The 2.22-metre (7.28-foot) decline could be down to lower precipitation during summer, said Jean des Garets, chief geometer in the Haute-Savoie department of southeastern France.
"Mont Blanc could well be much taller in two years" when it is next measured, he added, saying this was not the first time such a large change had been seen.
The mountain's rocky peak measures 4,792 metres above sea level, but its thick covering of ice and snow varies in height from year to year depending on wind and weather.
Researchers have been measuring it every two years since 2001, hoping to garner information about the impact of climate change on the Alps.
"We're gathering the data for future generations. We're not here to interpret them," des Garets said.
People shouldn't use the height measurement "to say any old thing", he urged.
Instead, "it's now up to the climatologists, glaciologists and other scientists to make use of all the data we've collected and come up with theories to explain" the shrinkage.
Mont Blanc's highest recorded summit was in 2007, at 4,810.90 metres.
A one-metre fall was measured in 2021 compared to 2017 -- after 2019's unusually low result was kept secret as experts judged it not representative.
- Vanishing glaciers -
"Mont Blanc's height has been fluctuating since time immemorial," the geometers say.
Within the year, strong winter winds usually scour away more snow than in summer, meaning the peak is higher as autumn begins than in early spring.
"We've learned a lot from these measurement campaigns. We know that the summit is constantly changing in altitude and position, with changes of up to five metres," des Garets said.
Luc Moreau, a glaciologist, told AFP that since the ice cap at Mont Blanc's summit was at a temperature of -15C (5F) it could not melt.
"So there is no link to global warming," he said. This could, however, change if climate change intensified, he said.
Faster melting has been observed in Alpine glaciers as a result of climate change.
European glaciers -- many at lower altitude than elsewhere on the globe -- are especially vulnerable to global warming.
They lost around one-third of their volume between 2000 and 2020, according to scientific data.
In 2022 alone, glaciologists believe up to seven percent of the remaining mass of the glaciers may have vanished.
But one of the Mont Blanc team members, Denis Borel, urged people to "stay humble" about climate's impact on the mountain.
People shouldn't "draw hasty conclusions about measurements that have only been made precisely since 2001", he added.
Around 20 people scaled the mountain in mid-September to carry out the measurements over several days, divided into eight parties equipped with high-tech tools and -- for the first time -- a drone.
Two Air India crashes and 20,000 climbers: Mont Blanc in figures
Paris (AFP) Oct 5, 2023 -
Nicknamed the "roof of Europe", the mighty Mont Blanc attracts thousands of climbers each year.
A study published Thursday found that it had shrunk by 2.22 metres (7.28 feet) since 2021, to 4,805,59 metres (15,766.4 feet).
Here are some key figures about the highest peak in the Alps:
- Two -
The number of Air India passenger planes to crash on Mont Blanc within a little over 15 years of each other in the 1950s and 1960s.
The first was the "Malabar Princess", a Lockheed Constellation airliner that was en route from Bombay to London when it went down on a glacier on the southwest face of Mont Blanc on November 3, 1950, killing all 48 passengers and crew.
On January 24, 1966, tragedy struck again when Air India's "Kangchenjunga", a Boeing 707 also travelling from Bombay to London, slammed into the same glacier.
All 117 passengers and crew, including the pioneer of India's nuclear programme, Homi Jehangir Bhabha, died.
- Three -
The number of guides who were swept away by an avalanche in the first fatal accident on Mont Blanc in 1820.
The trio was accompanying a Russian scientist, Joseph Hamel, who was sent by Tsar Alexander 1 up the mountain to conduct experiments.
The guides wanted to turn back because of bad weather but Hamel insisted on forging ahead, becoming forever associated with the tragedy which he survived.
- 10.4 -
The highest temperature in degrees Celsius (50.7 degrees Fahrenheit) ever recorded at the summit of Mont Blanc on June 18, 2022.
Temperatures at a weather station situated just below the peak, at 4,750 metres, remained above freezing for 11 consecutive hours while Europe baked in an unusually early heatwave.
In winter, the temperature at the summit regularly dips below -30C.
- 26 -
The number of years that passed before a French hunter won a competition launched by Swiss naturalist Horace Benedict de Saussure in 1760 to become the first person to conquer Mont Blanc.
Saussure, considered by many as the father of mountaineering, had promised a handsome reward to anyone who could find a way to the top.
Several failed attempts ensued before Jacques Balmat, a crystal hunter from Chamonix, and a local doctor Michel Paccard, reached the summit on August 8, 1786.
- 20,000 -
The upper range of the number of people who try to summit Mont Blanc each year, according to the prefecture (department) of Haute-Savoie where it is situated.
To avoid overcrowding, the town of Saint-Gervais has limited the number of climbers leaving each day along the most popular "Gouter" route to 214.
- 300,000 -
The estimated value in euros of the emeralds, rubies and sapphires found in a metal box near the summit in 2013, believed to have come from Air India's doomed "Kangchenjunga".
The local climber who found them was rewarded for turning his find into the police.
He was allowed to keep half the haul, with the other half going to the town of Chamonix.
- 4:57:40 -
The record time set by Kilian Jornet of Spain for summiting the mountain and then returning on foot to the church in Chamonix, situated at 1,037 metres, on July 11, 2013.
Jornet, 25, took 14 minutes off the previous record set by Swiss climber Pierre-Andre Gobet in 1990 to complete the climb, wearing shorts and trainers.
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