164-419: Paleoclimatology ( British spelling , palaeoclimatology ) is the scientific study of climates predating the invention of meteorological instruments , when no direct measurement data were available. As instrumental records only span a tiny part of Earth's history , the reconstruction of ancient climate is important to understand natural variation and the evolution of the current climate. Paleoclimatology uses
328-424: A cirque landform (alternatively known as a corrie or as a cwm ) – a typically armchair-shaped geological feature (such as a depression between mountains enclosed by arêtes ) – which collects and compresses through gravity the snow that falls into it. This snow accumulates and the weight of the snow falling above compacts it, forming névé (granular snow). Further crushing of the individual snowflakes and squeezing
492-583: A changing climate most likely evolved in ancient Egypt , Mesopotamia , the Indus Valley and China , where prolonged periods of droughts and floods were experienced. In the seventeenth century, Robert Hooke postulated that fossils of giant turtles found in Dorset could only be explained by a once warmer climate, which he thought could be explained by a shift in Earth's axis. Fossils were, at that time, often explained as
656-473: A climate sensitivity for the rest of the Phanerozoic which was calculated to be similar to today's modern range of values. The difference in global mean temperatures between a fully glacial Earth and an ice free Earth is estimated at 10 °C, though far larger changes would be observed at high latitudes and smaller ones at low latitudes. One requirement for the development of large scale ice sheets seems to be
820-447: A consequence of a biblical flood. Systematic observations of sunspots started by amateur astronomer Heinrich Schwabe in the early 19th century, starting a discussion of the Sun's influence on Earth's climate. The scientific study of paleoclimatology began to take shape in the early 19th century, when discoveries about glaciations and natural changes in Earth's past climate helped to understand
984-686: A consonant followed by an unstressed -re (pronounced /ə(r)/ ). In modern American English, most of these words have the ending -er . The difference is most common for words ending in -bre or -tre : British spellings calibre , centre , fibre , goitre , litre , lustre , manoeuvre , meagre , metre (length) , mitre , nitre , ochre , reconnoitre , sabre , saltpetre , sepulchre , sombre , spectre , theatre (see exceptions ) and titre all have -er in American spelling. In Britain, both -re and -er spellings were common before Johnson's 1755 dictionary
1148-464: A continually relatively warm surface during the complete early temperature record of Earth with the exception of one cold glacial phase about 2.4 billion years ago. In the late Archaean eon, an oxygen-containing atmosphere began to develop, apparently from photosynthesizing cyanobacteria (see Great Oxygenation Event ) which have been found as stromatolite fossils from 2.7 billion years ago. The early basic carbon isotopy ( isotope ratio proportions)
1312-468: A distinctive set of Canadian English spellings is viewed by many Canadians as one of the unique aspects of Canadian culture (especially when compared to the United States). In Australia, -or endings enjoyed some use throughout the 19th century and in the early 20th century. Like Canada, though, most major Australian newspapers have switched from " -or " endings to " -our " endings. The " -our " spelling
1476-490: A emium , and a enigma . In others, it is kept in all varieties: for example, phoenix , and usually subpoena , but Phenix in Virginia . This is especially true of names: Aegean (the sea), Caesar , Oedipus , Phoebe , etc., although "caesarean section" may be spelled as "cesarean section". There is no reduction of Latin -ae plurals (e.g., larv ae ); nor where the digraph <ae>/<oe> does not result from
1640-458: A eon , an a emia , an a esthesia , c a ecum , c a esium , c o eliac , diarrh o ea , encyclop a edia , f a eces , f o etal , gyn a ecology , h a emoglobin , h a emophilia , leuk a emia , o esophagus , o estrogen , orthop a edic , pal a eontology , p a ediatric , p a edophile . Oenology is acceptable in American English but is deemed
1804-463: A glacier is usually assessed by determining the glacier mass balance or observing terminus behavior. Healthy glaciers have large accumulation zones, more than 60% of their area is snow-covered at the end of the melt season, and they have a terminus with a vigorous flow. Following the Little Ice Age 's end around 1850, glaciers around the Earth have retreated substantially . A slight cooling led to
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#17330852291701968-595: A glacier may flow into a body of water, it forms only on land and is distinct from the much thinner sea ice and lake ice that form on the surface of bodies of water. On Earth, 99% of glacial ice is contained within vast ice sheets (also known as "continental glaciers") in the polar regions , but glaciers may be found in mountain ranges on every continent other than the Australian mainland, including Oceania's high-latitude oceanic island countries such as New Zealand . Between latitudes 35°N and 35°S, glaciers occur only in
2132-411: A glacier via moulins . Streams within or beneath a glacier flow in englacial or sub-glacial tunnels. These tunnels sometimes reemerge at the glacier's surface. Most of the important processes controlling glacial motion occur in the ice-bed contact—even though it is only a few meters thick. The bed's temperature, roughness and softness define basal shear stress, which in turn defines whether movement of
2296-408: A kilometer per year. Eventually, the ice will be surging fast enough that it begins to thin, as accumulation cannot keep up with the transport. This thinning will increase the conductive heat loss, slowing the glacier and causing freezing. This freezing will slow the glacier further, often until it is stationary, whence the cycle can begin again. The flow of water under the glacial surface can have
2460-404: A large effect on the motion of the glacier itself. Subglacial lakes contain significant amounts of water, which can move fast: cubic kilometers can be transported between lakes over the course of a couple of years. This motion is thought to occur in two main modes: pipe flow involves liquid water moving through pipe-like conduits, like a sub-glacial river; sheet flow involves motion of water in
2624-460: A lower heat conductance, meaning that the basal temperature is also likely to be higher. Bed temperature tends to vary in a cyclic fashion. A cool bed has a high strength, reducing the speed of the glacier. This increases the rate of accumulation, since newly fallen snow is not transported away. Consequently, the glacier thickens, with three consequences: firstly, the bed is better insulated, allowing greater retention of geothermal heat. Secondly,
2788-527: A minor variant of enology , whereas although archeology and ameba exist in American English, the British versions amoeba and archaeology are more common. The chemical haem (named as a shortening of h a emoglobin ) is spelled heme in American English, to avoid confusion with hem . Canadian English mostly follows American English in this respect, although it is split on gynecology (e.g. Society of Obstetricians and Gynaecologists of Canada vs.
