Krill are small crustaceans of the order Euphausiacea, are found in all the world's oceans. The name "krill" comes from the Norwegian word krill, meaning "small fry of fish", often attributed to species of fish. Krill are considered an important trophic level connection – near the bottom of the food chain – because they feed on phytoplankton and zooplankton, converting these into a form suitable for many larger animals for which krill make up the largest part of their diets. In the Southern Ocean, one species, the Antarctic krill, Euphausia superba, makes up an estimated biomass of around 379,000,000 tonnes, making it among the species with the largest total biomass. Of this, over half is eaten by whales, penguins and fish each year, is replaced by growth and reproduction. Most krill species display large daily vertical migrations, thus providing food for predators near the surface at night and in deeper waters during the day. Krill are fished commercially in the waters around Japan; the total global harvest amounts to 150,000–200,000 tonnes annually, most of this from the Scotia Sea.
Most of the krill catch is used for aquaculture and aquarium feeds, as bait in sport fishing, or in the pharmaceutical industry. In Japan, the Philippines, Russia, krill are used for human consumption and are known as okiami in Japan, they are eaten as camarones in Philippines. In the Philippines, krill are known as alamang and are used to make a salty paste called bagoong. Krill are the main prey of baleen whales, including the blue whale. Krill belong to the Crustacea; the most familiar and largest group of crustaceans, the class Malacostraca, includes the superorder Eucarida comprising the three orders, Euphausiacea and the planktonic Amphionidacea. The order Euphausiacea comprises two families; the more abundant Euphausiidae contains 10 different genera with a total of 85 species. Of these, the genus Euphausia is the largest, with 31 species; the lesser known family, the Bentheuphausiidae, has only one species, Bentheuphausia amblyops, a bathypelagic krill living in deep waters below 1,000 m.
It is considered the most primitive extant krill species. Well-known species of the Euphausiidae of commercial krill fisheries include Antarctic krill, Pacific krill and Northern krill; as of 2013, the order Euphausiacea is believed to be monophyletic due to several unique conserved morphological characteristics such as its naked filamentous gills and thin thoracopods and by molecular studies. There have been many theories of the location of the order Euphausiacea. Since the first description of Thysanopode tricuspide by Henri Milne-Edwards in 1830, the similarity of their biramous thoracopods had led zoologists to group euphausiids and Mysidacea in the order Schizopoda, split by Johan Erik Vesti Boas in 1883 into two separate orders. William Thomas Calman ranked the Mysidacea in the superorder Peracarida and euphausiids in the superorder Eucarida, although up to the 1930s the order Schizopoda was advocated, it was also proposed that order Euphausiacea should be grouped with the Penaeidae in the Decapoda based on developmental similarities, as noted by Robert Gurney and Isabella Gordon.
The reason for this debate is that krill share some morphological features of decapods and others of mysids. Molecular studies have not unambiguously grouped them due to the paucity of key rare species such as Bentheuphausia amblyops in krill and Amphionides reynaudii in Eucarida. One study supports the monophyly of Eucarida, another groups Euphausiacea with Mysida, while yet another groups Euphausiacea with Hoplocarida. No extant fossil can be unequivocally assigned to Euphausiacea; some extinct eumalacostracan taxa have been thought to be euphausiaceans such as Anthracophausia, Crangopsis – now assigned to the Aeschronectida – and Palaeomysis. All dating of speciation events were estimated by molecular clock methods, which placed the last common ancestor of the krill family Euphausiidae to have lived in the Lower Cretaceous about 130 million years ago. Krill occur worldwide in all oceans, although many individual species have endemic or neritic distributions. Bentheuphausia amblyops, a bathypelagic species, has a cosmopolitan distribution within its deep-sea habitat.
Species of the genus Thysanoessa occur in both Pacific oceans. The Pacific is home to Euphausia pacifica. Northern krill occur across the Atlantic from the Mediterranean Sea northward. Species with neritic distributions include the four species of the genus Nyctiphanes, they are abundant along the upwelling regions of the California, Humboldt and Canarias current systems. Another species having only neritic distribution is E. crystallorophias, endemic to the Antarctic coastline. Species with endemic distributions include Nyctiphanes capensis, which occurs only in the Benguela current, E. mucronata in the Humboldt current, the six Euphausia species native to the Southern Ocean. In the Antarctic, seven species are known, six in Euphausia; the Antarctic krill lives at depths reaching 100 m, whereas ice krill reach depth of 4,000 m, though they inhabit depths of at most 300–600 m. Both are found at latitudes south of 55° S, with E. crystallorophias dominating south of 74° S and in regions of pack ice.
