But ever more extreme climate change is taking a terrible toll on the century-old wooden refuge.

"When your hand fits into a crack in the foundation, there is need for action," said Georg Unterberger, who is in charge of mountain refuges and trails at the Austrian Alpine club.

The trails up to refuge on the 3,106-metre (10,190-foot) Sonnblick mountain are also suffering.

Experts say warmer temperatures across the Alps are accelerating glacier melt and thawing permafrost, the year-round ice that binds together giant slabs of rock.

This has increased the danger of sudden rockfalls and landslides, damaging paths and putting more pressure on the often ageing and neglected huts.

Austria's Alpine clubs are currently closing up to four huts a year as they become unsafe or too costly to maintain.

With the country heavily dependent on Alpine tourism, the cost of keeping up the trails "have doubled in the last five years", said Unterberger, who also works as a building surveyor.

Each year about one million people visit the more than 200 mountain huts the Austrian Alpine club -- the country's biggest -- operates.

- 'Struggling on all fours' -

The trail up to the Zittel in the Salzburg region was always a black-rated one, but "now it's even more dangerous", Unterberger told AFP.

Hikers may now need climbing gear to reach it as the retreating glacier that once went all the way up to the popular refuge has exposed steep rock faces and vast fields of scree.

"I've seen hikers struggle on all fours to make it," Unterberger said, adding that work on trails has dramatically increased in recent years, with ropes and steel rungs having to be put in place.

At the Zittel hut, the crumbling foundation and the weather-worn wooden shingles require urgent renovation and thermal insulation.

Thawing permafrost threatened the very existence of the hut and the adjacent observatory -- one of the world's oldest high-altitude weather stations -- already years ago, with the peak at risk of falling apart.

To keep it from disintegrating, workers rammed steel anchors 20 metres into the mountain top and further supported the summit with concrete braces.

For now the peak is stable, but further measures cannot be ruled out.

More money is needed to remedy the decaying infrastructure in the Austrian Alps, with experts saying 272 out of 429 mountain refuges, as well as 50,000 kilometres (31,000 miles) of trails, are in dire need of repair.

In a petition earlier this year, the cash-strapped Alpine clubs urged the government to provide an emergency fund of 95 million euros ($103 million).

So far the government has pledged just three million euros.

Unlike in neighbouring Switzerland, where the cantons are in charge of maintaining the trail network, Austrian Alpine clubs heavily rely on ever scarcer volunteers.

"Many of our 25,000 volunteers are more than 65 years old and recruiting young people has been a challenge," said Unterberger, observing a trend towards "micro-volunteering" for a few hours or a day, but not more.

- 'Critical situation' -

The Zittel refuge shares the summit with the Sonnblick observatory, which has been measuring and documenting the changes to climate since it opened in 1886.

Up on the mountain, the temperature has been recorded for 138 years straight, the longest uninterrupted high-altitude data anywhere in the world.

This data helps scientists worldwide to refine their climate models -- while it also offers a glimpse into the future.

Since the 1950s all high mountain regions like "the Alps, the Rocky Mountains, the Andes, the Himalayas -- have already seen an average annual temperature increase of more than two degrees Celsius", which is twice the global average, observatory head Elke Ludewig told AFP.

"As nice as it is to still see snow and glaciers, we really have a critical situation here in terms of the rate at which temperatures are increasing," she said.