2952-554: A mistaken etymology. The etymologically correct original spelling fetus reflects the Latin original and is the standard spelling in medical journals worldwide; the Oxford English Dictionary notes that "In Latin manuscripts both fētus and foetus are used". The Ancient Greek diphthongs <αι> and <οι> were transliterated into Latin as <ae> and <oe>. The ligatures æ and œ were introduced when
3116-416: A particular area. On a longer time scale, geologists must refer to the sedimentary record for data. On a longer time scale, the rock record may show signs of sea level rise and fall, and features such as "fossilised" sand dunes can be identified. Scientists can get a grasp of long-term climate by studying sedimentary rock going back billions of years. The division of Earth history into separate periods
3280-521: A shift from a reducing atmosphere to an oxidizing atmosphere. O 2 showed major variations until reaching a steady state of more than 15% by the end of the Precambrian. The following time span was the Phanerozoic eon, during which oxygen-breathing metazoan life forms began to appear. The amount of oxygen in the atmosphere has fluctuated over the last 600 million years, reaching a peak of 35% during
3444-660: A suffix for agentive ( reader , user , winner ) and comparative ( louder , nicer ) forms. One outcome is the British distinction of meter for a measuring instrument from metre for the unit of length. However, while " poetic metre " is often spelled as -re , pentameter , hexameter , etc. are always -er . Many other words have -er in British English. These include Germanic words, such as anger , mother , timber and water , and such Romance-derived words as danger , quarter and river . The ending -cre , as in acre , lucre , massacre , and mediocre ,
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#17330852291703608-484: A thin layer. A switch between the two flow conditions may be associated with surging behavior. Indeed, the loss of sub-glacial water supply has been linked with the shut-down of ice movement in the Kamb ice stream. The subglacial motion of water is expressed in the surface topography of ice sheets, which slump down into vacated subglacial lakes. The speed of glacial displacement is partly determined by friction . Friction makes
3772-410: A tremendous impact as the iceberg strikes the water. Tidewater glaciers undergo centuries-long cycles of advance and retreat that are much less affected by climate change than other glaciers. Thermally, a temperate glacier is at a melting point throughout the year, from its surface to its base. The ice of a polar glacier is always below the freezing threshold from the surface to its base, although
3936-404: A variety of proxy methods from Earth and life sciences to obtain data previously preserved within rocks , sediments , boreholes , ice sheets , tree rings , corals , shells , and microfossils . Combined with techniques to date the proxies, the paleoclimate records are used to determine the past states of Earth's atmosphere . The scientific field of paleoclimatology came to maturity in
4100-409: A wide variety of techniques to deduce ancient climates. The techniques used depend on which variable has to be reconstructed (this could be temperature , precipitation , or something else) and how long ago the climate of interest occurred. For instance, the deep marine record, the source of most isotopic data, exists only on oceanic plates, which are eventually subducted ; the oldest remaining material
4264-459: Is 200 million years old. Older sediments are also more prone to corruption by diagenesis . This is due to the millions of years of disruption experienced by the rock formations, such as pressure, tectonic activity, and fluid flowing. These factors often result in a lack of quality or quantity of data, which causes resolution and confidence in the data decrease over time. Specific techniques used to make inferences on ancient climate conditions are
4428-432: Is - /s/ for the noun and - /z/ for the verb). For licence / license or practice / practise , British English also keeps the noun–verb distinction graphically (although phonetically the two words in each pair are homophones with - /s/ pronunciation). On the other hand, American English uses license and practice for both nouns and verbs (with - /s/ pronunciation in both cases too). American English has kept
4592-414: Is a disadvantage to this method. Data of the climate only started being recorded in the mid-1800s. This means that researchers can only utilize 150 years of data. That is not helpful when trying to map the climate of an area 10,000 years ago. This is where more complex methods can be used. Mountain glaciers and the polar ice caps / ice sheets provide much data in paleoclimatology. Ice-coring projects in
4756-456: Is above or at freezing at the interface and is able to slide at this contact. This contrast is thought to a large extent to govern the ability of a glacier to effectively erode its bed , as sliding ice promotes plucking at rock from the surface below. Glaciers which are partly cold-based and partly warm-based are known as polythermal . Glaciers form where the accumulation of snow and ice exceeds ablation . A glacier usually originates from
4920-407: Is affected by factors such as slope, ice thickness, snowfall, longitudinal confinement, basal temperature, meltwater production, and bed hardness. A few glaciers have periods of very rapid advancement called surges . These glaciers exhibit normal movement until suddenly they accelerate, then return to their previous movement state. These surges may be caused by the failure of the underlying bedrock,
5084-440: Is analyzing how the varying concentrations of CO2 affect the overall climate. This is done by using various proxies to estimate past greenhouse gas concentrations and compare those to that of the present day. Researchers are then able to assess their role in progression of climate change throughout Earth’s history. The Earth's climate system involves the atmosphere , biosphere , cryosphere , hydrosphere , and lithosphere , and
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5248-411: Is because these peaks are located near or in the hyperarid Atacama Desert . Glaciers erode terrain through two principal processes: plucking and abrasion . As glaciers flow over bedrock, they soften and lift blocks of rock into the ice. This process, called plucking, is caused by subglacial water that penetrates fractures in the bedrock and subsequently freezes and expands. This expansion causes
5412-406: Is by basal sliding, where meltwater forms between the ice and the bed itself. Whether a bed is hard or soft depends on the porosity and pore pressure; higher porosity decreases the sediment strength (thus increases the shear stress τ B ). Porosity may vary through a range of methods. Bed softness may vary in space or time, and changes dramatically from glacier to glacier. An important factor
5576-434: Is called glaciology . Glaciers are important components of the global cryosphere . Glaciers are categorized by their morphology, thermal characteristics, and behavior. Alpine glaciers form on the crests and slopes of mountains. A glacier that fills a valley is called a valley glacier , or alternatively, an alpine glacier or mountain glacier . A large body of glacial ice astride a mountain, mountain range, or volcano
5740-416: Is called rock flour and is made up of rock grains between 0.002 and 0.00625 mm in size. Abrasion leads to steeper valley walls and mountain slopes in alpine settings, which can cause avalanches and rock slides, which add even more material to the glacier. Glacial abrasion is commonly characterized by glacial striations . Glaciers produce these when they contain large boulders that carve long scratches in
5904-399: Is dropped for other derivations, for example, central , fibrous , spectral . However, the existence of related words without e before the r is not proof for the existence of an -re British spelling: for example, entry and entrance come from enter , which has not been spelled entre for centuries. The difference relates only to root words; -er rather than -re is universal as
6068-490: Is generally preferred over oe and often over ae , but oe and ae are sometimes found in academic and scientific writing as well as government publications (for example, the fee schedule of the Ontario Health Insurance Plan ) and some words such as palaeontology or aeon . In Australia, it can go either way, depending on the word: for instance, medieval is spelled with the e rather than ae , following
6232-414: Is generally reflected by a greater or lesser thickness in growth rings. Different species however, respond to changes in climatic variables in different ways. A tree-ring record is established by compiling information from many living trees in a specific area. This is done by comparing the number, thickness, ring boundaries, and pattern matching of tree growth rings. The differences in thickness displayed in
6396-628: Is higher, and the mountains above 5,000 m (16,400 ft) usually have permanent snow. Even at high latitudes, glacier formation is not inevitable. Areas of the Arctic , such as Banks Island , and the McMurdo Dry Valleys in Antarctica are considered polar deserts where glaciers cannot form because they receive little snowfall despite the bitter cold. Cold air, unlike warm air, is unable to transport much water vapor. Even during glacial periods of
