Although there is no precise definition of “high altitude,” the term is generally applied to elevations above 2500–3000 m, as this is the altitude range at which as most individuals manifest characteristic anatomic, biochemical, clinical, and physiological changes. There is considerable interindividual variation, however, and some people are affected at altitudes as low as 2000 m. The purpose of this chapter is to describe the geography of some of the major mountainous areas of the world. After a brief description of the primary high altitude regions in which people reside, the chapter considers the people, terrain, climate, and socioeconomic features of these regions with a focus primarily on two of the major high altitude regions of the world in which people reside on a permanent basis, the Himalaya and Andes Mountains. The primary areas where it is possible to ascend above 3000 m are listed in Table 3.1. For sea-level visitors, an altitude of 4600–4900 m represents the highest acceptable level for permanent habitation, whereas for high altitude residents, 5800–6000 m is the highest so far recorded (West 1986). Indeed, the highest permanent human habitation is probably the town of La Rinconada (https://vis.sciencemag.org/hypoxia-city/), a mining city of close to 70,000 inhabitants (as of 2019) located at an altitude of 5100 m in southern Peru (Figure 3.1) (West 2002). Although even high altitude residents are affected by the altitude, the limit of permanent habitation is probably dictated by economic, rather than physiological, factors. Above 5000 m—even in the tropics—crops cannot be grown and animals cannot be pastured all year round. Nomadic and seminomadic peoples regularly take their flocks to pastures higher than 5000 m but do not reside in these areas on a permanent basis. Alaska Range Andes Mountains of South America Atlas Mountains of North Africa European Alps Ethiopian Highlands Himalayan Range Hindu Kush of Central Asia Mountains and Plateaus of Antarctica Mountains of East and South Africa Parts of the Southern Island of New Zealand Parts of New Guinea Pyrenees Mountains of France and Spain Rocky Mountains of the USA and Canada Sierra Madre of Mexico Sierra Nevada of the USA Tien Shan and Pamir Figure 3.1La Rinconada, Peru. At an elevation of 5100 m, it is likely the highest permanent habitation in the world. (Image courtesy of Axel Pittet and Sam Verges, Expédition 5300.) In addition to the European Alps and the Rocky Mountains of the United States and Canada, three of the major regions that support large populations are the Himalayas and Tibetan plateau, the Andes Mountains of South America, and the Ethiopian highlands. Each is considered in further detail below. The Himalayas form a topographically extremely complex region, extending 1500 miles from Nanga Parbat (8125 m) in the west to Namcha Barwa (7756 m) in the east. At their western extremity, they are part of a confused mass of peaks, passes, and glaciers where the western Kun Lun, Karakoram, Pir Panjal, and Pamirs form an area the size of France. The Himalayas contain the world’s highest mountain, Mount Everest (8848 m), and many other peaks over 7500 m. The main range forms the watershed between Central Asia and India, and there are middle ranges at intermediate altitudes. The outer Himalayas (up to 1500 m) form foothills rising from the plains of India. The Tibetan plateau is an area occupied by Tibetans, who have a well-defined culture. It extends in the south to the Himalayas and high Himalayan valleys. To the west, the plateau is demarcated by the northward curve of the Himalayas that continues into Kashmir, Baltistan, and then to Gilgit and the Karakoram. To the north, the peaks (up to 7700 m) of the Kun Lun range, 1500 miles long, mark off the plateau of Tibet from Xinjiang, a vast desert region. To the east, it extends to the Koko Nor or Qinghai Lake and, further south, the valleys of Qinghai and Sikiang and the gorge country of southeast Tibet. The area covers about 1.5 million square miles and is the largest and highest plateau in the world, much of it at an altitude of 4600–4900 m. It presents an enormous range of climate and topography. The major climatic contrast is between the southern side of the Himalayas and the high valleys exposed to the summer Indian monsoon with very high rainfall, particularly in the east, and the aridity and low rainfall of the Tibetan plateau. The change is so abrupt that in some passes in the eastern Himalayas, vegetation may change from tropical to subarctic within a few yards. Tibetans have been subject to influences from China, India, Central Asia, and the Middle East for many centuries (Stein 1972). Permanent buildings are found up to 3500 m with nomadic populations at higher levels. Neolithic human remains have been found near Lhasa (Ward 1990; Ward 1991). Aldenderfer (2011) suggested, “Although the data are sparse, both archaeology and genetics suggest that the plateau was occupied in the Late Pleistocene, perhaps as early as 30,000 yr ago.” Recent studies indicate the earliest habitation of the Tibetan plateau occurred 30,000 to 40,000 years before present (Zhang et al. 2018). There were almost certainly later migrations, with evidence of permanent settlements dating from about 6500, 5900, and 3750 years ago. With increasing numbers of Han Chinese immigrants, there are more than three million people living at or above more than 3000 m. It is estimated that the amount of land available