 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.