6560-586: Is known as the Pacific Parlour car, not Pacific Parlor . Proper names such as Pearl Harbor or Sydney Harbour are usually spelled according to their native-variety spelling vocabulary. The name of the herb savory is spelled thus everywhere, although the related adjective savo(u)ry , like savo(u)r , has a u in the UK. Honor (the name) and arbor (the tool) have -or in Britain, as mentioned above, as does
6724-521: Is largely based on visible changes in sedimentary rock layers that demarcate major changes in conditions. Often, they include major shifts in climate. Coral “rings'' share similar evidence of growth to that of trees, and thus can be dated in similar ways. A primary difference is their environments and the conditions within those that they respond to. Examples of these conditions for coral include water temperature, freshwater influx, changes in pH, and wave disturbances. From there, specialized equipment, such as
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6888-595: Is not replenished anymore and starts decaying. The proportion of 'normal' carbon and Carbon-14 gives information of how long the plant material has not been in contact with the atmosphere. Knowledge of precise climatic events decreases as the record goes back in time, but some notable climate events are known: The first atmosphere would have consisted of gases in the solar nebula , primarily hydrogen . In addition, there would probably have been simple hydrides such as those now found in gas giants like Jupiter and Saturn , notably water vapor, methane , and ammonia . As
7052-403: Is of limited use to study recent ( Quaternary , Holocene ) large climate changes since there are seldom discernible in the geomorphological record. The field of geochronology has scientists working on determining how old certain proxies are. For recent proxy archives of tree rings and corals the individual year rings can be counted, and an exact year can be determined. Radiometric dating uses
7216-593: Is sometimes used. The ratio between -ise and -ize stood at 3:2 in the British National Corpus up to 2002. The spelling -ise is more commonly used in UK mass media and newspapers, including The Times (which switched conventions in 1992), The Daily Telegraph , The Economist and the BBC . The Government of the United Kingdom additionally uses -ise , stating "do not use Americanisms" justifying that
7380-714: Is standard worldwide and complection is rare. However, the adjective complected (as in "dark-complected"), although sometimes proscribed, is on equal ground in the U.S. with complexioned. It is not used in this way in the UK, although there exists a rare alternative meaning of complicated . In some cases, words with "old-fashioned" spellings are retained widely in the U.S. for historical reasons (cf. connexionalism ). Many words, especially medical words, that are written with ae/æ or oe/œ in British English are written with just an e in American English. The sounds in question are /iː/ or /ɛ/ (or, unstressed, /i/ , /ɪ/ or /ə/ ). Examples (with non-American letter in bold ):
7544-528: Is taught in schools nationwide as part of the Australian curriculum. The most notable countrywide use of the -or ending is for one of the country's major political parties, the Australian Labor Party , which was originally called "the Australian Labour Party" (name adopted in 1908), but was frequently referred to as both "Labour" and "Labor". The "Labor" was adopted from 1912 onward due to
7708-897: Is termed an ice cap or ice field . Ice caps have an area less than 50,000 km (19,000 sq mi) by definition. Glacial bodies larger than 50,000 km (19,000 sq mi) are called ice sheets or continental glaciers . Several kilometers deep, they obscure the underlying topography. Only nunataks protrude from their surfaces. The only extant ice sheets are the two that cover most of Antarctica and Greenland. They contain vast quantities of freshwater, enough that if both melted, global sea levels would rise by over 70 m (230 ft). Portions of an ice sheet or cap that extend into water are called ice shelves ; they tend to be thin with limited slopes and reduced velocities. Narrow, fast-moving sections of an ice sheet are called ice streams . In Antarctica, many ice streams drain into large ice shelves . Some drain directly into
7872-413: Is the region where there is a net loss in glacier mass. The upper part of a glacier, where accumulation exceeds ablation, is called the accumulation zone . The equilibrium line separates the ablation zone and the accumulation zone; it is the contour where the amount of new snow gained by accumulation is equal to the amount of ice lost through ablation. In general, the accumulation zone accounts for 60–70% of
8036-402: Is the underlying geology; glacial speeds tend to differ more when they change bedrock than when the gradient changes. Further, bed roughness can also act to slow glacial motion. The roughness of the bed is a measure of how many boulders and obstacles protrude into the overlying ice. Ice flows around these obstacles by melting under the high pressure on their stoss side ; the resultant meltwater
8200-530: Is the usual form of the male given name, as a surname both the spellings Peter and Petre (the latter notably borne by a British lord ) are found. For British accoutre , the American practice varies: the Merriam-Webster Dictionary prefers the -re spelling, but The American Heritage Dictionary of the English Language prefers the -er spelling. More recent French loanwords keep
8364-552: Is then forced into the cavity arising in their lee side , where it re-freezes. As well as affecting the sediment stress, fluid pressure (p w ) can affect the friction between the glacier and the bed. High fluid pressure provides a buoyancy force upwards on the glacier, reducing the friction at its base. The fluid pressure is compared to the ice overburden pressure, p i , given by ρgh. Under fast-flowing ice streams, these two pressures will be approximately equal, with an effective pressure (p i – p w ) of 30 kPa; i.e. all of
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#17330852291708528-489: Is to study relict landforms to infer ancient climates. Being often concerned about past climates climatic geomorphology is considered sometimes to be a theme of historical geology . Evidence of these past climates to be studied can be found in the landforms they leave behind. Examples of these landforms are those such as glacial landforms (moraines, striations), desert features (dunes, desert pavements), and coastal landforms (marine terraces, beach ridges). Climatic geomorphology
8692-481: Is used in both British and American English to show that the c is pronounced /k/ rather than /s/ . The spellings euchre and ogre are also the same in both British and American English. Fire and its associated adjective fiery are the same in both British and American English, although the noun was spelled fier in Old and Middle English. Theater is the prevailing American spelling used to refer to both
8856-578: The -meter suffix (from Ancient Greek - μέτρον métron , via French -mètre ) normally had the -re spelling from earliest use in English but were superseded by -er . Examples include thermometer and barometer . The e preceding the r is kept in American-inflected forms of nouns and verbs, for example, fibers , reconnoitered , centering , which are fibres , reconnoitred , and centring respectively in British English. According to
9020-560: The -ise form as an alternative. Publications by Oxford University Press (OUP)—such as Henry Watson Fowler 's A Dictionary of Modern English Usage , Hart's Rules , and The Oxford Guide to English Usage —also recommend -ize . However, Robert Allan's Pocket Fowler's Modern English Usage considers either spelling to be acceptable anywhere but the U.S. American spelling avoids -ise endings in words like organize , realize and recognize . British spelling mostly uses -ise ( organise , realise , recognise ), though -ize
9184-846: The -ise form is preferred in Australian English at a ratio of about 3:1 according to the Macquarie Dictionary . Glacier A glacier ( US : / ˈ ɡ l eɪ ʃ ər / ; UK : / ˈ ɡ l æ s i ər , ˈ ɡ l eɪ s i ər / ) is a persistent body of dense ice that is constantly moving downhill under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries . It acquires distinguishing features, such as crevasses and seracs , as it slowly flows and deforms under stresses induced by its weight. As it moves, it abrades rock and debris from its substrate to create landforms such as cirques , moraines , or fjords . Although
9348-450: The -re spelling in American English. These are not exceptions when a French-style pronunciation is used ( /rə/ rather than /ə(r)/ ), as with double entendre , genre and oeuvre . However, the unstressed /ə(r)/ pronunciation of an -er ending is used more (or less) often with some words, including cadre , macabre , maître d' , Notre Dame , piastre , and timbre . The -re endings are mostly standard throughout
9512-952: The Andes , the Himalayas , the Rocky Mountains , the Caucasus , Scandinavian Mountains , and the Alps . Snezhnika glacier in Pirin Mountain, Bulgaria with a latitude of 41°46′09″ N is the southernmost glacial mass in Europe. Mainland Australia currently contains no glaciers, although a small glacier on Mount Kosciuszko was present in the last glacial period . In New Guinea, small, rapidly diminishing, glaciers are located on Puncak Jaya . Africa has glaciers on Mount Kilimanjaro in Tanzania, on Mount Kenya , and in
9676-483: The Canadian Medical Association 's Canadian specialty profile of Obstetrics/gynecology ). Pediatrician is preferred roughly 10 to 1 over paediatrician , while foetal and oestrogen are similarly uncommon. Words that can be spelled either way in American English include a esthetics and arch a eology (which usually prevail over esthetics and archeology ), as well as pal a estra , for which
