Exactly how tall is Mount Everest? That’s what scientists in India are trying to figure out.
This week, India’s surveyor general announced that the government is going to remeasure Mount Everest, in a bid to determine whether the world’s tallest peak shrank (or grew) following a devastating earthquake in 2015. The expedition is the first Indian survey of the mountain in more than 60 years, but experts say obtaining an accurate measurement will be a tall order, and determining the earthquake’s effect on it may be even tougher.
The recent confusion over Everest’s elevation stems from a 7.8-magnitude earthquake that struck Nepal in April 2015, killing nearly 9,000 people. Analysis of satellite imagery suggested that Everest shrank by about an inch following the earthquake, though other analyses suggest that it may have increased in height. China’s national surveying administration said in June 2015 that the earthquake shifted Everest northeast by about three centimeters, but that the mountain’s height did not change.
Swarna Subba Rao, India’s surveyor general, said this week that the Indian expedition aims to put the debate to rest. “Two years have passed since the major Nepal earthquake and there's doubt in the scientific community that it did in fact shrink,” Rao said at a conference in Hyderabad.
According to Rao, a team of 30 scientists will use two methods to measure the mountain. The first involves placing a GPS transistor at its summit to measure the distance from sea level; the second involves more traditional on-the-ground triangulation, whereby researchers would determine the distance to the summit based on its angle relative to two baseline points. Speaking to CNN, Rao said it may take “anywhere up to six months” to prepare for the expedition, and that once launched, it would take about 45 days to gather and analyze the data.
Everest’s height was a point of debate even prior to the Nepal earthquake. Everest’s official elevation of 29,029 feet above sea level is based on an Indian survey conducted in 1955. But a 2005 Chinese survey, which measured rock height rather than snow height, placed the mountain at 29,015 feet.
But like most of the Himalayas, Everest has probably been growing. The mountain range is where the Indian plate presses up against the Eurasia plate after a collision 50 million years ago. Measurements from 1994 suggest Everest grows about four millimeters (slightly more than one-tenth of an inch) every year. So if Everest did indeed shrink after the 2015 earthquake, that would be quite a reversal.
Much of the uncertainty surrounding Everest’s height is due to the imprecision of satellite imagery and GPS data. Romain Jolivet, an assistant professor of geology at the École Normale Supérieure in Paris, says that although GPS data can “measure millimeter displacements” along a horizontal axis, it’s a less precise measure of vertical displacements. Satellite imagery can provide accurate readings of displacements in areas close to an earthquake, but it becomes less reliable when analyzing areas that are farther away. (The epicenter of the Nepal earthquake was about 140 miles west of Everest.)
That’s why Jolivet and his colleagues did not include Everest in a study on the Nepal earthquake published earlier this year. Their model, based on satellite imagery and GPS data, found that some of the highest peaks in the Himalayas dropped by up to 60 centimeters immediately following the earthquake. If the model is expanded to include Everest, Jolivet says the mountain would have dropped by 0.4 millimeters, which he describes as “negligible” due to the imprecision of his measurements.
Ground surveys can provide more accurate mountain measurements, but they involve their own set of complications. Kari Craun, director of the National Geospatial Technical Operations Center at the US Geological Survey, says a key challenge to any ground survey is establishing stable control points — baseline elevations relative to a mountain’s summit. Craun says that could be especially difficult in a country like Nepal, whose topography was drastically changed following the earthquake. (The earthquake lifted Kathmandu by about three feet.)
“Everything moved, everything just literally moved,” Craun says. “So now all of that control information is no longer well understood, so you have to go back and re-establish that.”
There’s also the matter of scaling the mountain itself. “Actually getting that antenna and receiver to the top of Mount Everest poses a bigger logistical challenge than it would to go out and survey another point that’s more accessible,” Craun adds. “So that’s the big challenge.”
Even if the Indian team delivers an accurate measurement of Everest’s elevation, it may be difficult to interpret. If its altitude is higher, Jolivet says, that could be due to the ongoing collision of tectonic plates that have gradually lifted the Himalayas — which would make it difficult to determine what impact the earthquake had on it. A lower elevation could also be due to more advanced technology that gives more accurate readings, he adds, rather than the earthquake itself. That was the case in 2015, when the US government lowered the official height of Denali, North America’s highest peak, by 10 feet. The USGS said the decline likely had more to do with more precise measuring techniques than anything else.
Still, measuring Everest is very much worthwhile. A more accurate reading could shed light on how and why the Himalayas have continued to grow — even if the earthquake’s impact on that process may remain unclear.
“I assume it’s going to be a very tiny difference, and that would be very hard to interpret,” Jolivet says. “But still, at least we would know where the highest point on the Earth stands.”