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Mexico the United Mexican States, is a country in the southern portion of North America. It is bordered to the north by the United States. Covering 2,000,000 square kilometres, the nation is the fifth largest country in the Americas by total area and the 13th largest independent state in the world. With an estimated population of over 120 million people, the country is the eleventh most populous state and the most populous Spanish-speaking state in the world, while being the second most populous nation in Latin America after Brazil. Mexico is a federation comprising 31 states and Mexico City, a special federal entity, the capital city and its most populous city. Other metropolises in the state include Guadalajara, Puebla, Tijuana and León. Pre-Columbian Mexico dates to about 8000 BC and is identified as one of five cradles of civilization and was home to many advanced Mesoamerican civilizations such as the Olmec, Teotihuacan, Zapotec and Aztec before first contact with Europeans. In 1521, the Spanish Empire conquered and colonized the territory from its politically powerful base in Mexico-Tenochtitlan, administered as the viceroyalty of New Spain.
Three centuries the territory became a nation state following its recognition in 1821 after the Mexican War of Independence. The post-independence period was tumultuous, characterized by economic inequality and many contrasting political changes; the Mexican–American War led to a territorial cession of the extant northern territories to the United States. The Pastry War, the Franco-Mexican War, a civil war, two empires, the Porfiriato occurred in the 19th century; the Porfiriato was ended by the start of the Mexican Revolution in 1910, which culminated with the promulgation of the 1917 Constitution and the emergence of the country's current political system as a federal, democratic republic. Mexico has the 11th largest by purchasing power parity; the Mexican economy is linked to those of its 1994 North American Free Trade Agreement partners the United States. In 1994, Mexico became the first Latin American member of the Organisation for Economic Co-operation and Development, it is classified as an upper-middle income country by the World Bank and a newly industrialized country by several analysts.
The country is considered both a regional power and a middle power, is identified as an emerging global power. Due to its rich culture and history, Mexico ranks first in the Americas and seventh in the world for number of UNESCO World Heritage Sites. Mexico is an ecologically megadiverse country, ranking fourth in the world for its biodiversity. Mexico receives a huge number of tourists every year: in 2018, it was the sixth most-visited country in the world, with 39 million international arrivals. Mexico is a member of the United Nations, the World Trade Organization, the G8+5, the G20, the Uniting for Consensus group of the UN, the Pacific Alliance trade bloc. Mēxihco is the Nahuatl term for the heartland of the Aztec Empire, namely the Valley of Mexico and surrounding territories, with its people being known as the Mexica, it is believed to be a toponym for the valley which became the primary ethnonym for the Aztec Triple Alliance as a result, although it could have been the other way around.
In the colonial era, back when Mexico was called New Spain this territory became the Intendency of Mexico and after New Spain achieved independence from the Spanish Empire it came to be known as the State of Mexico with the new country being named after its capital: the City of Mexico, which itself was founded in 1524 on top of the ancient Mexica capital of Mexico-Tenochtitlan. Traditionally, the name Tenochtitlan was thought to come from Nahuatl tetl and nōchtli and is thought to mean "Among the prickly pears rocks". However, one attestation in the late 16th-century manuscript known as "the Bancroft dialogues" suggests the second vowel was short, so that the true etymology remains uncertain; the suffix -co is the Nahuatl locative, making the word a place name. Beyond that, the etymology is uncertain, it has been suggested that it is derived from Mextli or Mēxihtli, a secret name for the god of war and patron of the Mexica, Huitzilopochtli, in which case Mēxihco means "place where Huitzilopochtli lives".
