the third-largest deposit of ice and snow in the world, after Antarctica and the Arctic. The Himalayan range encompasses about 15,000 glaciers, which store about of fresh water. Its glaciers include the Gangotri and Yamunotri (Uttarakhand) and Khumbu glaciers (Mount Everest region), Langtang glacier (Langtang region) and Zemu (Sikkim). Owing to the mountains' latitude near the Tropic of Cancer, the permanent snow line is among the highest in the world at typically around . In contrast, equatorial mountains in New Guinea, the Rwenzoris and Colombia have a snow line some lower. The higher regions of the Himalayas are snowbound throughout the year, in spite of their proximity to the tropics, and they form the sources of several large perennial rivers. In recent years, scientists have monitored a notable increase in the rate of glacier retreat across the region as a result of climate change. For example, glacial lakes have been forming rapidly on the surface of debris-covered glaciers in the Bhutan Himalaya during the last few decades. Although the effect of this will not be known for many years, it potentially could mean disaster for the hundreds of millions of people who rely on the glaciers to feed the rivers during the dry seasons. The global climate change will affect the water resources and livelihoods of the Greater Himalayan region. Lakes The Himalayan region is dotted with hundreds of lakes. Pangong Tso, which is spread across the border between India and China, at far western end of Tibet, is among the largest with surface areas of . South of the main range, the lakes are smaller. Tilicho Lake in Nepal in the Annapurna massif is one of the highest lakes in the world. Other notable lakes include Rara Lake in western Nepal, She-Phoksundo Lake in the Shey Phoksundo National Park of Nepal, Gurudongmar Lake, in North Sikkim, Gokyo Lakes in Solukhumbu district of Nepal and Lake Tsongmo, near the Indo-China border in Sikkim. Some of the lakes present a danger of a glacial lake outburst flood. The Tsho Rolpa glacier lake in the Rowaling Valley, in the Dolakha District of Nepal, is rated as the most dangerous. The lake, which is located at an altitude of has grown considerably over the last 50 years due to glacial melting. The mountain lakes are known to geographers as tarns if they are caused by glacial activity. Tarns are found mostly in the