9840-518: The Carboniferous period, significantly higher than today's 21%. Two main processes govern changes in the atmosphere: plants use carbon dioxide from the atmosphere , releasing oxygen and the breakdown of pyrite and volcanic eruptions release sulfur into the atmosphere, which oxidizes and hence reduces the amount of oxygen in the atmosphere. However, volcanic eruptions also release carbon dioxide, which plants can convert to oxygen. The exact cause of
10004-636: The Faroe and Crozet Islands were completely glaciated. The permanent snow cover necessary for glacier formation is affected by factors such as the degree of slope on the land, amount of snowfall and the winds. Glaciers can be found in all latitudes except from 20° to 27° north and south of the equator where the presence of the descending limb of the Hadley circulation lowers precipitation so much that with high insolation snow lines reach above 6,500 m (21,330 ft). Between 19˚N and 19˚S, however, precipitation
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#173308522917010168-464: The Great Oxygenation Event , and its appearance is indicated by the end of the banded iron formations . Until then, any oxygen produced by photosynthesis was consumed by oxidation of reduced materials, notably iron. Molecules of free oxygen did not start to accumulate in the atmosphere until the rate of production of oxygen began to exceed the availability of reducing materials. That point was
10332-750: The Himalayas , Andes , and a few high mountains in East Africa, Mexico, New Guinea and on Zard-Kuh in Iran. With more than 7,000 known glaciers, Pakistan has more glacial ice than any other country outside the polar regions. Glaciers cover about 10% of Earth's land surface. Continental glaciers cover nearly 13 million km (5 million sq mi) or about 98% of Antarctica 's 13.2 million km (5.1 million sq mi), with an average thickness of ice 2,100 m (7,000 ft). Greenland and Patagonia also have huge expanses of continental glaciers. The volume of glaciers, not including
10496-534: The OED , centring is a "word ... of 3 syllables (in careful pronunciation)" (i.e., /ˈsɛntərɪŋ/ ), yet there is no vowel in the spelling corresponding to the second syllable ( /ə/ ). The OED third edition (revised entry of June 2016) allows either two or three syllables. On the Oxford Dictionaries Online website, the three-syllable version is listed only as the American pronunciation of centering . The e
10660-551: The Quaternary , Manchuria , lowland Siberia , and central and northern Alaska , though extraordinarily cold, had such light snowfall that glaciers could not form. In addition to the dry, unglaciated polar regions, some mountains and volcanoes in Bolivia, Chile and Argentina are high (4,500 to 6,900 m or 14,800 to 22,600 ft) and cold, but the relative lack of precipitation prevents snow from accumulating into glaciers. This
10824-521: The Rwenzori Mountains . Oceanic islands with glaciers include Iceland, several of the islands off the coast of Norway including Svalbard and Jan Mayen to the far north, New Zealand and the subantarctic islands of Marion , Heard , Grande Terre (Kerguelen) and Bouvet . During glacial periods of the Quaternary, Taiwan , Hawaii on Mauna Kea and Tenerife also had large alpine glaciers, while
10988-542: The Simplified Spelling Board in the early 20th century, but most were not adopted. In Britain, the influence of those who preferred the Norman (or Anglo-French ) spellings of words proved to be decisive. Later spelling adjustments in the United Kingdom had little effect on today's American spellings and vice versa. For the most part, the spelling systems of most Commonwealth countries and Ireland closely resemble
11152-508: The carbon cycle . The weathering sequesters CO 2 , by the reaction of minerals with chemicals (especially silicate weathering with CO 2 ) and thereby removing CO 2 from the atmosphere and reducing the radiative forcing. The opposite effect is volcanism , responsible for the natural greenhouse effect , by emitting CO 2 into the atmosphere, thus affecting glaciation (Ice Age) cycles. Jim Hansen suggested that humans emit CO 2 10,000 times faster than natural processes have done in
11316-534: The greenhouse effect . It was only in the 20th century that paleoclimatology became a unified scientific field. Before, different aspects of Earth's climate history were studied by a variety of disciplines. At the end of the 20th century, the empirical research into Earth's ancient climates started to be combined with computer models of increasing complexity. A new objective also developed in this period: finding ancient analog climates that could provide information about current climate change . Paleoclimatologists employ
11480-531: The theatre spelling. (The word "theater" in American English is a place where both stage performances and screenings of films take place, but in British English a "theatre" is where stage performances take place but not film screenings – these take place in a cinema, or "picture theatre" in Australia.) In the United States, the spelling theatre is sometimes used when referring to the art form of theatre, while
11644-515: The u has since been dropped: ambassadour , emperour , errour , governour , horrour , inferiour , mirrour , perturbatour , superiour , tenour , terrour , tremour . Johnson, unlike Webster, was not an advocate of spelling reform, but chose the spelling best derived, as he saw it, from among the variations in his sources. He preferred French over Latin spellings because, as he put it, "the French generally supplied us". English speakers who moved to
11808-435: The u : In American usage, derivatives and inflected forms are built by simply adding the suffix in all cases (for example, favorite , savory etc.) since the u is absent to begin with. American usage, in most cases, keeps the u in the word glamour , which comes from Scots , not Latin or French. Glamor is sometimes used in imitation of the spelling reform of other -our words to -or . Nevertheless,
11972-649: The ‑our/or group do not have a Latin counterpart that ends in ‑or ; for example, armo(u)r , behavio(u)r , harbo(u)r , neighbo(u)r ; also arbo(u)r , meaning "shelter", though senses "tree" and "tool" are always arbor , a false cognate of the other word. The word arbor would be more accurately spelled arber or arbre in the US and the UK, respectively, the latter of which is the French word for "tree". Some 16th- and early 17th-century British scholars indeed insisted that ‑or be used for words from Latin (e.g., color ) and ‑our for French loans; however, in many cases,
12136-441: The 17th and 18th centuries, whereas there are thousands of examples of their -our counterparts. One notable exception is honor . Honor and honour were equally frequent in Britain until the 17th century; honor only exists in the UK now as the spelling of Honor Oak , a district of London, and of the occasional given name Honor . In derivatives and inflected forms of the -our/or words, British usage depends on
12300-593: The 1970s, but had by then been overtaken by connection in regular usage (for example, in more popular newspapers). Connexion (and its derivatives connexional and connexionalism ) is still in use by the Methodist Church of Great Britain to refer to the whole church as opposed to its constituent districts, circuits and local churches, whereas the US-majority United Methodist Church uses Connection . Complexion (which comes from complex )
12464-448: The 1990s and 2000s. In a study using data from January 1993 through October 2005, more events were detected every year since 2002, and twice as many events were recorded in 2005 as there were in any other year. Ogives or Forbes bands are alternating wave crests and valleys that appear as dark and light bands of ice on glacier surfaces. They are linked to seasonal motion of glaciers; the width of one dark and one light band generally equals
12628-590: The 20th century. Notable periods studied by paleoclimatologists include the frequent glaciations that Earth has undergone, rapid cooling events like the Younger Dryas , and the rapid warming during the Paleocene–Eocene Thermal Maximum . Studies of past changes in the environment and biodiversity often reflect on the current situation, specifically the impact of climate on mass extinctions and biotic recovery and current global warming . Notions of
12792-500: The Advanced Very High Resolution Radiometer (AVHRR) instrument, can be used to derive the sea surface temperature and water salinity from the past few centuries. The δO of coralline red algae provides a useful proxy of the combined sea surface temperature and sea surface salinity at high latitudes and the tropics, where many traditional techniques are limited. Within climatic geomorphology , one approach
12956-704: The American National Theatre was referred to by The New York Times as the "American National Theater ", but the organization uses "re" in the spelling of its name. The John F. Kennedy Center for the Performing Arts in Washington, D.C. has the more common American spelling theater in its references to the Eisenhower Theater, part of the Kennedy Center. Some cinemas outside New York also use
13120-480: The American and British varieties of English. However, English-language spelling reform has rarely been adopted otherwise. As a result, modern English orthography varies only minimally between countries and is far from phonemic in any country. In the early 18th century, English spelling was inconsistent. These differences became noticeable after the publication of influential dictionaries . Today's British English spellings mostly follow Johnson's A Dictionary of
