Like astronauts and master sommeliers, mountaineers who have summited Mount Everest eschewing supplemental oxygen are a rarefied group. Now, inspired by these individuals who push human physiology to its limits, researchers have analyzed the availability of oxygen on the world’s highest peak (8,848 meters).
Weather-induced changes in air pressure alter the abundance of oxygen at Everest’s summit, which means that the mountain’s “perceived elevation” varies by up to hundreds of meters, the team showed. The difficulty of climbing the mountain is therefore far from constant, and it will, in fact, become incrementally easier to summit as the climate warms, the scientists concluded in a study published last month in iScience.
“A Gasping Lung”
Since 1979, just 177 people—8 women and 169 men—have summitted Everest without the aid of supplemental oxygen. (Breathing bottled oxygen, a common practice above approximately 7,000 meters, makes it feel like you’re at a lower elevation and also helps to keep your extremities warm.) Tom Matthews, a climate scientist at Loughborough University in the United Kingdom, and his colleagues set out to better understand just how closely that feat approaches the limits of human physiology.
One of the major difficulties of climbing Everest—and any other peak of significant height—is that air pressure decreases with elevation. And the number of oxygen molecules around is directly proportional to air pressure: Thinner air contains fewer oxygen molecules, so breathing is less efficient at elevation. “I am nothing more than a single, narrow, gasping lung” was how Reinhold Messner described climbing Everest without supplemental oxygen in his book Everest: Expedition to the Ultimate.
Lower or Higher, Depending on the Day
Using data from weather stations near the summit of Everest and ERA5 reanalysis data, Matthews and his colleagues reconstructed the air pressure on Everest’s summit from 1979 to 2019 on an hourly basis. They found values ranging from 309 to 343 hectopascals, roughly one third the value at sea level. That’s a significant range: Compared with the average air pressure measured on Everest in May, that span translates into a 737-meter difference in how high the summit feels from an oxygen availability standpoint. In other words, Everest’s “perceived elevation” changes by more than the height difference between Everest and K2, the world’s second-highest mountain at 8,610 meters. “Sometimes K2 is higher than Everest,” said Matthews.
Matthews and his collaborators showed that mountaineers who summited Everest without supplemental oxygen climbed within a much narrower range of air pressures, however: 329–340 hectopascals. “Humans haven’t sampled the full range of conditions,” said Matthews.
That’s not a surprise, the researchers suggest, because most climbs take place during narrow time windows dictated by good weather. Those air pressures are also far higher than the theoretical minimum air pressure necessary to climb the mountain without oxygen, which Matthews and his collaborators estimated to be 302 hectopascals.
The best time to climb Everest, purely based on oxygen availability, is the summertime because that’s when air pressures tend to be the highest, the team found. But that’s also the monsoon season, said Jeremy Windsor, a researcher of mountain medicine at the University of Sheffield in the United Kingdom not involved in the research. “You don’t climb Everest in July and August because it’s snowing so much there’s a risk of avalanches and snow storms and whiteouts.”
Most people climb Everest in May or October, when the weather is more settled. “Mountaineers use wind speed as the indicator of when to climb,” said Matthews. However, it so happens that air pressure—and therefore oxygen availability—is still generally high in the pre- and postmonsoon seasons.
Luck of the Draw in Winter
Wintertime ascents of Everest are rare because of the severe challenge of extreme cold and strong winds, and the new data now reveal how much oxygen availability can plunge, too: The arrival of the cold-bringing jet stream means that air pressure is much lower on average in the winter. (Just one person has summited Everest in the wintertime without supplemental oxygen: Ang Rita Sherpa, in 1987).
However, there’s significant variability to wintertime air pressures, Matthews and his colleagues found. “If you pick the right day, it can feel like climbing in spring,” said Matthews. “If you pick the wrong day, it’s well beyond the bounds of what anyone has done before.”
Shrinking the Mountain
Matthews and his collaborators also looked into the future. Climate change will affect the availability of oxygen at the top of Everest: As the atmosphere warms, it becomes less dense, so there’s more air above the summit. If the atmosphere around Everest warmed by 2°C, the mountain’s perceived elevation would decrease by roughly 100 meters, making it just a bit easier to climb. “Warming will shrink the mountain a little bit,” said Matthews.
That’s a sobering thought, the researchers suggest. Summiting Everest is an enormous challenge, and doing so without using supplemental oxygen is a phenomenal achievement, said Matthews. “The warming that we’re responsible for is eroding that challenge.”
—Katherine Kornei (@KatherineKornei), Science Writer