town to the adjacent Prarion plateau (). Heli-skiing gives ready access to many remote or off-piste ski routes in the Mont Blanc massif. Because heli-skiing is banned across France for environmental reasons, companies offering this service only operate on the Swiss and Italian sides of the range. Mountain huts Since the very early days of alpine mountaineering a number of high-altitude mountain refuges have been positioned strategically across the massif to give climbers easier access to the high summits by permitting an overnight stay. The majority are owned by national mountaineering clubs, and many are wardened during the summer months, although the smaller bivouac huts are unmanned and have very basic facilities. Those on the most popular routes, such as the Goûter Hut, now require all climbers to pre-book. Some tiny huts, such as the remote Eccles Hut, can also be extremely crowded during good mountaineering weather, and some climbers prefer to bivouac outside. Environmental protection Weather records show that since the 1960s there has been a trend of less snow at lower altitudes, whilst since the 1990s average mountain temperatures have increased more than at lower levels. Temperatures in Chamonix have risen by 1.5 °C over the last 75 years, and fresh snow build-up has halved there in the last 40 years, and there has been an increase in the melting and retreat of the massif's glaciers. The Mer de Glace has retreated in length since 1820 and, at Montenvers, has reduced in thickness by . In the last 20 years the glacier has been retreating at a rate of around every year. Since 1994 it has lost in length and in depth. To help counter these effects, in 2012 the various Chamonix-Mont Blanc authorities introduced a climate and energy action plan, committing the region to a 22% reduction in its greenhouse gas emissions by 2020. The plan included proposals to improve air quality by banning those lorries from using the Mont Blanc Tunnel which were deemed to be the most polluting and which, at times, had reduced air quality to levels more usually associated with the streets of Paris. Concerns over the state of the environment around the most popular parts of the Mont Blanc massif, and the need for visitors to better respect it, were reflected in a statement in 2014 by Jean-Marc Peillex, the mayor of Saint-Gervais, who said:Mont Blanc is a heap of garbage ...