How Tall Is Mount Everest? Hint: It's Changing

For three years, Roxanne Vogel trained, single-mindedly, with one number in mind: 29,029 feet.

She slept in a special tent, outside her home in California, that simulated high altitude. She summited dozens of peaks, on nearly every continent. And finally, last year, Vogel climbed up to 29,029 feet in the Himalayan mountains – the top of Mount Everest, the world's highest peak.

"That's the closest to heaven, or the closest to outer space, that I will ever get on this Earth," Vogel, 35, told NPR. "It's kind of life-changing, when you're up there."

(And Roxanne didn't just climb Everest; she set a speed record doing it. In May 2019, she traveled round-trip from her California home, to Everest's peak and back, in just 14 days.)

But that number — 29,029 feet, from sea level to summit – to which Vogel dedicated so many years of training, may not be the actual height of Everest – or at least not for long. Because the mountain is changing.

Scientists say Everest is getting taller, over time, because of plate tectonics. As the Indian plate slips under the Eurasian plate, it uplifts the Himalayas. But earthquakes can reduce their height in an instant. After a 7.8-magnitude quake in 2015 killed thousands, including climbers on Everest, scientists suspect the mountain got shorter.

So China and Nepal, on whose borders Everest stands, decided it's time to re-measure Everest.

This spring, with the climbing season canceled for COVID-19, China sent a survey team up to Everest's summit, carrying GPS receivers. Last year, Nepal did the same. The two countries have been analyzing their findings for months, and are expected to release them any day now – possibly as early as this weekend. Calculating that number has evolved as our technology has, but the science remains complicated.

SIR GEORGE EVEREST, AND AN INDIAN MATHEMATICIAN

Back in the 19^th century, when Sir George Everest – a Briton – was the Surveyor General of India, under colonial rule, they used trigonometry to measure mountains, with machines called theodolites. They're optical instruments – sort of a cross between a telescope and a compass – that are used to measure angles between visible points on the horizon, and vertical planes. Municipal surveyors still use tripod versions of them.

It was painstaking work – hauling theodolites out to plains around the Himalayas, waiting for clouds to clear — but surprisingly accurate.

In 1856, the Survey of India calculated the peak's height as 29,002 feet above sea level. That measurement held for nearly a century, until an Indian survey in 1955 concluded it was 29,029 feet – a height that's been the consensus ever since.

Sir George Everest, after he retired and moved back to Britain, got his name on the peak he helped measure. But it was actually an Indian mathematician and surveyor, Radhanath Sikdar, who did much of the work, and was first to discover it was the highest mountain in the world.

(And while Everest is now the peak's English name, the Nepalese have long called it Sagarmatha, and Tibetans call it Chomolungma – "Mother Goddess of the World.")

USING GPS TECHNOLOGY TO MEASURE MOUNTAINS

Instead of measuring peaks from afar, on the horizon, with theodolites, today's surveyors get help from satellites. They have to climb up Everest with a GPS receiver. It's not easy.

"It's a harsh environment there, very windy, and you can have battery power problems," explains Dinesh Manandhar, a GPS expert from Nepal who teaches at the University of Tokyo. "They can't stay there [atop Everest] for more than half an hour. They're already exhausted."

One of the Nepalese surveyors reportedly stayed at the peak for two hours, and lost a toe to frostbite, in last year's measuring expedition.

With the clock ticking, surveyors have to connect their GPS receiver to multiple satellites, because there are solar flares and interference at altitude – so they can't rely on just one satellite reading. They also have to measure the thickness of ice and snow underfoot, because they want a reading from the actual rock mountain, not the ice — and for that they need to also bring a ground-penetrating radar.

Fumbling with all of that equipment, in wind and with oxygen tanks depleting – that is the easy part, Manandhar says. Because all of that data from the top of Everest is only half of the story.

"You need a reference point, and that's the biggest problem," Manandhar says. "They need a sea reference somewhere, but we don't have a sea reference in Nepal, because Nepal is a landlocked country."

To know how high Everest is, you first need to know how low sea level is. But sea level varies across the globe.

DETERMINING SEA LEVEL

Sea level varies with climate change, the Earth's rotation, and gravity.

Here's how the Everest surveyors account for those variables: They first measure sea level in India at the Bay of Bengal, in China at the Yellow Sea, and at many other points – hundreds of them. Then they use those measurements to calculate the Earth's mean sea level.

Next, they try to figure out where sea level would be, if there were a sea right under Everest. But they have to take into account the Earth's ellipsoidal shape. As the Earth rotates on its axis, it bulges slightly around the equator. It's not a sphere.

In fact, if you were to measure mountains from the Earth's core – which some scientists think will increasingly be a safer constant, as climate change makes sea levels rise — there's a mountain in Ecuador that would be higher than Everest. The summit of Ecuador's Mount Chimborazo is more than 6,800 feet farther from the Earth's core, than Everest's summit is.

