109-879: Freshwater is any naturally occurring water except seawater and brackish water. Freshwater or Fresh Water may also refer to: Freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids . The term excludes seawater and brackish water , but it does include non-salty mineral-rich waters , such as chalybeate springs. Fresh water may encompass frozen and meltwater in ice sheets , ice caps , glaciers , snowfields and icebergs , natural precipitations such as rainfall , snowfall , hail / sleet and graupel , and surface runoffs that form inland bodies of water such as wetlands , ponds , lakes , rivers , streams , as well as groundwater contained in aquifers , subterranean rivers and lakes . Water
218-547: A desert climate often face physical water scarcity. Central Asia , West Asia , and North Africa are examples of arid areas. Economic water scarcity results from a lack of investment in infrastructure or technology to draw water from rivers, aquifers , or other water sources. It also results from weak human capacity to meet water demand. Many people in Sub-Saharan Africa are living with economic water scarcity. An important concern for hydrological ecosystems
327-529: A collection of 17 interlinked global goals designed to be a "blueprint to achieve a better and more sustainable future for all". Targets on fresh water conservation are included in SDG 6 (Clean water and sanitation) and SDG 15 (Life on land). For example, Target 6.4 is formulated as "By 2030, substantially increase water-use efficiency across all sectors and ensure sustainable withdrawals and supply of freshwater to address water scarcity and substantially reduce
436-668: A dominance of grasses over humid tropics trees. The tree Lophira alata and others may have spread out of the African forests during the AHP, and the Lactuca plants may have split into two species under the effects of the AHP and other climate changes in Africa during the Holocene. The Sahara climate did not become entirely homogeneous; its central-eastern parts were probably drier than
545-402: A drinking water supply it remains vital to protect due to its ability to carry contaminants and pollutants from the land into lakes and rivers, which constitute a significant percentage of other people's freshwater supply. It is almost ubiquitous underground, residing in the spaces between particles of rock and soil or within crevices and cracks in rock, typically within 100 m (330 ft) of
654-447: A dry to a "green" Sahara and back have threshold behaviour, with the change occurring once a certain level of insolation is exceeded; likewise, a gradual drop of insolation often leads to a sudden transition back to a dry Sahara. This is due to various feedback processes which are at work, and in climate models there is often more than one stable climate-vegetation state. Sea surface temperature and greenhouse gas changes synchronized
763-541: A gradual advance is supported by tephrochronological data. Likewise, in the Sahara there might have been a delay of about a millennium between the onset of the AHP and the full establishment of humid conditions, as vegetation growth and the filling of river systems took time. Lake Victoria reappeared and overflowed; Lake Albert also overflowed into the White Nile 15,000–14,500 years ago and so did Lake Tana , into
872-538: A large number of icebergs in the North Atlantic; the discharge of large amounts of such icebergs between 11,500 and 21,000 years before present coincided with droughts in the subtropics . Before the onset of the AHP, it is thought that Lake Victoria , Lake Albert , Lake Edward , Lake Turkana and the Sudd swamps had dried out. The White Nile had become a seasonal river whose course along with that of
981-428: A larger salt content. Freshwater habitats can be classified by different factors, including temperature, light penetration, nutrients, and vegetation. There are three basic types of freshwater ecosystems: Lentic (slow moving water, including pools , ponds , and lakes ), lotic (faster moving water, for example streams and rivers ) and wetlands (areas where the soil is saturated or inundated for at least part of
1090-476: A northward movement of the ITCZ and increasing moisture gradients between land and sea. Two temperature gradients, one between a cooler Atlantic during spring and an already warming African continent, the other between warmer temperatures north of 10° latitude and cooler south, may have assisted in this change. In Eastern Africa, ITCZ changes had relatively little effect on precipitation changes. The past position of
1199-645: A number of dry/wet periods have been defined for the Central Africa region. In general, these types of climate fluctuations between wetter and drier periods are known as " pluvials " and " interpluvials ", respectively. The term "Green Sahara" is frequently used to describe the AHPs. Because the AHP did not affect all of Africa and is thus not technically accurate, some scientists have instead used and recommended "North African humid period" and "Northern African humid period". Other terms that have been applied to
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#17327914998601308-503: A role in vegetation changes. Isotope ratios such as the hydrogen / deuterium ratio that have been used to reconstruct past precipitation values likewise are under the influence of various physical effects, which complicates their interpretation. Most records of Holocene precipitation in eastern Africa come from low altitudes. The term "African humid period" (AHP) was coined in 2000 by Peter B. de Menocal et al. Earlier humid periods are sometimes known as "African humid periods" and
1417-519: A single factor. Groundwater showed greater resilience to climate change than expected, and areas with an increasing threshold between 0.34 and 0.39 aridity index exhibited significant sensitivity to climate change. Land-use could affect infiltration and runoff processes. The years of most recharge coincided with the most precipitation anomalies, such as during El Niño and La Niña events. Three precipitation-recharge sensitivities were distinguished: in super arid areas with more than 0.67 aridity index, there