Another hypothesis suggests that Mēxihco derives from a portmanteau of the Nahuatl words for "moon" and navel. This meaning might refer to Tenochtitlan's position in the middle of Lake Texcoco; the system of interconnected lakes, of which Texcoco formed the center, had the form of a rabbit, which the Mesoamericans pareidolically associated with the moon rabbit. Still another hypothesis suggests that the word is derived from Mēctli, the name of the goddess of maguey; the name of the city-state was transliterated to Spanish as México with the phonetic value of the letter x in Medieval Spanish, which represented the voiceless postalveolar fricative. This sound, as well as the voiced postalveolar fricative, represented by a j, evolved into a voiceless velar fricative during the 16th century; this led to the use of the variant Méjico in many publications in Spanish, most notably in Spain, whereas in Mexico and most other Spanish–speaking countries, México was the preferred spelling. In recent years, the Real Academia Española, which regulates the Spanish l
An endangered species is a species, categorized as likely to become extinct. Endangered, as categorized by the International Union for Conservation of Nature Red List, is the second most severe conservation status for wild populations in the IUCN's schema after Critically Endangered. In 2012, the IUCN Red List featured 3,079 animal and 2,655 plant species as endangered worldwide; the figures for 1998 were 1,102 and 1,197. Many nations have laws that protect conservation-reliant species: for example, forbidding hunting, restricting land development or creating preserves. Population numbers and species' conservation status can be found at the lists of organisms by population; the conservation status of a species indicates the likelihood. Many factors are considered; the IUCN Red List of Threatened Species is the best-known worldwide conservation status listing and ranking system. Over 50% of the world's species are estimated to be at risk of extinction. Internationally, 199 countries have signed an accord to create Biodiversity Action Plans that will protect endangered and other threatened species.
In the United States, such plans are called Species Recovery Plans. Though labelled a list, the IUCN Red List is a system of assessing the global conservation status of species that includes "Data Deficient" species – species for which more data and assessment is required before their status may be determined – as well species comprehensively assessed by the IUCN's species assessment process; those species of "Near Threatened" and "Least Concern" status have been assessed and found to have robust and healthy populations, though these may be in decline. Unlike their more general use elsewhere, the List uses the terms "endangered species" and "threatened species" with particular meanings: "Endangered" species lie between "Vulnerable" and "Critically Endangered" species, while "Threatened" species are those species determined to be Vulnerable, Endangered or Critically Endangered; the IUCN categories, with examples of animals classified by them, include: Extinct no remaining individuals of the species Extinct in the wild Captive individuals survive, but there is no free-living, natural population.
Critically endangered Faces an high risk of extinction in the immediate future. Endangered Faces a high risk of extinction in the near future. Vulnerable Faces a high risk of endangerment in the medium term. Near-threatened May be considered threatened in the near future. Least concern No immediate threat to species' survival. A) Reduction in population size based on any of the following: An observed, inferred or suspected population size reduction of ≥ 70% over the last 10 years or three generations, whichever is the longer, where the causes of the reduction are reversible AND understood AND ceased, based on any of the following: direct observation an index of abundance appropriate for the taxon a decline in area of occupancy, extent of occurrence or quality of habitat actual or potential levels of exploitation the effects of introduced taxa, pathogens, competitors or parasites. An observed, inferred or suspected population size reduction of ≥ 50% over the last 10 years or three generations, whichever is the longer, where the reduction or its causes may not have ceased OR may not be understood OR may not be reversible, based on any of to under A1.
A population size reduction of ≥ 50%, projected or suspected to be met within the next 10 years or three generations, whichever is the longer, based on any of to under A1. An observed, inferred, projected or suspected population size reduction of ≥ 50% over any 10 year or three generation period, whichever is longer, where the time period must include both the past and the future, where the reduction or its causes may not have ceased OR may not be understood OR may not be reversible, based on any of to under A1. B) Geographic range in the form of either B1 OR B2 OR both: Extent of occurrence estimated to be less than 5,000 km², estimates indicating at least two of a-c: Severely fragmented or known to exist at no more than five locations. Continuing decline, observed or projected, in any of the following: extent of occurrence area of occupancy area, extent or quality of habitat number of locations or subpopulations number of mature individuals Extreme fluctuations in any of the following: extent of occurrence area of occupancy number of locations or subpopulations number of mature individuals Area of occupancy estimated to be less than 500 km², estimates indicating at least two of a-c: Severely fragmented or known to exist at no more than five locations.
