 The world has become far too hot for the aptly named Exit Glacier in Alaska. Like many low-altitude glaciers, it's steadily melting, shrinking two miles (3 kilometres) over the past 200 years as it tries to strike a new balance with rising temperatures. At the Kenai Fjords National Park south of Anchorage, managers have learned to follow the Exit and other glaciers, moving signs and paths to accommodate the ephemeral rivers of blue and white ice as they retreat up deeply carved valleys. "Some of the stuff is changing fast enough that we now have signs on moving pedestals," said Fritz Klasner, natural resource specialist at Kenai Fjords. The vast amounts of water stored in glaciers play crucial roles in river flows, hydropower generation and agricultural production, contributing to steady run-off for Ganges, Yangtze, Mekong and Indus rivers in Asia and elsewhere. But many are melting rapidly, with the pace picking up over the past decade, giving glaciers a central role in the debate over causes and impacts of climate change. That role has come even more sharply into focus after recent attacks on the U.N.'s climate panel, which included a wrong estimate for the pace of melting for Himalayan glaciers in a major 2007 report. The report said Himalayan glaciers could all melt by 2035, an apparent typographical error that stemmed from using literature not published in a scientific journal. Climate sceptics seized on the error and used it to question the panel's findings on climate change. The evidence for rapid glacial melting, though, is overwhelming. The problem is no one knows exactly what's occurring in the more remote Himalayas and parts of the Andes. Far better measurements are crucial to really understand the threat to millions of people downstream. "There is no serious information on the state of the melting of the glaciers in the Himalayan-Tibetan complex," Kurt Lambeck, President of the Australian Academy of Science, told a climate science media briefing in late February. The high altitude and remoteness of many glaciers in the Himalayas and Andes is the main reason. DATA IN A DEEP FREEZE To try to fill the gap, Indian Prime Minister Manmohan Singh said last month the government would establish a National Institute of Himalayan Glaciology in Dehra Dun in the north. In Europe and North America, glaciers are generally more accessible and there are more trained people to study them. Switzerland's Aletsch glacier, the largest in the Alps, has been retreating for about 150 years. But the glacier, which feeds the River Rhone, still stores an estimated 27 billion tonnes of ice, according to www.swissinfo.ch. That's about 12 million Olympic-sized swimming pools. In 2008, a total of 79 Swiss glaciers were in retreat, while 5 were advancing, the Swiss Glacier Monitoring network says. "There are a very small number of glaciers that are monitored," said veteran glaciologist Ian Allison, pointing to less than 100 globally for which there are regular "mass-balance" measurements that reflect how much a glacier grows or shrinks from one year to the next. Such measurements are the benchmark and several decades of data is regarded as the best way to build up an accurate picture of what's happening to a glacier. Glaciers originate on land and represent a sizeable accumulation of snow and ice over the years. They tend to carve their way through valleys as more and more ice accumulates until the point where more is lost through melting than is gained. THAT SHRINKING FEELING "We probably know less about the total volume of glaciers than we do about how much ice there is in the big ice sheets in Greenland and Antarctic because a lot of it is in small mass areas and a lot of it is inaccessible," said Allison, leader of the Australian Antarctic Division's ice, ocean, atmosphere and climate programme. The World Glacier Monitoring Service in Switzerland analyses mass balance data for just over 90 glaciers and says their average mass balance continues to decrease. Since 1980, cumulative thickness loss of the reference glacier group is about 12 metres of water equivalent, it says in its latest 2007/08 report. Estimates vary but glaciers and mountain caps could contribute about 70 cm (2.3 feet) to global sea levels, a 2009 report authored by Allison and other leading scientists says. The "Copenhagen Diagnosis" report from the Climate Change Research Centre at the University of New South Wales says there is widespread evidence of more rapid melting of glaciers and ice-caps since the mid-1990s. That means run-off from melting glaciers and ice-caps is raising sea levels by 1.2 millimetres a year, translating to up to 55 cm (1.8 feet) by 2100 if global warming accelerates. In Nepal, the International Centre for Integrated Mountain Development says "mass-balance" measurements would provide direct and immediate evidence of glacier volume increase or decrease. "But there are still no systematic measurements of glacial mass balance in the region although there are promising signs that this is changing," the centre said in a recent notice. It said that based on studies, the majority of glaciers in the region are in a general condition of retreat. "Small glaciers below 5,000 metres (16,500 feet) above sea level will probably disappear by the end of the century, whereas larger glaciers well above this level will still exist but be smaller," it said. Glaciers have almost vanished from New Guinea island and in Africa and many on Greenland, the Antarctic Peninsula and West Antarctica are also melting quickly, dumping large amounts of ice into the sea. BAMBOO STICKS Part of the problem is that glaciers are fickle things to measure, said Allison, and requires legwork and lots of bamboo stakes. These are placed in holes top to bottom, a potentially dangerous job, although satellites and lasers fitted to aircraft are changing this. After a year or so, stakes placed up high will have had snow build up on them, so you can estimate how much snow fell there. Those down low will have lost mass due to melt and evaporation, so there would be more of the canes sticking out. "So you can measure how much height is lowered down below, how much it's gained up top. You'll need to know the density of the snow and ice as well," Allison said. But he said glaciers in one region can all apparently behave differently in response to the same climate signal. "Because the fluctuations that occur in the front depend on how long it takes to transfer the mass from the top of the glacier to the bottom." "You might have an area where all the small glaciers are all rapidly retreating but big glaciers still coming forward because they are still integrating changes that happened maybe 50 years ago," he added. For the millions that live downstream, it is the impacts that are of most concern and among them is the threat of sudden bursting of lakes created as glaciers retreat. About 14 of the estimated 3,200 glaciers in Nepal are at risk of bursting their dams. Ang Tshering Sherpa, from Khumjung village in the shadows of Mount Everest, said the Imja glacial lake could burst its dam anytime and wash away villages. "When I was a child I used to take our yaks and mountain goats for grazing on grassy flat land overlooking Everest," Sherpa said. "What was a grazing ground for yaks in 1960 has now turned into the Imja due to melting of snow," Sherpa, now a trekking and climbing entrepreneur, said in Kathmandu. A glacial lake broke its dam 25 years ago destroying trekking trails, bridges and a hydroelectric plant in the region. Neighbouring Bhutan also faces the threat of bursting dams. Just how much water melting glaciers contribute to major rivers such as the Ganges and Brahmaputra, though, remains unknown. Richard Armstrong, a senior scientist of the National Snow and Ice Data Centre in Boulder, Colorado, said it was nonsense to think that if glaciers melted there would be no water in the Ganges, a lifeline for millions in northern India. "Even if the glaciers disappeared tomorrow it wouldn't have a huge impact on the water supply. The rest of the river flow comes from rain and melting seasonal snow." He said the centre has put in a proposal to NASA to use satellite data to build a better picture of the area and altitude of glaciers in the Himalayas. "What we want to look at is what's the contribution of melting glacier ice to the downstream hydrology," Armstrong said. "It's really what's of primary importance to the socio-economic impacts of retreating glaciers." Allison and Armstrong and many other scientists have dismissed the row over the U.N. climate panel error as overblown but said it served as a useful reminder of the gaps in global glacier monitoring and the need for a far better picture. "It certainly brought attention to the problem," said Armstrong.