And if you were to measure mountains from base to peak, rather than from sea level or from the Earth's core, yet another mountain would win that contest: Mauna Kea, in Hawaii, is the world's tallest mountain from base to peak. Most of it's just underwater!

A LUMPY ELLIPSOID & THE GEOID

Even after Everest's surveyors measure the mean sea level, and take into account the Earth's ellipsoidal shape, they still won't know exactly where sea level would be, if it were to exist directly under Everest. Because mountains themselves affect gravity, which in turn affects sea level. So the Earth at sea level – this invisible line, along the Earth's surface — is actually quite lumpy. Some scientists have likened it to a lumpy potato.

The next step, Manandhar says, is to map those lumps – or rather, variations in the Earth's gravitational force.

"You have to do a gravity survey, and this gravity survey will tell you, how this mean sea level is varying from place to place," he explains. "That's how you get the geoid."

The geoid is the shape of the Earth at sea level, taking into account gravity and the planet's rotation. If you follow that geoid to a point directly under Everest, that's what scientists use as a reference point to determine the mountain's height.

But it's still not that simple. Because the mountain itself is moving, with plate tectonics.

EVEREST'S UPLIFT, FROM PLATE TECTONICS

The Himalayas, including Everest, sit on the edge of two plates. The movement of the Indian plate slipping under the Eurasian plate is what created the mountain range in the first place, and continues to push it skyward.

By how much? That's what Sridevi Jade, an engineer and expert on Himalayan plate tectonics, has spent her career measuring.

Jade has taken measurements in the western Himalayas, and combined her findings with GPS data taken across the range. She calculates that the Indian plate is slipping under the Eurasian plate by about 5 cm per year. That lateral movement has translated, over the past 20 years, into a 1.4 mm uplift for Everest per year. Rounding down, to take into account erosion on the top of the mountain, Jade estimates that Everest is gaining about 1 cm every 10 years – or about a foot, every 300 years.

Other scientists say that's far too conservative, and the growth could be three times that much. But however fast Everest is rising, things can happen very quickly to change that: earthquakes.

Jade studied a 1934 quake that struck very close to Everest. She and other geoscientists have calculated it took about 60 cm off the mountain's height. That's at least 600 years of growth, erased in an instant.

As for how the 2015 quake in Nepal may have changed Everest, scientists are hoping the new Chinese and Nepalese surveys will answer that. Both countries say their calculations agree.

POLITICS AT PLAY

But they haven't always in the past. A 2005 Chinese survey concluded the mountain was 12 feet shorter than previously thought. So some climbers rushed to climb from the Nepal side, where they were still certifying the higher height. It may have cost China some mountaineering tourism.

China uses its own satellite system, called BeiDou, and has been accused of not sharing as much of its technology. So some scientists NPR interviewed were skeptical about China's earlier findings.

There's also nationalism involved. Most of the surveys of Everest over the centuries have been conducted by foreigners – British colonial rulers of India (18^th and 19^th centuries), Americans (1999), Italians (1987, 1992).

"Why don't we measure our own mountain?" asks Manandhar, the GPS expert from Nepal.

EVEREST'S NEW HEIGHT

There's little risk of Everest losing its title as the world's highest mountain. The new measurements may add or subtract inches from the peak. By contrast, the second-highest mountain in the world, K-2 in Pakistan, is nearly 800 feet shorter than Everest.

While some scientists believe the 2015 Nepal quake shortened Everest, there are reports that the "new" height about to be announced by Nepal and China may actually be taller.

That would be welcome news to Vogel, the California climber who summitted last year.

"I hope it's higher!" she says, laughing. Her 2019 climb was logged in the Himalayan Database, which records all expeditions to Everest's summit. But if the official height of the mountain changes, she might consider climbing again, she says – to get a new entry in the logbook.

Scientists say what matters to them is not how high Everest is, but how it's changing – and what that reveals about the Earth overall.

"It's the joint work — sharing knowledge," says B. Nagarajan, a geoscientist who was one of George Everest's successors at the Survey of India office, and went on to lead India's first geodesy lab. "The question is the learning, the teaching, how people understand, how much effort you put in, what model you used."

The GPS technology they're fine-tuning on Everest has practical applications, from agriculture to defense. Nagarajan's research has been used to make tsunami warning systems more accurate. Manandhar is honing GPS technology to guide seed-planting robots for farming.

If all that work gets more attention, because it involves the tallest mountain in the world, then that's a good thing for science, they say.

The Short Wave episode of this story was produced by Rebecca Ramirez, edited by Viet Le and fact-checked by Ariela Zebede. Special thanks to Daniel Shukhin, who was the audio engineer, NPR's India producer Sushmita Pathak and Brent Baughman.