1526-602: A widespread settlement of the Sahara and the Arabian Deserts, and had a profound effect on African cultures, such as the birth of the Ancient Egyptian civilization . People in the Sahara lived as hunter-gatherers and domesticated cattle, goats and sheep. They left archaeological sites and artifacts such as one of the oldest ships in the world , and rock paintings such as those in the Cave of Swimmers and in
1635-582: Is a climate period in Africa during the late Pleistocene and Holocene geologic epochs, when northern Africa was wetter than today. The covering of much of the Sahara desert by grasses, trees and lakes was caused by changes in the Earth's axial tilt ; changes in vegetation and dust in the Sahara which strengthened the African monsoon ; and increased greenhouse gases . During the preceding Last Glacial Maximum ,
1744-517: Is also known as " aqualithic " although substantial differences between the cultures of various places have been found. The greening of the Sahara led to a demographic expansion and especially in the Eastern Sahara human occupancy coincides with the AHP. Conversely occupation decreased along the Nile valley, perhaps due to the expansion of wetlands there and frequent large-scale flooding of
1853-407: Is an important factor in the precipitation increase. Specifically, increased precipitation increases the amount of vegetation; vegetation absorbs more sunlight and thus more energy is available for the monsoon. In addition, evapotranspiration from vegetation adds more moisture, although this effect is less pronounced than the albedo effect. Heat fluxes in the soil and evaporation are also altered by
1962-449: Is consumed through human activities than is naturally restored, this may result in reduced fresh water availability (or water scarcity ) from surface and underground sources and can cause serious damage to surrounding and associated environments. Water pollution also reduces the availability of fresh water. Where available water resources are scarce, humans have developed technologies like desalination and wastewater recycling to stretch
2071-416: Is critical to the survival of all living organisms . Many organisms can thrive on salt water, but the great majority of vascular plants and most insects , amphibians , reptiles , mammals and birds need fresh water to survive. Fresh water is the water resource that is of the most and immediate use to humans. Fresh water is not always potable water , that is, water safe to drink by humans . Much of
2180-452: Is extracted for human consumption. Agriculture uses roughly two thirds of all fresh water extracted from the environment. Fresh water is a renewable and variable, but finite natural resource . Fresh water is replenished through the process of the natural water cycle , in which water from seas, lakes, forests, land, rivers and reservoirs evaporates, forms clouds , and returns inland as precipitation. Locally, however, if more fresh water
2289-430: Is particularly crucial in Africa, where water resources are often scarce and climate change poses significant challenges. Saline water in oceans , seas and saline groundwater make up about 97% of all the water on Earth . Only 2.5–2.75% is fresh water, including 1.75–2% frozen in glaciers , ice and snow, 0.5–0.75% as fresh groundwater. The water table is the level below which all spaces are filled with water, while
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#17327914998602398-1076: Is possible that giant lakes only formed in the southern part of the Sahara. Other lakes are known from Adrar Bous in Niger , Era Kohor and Trou au Natron in the Tibesti Mountains , I-n-Atei in the Hoggar , at Ine Sakane and in Taoudenni in Mali , the Garat Ouda and Takarkori Lakes in the Acacus Mountains , Chemchane in Mauretania , at Guern El Louläilet in the Great Western Erg and Sebkha Mellala close to Ouargla , both in Algeria , at Wadi Shati and elsewhere in
2507-826: Is securing minimum streamflow , especially preserving and restoring instream water allocations . Fresh water is an important natural resource necessary for the survival of all ecosystems . Water pollution (or aquatic pollution) is the contamination of water bodies , with a negative impact on their uses. It is usually a result of human activities. Water bodies include lakes , rivers , oceans , aquifers , reservoirs and groundwater . Water pollution results when contaminants mix with these water bodies. Contaminants can come from one of four main sources. These are sewage discharges, industrial activities, agricultural activities, and urban runoff including stormwater . Water pollution may affect either surface water or groundwater . This form of pollution can lead to many problems. One
2616-525: Is supported by the frequent depiction of cattle in rock paintings . The relative importance of hunter-gatherer practices and pastoralism, and whether people were sedentary or migratory, is unclear. The Dufuna canoe , one of the oldest known ships in the world, appears to date to the Holocene humid period and implies that the waterbodies of that time were navigated by humans. The cultural units "Masara" and "Bashendi" existed in Dakhleh Oasis during
2725-586: Is the Cave of Swimmers in the Gilf Kebir mountains of Egypt; other well known sites are the Gabal El Uweinat mountains also of Egypt, Arabia and the Tassili n'Ajjer in Algeria where rock paintings from this time have been discovered. Humans also left artifacts such as Fesselsteine and ceramics in what today are inhospitable deserts. North Africa together with East Asia is one of
2834-420: Is the degradation of aquatic ecosystems . Another is spreading water-borne diseases when people use polluted water for drinking or irrigation . Water pollution also reduces the ecosystem services such as drinking water provided by the water resource . Uses of water include agricultural , industrial , household , recreational and environmental activities. The Sustainable Development Goals are
2943-518: Is the most profound climate change of the low latitudes during the last 100,000 years and stands out within the otherwise relatively climatically stable Holocene. It is part of the so-called Holocene climatic optimum and coincides with a global warm phase, the Holocene Thermal Maximum . Liu et al. 2017 subdivided the humid period into an "AHP I" which lasted until 8,000 years ago, and an "AHP II" from 8,000 years onward, with