for agriculture is only 5% of the total. The 1100 km Golmud to Lhasa rail link, three quarters of which is at altitudes above 4000 m, opened in July 2006. By making it possible to travel from all parts of China to Lhasa by rail, the railway has facilitated large increases in the number of Han Chinese tourists and residents in Tibet. The tourist population faces a risk of acute altitude illness, described further in Chapters 20–22, while those who relocate to Tibet are at risk for chronic forms of altitude illness described in Chapter 24. The highland zone of the Andes Mountains extends from Colombia in the north to central Chile in the south. It is flanked by an arid desert on its west with a deeply eroded escarpment to the east, which adjoins the Amazon basin. The central Andean region has three broadly defined areas running parallel with the Pacific Ocean: The cordillera occidental, the altiplano (a broad undulating plain at 4000 m in the middle), and the cordillera oriental in the east. The earliest archeological evidence for human occupation dates back 12,000 years (MacNeish 1971) and has been found at Ayacucho, Peru, at 2900 m; other early finds are recorded in central Chile, Venezuela, and Argentina. However, this date and the evidence for it have been questioned by Lynch (1990), who suggests a maximum date of 12,000 years ago, which is supported by more recent studies indicating humans reached high altitude regions in the Andes shortly after the arrival of modern humans in South America (Rademaker et al. 2014). The skeleton of a man who lived 9500 years ago has been found at Lauricocha (4200 m) in Peru (Hurtado 1971). It is widely accepted that South American indigenous peoples migrated into the American continent from the Mongolian area via the Bering land bridge. The pre-Inca civilizations were situated mainly along the Pacific coast and the population subsisted mainly on seafood. Little is known of the highland population during this period. Both agriculture and stock raising dominate the subsistence economy, with the upper limit of agriculture at 4000 m and the upper limit of vegetation at 4600 m. Mining is carried out at even greater altitudes, as described in Chapter 5, and tourism is a major source of visitors and revenue for the region. No well-circumscribed highland zone exists in the Ethiopian highlands. The country is intersected by many rift valley systems, establishing a connection between the African Rift Valley in the south and the Red Sea. The valley systems divide the country into three reasonably well-defined regions: The western highlands, the eastern highlands, and the rift valley itself with the lowland area. Some of the earliest hominids occupied wooded habitats between the Main Ethiopian Rift (MER) and the Afar Rift between 5.54 and 5.77 million years ago, providing important clues into the impact of climate and savannah habitat on hominid origins (WoldeGabriel et al. 2001). It appears that modern human populations have migrated in and out of the Ethiopian highlands for possibly the past 70,000 years (Hassen 1990), with recent archaeological evidence suggesting high altitude human settlements (above 4000 m) dated to 47,000 to 31,000 years ago in the Bale Mountains (Ossendorf et al. 2019). The northern part of the western highlands, the Amhara highlands, attains the greatest altitude (2400–3700 m) and is the home to some species that are not located anywhere else in the world: Geladas, the Ethiopian wolf, and ibex. Much of Ethiopian history centers on this area, which has been settled for many centuries. It is inhabited by the largest of Ethiopia’s many population groups, the Amharas and Tigraeans, who are the descendants of people who came from southern Arabia prior to 1000 BC (Sellassie 1972). Much of the population of Ethiopia lives above 2000 m. Gondar (3000 m), in the Amhara highlands with a population of about 200,000, became the second largest city in Africa, and it remained the capital of Ethiopia until the middle of the first century, when Addis Ababa was founded. Teff, a type of grass that produces a small seed, is grown up to 3000 m and is the mainstay of the agricultural economy. Ethiopian, as well as Kenyan, distance runners are among the most elite in the world (Wilber et al. 2012). Efforts to understand the environmental and genetic contributions to endurance running in these populations are under way (Georgiades et al. 2003). Numerous high altitude training centers in Ethiopia and those at moderate altitudes in Kenya (e.g., Iten and Eldoret, between 2100 and 2700 m) showcase the extraordinary performance of individuals in these populations and attract visitors from throughout the world interested in high altitude training. Most of the high altitude areas of the world are less-developed economically and for this reason population numbers in relation to altitude are difficult to obtain. Whereas, as of 2004, about 89.3 million people were living at altitudes above 2500 m, including 25.9 million and 5.4 million living more than 3500 m and 4500 m above sea level, respectively (Beall 2014), it is now estimated that over 200 million people live above 2500 m (Moore et al. 1998; WHO 2019). In South America, large populations have lived at high altitude since prehistory, and the Andean population at the time of the Spanish conquest was estimated between 4.5 and 7.5 million. In 1980, it was considered that between 10 and 17 million were living at more than 2500 m