hunger as a consequence of these impacts. Climate change also impacts fish populations. Globally, less will be available to be fished. Regions dependent on glacier water, regions that are already dry, and small islands have a higher risk of water stress due to climate change.


title: Climate change

text: Economic damages due to climate change may be severe and there is a chance of disastrous consequences. Climate change has likely already increased global economic inequality, and this trend is projected to continue. Most of the severe impacts are expected in sub-Saharan Africa, where most of the local inhabitants are dependent upon natural and agricultural resources, and South-East Asia. The World Bank estimates that climate change could drive over 120 million people into poverty by 2030.


title: Climate change

text: Current inequalities based on wealth and social status have worsened due to climate change. Major difficulties in mitigating, adapting, and recovering to climate shocks are faced by marginalized people who have less control over resources. Indigenous people, who are subsistent on their land and ecosystems, will face endangerment to their wellness and lifestyles due to climate change. An expert elicitation concluded that the role of climate change in armed conflict has been small compared to factors such as socio-economic inequality and state capabilities.


title: Climate change

text: Low-lying islands and coastal communities are threatened by sea level rise, which makes flooding more common. Sometimes, land is permanently lost to the sea. This could lead to statelessness for people in island nations, such as the Maldives and Tuvalu. In some regions, the rise in temperature and humidity may be too severe for humans to adapt to. With worst-case climate change, models project that almost one-third of humanity might live in extremely hot and uninhabitable climates, similar to the current climate found in the Sahara. These factors can drive environmental migration, both within and between countries. More people are expected to be displaced because of sea level rise, extreme weather and conflict from increased competition over natural resources. Climate change may also increase vulnerability, leading to "trapped populations" who are not able to move due to a lack of resources.


title: Climate change

text: Climate change can be mitigated by reducing greenhouse gas emissions and enhancing sinks that absorb greenhouse gases from the atmosphere. In order to limit global warming to less than 1.5 °C global greenhouse gas emissions needs to be net-zero by 2050, or by 2070 with a 2 °C target. This requires far-reaching, systemic changes on an unprecedented scale in energy, land, cities, transport, buildings, and industry. The United Nations Environment Programme estimates that countries need to triple their pledges under the Paris Agreement within the next decade to limit global warming to 2 °C. An even greater level of reduction is required to meet the 1.5 °C goal. With pledges made under the Agreement as of October 2021, global warming would still have a 66% chance of reaching about 2.7 °C (range: 2.2–3.2 °C) by the end of the century.