3052-486: The Acacus Mountains . Earlier humid periods in Africa were postulated after the discovery of these rock paintings in now-inhospitable parts of the Sahara. When the period ended, humans gradually abandoned the desert in favour of regions with more secure water supplies, such as the Nile Valley and Mesopotamia , where they gave rise to early complex societies . Herodotus in 440 BC and Strabo in 23 AD discussed
3161-661: The Air Mountains . Ephemeral lakes developed between dunes, and a "freshwater archipelago" appears to have existed in the Murzuq basin. All these lake systems left fossils such as fish, limnic sediments and fertile soils that were later used for agriculture (El Deir, Kharga Oasis ). Finally, crater lakes formed in volcanic fields such as Trou au Natron and Era Kohor in the Tibesti, and sometimes survive to this day as smaller remnant lakes such as Malha crater in
3270-537: The Amazon River . The atmosphere contains 0.04% water. In areas with no fresh water on the ground surface, fresh water derived from precipitation may, because of its lower density, overlie saline ground water in lenses or layers. Most of the world's fresh water is frozen in ice sheets . Many areas have very little fresh water, such as deserts . Water is a critical issue for the survival of all living organisms. Some can use salt water but many organisms including
3379-584: The Blue Nile . The White Nile flooded part of its valley and reconnected to the main Nile. In Egypt widespread flooding by the "Wild Nile" took place; this "Wild Nile" period led to the largest recorded floods on this river, sedimentation in floodplains, and probably also impacted human populations along the river. Even earlier, 17,000–16,800 years ago, meltwater from glaciers in Ethiopia – which were retreating at that time – may have begun to increase
Freshwater (disambiguation) - Misplaced Pages Continue
3488-562: The Bodélé Depression and perhaps as much as 8% of the present-day Sahara desert. It influenced the climate itself; for example rainfall would have been reduced at the centre of the lake and increased at its margins. Lake Chad was possibly fed from the north by rivers draining the Hoggar (Taffassasset drainage) and Tibesti Mountains, from the Ennedi Mountains in the east through the "Eastern palaeorivers" and from
3597-616: The Congo Air Boundary or increased convergence along this boundary may have contributed; the Congo Air Boundary would have been shifted east by the stronger westerly winds directed by lower atmospheric pressure over Northern Africa, allowing additional moisture from the Atlantic to reach East Africa. The parts of East Africa that were isolated from Atlantic moisture did not become significantly wetter during
3706-801: The Grand Erg Occidental . Wetlands also expanded during the AHP, but both their expansion and subsequent retreat were slower than that of lakes. The Niger River , which had been dammed by dunes during the LGM, formed a lake in the Timbuktu region that eventually overflowed and drained at some point during the AHP. In some parts of the Sahara ephemeral lakes formed such as at Abu Ballas , Bir Kiseiba , Bir Sahara , Bir Tarfawi and Nabta Playa in Egypt, which may relate to later Egyptian religions, or swamp -lakes such as at Adrar Bous close to
3815-663: The Green Sahara periods) and are not appreciably replenished under current climatic conditions - at least compared to drawdown, these aquifers form essentially non-renewable resources comparable to peat or lignite, which are also continuously formed in the current era but orders of magnitude slower than they are mined. Fresh water can be defined as water with less than 500 parts per million (ppm) of dissolved salts . Other sources give higher upper salinity limits for fresh water, e.g. 1,000 ppm or 3,000 ppm. Fresh water habitats are classified as either lentic systems , which are
3924-588: The Holocene AHP or correlative climate phases are "Holocene humid period", which also covers an analogous episode in Arabia and Asia; "early to mid-Holocene humid episode"; "Holocene Pluvial"; "Holocene Wet Phase"; " Kibangien A " in Central Africa; "Makalian" for the Neolithic period of northern Sudan; "Nabtian Pluvial", "Nabtian Wet Phase" or "Nabtian period" for the 14,000–6,000 humid period over
4033-684: The Mayo Kebbi and the Benue River , eventually reaching the Gulf of Guinea . Older dune systems were submerged by Lake Chad. Among the large lakes which may have formed in the Sahara are Lake Megafezzan in Libya and Lake Ptolemy in Sudan. Quade et al. 2018 raised some doubts about the size and existence of some of these lakes such as Lake Ptolemy, Lake Megafezzan, Lake Ahnet-Mouydir ; it
4142-517: The Meidob volcanic field . Potentially, the increased availability of water during the AHP may have facilitated the onset of phreatomagmatic eruptions such as maar formation in the Bayuda volcanic field , although the chronology of volcanic eruptions there is not well known enough to substantiate a link to the AHP. Increased precipitation resulted in the formation or reactivation of river systems in
4251-509: The Nile crocodile and the fish Clarias gariepinus and Tilapia zillii . It is possible that the name Tassili n'Ajjer , which means "plateau of the rivers" in Berber , is a reference to past river flows. On the other hand, intense flows of these rivers may have made their shores dangerous to humans and thus created additional impetus for human movement. Now-dry river valleys from the AHP in
4360-732: The North Atlantic and the Mediterranean Sea towards the Mediterranean coasts of Africa. There were complex interactions with the atmospheric circulation of the extratropics and between moisture coming from the Atlantic Ocean and the Indian Ocean , and an increased overlap between the areas wetted by the monsoon and those wetted by extratropical cyclones . Climate models indicate that changes from
4469-658: The Northern Hemisphere while decreasing it in the Southern Hemisphere . According to climate modelling , orbital changes by themselves cannot increase precipitation over Africa enough to explain the formation of the large desert lakes such as 330,000 square kilometres (130,000 sq mi) Lake Megachad , the climate proxies for precipitation, or the northward expansion of vegetation unless ocean and land surface changes are factored in. Decreasing albedo resulting from vegetation changes
