Glaciers throughout Alaska are shrinking more and more rapidly, and scientists comparing old photos taken up to a century ago with digital images made during climbing expeditions today say the pictures provide the most dramatic evidence yet that global warming is real.

And it's not only the glaciers reflecting the climate change. Everywhere on the treeless tundra north of the jagged slopes of Alaska's Brooks Range, explosive bursts of vegetation -- willows, alders, birch and many shrubs -- are thriving where permafrost once kept the tundra surface frozen in winter.

Two geophysicists and a government geologist who spend much of their working lives exploring changes in the Arctic displayed dozens of photographs from the thousands in their files Thursday at the annual meeting of the American Geophysical Union in San Francisco.

"You don't need science to prove the point," said Matt Nolan of the University of Alaska in Fairbanks. "This evidence is visual, and it's real.

"All the glaciers in the Arctic National Wildlife Refuge are retreating from their most extended positions thousands of years ago, and the only scientific explanation for their retreat is a change in climate. There's no doubt at all, and the loss of glacial volume is accelerating."

Bruce Molnia, a geologist with the U.S. Geological Survey, has gathered more than 200 glacier photos taken from the 1890s to the late 1970s and has visited more than 1,000 Alaskan glaciers in the past four years to photograph them from precisely the same locations and pointing in the same directions as the older ones.

Where masses of ice were once surging down wide mountain passes into the sea, or were hanging from high and perilously steep faces, the surfaces in Molnia's images now stand bare. What remains from many of the retreating glaciers are stretches of open water or broad, snow-free layers of sediment.

"And as the glaciers disappear," Molnia said, "you get the amazing appearance of vegetation."

As certain as the scientists are that global warming is responsible for Alaska's changing landscape, they hesitated to blame it all on the increasing levels of greenhouse gases from industry that have marked the past century and have resulted in the 1997 Kyoto Protocol, now ratified by 126 nations but which the Bush administration has rejected.

"The rapid melting of the glaciers, the increasing vegetation in the high Arctic and the invasions of insects where insects were once unknown are all happening," Molnia said, "and I would not question that a significant component of the change is due to the heat-trapping greenhouse effect -- certainly a human-caused issue -- but I wouldn't say it's all caused by global industries."

The increasing pace of change is clear in the glaciers he has explored, Molnia said. Many photos he has recovered from the 1890s, the 1940s and the 1970s show how fast the glaciers have been retreating; in a few cases, however, where warming temperatures have increased precipitation at higher altitudes, some glaciers actually advanced, he said.

Geophysicist Ken Tape, of the University of Washington, has been exploring the Brooks Range in the far north of Alaska as well as the wide stretches of treeless tundra between the mountains and the Beaufort Sea along the state's north coast.

The growth of shrubs across the tundra has increased by 40 percent in less than 60 years, Tape said, "and that perturbation is certainly due to the changing climate."