13284-413: The American usage along with numerous other words such as eon or fetus , while other words such as oestrogen or paediatrician are spelled the British way. The Macquarie Dictionary also notes a growing tendency towards replacing ae and oe with e worldwide and with the exception of manoeuvre, all British or American spellings are acceptable variants. Elsewhere, the British usage prevails, but
13448-581: The Anglo-French spelling for defense and offense , which are defence and offence in British English. Likewise, there are the American pretense and British pretence ; but derivatives such as defensive , offensive , and pretension are always thus spelled in both systems. Australian and Canadian usages generally follow British usage. The spelling connexion is now rare in everyday British usage, its use lessening as knowledge of Latin attenuates, and it has almost never been used in
13612-440: The British system. In Canada, the spelling system can be said to follow both British and American forms, and Canadians are somewhat more tolerant of foreign spellings when compared with other English-speaking nationalities. Australian English mostly follows British spelling norms but has strayed slightly, with some American spellings incorporated as standard. New Zealand English is almost identical to British spelling, except in
13776-637: The British usage of -our . This coincided with a renewed interest in Canadian English, and the release of the updated Gage Canadian Dictionary in 1997 and the first Canadian Oxford Dictionary in 1998. Historically, most libraries and educational institutions in Canada have supported the use of the Oxford English Dictionary rather than the American Webster's Dictionary . Today, the use of
13940-465: The Commonwealth. The -er spellings are recognized as minor variants in Canada, partly due to United States influence. They are sometimes used in proper names (such as Toronto's controversially named Centerpoint Mall ). For advice / advise and device / devise , American English and British English both keep the noun–verb distinction both graphically and phonetically (where the pronunciation
14104-524: The Earth's current oxygen level. At the end of the Proterozoic, there is evidence of global glaciation events of varying severity causing a ' Snowball Earth '. Snowball Earth is supported by different indicators such as, glacial deposits, significant continental erosion called the Great Unconformity , and sedimentary rocks called cap carbonates that form after a deglaciation episode. Major drivers for
14268-428: The Earth's surface. Dependent on the radiative balance of incoming and outgoing energy, the Earth either warms up or cools down. Earth radiative balance originates from changes in solar insolation and the concentrations of greenhouse gases and aerosols . Climate change may be due to internal processes in Earth sphere's and/or following external forcings. One example of a way this can be applied to study climatology
14432-477: The Earth’s climate system. These estimates include the evidence for systems such as long term climate variability (eccentricity, obliquity precession), feedback mechanisms (Ice-Albedo Effect), and anthropogenic influence. Examples: On timescales of millions of years, the uplift of mountain ranges and subsequent weathering processes of rocks and soils and the subduction of tectonic plates , are an important part of
14596-512: The English Language (1755), while many American English spellings follow Webster's An American Dictionary of the English Language ("ADEL", "Webster's Dictionary", 1828). Webster was a proponent of English spelling reform for reasons both philological and nationalistic. In A Companion to the American Revolution (2008), John Algeo notes: "it is often assumed that characteristically American spellings were invented by Noah Webster. He
14760-482: The Greek-style ligature as, for example, in maelstrom or toe ; the same is true for the British form aeroplane (compare other aero- words such as aerosol ) . The now chiefly North American airplane is not a respelling but a recoining, modelled after airship and aircraft . The word airplane dates from 1907, at which time the prefix aero- was trisyllabic, often written aëro- . In Canada, e
14924-504: The Precambrian climate is difficult for various reasons including the low number of reliable indicators and a, generally, not well-preserved or extensive fossil record (especially when compared to the Phanerozoic eon). Despite these issues, there is evidence for a number of major climate events throughout the history of the Precambrian: The Great Oxygenation Event , which started around 2.3 Ga ago (the beginning of
15088-608: The Proterozoic) is indicated by biomarkers which demonstrate the appearance of photosynthetic organisms. Due to the high levels of oxygen in the atmosphere from the GOE, CH 4 levels fell rapidly cooling the atmosphere causing the Huronian glaciation. For about 1 Ga after the glaciation (2-0.8 Ga ago), the Earth likely experienced warmer temperatures indicated by microfossils of photosynthetic eukaryotes, and oxygen levels between 5 and 18% of
15252-515: The US: the more common connection has become the standard worldwide. According to the Oxford English Dictionary , the older spelling is more etymologically conservative, since the original Latin word had -xio- . The American usage comes from Webster , who abandoned -xion and preferred -ction . Connexion was still the house style of The Times of London until the 1980s and was still used by Post Office Telecommunications for its telephone services in
15416-491: The United States took these preferences with them. In the early 20th century, H. L. Mencken notes that " honor appears in the 1776 Declaration of Independence , but it seems to have been put there rather by accident than by design". In Jefferson 's original draft it is spelled "honour". In Britain, examples of behavior , color , flavor , harbor , and neighbor rarely appear in Old Bailey court records from
15580-423: The United States use Centre in their names. Examples include the villages of Newton Centre and Rockville Centre , the city of Centreville , Centre County and Centre College . Sometimes, these places were named before spelling changes but more often the spelling serves as an affectation. Proper names are usually spelled according to their native-variety spelling vocabulary; so, for instance, although Peter
15744-476: The United States. A "British standard" began to emerge following the 1755 publication of Samuel Johnson 's A Dictionary of the English Language , and an "American standard" started following the work of Noah Webster and, in particular, his An American Dictionary of the English Language , first published in 1828. Webster's efforts at spelling reform were effective in his native country, resulting in certain well-known patterns of spelling differences between
15908-601: The adjective glamorous often drops the first "u". Saviour is a somewhat common variant of savior in the US. The British spelling is very common for honour (and favour ) in the formal language of wedding invitations in the US. The name of the Space Shuttle Endeavour has a u in it because the spacecraft was named after British Captain James Cook 's ship, HMS Endeavour . The (former) special car on Amtrak 's Coast Starlight train
16072-412: The advance of many alpine glaciers between 1950 and 1985, but since 1985 glacier retreat and mass loss has become larger and increasingly ubiquitous. Glaciers move downhill by the force of gravity and the internal deformation of ice. At the molecular level, ice consists of stacked layers of molecules with relatively weak bonds between layers. When the amount of strain (deformation) is proportional to
16236-520: The air from the snow turns it into "glacial ice". This glacial ice will fill the cirque until it "overflows" through a geological weakness or vacancy, such as a gap between two mountains. When the mass of snow and ice reaches sufficient thickness, it begins to move by a combination of surface slope, gravity, and pressure. On steeper slopes, this can occur with as little as 15 m (49 ft) of snow-ice. In temperate glaciers, snow repeatedly freezes and thaws, changing into granular ice called firn . Under
16400-430: The amount of melting at surface of the glacier, the faster the ice will flow. Basal sliding is dominant in temperate or warm-based glaciers. The presence of basal meltwater depends on both bed temperature and other factors. For instance, the melting point of water decreases under pressure, meaning that water melts at a lower temperature under thicker glaciers. This acts as a "double whammy", because thicker glaciers have
16564-713: The annual movement of the glacier. Ogives are formed when ice from an icefall is severely broken up, increasing ablation surface area during summer. This creates a swale and space for snow accumulation in the winter, which in turn creates a ridge. Sometimes ogives consist only of undulations or color bands and are described as wave ogives or band ogives. Glaciers are present on every continent and in approximately fifty countries, excluding those (Australia, South Africa) that have glaciers only on distant subantarctic island territories. Extensive glaciers are found in Antarctica, Argentina, Chile, Canada, Pakistan, Alaska, Greenland and Iceland. Mountain glaciers are widespread, especially in
