Heat is rewriting the Mont Blanc season — 2015 to 2026, and how to pick your window

On 20 July 2026, the guide society of Courmayeur collectively stopped guiding Mont Blanc's Italian normal route. "With the heat we had in June, conditions deteriorated quickly. It's no longer safe enough to take clients up there," said its president, Alex Campedelli. No prefect ordered it and no route was formally closed — the professionals simply refused the conditions (SnowBrains).

Two days earlier, on the French side, Czech UIAGM guide Pavel Zofka (57) and his 49-year-old client were killed by rockfall in the Grand Couloir du Goûter. They had left Tête Rousse in the dark and were hit at around 02:30 — the coldest hour, the hour the rulebook says is correct (ExplorersWeb).

Neither event is an anomaly. Heat suspensions of Mont Blanc's normal routes now recur roughly every hot summer, and the record goes back at least to 2015. If you are planning an ascent, the question is no longer just "which year" but "which weeks" — and the calendar answer has been quietly shifting to the edges of the season.

---

The record: 2015 to 2026

Note the irony inside the table: in 2015, the Italian side was the recommended escape from the overheated Goûter. In 2026, the Italian side is the one the professionals walked away from. Heat does not respect the diversion plans; it degrades every aspect of the mountain at once.

The stakes are concentrated in one place. The Grand Couloir — a 30-second to 2-minute exposed crossing at ~3,270 m between Tête Rousse and the Goûter ridge — accounted for 347 rescue operations, 102 deaths and 230 injured between 1990 and 2017 (Fondation Petzl). Anything that makes the couloir worse makes the whole Goûter route worse.

---

The mechanism: permafrost is the glue

Why does heat translate so directly into rockfall? Because on the upper massif, thawing permafrost is what holds the rock together. Heatwaves melt it along with the residual snow, and both couloir rockfall and serac behaviour degrade — by day and by night (UIAA / Mourey study).

The instrumented picture comes from the EDYTEM/ISTerre seismic study led by Jacques Mourey (Petzl Foundation funding, published 2020). Sensors placed in the couloir over the 2018-2019 campaigns detected rockfalls down to ~100 kg, and the pattern they recorded is precise:

• During critical hours, one rockfall every 24 minutes on average, with peak activity between 18:00 and 20:00.
• The statistically safest crossing window is 09:00-10:00 — after the overnight refreeze has done its work.
• Falls spike when snow melts in the couloir or after rainfall. Heat and wet weather are the triggers, not the clock.

This produces what we call the timing paradox. Guide practice is to leave Tête Rousse in the dark and cross at the coldest hour; the sensor data says mid-morning is statistically safest and evening worst. Both are rational — the pre-dawn habit minimises heat-of-day melt, while the 09:00-10:00 figure reflects a completed overnight refreeze.

But the paradox has a fatal edge case: in a heatwave, the refreeze never happens. The 18 July 2026 team crossed at ~02:30 — textbook-correct timing — and was killed anyway, because after consecutive hot days the rock is unglued around the clock. In hot spells there is no safe hour. The only lever left is to not be there: divert or wait (ExplorersWeb, UIAA).

One caution about the standard diversion: the Trois Monts route is a full grade harder (AD- vs PD) and its Tacul and Maudit serac hazard is also heat-loaded — see the May and August 2025 entries in the table above. It is an alternative for competent parties, not an escape hatch for displaced beginners.

---

Why June and September are becoming the smart windows

The official summer mountaineering window is June-September, when the huts are staffed and the lifts run (La Chamoniarde). But the de facto good window has been sliding to the edges of that period. Mid-summer heatwaves now repeatedly close or degrade the Goûter route; early season (June) and late season (September) increasingly carry the best rockfall odds — at the cost of colder, more wintry ridges.

