All further melting between now and October will see the size of glaciers in the Swiss Alps shrink, according to Glacier Monitoring in Switzerland (GLAMOS).

This century, the tipping point, on average, has been reached in mid-August -- itself already bad news for the nation's 1,400 glaciers, which are shrinking at a staggering rate.

Its arrival several weeks earlier on July 4 is "another alarm call", GLAMOS chief Matthias Huss told AFP.

"It's like the glaciers are shouting out: 'We're disappearing. Help us.'"

Glaciers in the Swiss Alps began to retreat about 170 years ago.

The retreat was initially modest but in recent decades, melting has accelerated significantly as the climate warms.

The volume of Swiss glaciers shrank by 38 percent between 2000 and 2024.

- Summer of destruction -

"If we have a glacier loss day, it means that the glacier is losing mass," said Huss.

"For a glacier that is healthy, the day would occur at the end of September, or in October -- or not at all".

With no glacier loss day, the summer would simply melt away only the snow that accumulated over the previous winter.

This would be "the ideal case -- a glacier in equilibrium with the climate", said Huss.

Its arrival on July 4 means that "critically, we have the whole summer left to destroy the ice".

"Moving this day forward by five to six weeks before the normal date over the last 20 years means we're just prolonging this mass loss season dramatically," he said.

The assessment is made using 12 reference glaciers.

Last winter saw low levels of snowfall, and June was the second warmest on record, contributing to the day's early arrival this year.

In data going back to 2000, the only time that the tipping point arrived even earlier was in 2022, when it came on June 26.

"That was really a game-changer for us glaciologists because it was the first year when we saw absolutely extreme melting.

"Everything that we knew before about glacier melting changed," said Huss.

Experts thought 2022 was a complete outlier and although a warming climate meant other such years would be coming down the line, they did not expect to see the next very early glacier loss day coming so soon afterwards.

- Feedback effect -

Huss noted that extreme melting produces an accelerating feedback effect, worsening the situation even further.

Once the reflective white snow coverage from winter is gone from the top of the glacier, the darker, more absorbent grey surface of the bare ice is exposed.

"With the same amount of solar radiation, we can now melt more ice," Huss said.

With the European heatwave over the past week and the possibility of further heatwaves in July and August, "it is very like that again it is a very bad year for Swiss glaciers", he said.

Melting glaciers threatens the long-term water security for millions of people downstream who rely on them for fresh water.

Much of the water that flows into the Rhine and the Rhone, two of Europe's major rivers, comes from the Alpine glaciers.