The Tibetan Plateau contains theheadwaters of thedrainage basins of most of thestreams andrivers in surroundingregions. This includes the three longest rivers inAsia (theYellow,Yangtze, andMekong). Its tens of thousands ofglaciers and other geographical and ecological features serve as a "water tower" storing water and maintainingflow. It is sometimes termed theThird Pole because itsice fields contain the largest reserve of fresh water outside the polar regions. The impact ofclimate change on the Tibetan Plateau is of ongoing scientific interest.[10][11][12][13]
The Tibetan Plateau is bounded in the north by a broad escarpment where the altitude drops from around 5,000 metres (16,000 ft) to 1,500 metres (4,900 ft) over a horizontal distance of less than 150 kilometres (93 mi). Along the escarpment is a range of mountains. In the west, theKunlun Mountains separate the plateau from the Tarim Basin. About halfway across the Tarim the bounding range becomes theAltyn-Tagh and the Kunluns, by convention, continue somewhat to the south. In the 'V' formed by this split is the western part of theQaidam Basin. The Altyn-Tagh ends near the Dangjin pass on theDunhuang–Golmud road. To the west are short ranges called the Danghe, Yema, Shule, and Tulai Nanshans. The easternmost range is the Qilian Mountains. The line of mountains continues east of the plateau as theQinling, which separates theOrdos Plateau from Sichuan. North of the mountains runs the Gansu orHexi Corridor which was the main silk-road route fromChina proper to the West.
The plateau is a high-altitude aridsteppe interspersed with mountain ranges and largebrackish lakes. Annual precipitation ranges from 100 to 300 millimetres (3.9 to 11.8 in) and falls mainly ashail. The southern and eastern edges of the steppe have grasslands that can sustainably support populations of nomadic herdsmen, although frost occurs for six months of the year.Permafrost occurs over extensive parts of the plateau. Proceeding to the north and northwest, the plateau becomes progressively higher, colder, and drier, until reaching the remoteChangtang region in the northwestern part of the plateau. Here the average altitude exceeds 5,000 metres (16,000 ft) and winter temperatures can drop to −40 °C (−40 °F). As a result of this extremely inhospitable environment, the Changtang region (together with the adjoining Kekexili region) is the least populous region in Asia and the third least populous area in the world after Antarctica and northern Greenland.
The collision began in theUpper Cretaceous period about 70 million years ago, when the north-movingIndo-Australian Plate, moving at about 15 cm (6 in) per year, collided with theEurasian Plate. About 50 million years ago, this fast-moving Indo-Australian plate had completely closed theTethys Ocean, the existence of which has been determined bysedimentary rocks settled on the ocean floor, and thevolcanoes that fringed its edges. Since these sediments were light, they crumpled into mountain ranges rather than sinking to the floor. During this early stage of its formation in the Late Palaeogene, Tibet consisted of a deep palaeovalley bounded by multiple mountain ranges rather than the more topographically uniform elevated flatland that it is today.[17] The Tibetan Plateau's mean elevation continued to vary since its initial uplift in the Eocene; isotopic records show the plateau's altitude was around 3,000 metres above sea level around the Oligocene-Miocene boundary and that it fell by 900 metres between 25.5 and 21.6 million years ago, attributable to tectonic unroofing from east–west extension or to erosion from climatic weathering. The plateau subsequently rose by 500 to 1,000 metres between 21.6 and 20.4 million years ago.[18]
Natural-colour satellite image of the Tibetan Plateau.