Continuing decline, observed or projected, in any of the following: extent of occurrence area of occupancy area, extent or quality of habitat number of locations or subpopulations number of mature individuals Extreme fluctuations in any of the following: extent of occurrence area of occupancy number of locations or subpopulations number of mature individualsC) Population estimated to number fewer than 2,500 mature individuals and either: An estimated continuing decline of at least 20% within five years or two generations, whichever is longer, OR A continuing decline, projected
San Francisco the City and County of San Francisco, is the cultural and financial center of Northern California. San Francisco is the 13th-most populous city in the United States, the fourth-most populous in California, with 884,363 residents as of 2017, it covers an area of about 46.89 square miles at the north end of the San Francisco Peninsula in the San Francisco Bay Area, making it the second-most densely populated large US city, the fifth-most densely populated U. S. county, behind only four of the five New York City boroughs. San Francisco is part of the fifth-most populous primary statistical area in the United States, the San Jose–San Francisco–Oakland, CA Combined Statistical Area; as of 2017, it was the seventh-highest income county in the United States, with a per capita personal income of $119,868. As of 2015, San Francisco proper had a GDP of $154.2 billion, a GDP per capita of $177,968. The San Francisco CSA was the country's third-largest urban economy as of 2017, with a GDP of $907 billion.
Of the 500+ primary statistical areas in the US, the San Francisco CSA had among the highest GDP per capita in 2017, at $93,938. San Francisco was ranked 14th in the world and third in the United States on the Global Financial Centres Index as of September 2018. San Francisco was founded on June 29, 1776, when colonists from Spain established Presidio of San Francisco at the Golden Gate and Mission San Francisco de Asís a few miles away, all named for St. Francis of Assisi; the California Gold Rush of 1849 brought rapid growth, making it the largest city on the West Coast at the time. San Francisco became a consolidated city-county in 1856. San Francisco's status as the West Coast's largest city peaked between 1870 and 1900, when around 25% of California's population resided in the city proper. After three-quarters of the city was destroyed by the 1906 earthquake and fire, San Francisco was rebuilt, hosting the Panama-Pacific International Exposition nine years later. In World War II, San Francisco was a major port of embarkation for service members shipping out to the Pacific Theater.
It became the birthplace of the United Nations in 1945. After the war, the confluence of returning servicemen, significant immigration, liberalizing attitudes, along with the rise of the "hippie" counterculture, the Sexual Revolution, the Peace Movement growing from opposition to United States involvement in the Vietnam War, other factors led to the Summer of Love and the gay rights movement, cementing San Francisco as a center of liberal activism in the United States. Politically, the city votes along liberal Democratic Party lines. A popular tourist destination, San Francisco is known for its cool summers, steep rolling hills, eclectic mix of architecture, landmarks, including the Golden Gate Bridge, cable cars, the former Alcatraz Federal Penitentiary, Fisherman's Wharf, its Chinatown district. San Francisco is the headquarters of five major banking institutions and various other companies such as Levi Strauss & Co. Gap Inc. Fitbit, Salesforce.com, Reddit, Inc. Dolby, Weebly, Pacific Gas and Electric Company, Pinterest, Uber, Mozilla, Wikimedia Foundation and Weather Underground.
It is home to a number of educational and cultural institutions, such as the University of San Francisco, University of California, San Francisco, San Francisco State University, the De Young Museum, the San Francisco Museum of Modern Art, the California Academy of Sciences. As of 2019, San Francisco is the highest rated American city on world liveability rankings; the earliest archaeological evidence of human habitation of the territory of the city of San Francisco dates to 3000 BC. The Yelamu group of the Ohlone people resided in a few small villages when an overland Spanish exploration party, led by Don Gaspar de Portolà, arrived on November 2, 1769, the first documented European visit to San Francisco Bay. Seven years on March 28, 1776, the Spanish established the Presidio of San Francisco, followed by a mission, Mission San Francisco de Asís, established by the Spanish explorer Juan Bautista de Anza. Upon independence from Spain in 1821, the area became part of Mexico. Under Mexican rule, the mission system ended, its lands became privatized.
In 1835, Englishman William Richardson erected the first independent homestead, near a boat anchorage around what is today Portsmouth Square. Together with Alcalde Francisco de Haro, he laid out a street plan for the expanded settlement, the town, named Yerba Buena, began to attract American settlers. Commodore John D. Sloat claimed California for the United States on July 7, 1846, during the Mexican–American War, Captain John B. Montgomery arrived to claim Yerba Buena two days later. Yerba Buena was renamed San Francisco on January 30 of the next year, Mexico ceded the territory to the United States at the end of the war. Despite its attractive location as a port and naval base, San Francisco was still a small settlement with inhospitable geography; the California Gold Rush brought a flood of treasure seekers. With their sourdough bread in tow, prospectors accumulated in San Francisco over rival Benicia, raising the population from 1,000 in 1848 to 25,000 by December 1849; the promise of great wealth was so strong that crews on arriving vessels deserted and rushed off to the gold fields, leaving behind a forest of masts in San Francisco harbor.