Freshwater (disambiguation) - Misplaced Pages Continue
4578-565: The Ténéré desert a cemetery has been found, which has been used to reconstruct the lifestyle of these former inhabitants of the Sahara, and at Lake Ptolemy in Nubia humans settled close to the lake shore, using its resources and perhaps even engaging in leisure activities. At that time, many humans appear to have depended on water-bound resources, seeing as many of the tools left by the early humans are associated with fishery ; hence this culture
4687-514: The earth 's fresh water (on the surface and groundwater) is to a substantial degree unsuitable for human consumption without treatment . Fresh water can easily become polluted by human activities or due to naturally occurring processes, such as erosion. Fresh water makes up less than 3% of the world's water resources, and just 1% of that is readily available. About 70% of the world's freshwater reserves are frozen in Antarctica . Just 3% of it
4796-528: The last glacial maximum , the latter is equivalent to the "Kanemian"; "Kanemian dry period" refers to a dry period between 20,000 and 13,000 years before present in the Lake Chad area. The African humid period took place in the late Pleistocene and early-middle Holocene , and saw increased precipitation in Northern and Western Africa due to a northward migration of the tropical rainbelt. The AHP
4905-970: The rain shadows of mountains and could have supported arid climate vegetation, explaining the presence of its pollen in sediment cores . In addition, north–south gradations in vegetation patterns have been reconstructed from charcoal and pollen data. Fossils record changes in the animal fauna of the Sahara. This fauna included antelopes , baboons , cane rats , catfish , clams , cormorants , crocodiles, elephants, frogs, gazelles , giraffes , hartebeest , hares , hippos , molluscs , Nile perches , pelicans , rhinoceroses , snake-eagles , snakes, tilapia , toads , turtles and many more animals, and in Egypt there were African buffaloes , spotted hyenas , warthogs , wildebeest and zebra . Additional birds include brown-necked raven , coot , common moorhen , crested grebe , glossy ibis , long-legged buzzard , rock dove , spur-winged goose and tufted duck . Large herds of animals lived in
5014-549: The AHP although at one site in Somalia the seasonality of precipitation may or may not have decreased. Various contributing factors may have led to the increased humidity in East Africa, not all of which were necessarily operating simultaneously during the AHP. That the "African humid period" reached this part of Africa has been doubted. Finally, increased greenhouse gas concentrations may have been involved in directing
5123-501: The AHP has been described as a mosaic between various vegetation types of semi-desert and humid origin rather than a simple northward displacement of plant species, and some brown or yellow vegetation communities persisted. There was no southward displacement of Mediterranean plants during the Holocene and on the Tibesti Mountains cold temperatures may have restricted the expansion of tropical plants. Pollen data often show
5232-543: The AHP has been used as an experiment in the Paleoclimate Modelling Intercomparison Project . Most recently, the effects of the Sahara greening on other continents has drawn scientific attention. The concept of a Sahara significantly different than today, and the rich record it left, has driven the imagination of the public and scientists alike. While the precipitation changes since the last glacial cycle are well established,
5341-483: The AHP. In Egypt, some rivers active during the AHP are now gravel ridges. In the Air , Hoggar and Tibesti Mountains, the so-called "Middle Terrace " was emplaced at this time. The rivers of the Sahara, lakes and their watersheds may have acted as pathways for the spread of humans and animals; the rivers were often connected to each other by alluvial fans . Proposed examples of animals that spread through rivers are
5450-522: The AHP. In the Acacus Mountains, several cultural horizons known as Early and Late Acacus and Early, Middle, Late and Final Pastoral have been identified while in Niger the Kiffian culture has been related to the beginning of the AHP. Ancient civilizations thrived, with farming and animal husbandry taking place in Neolithic settlements. Possibly, the domestication of plants in Africa
5559-611: The Eastern Mediterranean and Levant ; " Neolithic pluvial "; "Neolithic Subpluvial"; "Neolithic wet phase"; " Nouakchottien " of the Western Sahara 6,500 – 4,000 years before present; "Subpluvial II" and " Tchadien " in the Central Sahara 14,000 – 7,500 years before present. The terms "Big Dry", " Léopoldvillien " and Ogolien [ fr ] have been applied to the dry period in
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#17327914998605668-557: The Fezzan in Libya , at Bilma, Dibella, Fachi and Gobero in the Ténéré , Seeterrassental in Niger and at "Eight Ridges", El Atrun, Lake Gureinat, Merga, "Ridge", Sidigh, at Wadi Mansurab, Selima and Oyo in Sudan. The lakes of Ounianga merged into two large lakes and overflowed, either above surface or underground. Mosaics of small lakes developed in some regions, such as
5777-485: The ITCZ in Arabia is also contentious. The African humid period that took place in East Africa appears to have been caused by different mechanisms. Among the proposed mechanisms are decreased seasonality of precipitation due to increased dry season precipitation, shortening of the dry season, increased precipitation and increased inflow of moisture from the Atlantic and Indian Oceans. The Atlantic moisture inflow
5886-784: The Jebel Marra and Tibesti Mountains between 15,000 and 14,000 years ago and the youngest stage of glaciation in the High Atlas mountains took place at the same time as the Younger Dryas and early African humid period. Around 14,500 years ago, lakes started to appear in the arid areas. The humid period began about 15,000 –14,500 years ago. The onset of the humid period took place almost simultaneously over all of Northern and Tropical Africa, with impacts as far as Santo Antão on Cape Verde . In Arabia, wet conditions apparently took about two millennia to advance northward,