and, in Peru, 30% to 40% of the population of 4 million lived at or above this height, with 1.5% living at more than 4000 m. The highest capital city in the world, La Paz, Bolivia (3650 m), has a population of 2.7 million; other large cities such as El Alto (4150 m) and the constitutional capital of Bolivia, Sucre (2810 m), have populations of 970,000 and 300,000, respectively. In Asia and Africa, the estimates are less accurate. On the Tibetan plateau, which consists of the autonomous region of Tibet (Xizang) and Qinghai province, the population has been estimated between 4 and 5 million. Lhasa (3658 m) was estimated to have 130,000 inhabitants, most of whom were Tibetan in 1986, but recent immigration of Han Chinese has increased this number to roughly 900,000 people in 2015. Relatively small groups, nomads (at up to 5450 m) and miners (at up to 6000 m), live at higher levels. Fairly large numbers live at altitudes exceeding 3000 m in the upper valleys of eastern Tibet, and in Nepal about 60,000 live above this level, with a number of villages in Dolpo located at 5000 m (Snellgrove 1961). About 50% of the total Ethiopian population of 110 million live above 2000 m. Small populations in Mexico, the United States, and the former Soviet Union, for instance in Kyrgyzstan, live above 3000 m. In tropical latitudes, permanent settlements are usually placed where both pasture and timber can be used; the upper limit of habitation may fall between the two. Further from the equator, the upper limit falls below the timber line and variation in temperature becomes seasonal; the upper pasturelands are thus used for a seminomadic economy. Permanently inhabited villages are found at lower levels, with isolated groups of buildings or shelters on the pastures occupied for the grazing season and evacuated during the winter. Considerable migration may occur and part of the population may always be on the move. One mine, now closed, was worked at 5950 m in South America; although the miners lived at rather lower altitudes, the caretakers lived there permanently (West 1986a). Individuals who travel between moderate and high altitude work sites are discussed in Chapter 5. Highland populations, being strategically placed between prosperous lowland centers, play a vital role in trade. Because they are physiologically well adapted, they have been historically capable of crossing high mountain passes with heavy loads and have also used their animals to carry goods. Major mountain passes, including the Silk Road (Lu et al. 2016), have for centuries been arteries for trade, the movement of people and ideas, and the dissemination of disease. Increasing road construction in the Himalaya and elsewhere is changing many of these dynamics and facilitating both easier migration, as well as trade and commerce. Although mountain country varies widely, there are two distinct types: The high, flat, plateaus characteristic of Tibet and the altiplano of South America and the deep valleys seen in the Himalayas and Andes (Figure 3.2) as well as Ethiopia. Figure 3.2Contrasting terrain and climate at altitude. (A) Typical mountainous country on southern slopes of the Himalayas. (B) The volcanoes Pomerape (left) and Parinacota rising above the arid altiplano near the border of northern Chile and Bolivia. (Images courtesy of Andrew M. Luks.) Plateaus can support large populations and large towns, but they may be isolated by virtue of distance from lowland cities, which are usually the center of government, commerce, and industry. Extensive road construction in northern India and Nepal is an example of recent efforts to counteract this isolation. Increasing ease of air travel and railway has served a similar purpose. In mountain valleys, where flat ground is at a premium, populations tend to be smaller, with groups perched on slopes and ridges far from one another. The placing of houses in sunny positions is more difficult and isolation within the community is common. Communications, roadways, and trails are easily severed by landslides, avalanches, and other natural disasters, as evidenced by the damage suffered in mountainous areas during the large 2015 earthquake in Nepal. The funneling effect of valleys on wind may increase its velocity with an ensuing stunting effect on vegetation and trees. This also restricts the placement of houses, as does the availability of water and the possibility of natural disasters. The climate near the ground at high altitude has several basic features. At any given latitude, seasonal variation of monthly temperature is less at high altitude than at sea level and, as the equator is reached, seasonal variation virtually disappears. Diurnal variations are considerable, with temperatures varying as much as 30°C. This is because of high levels of long-wave radiation that occur in cloudless skies during the day and escape to clear skies at night. Diurnal variation decreases in overcast conditions. Climate change has become an increasing concern in high altitude regions. Reports from 2005 indicate that glaciers covered more than 100,000 km2 in Asia, mostly in the Himalayas, and more than 4000 km2 combined in Argentina, Bolivia, Chile, and Peru in South America (WHO 2005). These values have declined in many areas since the time of that report, although areas of glacial growth, such as in the Karakoram range, have been documented (Gardelle et al. 2012). Evidence for altitude amplification (increased rate of warming with increased