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Climbing a Triassic Mount Everest: Into Thinner Air
To the Editor: At a height of 8850 m, Mount Everest has long been a magnet to Himalayan mountaineers, and its summit has been reached 2251 times through 2004.1 Because 130 of those ascents were made without supplemental oxygen,1 contemporary humans are undoubtedly capable of climbing higher than 8850 m without supplemental oxygen, if a higher summit were available. The upper limit for mountaineers has probably varied over time because atmospheric oxygen concentrations (currently 20.9%) have changed drastically over the past 570 million years.2 We simulated how these oxygen shifts would have affected the maximum altitude reachable by hypothetical "paleo-mountaineers."
Methods
To estimate past maximum potential altitudes, we first determined the maximum altitude reachable in today’s atmosphere. West3 calculated that Mount Everest’s summit should be close to the limit of human climbing without supplementary oxygen. Consistent with this, we used 9.0 km as a conservative limit. This is feasible because it approximates the "physiological" altitude reached by Sherpa Ang Rita when he summited Mount Everest without using supplemental oxygen on December 22, 1987. The estimated summit partial pressure of inspired oxygen (PIO2) on that winter day was physiologically equivalent to 9.0 km during the customary spring climbing season when PIO2 is higher.4 Although Bailey5 recently proposed a much higher limit (9972.7 m), his estimate was incorrectly based on a linear (rather than curvilinear) regression model and is unrealistic given slow climbing rates at extreme altitude.
We used a model atmosphere equation for barometric pressure vs altitude4 to compute present-day PIO2 at 9.0 km, and then used this amount as the minimum level tolerable by both present-day and hypothetical paleo-mountaineers. We next expanded an equation4 for PIO2 as a function of percent oxygen and of the summed partial pressures of oxygen and of nitrogen (including minor gasses, all assumed constant over time2). We then corrected the PIO2 for percent oxygen, and solved for altitude.6 This estimated the maximum altitude reachable under a given percent oxygen. We assumed that maximum altitude is determined only by PIO2 and ignored minor effects of concurrent climate change6 and of uncertainty in percent oxygen.2
Results
During the mid-Permian era, oxygen was relatively abundant2 and PIO2 is thought to have reached approximately 30 percent (Figure). By the early Triassic era, however, PIO2 fell to approximately 12%.2 Shifts in oxygen concentration would have dramatically altered the maximum climbable altitude over time (Figure). During the Permian high oxygen concentration, hypothetical paleo-mountaineers would have been aerobically capable of reaching nearly 12 km, about one third above the current summit of Mount Everest. During the Triassic low oxygen concentration, climbers would have been stopped at 4.5 km, below the summit of Mount Whitney (4.4 km). A prehistoric Ang Rita would have been incapable of reaching a Triassic base camp on Mount Everest (5.3 km).
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Figure. Change in Percent Oxygen and in the Maximum Climbable Altitude Reachable by "Paleo-Mountaineers" Over Geological Time
Geological periods:C–, Cambrian; O, Ordovician; S, Silurian; D, Devonian; C, Carboniferous; P, Permian; Tr, Triassic; J, Jurassic; K, Cretaceous; T, Tertiary. Blue shading indicates times with sufficient oxygen for ascents of a peak the height of Mount Everest to be summited without using supplemental oxygen. The intervals on the right vertical axis (maximum climbable altitude) are not equal because the relationship between percent oxygen and maximum climbable altitude is nonlinear.
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Comment
On a geological scale, neither Mount Everest nor humans existed until recently. Nevertheless, our findings add a novel, deep-time perspective on high-altitude physiology and medicine. Our analysis suggests that peaks as high as Mount Everest would have been physiologically reachable by humans during less than one third of the past 570 million years. Thus, it is only through a fortunate accident of geology and biology that humans evolved and have always lived during a time in which oxygen levels have been sufficiently high to allow (a few of) us to reach the highest summit on Earth.
Financial Disclosures: None reported.
Author Contributions: Dr Huey had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.
Funding/Support: This research was supported by the National Science Foundation (Dr Huey) and the NASA Astrobiology Institute (University of Washington Node, Dr Ward, principal investigator).
Role of the Sponsors: The funding agencies had no role in the design and conduct of this study, in the collection, analysis, and interpretation of the data, or in the preparation, review, and approval of the manuscript.
Raymond B. Huey, PhD
hueyrb{at}u.washington.edu
Peter D. Ward, PhD
Department of Biology
University of Washington
Seattle
1. Salisbury R. The Himalayan Database: The Expedition Archives of Elizabeth Hawley. Golden, Colo: The American Alpine Club; 2004.
2. Berner RA. Examination of hypotheses for the Permo-Triassic boundary extinction by carbon cycle modeling. Proc Natl Acad Sci U S A. 2002;99:4172-4177.
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3. West JB. Climbing Mt. Everest without oxygen: an analysis of maximal exercise during extreme hypoxia. Respir Physiol. 1983;52:265-279.
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4. Ward MP, Milledge JS, West JB. High Altitude Medicine and Physiology. London, England: Arnold; 2000.
5. Bailey DM. The last "oxygenless" ascent of Mt Everest. Br J Sports Med. 2001;35:294-296.
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6. Huey RB, Ward PD. Hypoxia, global warming, and terrestrial Late Permian extinctions. Science. 2005;308:398-401.
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Letters Section Editor: Robert M. Golub, MD, Senior Editor.
JAMA. 2005;294:1761-1762.
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