Some of these 500 abandoned ships were used at times as storeships and hotels.
Global warming is a long-term rise in the average temperature of the Earth's climate system, an aspect of climate change shown by temperature measurements and by multiple effects of the warming. Though earlier geological periods experienced episodes of warming, the term refers to the observed and continuing increase in average air and ocean temperatures since 1900 caused by emissions of greenhouse gasses in the modern industrial economy. In the modern context the terms global warming and climate change are used interchangeably, but climate change includes both global warming and its effects, such as changes to precipitation and impacts that differ by region. Many of the observed warming changes since the 1950s are unprecedented in the instrumental temperature record, in historical and paleoclimate proxy records of climate change over thousands to millions of years. In 2013, the Intergovernmental Panel on Climate Change Fifth Assessment Report concluded, "It is likely that human influence has been the dominant cause of the observed warming since the mid-20th century."
The largest human influence has been the emission of greenhouse gases such as carbon dioxide and nitrous oxide. Climate model projections summarized in the report indicated that during the 21st century, the global surface temperature is to rise a further 0.3 to 1.7 °C to 2.6 to 4.8 °C depending on the rate of greenhouse gas emissions and on climate feedback effects. These findings have been recognized by the national science academies of the major industrialized nations and are not disputed by any scientific body of national or international standing. Future climate change effects are expected to include rising sea levels, ocean acidification, regional changes in precipitation, expansion of deserts in the subtropics. Surface temperature increases are greatest in the Arctic, with the continuing retreat of glaciers and sea ice. Predicted regional precipitation effects include more frequent extreme weather events such as heat waves, wildfires, heavy rainfall with floods, heavy snowfall. Effects directly significant to humans are predicted to include the threat to food security from decreasing crop yields, the abandonment of populated areas due to rising sea levels.
Environmental impacts appear to include the extinction or relocation of ecosystems as they adapt to climate change, with coral reefs, mountain ecosystems, Arctic ecosystems most threatened. Because the climate system has a large "inertia" and greenhouse gases will remain in the atmosphere for a long time, climatic changes and their effects will continue to become more pronounced for many centuries if further increases to greenhouse gases stop. Possible societal responses to global warming include mitigation by emissions reduction, adaptation to its effects, possible future climate engineering. Most countries are parties to the United Nations Framework Convention on Climate Change, whose ultimate objective is to prevent dangerous anthropogenic climate change. Parties to the UNFCCC have agreed that deep cuts in emissions are required and that global warming should be limited to well below 2.0 °C compared to pre-industrial levels, with efforts made to limit warming to 1.5 °C. Some scientists call into question climate adaptation feasibility, with higher emissions scenarios, or the two degree temperature target.
Public reactions to global warming and concern about its effects are increasing. A 2015 global survey showed that a median of 54% of respondents consider it "a serious problem", with significant regional differences: Americans and Chinese are among the least concerned. Multiple independently produced datasets confirm that between 1880 and 2012, the global average surface temperature increased by 0.85 °C. Since 1979 the rate of warming has doubled. Climate proxies show the temperature to have been stable over the one or two thousand years before 1850, with regionally varying fluctuations such as the Medieval Warm Period and the Little Ice Age. Although the increase of the average near-surface atmospheric temperature is used to track global warming, over 90% of the additional energy stored in the climate system over the last 50 years has accumulated in the oceans; the rest warmed the continents and the atmosphere. The warming evident in the instrumental temperature record is consistent with a wide range of observations, as documented by many independent scientific groups.
Examples include sea level rise, widespread melting of snow and land ice, increased heat content of the oceans, increased humidity, the earlier timing of spring events, e.g. the flowering of plants. Global warming refers with the amount of warming varying by region. Since 1979, global average land temperatures have increased about twice as fast as global average ocean temperatures; this is due to the larger heat capacity of the oceans and because oceans lose more heat by evaporation. Where greenhouse gas emissions occur does not impact the location of warming because the major greenhouse gases persist long enough to diffuse across the planet, although localized black carbon deposits on snow and ice do contribute to Arctic warming; the Northern Hemisphere and North Pole have heated much faster than the South Pole and Southern Hemisphere. The Northern Hemisphere not only has much more land, its arrangement around the Arctic Ocean has resulted in the maximum surface area flipping from reflective snow and ice cover to ocean and land surfaces that absorb more sunlight.