There is a second, quieter argument for the shoulder weeks: booking pressure. The reservation system caps legal two-day ascents at 214 climbers per day (Goûter 120 + Tête Rousse 74 + Nid d'Aigle 20 beds — Voyageons Autrement), tour operators block-book, and the mayor of Saint-Gervais has publicly complained that agencies "monopolize reservations a year in advance" (Outside.fr). The July-August core sells out fast at the public opening. Date flexibility toward June and September is worth more than any "book early" advice.

Be honest about what June buys you, though. It is a bet on reliable overnight refreeze, not a guarantee — the June 2026 heat is precisely what wrecked the Italian route by mid-July. The forecast, not the month, has the final word.

The 2026 infrastructure calendar

Read the edges of that table carefully: the Goûter-route huts open in late May, before the tramway starts running on 13 June, and they stay open into early October, after the tramway stops on 27 September. The hut season outlives the lift season at both ends.

---

Month by month: picking your window

The practical decision rules, compressed:

1. Aim for June or September. Better statistical rockfall odds, and realistic hut availability under the 214-bed daily ceiling.
2. Treat a forecast heatwave as a no-go, whatever your dates. The standing 2026 advice on the French side is to cross the couloir before 06:00, treat heat spikes as a no-go or a Trois Monts trigger, and re-check with the Office de Haute Montagne before committing (Global Summit Guide).
3. Watch for the refreeze, not the clock. After consecutive hot days with no overnight refreeze, there is no safe crossing hour — the 2026 accident happened at 02:30.
4. Do not treat the Trois Monts as a beginner's escape. It is harder (AD- vs PD) and serac-loaded in the same heat.

---

The 2027 wave: regulation is spreading downhill

The heat story and the regulation story are converging. The ascent itself has been a de facto permit peak since the 2019/2020 prefectural decrees — nominative hut booking, a bivouac ban, patrol checks. What is new is where the regulatory frontier is moving: downhill, to the trekking circuit.

Mayor Peillex — the man behind the 2019 reservation system and the 2022 deposit statement — is now driving a plan to regulate the Tour du Mont-Blanc trek from summer 2027: reservations capped to actual hut and bivouac capacity, mandatory bivouac booking, a "green brigade" mirroring the ascent's white brigade, and luggage-transport rules, with the French communes coordinating and Italy and Switzerland being courted to join (saintgervais.com, Outside.fr).

For climbers, the read-through is simple: expect the ascent rules to stay and tighten at the margins, not loosen. For trekkers, 2026 may be the last unregulated TMB summer. Either way, the era of showing up in Chamonix or Les Contamines and improvising is closing — on the summit routes it already has.

Heat compressed the season; regulation is now formalising the compression. Pick your window accordingly: early, late, flexible — and always ready to not go.

---

Sources

• SnowBrains — Italian guides suspend tours on Mont Blanc (2026)
• ExplorersWeb — Two dead in Mont Blanc's "Death Couloir" (2026)
• Chamonix.net — Goûter route closed until further notice (2015)
• ExplorersWeb — Danger: do not climb Mont Blanc (2022)
• BBC — Mont Blanc mayor's €15,000 deposit statement (2022)
• The Local France — Why Mont Blanc's mayor wants to charge climbers
• Chamonix.net — Spectacular serac fall on Mont Blanc du Tacul (2025)
• ExplorersWeb — One dead, four injured from serac fall near Mont Blanc (2025)
• UIAA — When and why do rocks fall in the Couloir du Goûter (Mourey/EDYTEM study)
• Fondation Petzl — Limiting accidents in the Goûter couloir
• La Chamoniarde — Mont Blanc ascent
• FFCAM — Refuge du Goûter, 2026 rates and season
• FFCAM — Refuge de Tête Rousse, 2026 rates and season
• Refuge des Cosmiques — 2026 season
• Tramway du Mont-Blanc — prices and schedules 2026
• Aiguille du Midi — timetables and rates 2026
• saintgervais.com — Peillex's call to regulate the Tour du Mont-Blanc
• Outside.fr — Peillex details his TMB regulation plan (2027)
• Global Summit Guide — Goûter route conditions 2026
• Voyageons Autrement — Mont Blanc access limited to 214 climbers per day