5995-690: The Mediterranean coast and the Nile Valley. The aridity during the Last Glacial Maximum appears to have been the consequence of the colder climate and larger polar ice sheets , which squeezed the monsoon belt to the equator and weakened the West African Monsoon. The atmospheric water cycle and the Walker and Hadley circulations were weaker as well. Exceptional dry phases are linked to Heinrich events when there are
6104-420: The Nile delta. Humans were hunting large animals with weapons that have been found in archaeological sites and wild cereals occurring in the Sahara during the AHP such as brachiaria , sorghum and urochloa were an additional source of food. Humans also domesticated cattle , goats and sheep . Cattle domestication may have occurred especially in the more environmentally variable Eastern Sahara, where
6213-659: The Sahara and consisted of an open grass savannah with shrubs and trees, with a moist savanna vegetation getting established in the mountains. In general, the vegetation expanded northward to 27 – 30° northern latitude in West Africa with a Sahel boundary at about 23° north, as the Sahara was populated by plants that today often occur about 400–600 kilometres (250–370 mi) farther south. The northward movement of vegetation took some time and some plant species moved faster than others. Plants that perform C3 carbon fixation became more common. The fire regime of
6322-443: The Sahara contained extensive dune fields and was mostly uninhabited. It was much larger than today, and its lakes and rivers such as Lake Victoria and the White Nile were either dry or at low levels. The humid period began about 14,600–14,500 years ago at the end of Heinrich event 1 , simultaneously to the Bølling–Allerød warming . Rivers and lakes such as Lake Chad formed or expanded, glaciers grew on Mount Kilimanjaro and
6431-431: The Sahara retreated. Two major dry fluctuations occurred; during the Younger Dryas and the short 8.2 kiloyear event . The African humid period ended 6,000–5,000 years ago during the Piora Oscillation cold period. While some evidence points to an end 5,500 years ago, in the Sahel , Arabia and East Africa, the end of the period appears to have taken place in several steps, such as the 4.2-kiloyear event . The AHP led to
6540-420: The Sahara. Some animals expanded over the whole desert, while others were limited to places with deep water. Earlier humid periods in the Sahara may have allowed species to cross the now-desert. A reduction in open grasslands at the beginning of the AHP may explain the decline of the populations of some mammals during and a population bottleneck in cheetahs at the start of the humid period, while leading to
6649-490: The Sahara. The large Tamanrasset River flowed from the Atlas Mountains and Hoggar westward towards the Atlantic and entered it in the Bay of Arguin in Mauritania . It once formed the 12th largest watershed in the world and left a submarine canyon and riverine sediments. Together with other rivers it formed estuaries and mangroves in the Bay of Arguin. Other rivers in the same area also formed submarine canyons, and sediment patterns in marine sediment cores and
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#17327914998606758-656: The Senegal River expanded its riverbed, breached dunes and re-entered the Atlantic Ocean. During the African humid period, lakes, rivers, wetlands and vegetation including grass and trees covered the Sahara and Sahel creating a "Green Sahara" with a land cover that has no modern analogues. Evidence includes pollen data, archaeological sites, evidence of faunal activity such as diatoms , mammals , ostracods , reptiles and snails , buried river valleys , organic-rich mats , mudstones , evaporites as well as travertines and tufas deposited in subaqueous environments. The vegetation cover then extended over almost all of
6867-494: The area above this level, where spaces in the rock and soil contain both air and water, is known as the unsaturated zone. The water in this unsaturated zone is referred to as soil moisture. Below the water table, the entire region is known as the saturated zone, and the water in this zone is called groundwater. Groundwater plays a crucial role as the primary source of water for various purposes including drinking, washing, farming, and manufacturing, and even when not directly used as
6976-421: The available supply further. However, given the high cost (both capital and running costs) and - especially for desalination - energy requirements, those remain mostly niche applications. A non-sustainable alternative is using so-called " fossil water " from underground aquifers . As some of those aquifers formed hundreds of thousands or even millions of years ago when local climates were wetter (e.g. from one of
7085-428: The beginning of the AHP across Africa. The African humid period has been explained by increased insolation during Northern Hemisphere summer. Due to precession , the season at which Earth passes closest to the Sun on its elliptical orbit – the perihelion – changes, with maximum summer insolation occurring when this happens during Northern Hemisphere summer. Between 11,000 and 10,000 years ago, Earth passed through
7194-420: The concept of a Green Sahara in the 1930s. Later in the 20th century, conclusive evidence of a past greener Sahara, the existence of lakes and higher Nile flow levels was increasingly reported and it was recognized that the Holocene featured a humid period in the Sahara. The idea that changes in Earth's orbit around the Sun influence the strength of the monsoons was already advanced in 1921, and while
7303-444: The continent. A similar wet episode took place in the tropical Americas, China, Asia, India , the Makran region, the Middle East and the Arabian Peninsula and appears to relate to the same orbital forcing as the AHP. An early Holocene monsoonal episode extended as far as the Mojave Desert in North America. In contrast, a drier episode is recorded from much of South America where Lake Titicaca , Lake Junin ,