Sea level rise
Since at least the start of the 20th century, the average global sea level has been rising. Between 1900 and 2016, the sea level rose by 16–21 cm. More precise data gathered from satellite radar measurements reveal an accelerating rise of 7.5 cm from 1993 to 2017, a trend of 30 cm per century. This acceleration is due to human-caused global warming, driving thermal expansion of seawater and the melting of land-based ice sheets and glaciers. Between 1993 and 2018, thermal expansion of the oceans contributed 42% to sea level rise. Climate scientists expect the rate to further accelerate during the 21st century. Projecting future sea level is challenging, due to the complexity of many aspects of the climate system; as climate research into past and present sea levels leads to improved computer models, projections have increased. For example, in 2007 the Intergovernmental Panel on Climate Change projected a high end estimate of 60 cm through 2099, but their 2014 report raised the high-end estimate to about 90 cm.
A number of studies have concluded that a global sea level rise of 200 to 270 cm this century is "physically plausible". A conservative estimate of the long-term projections is that each Celsius degree of temperature rise triggers a sea level rise of 2.3 metres over a period of two millennia: an example of climate inertia. The sea level will not rise uniformly everywhere on Earth, it will drop in some locations. Local factors include tectonic effects and subsidence of the land, tides and storms. Sea level rises can influence human populations in coastal and island regions. Widespread coastal flooding is expected with several degrees of warming sustained for millennia. Further effects are higher storm-surges and more dangerous tsunamis, displacement of populations and degradation of agricultural land and damage in cities. Natural environments like marine ecosystems are affected, with fish and plants losing parts of their habitat. Societies can respond to sea level rise in three different ways: to retreat, to accommodate and to protect.
Sometimes these adaptation strategies go hand in hand, but at other times choices have to be made among different strategies. Ecosystems that adapt to rising sea levels by moving inland might not always be able to do so, due to natural or man-made barriers. Understanding past sea level is important for the analysis of current and future changes. In the recent geological past, changes in land ice and thermal expansion from increased temperatures are the dominant reasons of sea level rise; the last time the Earth was 2 °C warmer than pre-industrial temperatures, sea levels were at least 5 metres higher than now: this was when warming because of changes in the amount of sunlight due to slow changes in the Earth's orbit caused the last interglacial. The warming was sustained over a period of thousands of years and the magnitude of the rise in sea level implies a large contribution from the Antarctic and Greenland ice sheets. Since the last glacial maximum about 20,000 years ago, the sea level has risen by more than 125 metres, with rates varying from less than a mm/year to 40+ mm/year, as a result of melting ice sheets over Canada and Eurasia.
Rapid disintegration of ice sheets led to so called'meltwater pulses', periods during which sea level rose rapidly. The rate of rise started to slow down about 8,200 years before present. Sea level changes can be driven either by variations in the amount of water in the oceans, the volume of the ocean or by changes of the land compared to the sea surface; the different techniques used to measure changes in sea level do not measure the same. Tide gauges can only measure relative sea level, whilst satellites can measure absolute sea level changes. To get precise measurements for sea level, researchers studying the ice and the oceans on our planet factor in ongoing deformations of the solid Earth, in particular due to landmasses still rising from past ice masses retreating, the Earth's gravity and rotation. Since the 1992 launch of TOPEX/Poseidon, altimetric satellites have been recording the change in sea level; those satellites can measure the hills and valleys in the sea caused by currents and detect trends in their height.
