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 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 UN'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.