7412-544: The desert such as highlands like the Red Sea Hills were reached by the westerlies or weather systems associated with the subtropical jet stream —and thus received precipitation—is contentious. It is only clearly supported for the Maghreb in northwestern Africa and parts of northeastern Africa, though river flow / terrace formation and lake development in the Tibesti and Jebel Marra mountains and residual Nile flow may be explained in this way. The highlands of Africa appear to have been less affected by drought during
7521-425: The discharge of the Amazon River and water availability in the Atacama were lower. The discharge of the Congo , Niger , Nile , Ntem , Rufiji , and Sanaga rivers increased. Runoff from Algeria , equatorial Africa, northeastern Africa and the western Sahara was also larger. Changes in the morphology of the river systems and their alluvial plains occurred in response to the increased discharge, and
7630-405: The eastern Sahara have been used as analogues for former river systems on Mars . Conditions and resources were ripe for first hunter-gatherers , fishermen and, later, pastoralists ; the exact chronology – when humans returned in the Sahara after the onset of the AHP – is disputed. They may have come either from the north ( Maghreb or Cyrenaica ) where the Capsian culture was located,
7739-422: The equator, and rainforests had retreated in favour of afromontane and savannah landscapes as temperatures, rainfall, and humidity decreased. There is little and often equivocal evidence of human activity in the Sahara or Arabia at that time, reflecting its drier nature; in the Acacus Mountains the last human presence was recorded 70,000–61,000 years ago and by the LGM humans had largely retreated to
7848-650: The existence of a greener Sahara, although their reports were at first questioned owing to their anecdotal nature. In 1850 the researcher Heinrich Barth discussed the possibility of past climate change leading to increased wetness in the Sahara after discovering petroglyphs in the Murzuq Desert , as did Ahmed Hassanein following his 1923 exploration of the Libyan Desert when he saw depictions of savanna animals at Gabal El Uweinat . Further discoveries of petroglyphs led desert explorer László Almásy to coin
7957-504: The expansion of the population of other animals such as Hubert's multimammate mouse and Natal multimammate mouse . A number of lakes formed or expanded in the Sahara and the Hoggar and Tibesti Mountains . The largest of them was Lake Chad which increased to at least ten times its present-day size to form Lake Megachad. This enlarged Lake Chad reached dimensions of 1,000 by 600 kilometres (620 mi × 370 mi) in north–south and east–west direction respectively, covering
8066-510: The fertilizing effects of increased carbon dioxide concentrations in the atmosphere. Other sources of albedo changes: Warmer extratropics during summer may have drawn the Intertropical Convergence Zone (ITCZ) northward by about five or seven degrees latitude , resulting in precipitation changes. Sea surface temperatures off North Africa warmed under orbital effects and through weaker trade winds , leading to
8175-584: The flow of water and sediment in the Nile. In the East African Rift water levels in lakes began to rise by about 15,500/15,000 -12,000 years ago; Lake Kivu began overflowing into Lake Tanganyika by about 10,500 years ago. About the same time that the AHP started, the cold glacial climate in Europe associated with Heinrich event 1 ended with climate changing as far as Australasia . A warming and retreat of sea ice around Antarctica coincides with
8284-473: The former being wetter than the latter. The African humid period was not the first such phase; evidence for about 230 older such "green Sahara"/wet periods exist going back perhaps to the first appearance of the Sahara 7–8 million years ago, for example during Marine Isotope Stage 5 a and c. Earlier humid periods appear to have been more intense than the AHP of the Holocene, including the exceptionally intense Eemian humid period. This humid period provided
8393-483: The freshwater flow to be measurably contaminated both by insoluble solids but also by the soluble components of those soils. Significant quantities of iron may be transported in this way including the well-documented transfer of iron-rich rainfall falling in Brazil derived from sand-storms in the Sahara in north Africa . In Africa, it was revealed that groundwater controls are complex and do not correspond directly to
8502-547: The great majority of higher plants and most mammals must have access to fresh water to live. Some terrestrial mammals, especially desert rodents , appear to survive without drinking, but they do generate water through the metabolism of cereal seeds, and they also have mechanisms to conserve water to the maximum degree. Freshwater ecosystems are a subset of Earth's aquatic ecosystems . They include lakes , ponds , rivers , streams , springs , bogs , and wetlands . They can be contrasted with marine ecosystems , which have
8611-476: The lack of lakes (cattle having high requirements of drinking water ) may however have limited the occurrence of cattle. Animal husbandry picked up in earnest around 7,000 years ago when domestic animals came to the Sahara, and a population boom may be linked to this change in cultural practice; cattle and goats spread southwestwards from northeasternmost Africa from 8,000 years before present. Dairying has been demonstrated in some locations and cattle-husbandry
8720-529: The last glacial maximum. The end of the glacial drought occurred between 17,000 and 11,000 years ago, with an earlier beginning noted in the Acacus and Saharan mountains 26,500–22,500 and (possibly) 18,500 years ago, respectively. In southern and central Africa earlier starts 17,000 and 17,500 years ago, respectively, may be linked to Antarctic warming, while Lake Malawi appears to have been low until about 10,000 years ago. High lake levels occurred in
8829-412: The magnitude and timing of the changes are unclear. Depending on how and where measurements and reconstructions are made, different beginning dates, ending dates, durations and precipitation levels have been determined for the African humid period. The amounts of precipitation reconstructed from paleoclimate records and simulated by climate modelling are often inconsistent with each other; in general,
