Geoscience Reference
In-Depth Information
Living at altitude
HUMAN IMPACT
No landsystem inspires human emotions of awe and spirituality quite like mountains, which are hostile, rugged and
remote in their physical character. Often thought of as the abode of gods, they are central to a number of religions
but peripheral to most people's lives. Mountains form ethnic and political boundaries dividing peoples and yet are
home to others. The Incas and Tihuanacos of South America and the Kurds, Tibetans and Nepalese of Asia suffered
persecution by intolerant societies, and mountains have provided a refuge or barrier against political storms throughout
history. The mountains of Afghanistan, Kashmir and Iraq are still among Earth's most intractable battlegrounds.
Mountains form 20 per cent of continental land surfaces but house only 10 per cent of world population. They influence
a further 40-50 per cent of us indirectly through their resources and role as global 'weather makers' and 'water
towers' - denoting their impact on planetary and synoptic meteorology and hydrology - and sustain 24 per cent of
global tourism.
Atmosphere and climate at high altitude pose a range of problems for human living and socio-economic activity,
caused by lower atmospheric pressure and extreme cold; the latter is shared with polar environments. They cause
problems for
human biometeorology
, although the human body also displays short-term, facilitative adaptations
and longer-term (perhaps genetic) acclimatization. The partial pressure of oxygen falls with altitude, in line with general
atmospheric pressure, leading to hypoxaemia or reduced blood O
2
concentration. The quantity of O
2
inhaled by our
lungs and bound by haemoglobinin red blood cells determines our aerobic working capacity. This falls by 10 per cent
km
-1
above 1,500 m, as reduced O
2
flow to body tissues causes anoxia and its more severe form, hypoxia.
Apart from making any activity more tiring, they can trigger more serious disabling consequences - often exacerbated
by other mountain weather phenomena. Acute mountain sicknessis often first to appear, caused by a slow leakage
of fluids into the brain and resultant swelling as the heart tries to compensate by pumping more blood. Rapid ascent
heightens the risk above 3ยท5 km and, if not redressed by equally rapid descent to lower altitudes, may seriously
impair mental judgement and develop life-threatening high-altitude cerebral oedema. Fluid may also build up in the
lungs (enhanced by water vapour condensed from cold inhaled air) and cause high-altitude pulmonary oedema, which
reduces O
2
intake further still and can also prove fatal. Conversely, cold dry air can cause coughing severe enough
to crack ribs.
Enabling responses may prevent or mitigate some of these conditions. Even at low altitude, most of us experience
a faster pulse, raised cardiac output and hyperventilation on physical exertion as the heart seeks to raise its output
of oxygenated blood or the lungs increase ventilation (breathing volume). Two other responses involve the
bloodstream. In haemoconcentrationblood plasma (fluid) levels fall and thereby raise the red blood cell concentrations,
whilst in polycaethaemia bone marrow actually produces more red blood cells. These are clearly longer-term
responses and may become part of the genetic adaptations which many mountain peoples possess - and explains
why they produce such good long-distance athletes!
km through the Americas-Transantarctic mountains,
from pole towards pole and through every latitude across
the equator. By comparison, Tethyan orogens extend
across 150
The Americas: Andes, Rocky Mountains and
coast ranges
Differences between individual western American
cordillera are explained by asymmetrical motion of the
American and Pacific basin plates noted earlier (
Chapters
late Mesozoic to Quaternary age but include exposed or
rejuvenated elements of Proterozoic terranes, up to 2 Ga
old in the central Andes. The type-cordilleran Andes were
formed by 'head-on' motion of the Nazca plate after
approximately 190 Ma ago. Older, late Panthalassic Ocean
of longitude from Morocco through south
central Europe, the Middle East and central Asia to
Malaysia and Indonesia.
Altitude
simulates
latitude
,
replicating the eco-climatic gradient of 90
of latitude in
just seven vertical kilometres in the equatorial Andes.
Their spatial distribution is set by the supercontinental
cycle, and its impact on representative mountain systems
is examined now.