16728-612: The arrangement of continental land masses at or near the poles. The constant rearrangement of continents by plate tectonics can also shape long-term climate evolution. However, the presence or absence of land masses at the poles is not sufficient to guarantee glaciations or exclude polar ice caps. Evidence exists of past warm periods in Earth's climate when polar land masses similar to Antarctica were home to deciduous forests rather than ice sheets. The relatively warm local minimum between Jurassic and Cretaceous goes along with an increase of subduction and mid-ocean ridge volcanism due to
16892-494: The auspices of International Partnerships in Ice Core Sciences (IPICS), defined a priority project to obtain the oldest possible ice core record from Antarctica, an ice core record reaching back to or towards 1.5 million years ago. Climatic information can be obtained through an understanding of changes in tree growth. Generally, trees respond to changes in climatic variables by speeding up or slowing down growth, which in turn
17056-432: The bedrock has frequent fractures on the surface, glacial erosion rates tend to increase as plucking is the main erosive force on the surface; when the bedrock has wide gaps between sporadic fractures, however, abrasion tends to be the dominant erosive form and glacial erosion rates become slow. Glaciers in lower latitudes tend to be much more erosive than glaciers in higher latitudes, because they have more meltwater reaching
17220-445: The bedrock. By mapping the direction of the striations, researchers can determine the direction of the glacier's movement. Similar to striations are chatter marks , lines of crescent-shape depressions in the rock underlying a glacier. They are formed by abrasion when boulders in the glacier are repeatedly caught and released as they are dragged along the bedrock. The rate of glacier erosion varies. Six factors control erosion rate: When
17384-600: The breakup of the Pangea supercontinent . Superimposed on the long-term evolution between hot and cold climates have been many short-term fluctuations in climate similar to, and sometimes more severe than, the varying glacial and interglacial states of the present ice age . Some of the most severe fluctuations, such as the Paleocene-Eocene Thermal Maximum , may be related to rapid climate changes due to sudden collapses of natural methane clathrate reservoirs in
17548-507: The building itself, as noted above, generally is spelled theater . For example, the University of Wisconsin–Madison has a "Department of Theatre and Drama", which offers courses that lead to the "Bachelor of Arts in Theatre ", and whose professed aim is "to prepare our graduate students for successful 21st Century careers in the theatre both as practitioners and scholars". Some placenames in
17712-526: The climate and how they affected the surrounding species. Older intact wood that has escaped decay can extend the time covered by the record by matching the ring depth changes to contemporary specimens. By using that method, some areas have tree-ring records dating back a few thousand years. Older wood not connected to a contemporary record can be dated generally with radiocarbon techniques. A tree-ring record can be used to produce information regarding precipitation, temperature, hydrology, and fire corresponding to
17876-426: The close historic, economic, and cultural relationship with the United States, -or endings are also sometimes used. Throughout the late 19th and early to mid-20th century, most Canadian newspapers chose to use the American usage of -or endings, originally to save time and money in the era of manual movable type . However, in the 1990s, the majority of Canadian newspapers officially updated their spelling policies to
18040-559: The created ice's density. The word glacier is a loanword from French and goes back, via Franco-Provençal , to the Vulgar Latin glaciārium , derived from the Late Latin glacia , and ultimately Latin glaciēs , meaning "ice". The processes and features caused by or related to glaciers are referred to as glacial. The process of glacier establishment, growth and flow is called glaciation . The corresponding area of study
18204-467: The deep profile of fjords , which can reach a kilometer in depth as ice is topographically steered into them. The extension of fjords inland increases the rate of ice sheet thinning since they are the principal conduits for draining ice sheets. It also makes the ice sheets more sensitive to changes in climate and the ocean. Although evidence in favor of glacial flow was known by the early 19th century, other theories of glacial motion were advanced, such as
18368-483: The deformation to become a plastic flow rather than elastic. Then, the glacier will begin to deform under its own weight and flow across the landscape. According to the Glen–Nye flow law , the relationship between stress and strain, and thus the rate of internal flow, can be modeled as follows: where: The lowest velocities are near the base of the glacier and along valley sides where friction acts against flow, causing
18532-403: The dramatic arts and buildings where stage performances and screenings of films take place (i.e., " movie theaters "); for example, a national newspaper such as The New York Times would use theater in its entertainment section. However, the spelling theatre appears in the names of many New York City theatres on Broadway (cf. Broadway theatre) and elsewhere in the United States. In 2003,
18696-703: The ending became ‑our to match the later Old French spelling. The ‑our ending was used not only in new English borrowings, but was also applied to the earlier borrowings that had used ‑or . However, ‑or was still sometimes found. The first three folios of Shakespeare 's plays used both spellings before they were standardised to ‑our in the Fourth Folio of 1685. After the Renaissance , new borrowings from Latin were taken up with their original ‑or ending, and many words once ending in ‑our (for example, chancellour and governour ) reverted to ‑or . A few words of
18860-418: The essentially correct explanation in the 1840s, although it was several decades before it was fully accepted. The top 50 m (160 ft) of a glacier are rigid because they are under low pressure . This upper section is known as the fracture zone and moves mostly as a single unit over the plastic-flowing lower section. When a glacier moves through irregular terrain, cracks called crevasses develop in
19024-406: The etymology was not clear, and therefore some scholars advocated ‑or only and others ‑our only. Webster's 1828 dictionary had only -or and is given much of the credit for the adoption of this form in the United States. By contrast, Johnson's 1755 (pre-U.S. independence and establishment) dictionary used -our for all words still so spelled in Britain (like colour ), but also for words where
19188-475: The fracture zone. Crevasses form because of differences in glacier velocity. If two rigid sections of a glacier move at different speeds or directions, shear forces cause them to break apart, opening a crevasse. Crevasses are seldom more than 46 m (150 ft) deep but, in some cases, can be at least 300 m (1,000 ft) deep. Beneath this point, the plasticity of the ice prevents the formation of cracks. Intersecting crevasses can create isolated peaks in
19352-453: The glacier to melt, creating a water source that is especially important for plants, animals and human uses when other sources may be scant. However, within high-altitude and Antarctic environments, the seasonal temperature difference is often not sufficient to release meltwater. Since glacial mass is affected by long-term climatic changes, e.g., precipitation , mean temperature , and cloud cover , glacial mass changes are considered among
19516-428: The glacier will be accommodated by motion in the sediments, or if it'll be able to slide. A soft bed, with high porosity and low pore fluid pressure, allows the glacier to move by sediment sliding: the base of the glacier may even remain frozen to the bed, where the underlying sediment slips underneath it like a tube of toothpaste. A hard bed cannot deform in this way; therefore the only way for hard-based glaciers to move
19680-504: The glacier's surface area, more if the glacier calves icebergs. Ice in the accumulation zone is deep enough to exert a downward force that erodes underlying rock. After a glacier melts, it often leaves behind a bowl- or amphitheater-shaped depression that ranges in size from large basins like the Great Lakes to smaller mountain depressions known as cirques . The accumulation zone can be subdivided based on its melt conditions. The health of
19844-411: The growth rings in trees can often indicate the quality of conditions in the environment, and the fitness of the tree species evaluated. Different species of trees will display different growth responses to the changes in the climate. An evaluation of multiple trees within the same species, along with one of trees in different species, will allow for a more accurate analysis of the changing variables within
20008-614: The ice at the bottom of the glacier move more slowly than ice at the top. In alpine glaciers, friction is also generated at the valley's sidewalls, which slows the edges relative to the center. Mean glacial speed varies greatly but is typically around 1 m (3 ft) per day. There may be no motion in stagnant areas; for example, in parts of Alaska, trees can establish themselves on surface sediment deposits. In other cases, glaciers can move as fast as 20–30 m (70–100 ft) per day, such as in Greenland's Jakobshavn Isbræ . Glacial speed