8938-469: The main Nile may have been dammed by dunes. The Nile Delta was partially dry, with sandy plains extending between ephemeral channels and exposed seafloor, and it became a source of sand for ergs farther east. Other lakes across Africa, such as Lake Chad and Lake Tanganyika , also had shrunk during this time, and both the Niger River and Senegal River were stunted. Whether some parts of
9047-531: The monsoon is unclear. During summer, solar heating is stronger over the North African land than over the ocean, forming a low pressure area that draws moist air and precipitation in from the Atlantic Ocean. This effect was strengthened by the increased summer insolation, leading to a stronger monsoon that also reached farther north. The effects of these circulation changes reached as far as
9156-485: The northward expansion of the monsoon. There is not universal agreement on the effects of dust on precipitation in the Sahel, however, in part because the effects of dust on precipitation may be dependent on its size. In addition to raw precipitation changes, changes in precipitation seasonality such as the length of dry seasons need to be considered when assessing the effects of climate change on vegetation, as well as
9265-446: The number of people suffering from water scarcity ." Another target, Target 15.1, is: "By 2020, ensure the conservation, restoration and sustainable use of terrestrial and inland freshwater ecosystems and their services, in particular forests, wetlands , mountains and drylands , in line with obligations under international agreements." Subnotes Green Sahara The African humid period (AHP; also known by other names )
9374-604: The occurrence of submarine landslides in the area have been related to the activity of these rivers. Rivers such as the Irharhar in Algeria , Libya and Tunisia and the Sahabi and Kufra rivers in Libya were active during this time although there is some doubt that they had perennial flow; they appear to have been more important in earlier humid periods. Small watersheds, wadis and rivers discharging into endorheic basins such as Wadi Tanezzuft also carried water during
9483-483: The onset of the AHP in tropical southeastern Africa; there, orbital changes would be expected to lead to climate variations opposite to those in the Northern Hemisphere. The pattern of humidity changes in south-eastern Africa are complex. The African humid period extended over most of Africa: The Sahara and eastern, southeastern and equatorial Africa. In general, forests and woodlands expanded through
9592-725: The onset of the African humid period as well as to increased humidity in Arabia. Later, in the Blytt–Sernander sequence the humid period coincides with the Atlantic period . During the Last Glacial Maximum , the Sahara and Sahel had been extremely dry with even less precipitation than today as reflected by the extent of dune sheets and water levels in closed lakes . The Sahara was much larger, extending 500–800 kilometres (310–500 mi) farther south to about 12° northern latitude. Dunes were active much closer to
9701-527: The original description was partly inaccurate, later widespread evidence for such orbital controls on climate was found. At first it was believed that humid periods in Africa correlate with glacial stages (" pluvial hypothesis ") before radiocarbon dating became widespread. The development and existence of the African humid period has been investigated with archaeology , climate modelling and paleoproxies , with archaeological sites , dunes and deposits left by lakes, aeolian deposits and leaf wax in
9810-413: The others as well. Water scarcity (closely related to water stress or water crisis) is the lack of fresh water resources to meet the standard water demand. There are two types of water scarcity. One is physical. The other is economic water scarcity . Physical water scarcity is where there is not enough water to meet all demands. This includes water needed for ecosystems to function. Regions with
9919-576: The pathways for early humans to cross Arabia and Northern Africa and which, together with later moist periods, has been linked to expansions of the Aterian populations and the speciation of insect species. Such humid periods are usually associated with interglacials , while glacial stages correlate to dry periods; they occur during precession minima, unless large ice sheets or insufficient greenhouse gas concentrations suppress their onset. The Bølling–Allerød warming appears to be synchronous with
10028-437: The perihelion at the time of summer solstice , increasing the amount of solar radiation by about 8%, resulting in the African monsoon becoming both stronger and reaching farther north. Between 15,000 and 5,000 years ago, summer insolation was at least 4% higher than today. The obliquity also decreased during the Holocene but the effect of obliquity changes on the climate is focused on the high latitudes and its influence on
10137-462: The sea and wetlands playing an important role. Pollen , lake deposits and former levels of lakes have been used to study the ecosystems of the African humid period, and charcoal and leaf impressions have been used to identify vegetation changes. Questions in AHP research are its beginning, cause, intensity, end, land feedbacks and fluctuations during the AHP. The time 6,000 years ago has received particular attention, especially since that period of
10246-421: The sea if windy conditions have lifted drops of seawater into the rain-bearing clouds. This can give rise to elevated concentrations of sodium , chloride , magnesium and sulfate as well as many other compounds in smaller concentrations. In desert areas, or areas with impoverished or dusty soils, rain-bearing winds can pick up sand and dust and this can be deposited elsewhere in precipitation and causing
10355-499: The simulation of the Green Sahara is considered a problem for earth system models. There is more evidence of the late phase of the AHP than its beginning. Erosion of lake sediments and carbon reservoir effects make it difficult to date when they dried up. Vegetation changes by themselves do not necessarily indicate precipitation changes, as changes in seasonality, plant species composition and changes in land use also play