Palaeobotanical evidence indicates that the Nujiang Suture Zone and the Yarlung-Tsangpo Suture Zone remained tropical or subtropicallowlands until the latestOligocene orEarly Miocene, enabling biotic interchange across Tibet.[19] The age of east–west grabens in the Lhasa and Himalaya terranes suggests that the plateau's elevation was close to its modern altitude by around 14 to 8 million years ago.[20] Erosion rates in Tibet decreased significantly around 10 million years ago.[21] The Indo-Australian plate continues to be driven horizontally below the Tibetan Plateau, which forces the plateau to move upwards; the plateau is stillrising at a rate of approximately 5 mm (0.2 in) per year (although erosion reduces the actual increase in height).[22]
Much of the Tibetan Plateau is of relatively low relief. The cause of this is debated among geologists. Some argue that the Tibetan Plateau is an upliftedpeneplain formed at low altitude, while others argue that the low relief stems fromerosion andinfill of topographic depressions that occurred at already high elevations.[23] The current tectonics of the plateau are also debated. The best-regarded explanations are provided by the block model and the alternative continuum model. According to the former, the crust of the plateau is formed of several blocks with little internal deformation separated by majorstrike-slip faults. In the latter, the plateau is affected by distributed deformation resulting from flow within the crust.[24]
The Tibetan Plateau supports a variety of ecosystems, most of them classified asmontane grasslands. While parts of the plateau feature analpine tundra-like environment, other areas feature monsoon-influenced shrublands and forests.Species diversity is generally reduced on the plateau due to the elevation and low precipitation. The Tibetan Plateau hosts theTibetan wolf,[25] and species ofsnow leopard,wild yak,wild ass, cranes, vultures, hawks, geese, snakes, andwater buffalo. One notable animal is thehigh-altitude jumping spider, that can live at elevations of over 6,500 metres (21,300 ft).[26]
TheSoutheast Tibet shrub and meadows cover the southeastern and eastern parts of the plateau and are generally rainier than the other high-altitude Tibetan Plateau regions
Extinct humans (Denisovans) lived on the Tibetan plateau from around 200,000 to 40,000 years ago, according to a study published inNature.[27]
Nomads on the Tibetan Plateau and in theHimalayas are the remainders of nomadic practices historically once widespread in Asia and Africa.[28] Pastoral nomads constitute about 40% of the ethnicTibetan population.[29] The presence of nomadic peoples on the plateau is predicated on their adaptation to survival on the world'sgrassland by raising livestock rather than crops, which are unsuitable to the terrain. Archaeological evidence suggests that the earliest human occupation of the plateau occurred between 30,000 and 40,000 years ago.[30] Since colonization of the Tibetan Plateau, Tibetan culture has adapted and flourished in the western, southern, and eastern regions of the plateau. The northern portion, theChangtang, is generally too high and cold to support permanent population.[31] One of the most notable civilizations to have developed on the Tibetan Plateau is theTibetan Empire from the 7th century to the 9th century AD.
Monsoons are caused by the different amplitudes of surface-temperature seasonal cycles between land and oceans. This differential warming occurs because heating rates differ between land and water. Ocean heating is distributed vertically through a "mixed layer" that may be 50 meters deep through the action of wind and buoyancy-generatedturbulence, whereas the land surface conducts heat slowly, with the seasonal signal penetrating only a meter or so. Additionally, thespecific heat capacity of liquid water is significantly greater than that of most materials that make up land. Together, these factors mean that the heat capacity of the layer participating in the seasonal cycle is much larger over the oceans than over land, with the consequence that the land warms and cools faster than the ocean. In turn, air over the land warms faster and reaches a higher temperature than does air over the ocean.[32] The warmer air over land tends to rise, creating an area oflow pressure. The pressure anomaly then causes a steady wind to blow toward the land, which brings the moist air over the ocean surface with it. Rainfall is then increased by the presence of the moist ocean air. The rainfall is stimulated by a variety of mechanisms, such as low-level air being lifted upwards by mountains, surface heating, convergence at the surface, divergence aloft, or from storm-produced outflows near the surface. When such lifting occurs, the air cools due to expansion in lower pressure, which in turn producescondensation and precipitation.
The Himalayas as seen from space looking south from over the Tibetan Plateau.
In winter, the land cools off quickly, but the ocean maintains the heat longer. The hot air over the ocean rises, creating a low-pressure area and a breeze from land to ocean while a large area of drying high pressure is formed over the land, increased by wintertime cooling.[32] Monsoons are similar tosea and land breezes, a term usually referring to the localized,diurnal cycle of circulation near coastlines everywhere, but they are much larger in scale, stronger and seasonal.[33] The seasonal monsoon wind shift and weather associated with the heating and cooling of the Tibetan plateau is the strongest such monsoon on Earth.