To measure the distance to the sea surface, the satellite sends a microwave pulse to the ocean's surface and records the time it takes to return. A microwave radiometer corrects any delay. Combining this data with the precise location of the spacecraft makes it possible to determine sea-surface height to within a few centimeters. Current rates of sea level rise from satellite altimetry have been estimated to be 3.0 ± 0.4 millimetres per year for the period 1993–2017. Earlier satellite measurements were at odds with tide gauge measurements. A small calibration error for the Topex/Poseidon satellite discovered in 2015 was identified as the cause of this mismatch, it had caused a slight overestimation of the 1992–2005 sea levels, which masked the ongoing sea level rise acceleration. Satellites are useful for measuring regional variations in sea level, such as the substantial rise between 1993 and 2012 in the western tropical Pacific; this sharp rise has been linked to increasing trade winds, which occur when the Pacific Decadal Oscillation and the El Niño–Southern Oscillation change from one state to t
A cephalopod is any member of the molluscan class Cephalopoda such as a squid, octopus or nautilus. These marine animals are characterized by bilateral body symmetry, a prominent head, a set of arms or tentacles modified from the primitive molluscan foot. Fishermen sometimes call; the study of cephalopods is a branch of malacology known as teuthology. Cephalopods became dominant during the Ordovician period, represented by primitive nautiloids; the class now contains two, only distantly related, extant subclasses: Coleoidea, which includes octopuses and cuttlefish. In the Coleoidea, the molluscan shell has been internalized or is absent, whereas in the Nautiloidea, the external shell remains. About 800 living species of cephalopods have been identified. Two important extinct taxa are the Belemnoidea. There are over 800 extant species of cephalopod. An estimated 11,000 extinct taxa have been described, although the soft-bodied nature of cephalopods means they are not fossilised. Cephalopods are found in all the oceans of Earth.
None of them can tolerate freshwater, but the brief squid, Lolliguncula brevis, found in Chesapeake Bay, is a notable partial exception in that it tolerates brackish water. Cephalopods are thought to be unable to live in freshwater due to multiple biochemical constraints, in their +400 million year existence have never ventured into freshwater habitats. Cephalopods occupy most of the depth of the ocean, from the abyssal plain to the sea surface, their diversity is decreases towards the poles. Cephalopods are regarded as the most intelligent of the invertebrates, have well developed senses and large brains; the nervous system of cephalopods is the most complex of the invertebrates and their brain-to-body-mass ratio falls between that of endothermic and ectothermic vertebrates. Captive cephalopods have been known to climb out of their aquaria, maneuver a distance of the lab floor, enter another aquarium to feed on the crabs, return to their own aquarium; the brain is protected in a cartilaginous cranium.
The giant nerve fibers of the cephalopod mantle have been used for many years as experimental material in neurophysiology. Many cephalopods are social creatures; some cephalopods are able to fly through the air for distances of up to 50 m. While cephalopods are not aerodynamic, they achieve these impressive ranges by jet-propulsion; the animals spread their fins and tentacles to form wings and control lift force with body posture. One species, Todarodes pacificus, has been observed spreading tentacles in a flat fan shape with a mucus film between the individual tentacles while another, Sepioteuthis sepioidea, has been observed putting the tentacles in a circular arrangement. Cephalopods have advanced vision, can detect gravity with statocysts, have a variety of chemical sense organs. Octopuses use their arms to explore their environment and can use them for depth perception. Most cephalopods rely on vision to detect predators and prey, to communicate with one another. Cephalopod vision is acute: training experiments have shown that the common octopus can distinguish the brightness, size and horizontal or vertical orientation of objects.
The morphological construction gives cephalopod eyes the same performance as sharks'. Cephalopods' eyes are sensitive to the plane of polarization of light. Unlike many other cephalopods, nautiluses do not have good vision, they have a simple "pinhole" eye. Instead of vision, the animal is thought to use olfaction as the primary sense for foraging, as well as locating or identifying potential mates. Given their ability to change color, all octopodes and most cephalopods are considered to be color blind. Coleoid cephalopods have a single photoreceptor type and lack the ability to determine color by comparing detected photon intensity across multiple spectral channels; when camouflaging themselves, they use their chromatophores to change brightness and pattern according to the background they see, but their ability to match the specific color of a background may come from cells such as iridophores and leucophores that reflect light from the environment. They produce visual pigments throughout their body, may sense light levels directly from their body.
Evidence of color vision has been found in the sparkling enope squid, which achieves color vision by the use of three distinct retinal molecules which bind to its opsin. In 2015, a novel mechanism for spectral discrimination in cephalopods was described; this relies on the exploitation of chromatic aberration. Numerical modeling shows that chromatic aberration can yield useful chromatic information through the dependence of image acuity on accommodation; the unusual off-axis slit and annular pupil sha