Mount Everest is the tallest mountain in the world — but sometimes, it feels like the second-tallest, according to a story reported in the American Geophysical Union’s news blog Eos.
That’s because the mountain’s air pressure fluctuates significantly throughout the year, a recent study found, causing the summit’s “perceived elevation” to occasionally dip below that of its less-lofty rival, K2 — the second-tallest mountain in the world.
“Sometimes K2 is higher than Everest,” lead study author Tom Matthews, a climate scientist at Loughborough University in the United Kingdom, told Eos.
Air pressure is closely tied to oxygen availability on Everest; when air pressure decreases, there are fewer oxygen molecules in the air, making the simple act of breathing much more strenuous, according to Eos. For this reason, many who choose to hike Everest rely on supplemental oxygen to stay on their feet as they scale to higher elevations where the air is thinner. (Only 169 men and eight women have ever summited Everest without the use of supplemental oxygen, the study authors noted.)
But while air pressure reliably decreases with elevation, it also fluctuates with the weather, the study authors found. From 1979 to 2019, the air pressure near the peak of Everest ranged anywhere from 309 to 343 hectopascals — roughly one-third the pressure at sea level — depending on the season.
The researchers also found that air pressure on Everest was consistently highest in the summertime, making that the best season to scale the mountain based purely on oxygen availability. As Earth’s atmosphere continues to warm due to climate change, there could even be a permanent decrease in the mountain’s perceived elevation, the researchers found.
“Warming will shrink the mountain a little bit,” Matthews told Eos.