Ice of the plateau provides a valuable window to the past. In 2015, researchers studying the Plateau reached the top of the Guliyaglacier, with ice thickness of 310 m (1,020 ft), and drilled to a depth of 50 m (160 ft) in order to recoverice core samples. Due to the extremely lowbiomass in those 15,000-year-old samples, it had taken around 5 years of research to extract 33 viruses, of which 28 were new to science. None had survived the extraction process.Phylogenetic analysis suggests those viruses infectedplants or other microorganisms.[34][35]
The Tibetan Plateau contains the world's third-largest store of ice. Qin Dahe, the former head of theChina Meteorological Administration, issued the following assessment in 2009:
Temperatures are rising four times faster than elsewhere in China, and the Tibetan glaciers are retreating at a higher speed than in any other part of the world. In the short term, this will cause lakes to expand and bring floods and mudflows. In the long run, the glaciers are vital lifelines for Asian rivers, including theIndus and theGanges. Once they vanish, water supplies in those regions will be in peril.[36]
The Tibetan Plateau contains the largest area of low-latitude glaciers and is particularly vulnerable to global warming. Over the past five decades, 80% of the glaciers in the Tibetan Plateau have retreated, losing 4.5% of their combined areal coverage.[37]
This region is also liable to suffer damages from permafrost thaw caused by climate change.
Detailed map of Qinghai–Tibet Plateau infrastructure at risk from permafrost thaw under the SSP2-4.5 scenario.
Outside of the Arctic,Qinghai–Tibet Plateau (sometimes known as "the Third Pole"), also has an extensive permafrost area. It is warming at twice the global average rate, and 40% of it is already considered "warm" permafrost, making it particularly unstable. Qinghai–Tibet Plateau has a population of over 10 million people – double the population of permafrost regions in the Arctic – and over 1 million m2 of buildings are located in its permafrost area, as well as 2,631 km ofpower lines, and 580 km of railways.[38] There are also 9,389 km of roads, and around 30% are already sustaining damage from permafrost thaw.[39] Estimates suggest that under the scenario most similar to today,SSP2-4.5, around 60% of the current infrastructure would be at high risk by 2090 and simply maintaining it would cost $6.31 billion, with adaptation reducing these costs by 20.9% at most. Holding the global warming to 2 °C (3.6 °F) would reduce these costs to $5.65 billion, and fulfilling the optimisticParis Agreement target of 1.5 °C (2.7 °F) would save a further $1.32 billion. In particular, fewer than 20% of railways would be at high risk by 2100 under 1.5 °C (2.7 °F), yet this increases to 60% at 2 °C (3.6 °F), while under SSP5-8.5, this level of risk is met by mid-century.[38]
Webb, Andrew Alexander Gordon (2007).Contractional and Extensional Tectonics During the India-Asia Collision.ProQuest LLC. p. 137.ISBN978-0-549-50627-0.
^Sanyal, Sanjeev (10 July 2013).Land of the seven rivers: a brief history of India's geography. Penguin Books.ISBN978-0-14-342093-4.OCLC855957425.
^Lia, Jijun; Ma, Zhenhua; Li, Xiaomiao; Peng, Tingjiang; Guo, Benhong; Zhang, Jun; Song, Chunhui; Liu, Jia; Hui, Zhengchuang; Yu, Hao; Ye, Xiyan; Liu, Shanpin; Wang Xiuxi (2017). "Late Miocene-Pliocene geomorphological evolution of the Xiaoshuizi peneplain in the Maxian Mountains and its tectonic significance for the northeastern Tibetan Plateau".Geomorphology.295:393–405.Bibcode:2017Geomo.295..393L.doi:10.1016/j.geomorph.2017.07.024.
![[icon]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aWiki_letter_w_cropped.svg)