20172-624: The ice caps of Greenland and Antarctica have yielded data going back several hundred thousand years, over 800,000 years in the case of the EPICA project. A multinational consortium, the European Project for Ice Coring in Antarctica (EPICA), has drilled an ice core in Dome C on the East Antarctic ice sheet and retrieved ice from roughly 800,000 years ago. The international ice core community has, under
20336-420: The ice sheets of Antarctica and Greenland, has been estimated at 170,000 km . Glacial ice is the largest reservoir of fresh water on Earth, holding with ice sheets about 69 percent of the world's freshwater. Many glaciers from temperate , alpine and seasonal polar climates store water as ice during the colder seasons and release it later in the form of meltwater as warmer summer temperatures cause
20500-544: The ice to act as a lever that loosens the rock by lifting it. Thus, sediments of all sizes become part of the glacier's load. If a retreating glacier gains enough debris, it may become a rock glacier , like the Timpanogos Glacier in Utah. Abrasion occurs when the ice and its load of rock fragments slide over bedrock and function as sandpaper, smoothing and polishing the bedrock below. The pulverized rock this process produces
20664-488: The ice, called seracs . Crevasses can form in several different ways. Transverse crevasses are transverse to flow and form where steeper slopes cause a glacier to accelerate. Longitudinal crevasses form semi-parallel to flow where a glacier expands laterally. Marginal crevasses form near the edge of the glacier, caused by the reduction in speed caused by friction of the valley walls. Marginal crevasses are largely transverse to flow. Moving glacier ice can sometimes separate from
20828-411: The idea that meltwater, refreezing inside glaciers, caused the glacier to dilate and extend its length. As it became clear that glaciers behaved to some degree as if the ice were a viscous fluid, it was argued that "regelation", or the melting and refreezing of ice at a temperature lowered by the pressure on the ice inside the glacier, was what allowed the ice to deform and flow. James Forbes came up with
20992-418: The increased pressure can facilitate melting. Most importantly, τ D is increased. These factors will combine to accelerate the glacier. As friction increases with the square of velocity, faster motion will greatly increase frictional heating, with ensuing melting – which causes a positive feedback, increasing ice speed to a faster flow rate still: west Antarctic glaciers are known to reach velocities of up to
21156-669: The influence of the American labor movement and King O'Malley . On top of that, some place names in South Australia such as Victor Harbor , Franklin Harbor or Outer Harbor are usually spelled with the -or spellings. Aside from that, -our is now almost universal in Australia but the -or endings remain a minority variant. New Zealand English , while sharing some words and syntax with Australian English , follows British usage. In British English, some words from French, Latin or Greek end with
21320-423: The infrared OH stretching mode of the water molecule. (Liquid water appears blue for the same reason. The blue of glacier ice is sometimes misattributed to Rayleigh scattering of bubbles in the ice.) A glacier originates at a location called its glacier head and terminates at its glacier foot, snout, or terminus . Glaciers are broken into zones based on surface snowpack and melt conditions. The ablation zone
21484-444: The left shows the temperature change over the past 12,000 years, from various sources; the thick black curve is an average. Climate forcing is the difference between radiant energy ( sunlight ) received by the Earth and the outgoing longwave radiation back to space. Such radiative forcing is quantified based on the CO 2 amount in the tropopause , in units of watts per square meter to
21648-679: The most deformation. Velocity increases inward toward the center line and upward, as the amount of deformation decreases. The highest flow velocities are found at the surface, representing the sum of the velocities of all the layers below. Because ice can flow faster where it is thicker, the rate of glacier-induced erosion is directly proportional to the thickness of overlying ice. Consequently, pre-glacial low hollows will be deepened and pre-existing topography will be amplified by glacial action, while nunataks , which protrude above ice sheets, barely erode at all – erosion has been estimated as 5 m per 1.2 million years. This explains, for example,
21812-445: The most sensitive indicators of climate change and are a major source of variations in sea level . A large piece of compressed ice, or a glacier, appears blue , as large quantities of water appear blue , because water molecules absorb other colors more efficiently than blue. The other reason for the blue color of glaciers is the lack of air bubbles. Air bubbles, which give a white color to ice, are squeezed out by pressure increasing
21976-414: The nature of the suffix used. The u is kept before English suffixes that are freely attachable to English words (for example in humourless , neighbourhood , and savoury ) and suffixes of Greek or Latin origin that have been adopted into English (for example in behaviourism , favourite , and honourable ). However, before Latin suffixes that are not freely attachable to English words,
22140-529: The oceans. A similar, single event of induced severe climate change after a meteorite impact has been proposed as reason for the Cretaceous–Paleogene extinction event . Other major thresholds are the Permian-Triassic , and Ordovician-Silurian extinction events with various reasons suggested. The Quaternary geological period includes the current climate. There has been a cycle of ice ages for
22304-455: The past 2.2–2.1 million years (starting before the Quaternary in the late Neogene Period). Note in the graphic on the right the strong 120,000-year periodicity of the cycles, and the striking asymmetry of the curves. This asymmetry is believed to result from complex interactions of feedback mechanisms. It has been observed that ice ages deepen by progressive steps, but the recovery to interglacial conditions occurs in one big step. The graph on
22468-483: The past. Ice sheet dynamics and continental positions (and linked vegetation changes) have been important factors in the long term evolution of the Earth's climate. There is also a close correlation between CO 2 and temperature, where CO 2 has a strong control over global temperatures in Earth's history. American and British English spelling differences Despite the various English dialects spoken from country to country and within different regions of
22632-721: The pooling of meltwater at the base of the glacier — perhaps delivered from a supraglacial lake — or the simple accumulation of mass beyond a critical "tipping point". Temporary rates up to 90 m (300 ft) per day have occurred when increased temperature or overlying pressure caused bottom ice to melt and water to accumulate beneath a glacier. In glaciated areas where the glacier moves faster than one km per year, glacial earthquakes occur. These are large scale earthquakes that have seismic magnitudes as high as 6.1. The number of glacial earthquakes in Greenland peaks every year in July, August, and September and increased rapidly in
22796-407: The preindustrial ages have been variations of the Sun, volcanic ashes and exhalations, relative movements of the Earth towards the Sun, and tectonically induced effects as for major sea currents, watersheds, and ocean oscillations. In the early Phanerozoic, increased atmospheric carbon dioxide concentrations have been linked to driving or amplifying increased global temperatures. Royer et al. 2004 found
22960-410: The pressure of the layers of ice and snow above it, this granular ice fuses into denser firn. Over a period of years, layers of firn undergo further compaction and become glacial ice. Glacier ice is slightly more dense than ice formed from frozen water because glacier ice contains fewer trapped air bubbles. Glacial ice has a distinctive blue tint because it absorbs some red light due to an overtone of
23124-416: The properties of radioactive elements in proxies. In older material, more of the radioactive material will have decayed and the proportion of different elements will be different from newer proxies. One example of radiometric dating is radiocarbon dating . In the air, cosmic rays constantly convert nitrogen into a specific radioactive carbon isotope, C . When plants then use this carbon to grow, this isotope
23288-506: The same country, there are only slight regional variations in English orthography , the two most notable variations being British and American spelling. Many of the differences between American and British or Commonwealth English date back to a time before spelling standards were developed. For instance, some spellings seen as "American" today were once commonly used in Britain, and some spellings seen as "British" were once commonly used in
23452-509: The sea, often with an ice tongue , like Mertz Glacier . Tidewater glaciers are glaciers that terminate in the sea, including most glaciers flowing from Greenland, Antarctica, Baffin , Devon , and Ellesmere Islands in Canada, Southeast Alaska , and the Northern and Southern Patagonian Ice Fields . As the ice reaches the sea, pieces break off or calve, forming icebergs . Most tidewater glaciers calve above sea level, which often results in
23616-475: The simplified form palestra is described by Merriam-Webster as "chiefly Brit[ish]." This is a reverse of the typical rule, where British spelling uses the ae / oe and American spelling simply uses e . Words that can be spelled either way in British English include cham a eleon , encyclop a edia , hom o eopathy , medi a eval (a minor variant in both AmE and BrE ), f o etid and f o etus . The spellings f o etus and f o etal are Britishisms based on