elevation) suggests mountain regions may experience even faster temperature changes in the coming decades (Pepin et al. 2015; Urrutia and Vuille 2009). Such changes present various challenges to highland communities. For example, glacial melting due to climate change leads to increased accumulation of water in large glacial lakes. The frequency of glacial lake outburst floods, which lead to the sudden discharge of large volumes of water and debris, have increased in the Himalayas since the latter half of the 20th century and are of major concern for nearby communities. Changes in snow and vegetation cover due to increasing temperature also affect hydropower and agriculture, which can impact resources available to and the lifestyle of highland populations. In the Andes, most freshwater comes from glaciers, and glacial melt can expose minerals that contaminate water sources, thereby further threatening surrounding populations. These issues are of increasing concern as temperatures are predicted to rise 2–5°C, for example, in the Andes by the end of the 21st century (Cabré et al. 2016; Hijmans et al. 2005). The Research Centre for Alpine Ecosystems reports the 2°C temperature increase in the European Alps during the 20th century is double that of the northern hemisphere. Glacier melt in the European Alps has accelerated in the past 40 years, with a 50-cm average loss per year of glacier mass recorded between 2006 and 2015. A “Call for Action” was made by the World Meteorological Organization at the High Mountain Summit in October 2019 to create changes necessary to mitigate the decline of glacial melting and impacts on the ecosystem and the security, economy, food, and water resources for highland communities. In Asia, the monsoon flows from east to west across India, cooling as it is forced to ascend by the Himalayas. Water vapor condenses and falls as rain, and as it passes to the west, the monsoon becomes depleted of water; the eastern Himalayas are thus very wet, while the western part of the range is dry. Whereas in Darjeeling, the annual rainfall is 2000–3000 mm a year, it is 1500 mm in the central Himalayas at Simla and only 75 mm in the west at Ladakh. The Karakoram is arid, whereas the eastern Himalayan region is tropical. There is also considerable north–south variation with subarctic species on the Tibetan plateau and tropical species often only a few hundred yards away to the south. This is particularly marked on some passes in the eastern Himalayas. On the plateau, although monsoon clouds are seen on the Tangulla range, about 700 km north of Lhasa, precipitation is small. In the deserts of the Tarim basin and Tsaidam to the north of the Tibetan plateau, annual rainfall may be less than 100 mm. In the Andes, the Pacific coastal strip is desert. The western slopes of the Andes are dry, cacti and eucalyptus trees flourish, and only a few high mountains are snow-covered. The eastern slopes, which descend to the Amazon basin, become progressively more humid and tree-covered as one moves down in elevation. The Ethiopian highlands tend to have temperatures much lower than surrounding regions. Highland residents more than 2000 m above sea level may experience average temperatures near 16°C, a stark contrast to neighboring areas only hundreds of miles away, such as the Danakil Depression (100 m below sea level), with temperatures that may reach 50°C. Despite proximity to the equator, tropical monsoons that are common from June through September are also much cooler due to increased elevation. As noted in Chapter 2, temperature falls with increasing elevation. There is no uniform value for this decline, but a rate of 1°C for every 150 m increase in elevation is often given. Mountain temperatures vary between ranges based on the location relative to the equator and other factors. Temperatures as low as –73°C have been recorded on Denali (Dixon 1938) and −88.2°C in Antarctica. Mountain ranges located at far lower elevations, however, can also have quite low temperatures depending on local weather patterns. For example, on the summit of Ben Nevis in Scotland, a peak of only 1300 m, the mean temperature over a 17-year period from 1884 to 1901 was –0.1°C with a range from −17°C to +19°C. At a given location on a particular mountain, temperatures can also vary significantly over the course of the year. For example, in Antarctica, the temperatures ranged from the low of –88.2°C, as noted previously, to as high as 15°C, whereas at the South Col on Mount Everest (7896 m), monthly median temperatures of –11.9°C in July to –26.3°C in December have been documented. When accounting for wind chill, these values fall to –18.7°C and –39.3°C, respectively (Moore et al. 2012). These low temperatures are of great clinical relevance as they significantly increase the risk of cold injuries including hypothermia and frostbite, which are described further in Chapter 27. As global temperatures rise, it is expected that temperatures in highland regions will increase as well. Climate-sensitive diseases are impacted by temperature, as is the case with malaria, whereby temperatures below a specific threshold prohibit survival of parasites (18°C for Plasmodium falciparum and 15°C for P. vivax
Introduction
Major High Altitude Regions of the World
Himalayas and Tibetan plateau
Andean Mountains of South America
Ethiopian highlands
Population
Terrain
Climate
Rainfall
Temperature
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