10464-751: The south ( Sub-Saharan Africa ), or the east ( Nile Valley ). The human population in the Sahara rapidly increased during the AHP, interrupted by a brief decline between 7,600 and 6,700 years ago. Traces of human activity have been found in the Acacus Mountains where caves and rock shelters were used as basecamps for humans, such as the Uan Afuda cave and the Uan Tabu and Takarkori rock shelters. The first occupation in Takarkori took place between 10,000 and 9,000 years ago; about five millennia of human cultural evolution are recorded there. At Gobero in
10573-605: The south by the Chari - Logone and Komadugu Rivers . The Chari River was the main tributary while the rivers draining the Tibesti formed alluvial fans /the Angamma river delta at their entry into northern Lake Chad. Skeletons of elephants, hippos and hominins have been found in the Angamma delta, which is the dominant shoreline feature of northern Lake Chad. The lake overflowed into the Niger River during highstand through
10682-505: The start of the African humid period, although the Antarctic Cold Reversal also falls into this time and may relate to a drought interval recorded in the Gulf of Guinea . The African humid period was caused by a stronger West African Monsoon directed by changes in solar irradiance and in albedo feedbacks. These led to increased moisture import from both the equatorial Atlantic into West Africa, as well as from
10791-420: The stillwaters including ponds , lakes, swamps and mires ; lotic which are running-water systems; or groundwaters which flow in rocks and aquifers . There is, in addition, a zone which bridges between groundwater and lotic systems, which is the hyporheic zone , which underlies many larger rivers and can contain substantially more water than is seen in the open channel. It may also be in direct contact with
10900-486: The subtropics. Obliquity and precession are responsible for two of the foremost Milankovich cycles and are responsible not only for the onset and cessation of ice ages but also for monsoon strength variations. Southern Hemisphere monsoons are expected to have the opposite response of Northern Hemisphere monsoons to precession, as the insolation changes are reversed; this observation is borne out by data from South America. The precession change increased seasonality in
11009-666: The surface, and soil moisture, and less than 0.01% of it as surface water in lakes , swamps and rivers . Freshwater lakes contain about 87% of this fresh surface water, including 29% in the African Great Lakes , 22% in Lake Baikal in Russia, 21% in the North American Great Lakes , and 14% in other lakes. Swamps have most of the balance with only a small amount in rivers, most notably
11118-469: The time). Freshwater ecosystems contain 41% of the world's known fish species. The increase in the world population and the increase in per capita water use puts increasing strains on the finite resources availability of clean fresh water. The response by freshwater ecosystems to a changing climate can be described in terms of three interrelated components: water quality, water quantity or volume, and water timing. A change in one often leads to shifts in
11227-657: The underlying underground water. The original source of almost all fresh water is precipitation from the atmosphere , in the form of mist , rain and snow . Fresh water falling as mist, rain or snow contains materials dissolved from the atmosphere and material from the sea and land over which the rain bearing clouds have traveled. The precipitation leads eventually to the formation of water bodies that humans can use as sources of freshwater: ponds , lakes , rainfall , rivers , streams , and groundwater contained in underground aquifers . In coastal areas fresh water may contain significant concentrations of salts derived from
11336-504: The vegetation changed; in the desert the expansion of vegetation facilitated fire activity, while in the savanna the increased prevalence of woody vegetation reduced fire activity. Forests and plants from the humid tropics were concentrated around lakes, rivers and the Atlantic Ocean coast of Senegal ; waterbodies were also settled by aquatic and partially aquatic plants and the Senegalese coast by mangroves . The landscape during
11445-466: The vegetation. Reduced dust generation from a wetter Sahara, where major dust-generating regions were submerged by lakes, influences the climate by reducing the amount of light absorbed by dust. Decreased dust emissions also modify cloud properties, making them less reflective and more efficient at inducing precipitation. In climate models, reduced amounts of dust in the troposphere together with vegetation changes can often but not always explain
11554-540: The western and central sectors and the Libyan sand sea was still a desert although pure desert areas retreated or became arid / semiarid . An arid belt may have existed north of 22° latitude and towards the Nile Delta, or the vegetation and the African monsoon might have reached 28–31° northern latitude; in general conditions between 21° and 28° northern latitude are poorly known. Dry areas may have persisted in
11663-461: Was constant recharge with little variation with precipitation; in most sites (arid, semi-arid, humid), annual recharge increased as annual precipitation remained above a certain threshold; and in complex areas down to 0.1 aridity index (focused recharge), there was very inconsistent recharge (low precipitation but high recharge). Understanding these relationships can lead to the development of sustainable strategies for water collection. This understanding
11772-405: Was delayed by the increased food availability during the AHP, it only took place around 2,500 BC . Humans created rock art such as petroglyphs and rock paintings in the Sahara, perhaps the largest density of such creations in the world. Scenes include animals and everyday life such as swimming which supports the presence of past wetter climates. One well-known such petroglyph location
11881-506: Was in part triggered by a stronger West African and Indian monsoon, perhaps explaining why the effects of the AHP extended into the Southern Hemisphere. The behaviour of the easterly trade winds is unclear; increased moisture transport by easterly trade winds may have aided in the development of the AHP but alternatively a stronger Indian Monsoon that draws easterly winds away from East Africa may have occurred. Changes in
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