23780-562: The solar nebula dissipated, the gases would have escaped, partly driven off by the solar wind . The next atmosphere, consisting largely of nitrogen , carbon dioxide , and inert gases, was produced by outgassing from volcanism , supplemented by gases produced during the late heavy bombardment of Earth by huge asteroids . A major part of carbon dioxide emissions were soon dissolved in water and built up carbonate sediments. Water-related sediments have been found dating from as early as 3.8 billion years ago. About 3.4 billion years ago, nitrogen
23944-410: The sounds became monophthongs , and later applied to words not of Greek origin, in both Latin (for example, cœli ) and French (for example, œuvre ). In English, which has adopted words from all three languages, it is now usual to replace Æ/æ with Ae/ae and Œ/œ with Oe/oe . In many words, the digraph has been reduced to a lone e in all varieties of English: for example, o economics , pr
24108-672: The spelling "is often seen as such". The -ize form is known as Oxford spelling and is used in publications of the Oxford University Press, most notably the Oxford English Dictionary , and of other academic publishers such as Nature , the Biochemical Journal and The Times Literary Supplement . It can be identified using the IETF language tag en-GB-oxendict (or, historically, by en-GB-oed ). In Ireland, India, Australia, and New Zealand -ise spellings strongly prevail:
24272-608: The spellings with just e are increasingly used. Manoeuvre is the only spelling in Australia, and the most common one in Canada, where maneuver and manoeuver are also sometimes found. The -ize spelling is often incorrectly seen in Britain as an Americanism. It has been in use since the 15th century, predating the -ise spelling by over a century. The verb-forming suffix -ize comes directly from Ancient Greek -ίζειν ( -ízein ) or Late Latin -izāre , while -ise comes via French -iser . The Oxford English Dictionary ( OED ) recommends -ize and lists
24436-409: The stagnant ice above, forming a bergschrund . Bergschrunds resemble crevasses but are singular features at a glacier's margins. Crevasses make travel over glaciers hazardous, especially when they are hidden by fragile snow bridges . Below the equilibrium line, glacial meltwater is concentrated in stream channels. Meltwater can pool in proglacial lakes on top of a glacier or descend into the depths of
24600-423: The stress being applied, ice will act as an elastic solid. Ice needs to be at least 30 m (98 ft) thick to even start flowing, but once its thickness exceeds about 50 m (160 ft) (160 ft), stress on the layer above will exceeds the inter-layer binding strength, and then it'll move faster than the layer below. This means that small amounts of stress can result in a large amount of strain, causing
24764-408: The sum of these processes from Earth's spheres is what affects the climate. Greenhouse gasses act as the internal forcing of the climate system. Particular interests in climate science and paleoclimatology focus on the study of Earth climate sensitivity , in response to the sum of forcings. Analyzing the sum of these forcings contributes to the ability of scientists to make broad conclusive estimates on
24928-438: The surface snowpack may experience seasonal melting. A subpolar glacier includes both temperate and polar ice, depending on the depth beneath the surface and position along the length of the glacier. In a similar way, the thermal regime of a glacier is often described by its basal temperature. A cold-based glacier is below freezing at the ice-ground interface and is thus frozen to the underlying substrate. A warm-based glacier
25092-458: The time of the dinosaur extinction, "Hothouse", endured from 56 Mya to 47 Mya and was ~14 °C warmer than average modern temperatures. The Precambrian took place between the time when Earth first formed 4.6 billion years ( Ga ) ago, and 542 million years ago. The Precambrian can be split into two eons, the Archean and the Proterozoic, which can be further subdivided into eras. The reconstruction of
25256-464: The use of lake sediment cores and speleothems. These utilize an analysis of sediment layers and rock growth formations respectively, amongst element-dating methods utilizing oxygen, carbon and uranium. The Direct Quantitative Measurements method is the most direct approach to understand the change in a climate. Comparisons between recent data to older data allows a researcher to gain a basic understanding of weather and climate changes within an area. There
25420-601: The variation of the amount of oxygen in the atmosphere is not known. Periods with much oxygen in the atmosphere are associated with rapid development of animals. Today's atmosphere contains 21% oxygen, which is high enough for rapid development of animals. In 2020 scientists published a continuous, high-fidelity record of variations in Earth's climate during the past 66 million years and identified four climate states , separated by transitions that include changing greenhouse gas levels and polar ice sheets volumes. They integrated data of various sources. The warmest climate state since
25584-412: The vowel is unreduced in pronunciation (e.g., devour , contour , flour , hour , paramour , tour , troubadour , and velour ), the spelling is uniform everywhere. Most words of this kind came from Latin, where the ending was spelled ‑or . They were first adopted into English from early Old French , and the ending was spelled ‑our , ‑or or ‑ur . After the Norman conquest of England ,
25748-417: The weight of the ice is supported by the underlying water, and the glacier is afloat. Glaciers may also move by basal sliding , where the base of the glacier is lubricated by the presence of liquid water, reducing basal shear stress and allowing the glacier to slide over the terrain on which it sits. Meltwater may be produced by pressure-induced melting, friction or geothermal heat . The more variable
25912-624: The word fiord (instead of fjord ) . There is an increasing use of macrons in words that originated in Māori and an unambiguous preference for -ise endings (see below). Most words ending in an unstressed ‑our in British English (e.g., behaviour , colour , favour , flavour , harbour , honour , humour , labour , neighbour , rumour , splendour ) end in ‑or in American English ( behavior , color , favor , flavor , harbor , honor , humor , labor , neighbor , rumor , splendor ). Wherever
26076-428: The word pallor . As a general noun, rigour / ˈ r ɪ ɡ ər / has a u in the UK; the medical term rigor (sometimes / ˈ r aɪ ɡ ər / ) does not, such as in rigor mortis , which is Latin. Derivations of rigour / rigor such as rigorous , however, are typically spelled without a u , even in the UK. Words with the ending -irior , -erior or similar are spelled thus everywhere. The word armour
26240-428: Was once somewhat common in American usage but has disappeared except in some brand names such as Under Armour . The agent suffix -or ( separator , elevator , translator , animator , etc.) is spelled thus both in American and British English. Commonwealth countries normally follow British usage. Canadian English most commonly uses the -our ending and -our- in derivatives and inflected forms. However, owing to
26404-929: Was published. Following this, -re became the most common usage in Britain. In the United States, following the publication of Webster's Dictionary in the early 19th century, American English became more standardized, exclusively using the -er spelling. In addition, spelling of some words have been changed from -re to -er in both varieties. These include September , October , November , December , amber, blister , cadaver , chamber , chapter , charter , cider , coffer , coriander , cover , cucumber , cylinder , diaper , disaster , enter , fever , filter , gender , leper , letter , lobster , master , member , meter (measuring instrument) , minister , monster , murder , number , offer , order , oyster , powder , proper , render , semester , sequester , sinister , sober , surrender , tender , and tiger . Words using
26568-456: Was the major part of the then stable "second atmosphere". An influence of life has to be taken into account rather soon in the history of the atmosphere because hints of early life forms have been dated to as early as 3.5 to 4.3 billion years ago. The fact that it is not perfectly in line with the 30% lower solar radiance (compared to today) of the early Sun has been described as the " faint young Sun paradox ". The geological record, however, shows
26732-437: Was very influential in popularizing certain spellings in the United States, but he did not originate them. Rather [...] he chose already existing options such as center, color and check for the simplicity, analogy or etymology". William Shakespeare 's first folios , for example, used spellings such as center and color as much as centre and colour . Webster did attempt to introduce some reformed spellings, as did
26896-442: Was very much in line with what is found today, suggesting that the fundamental features of the carbon cycle were established as early as 4 billion years ago. The constant rearrangement of continents by plate tectonics influences the long-term evolution of the atmosphere by transferring carbon dioxide to and from large continental carbonate stores. Free oxygen did not exist in the atmosphere until about 2.4 billion years ago, during
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