The Mendenhall Glacier is retreating so quickly that a recently revealed rock outcropping on its east side is expected to become a large wall that will separate the river of ice from Mendenhall Lake within the next 20 to 30 years.
About 20 miles to the east, the Taku Glacier is charging forward - bulldozing trees and pushing sediment into a barrier between the moving ice and Taku Inlet.
The fact that these two glaciers are behaving so differently, and at such rapid speeds in recent years, brings scientists from around the state and the world to Juneau every summer. Their studies can help predict how the landscape of Southeast Alaska will change in the coming years, and they can help document how the region's climate has changed over millennia. Some scientists believe the glaciers can indicate what's happening to the global climate today.
The glaciers flowing out of the Juneau Icefield are "the most delicate indicator of climate change that we have anywhere on the planet," said Maynard Miller, a scientist with the University of Idaho. He founded the Juneau Icefield Research Project in 1946, and has sent students and scientists to the ice every summer since to study the size and movement of its glaciers.
The Juneau Icefield is ideal because it is situated right where the low-pressure weather patterns over the Gulf of Alaska meet the high-pressure systems over the continent, Miller said. The arctic front, the name given for the area where these two systems meet, is where the most snow falls over the icefield.
In recent years, the arctic front has moved further east over the continent - a trend that, Miller said, is indicative of a global warming trend.
"One, we're observing the intensity of the changes due to global warming and, two, the rapidity of those changes," said Miller.
Not every scientist agrees entirely with Miller's belief in the value of the Juneau Icefield as an indicator of global climate change. But the icefield is old enough and dynamic enough to make a major contribution to our knowledge of Earth's climate history, scientists said.
An icy beginning
Starting about 1.8 million years ago and as recently as 20,000 years ago, sheets of glacial ice covered much of Alaska and Canada, and some of the northern Lower 48. Geologists know this era, which consisted of a series of warming and cooling periods, as the Pleistocene epoch. But it is known to most people as the last great ice age - remnants of which Juneau residents can see on their daily commute in views of the Mendenhall Glacier, as well as the Thomas Glacier above Lemon Creek.
The Pleistocene epoch lasted until about 11,000 years ago, when glaciers everywhere, including those flowing from the Juneau Icefield, began a retreat that continues, for the most part, today.
The glaciers have gone through several periods of growth and retreat in the last several thousand years, though, said Roman Motyka, a professor of geology and geophysics at the University of Alaska Southeast. He has studied the glaciers of Southeast Alaska for more than 20 years.
A minor glacial advance, known as the Little Ice Age, started in the mid-13th century. The Little Ice Age reached its maximum in the mid-18th century, when the glaciers began the retreat most are exhibiting today.
Glaciers can give scientists an indication of what climate was like centuries ago, before people began systematically measuring climate, Motyka said.
"The temperature record is uneven, and it doesn't go back very far," he said. "We can tell through glaciers what it was like hundreds of years ago."
By studying the geologic formations a glacier reveals or leaves in its wake as it retreats, as well as the age of the flora growing on once-glaciated land, a scientist can estimate where the glacier was at a particular point in time, how fast it retreated and whether it advanced and retreated multiple times.
These advances and retreats serve as a record of climate fluctuations through the centuries.
In addition to telling scientists what happened to Earth's climate in centuries past, glaciers can indicate current climatic trends on a regional and global scale, Motyka said.
"Most glaciers tell us that there's been significant warming in recent decades," he said. In Juneau, that warming has been recorded since the mid-1970s.
The warming could be part of a "so-called global warming" trend, Motyka said, or shorter weather patterns caused by El Nino or other climatic events.
About 90 percent of the glaciers in Alaska are retreating, Motyka said. Only one glacier on the Juneau Icefield bucks this trend: the Taku.
"The Taku is the maverick," said Motyka.
The surging Taku
About 100 years ago, when the Taku was about four miles further up the valley from its present location, it was a tidewater glacier, meaning the ocean and the glacier's face were in direct contact. Icebergs frequently broke off the glacier, a process called calving, and were carried by currents and tides down Taku Inlet and up into Gastineau Channel.
As a tidewater glacier, the Taku moved much more quickly than glaciers that aren't in direct contact with water. But glacier dynamics eventually slowed that movement.
"As (glaciers) advance, they build themselves a moraine, a pile of debris at the terminus," Motyka said. "A tidewater glacier does this submarine. It keeps pushing the debris up, and pretty soon it's got a wall there that protects it from the ocean water, so it stabilizes."
Motyka and his colleagues recorded this stabilization in the 1980s. But while there was little movement of the terminus of the glacier, the thickness of the ice at the terminus was increasing. In the summer of 2001, Motyka returned to the glacier and found it moving.
"We knew that the glacier had to start advancing again sometime because it was getting so thick," Motyka said.
One of Motyka's theories for the Taku's advance is that the high amount of precipitation received at the head of the glacier, on the Juneau Icefield, is overriding the melting at the terminus of the glacier caused by the higher temperatures.
Using the movement of the Taku Glacier to prove or refute any global warming theory is too simplistic, though, Motyka said.
"A tidewater glacier can't be used to study climate change," he said.
Motyka and co-principal investigator, Martin Truffer, a scientist with the Geophysical Institute at the University of Alaska Fairbanks, are tracking the glacier's movement again this summer and will analyze the results of their measurements this fall.
Miller of the Juneau Icefield Research Project also has been studying the movement of the Taku Glacier.
"For the last five to six years it's had some very pronounced thrusting," Miller said.
Rather than acting as proof against a pattern of global warming, the advancement of the Taku furthers the global warming argument, Miller said.
As the climate warms, snow falls at higher elevations on the icefield, where previously temperatures were too cold to allow snowfall. Unlike other glaciers on the icefield, the Taku is "fed" at these higher levels. As more snow falls at these higher elevations, the glacier grows and pushes forward into the inlet, Miller said.
"It's a very complex problem," he said. "The other glaciers that are receding are doing so because the freezing level has gone above their main source of nourishment."
Some climatologists disagree with Miller's presumption that what happens on the Juneau Icefield is indicative of what is happening to the global climate.
"The icefield is a good place to study in that it's large and it's consistent and any large changes are going to be noticeable," said Brian Hartmann, a climatologist for the Alaska Climate Research Center. "... But so many interacting forces are in play there."
Climate change certainly plays a part in glacier dynamics, but exactly what part and its size are debatable, Hartmann said.
Hartmann takes issue with Miller's assertion that the Juneau Icefield's location at the border of high- and low-pressure systems makes it an ideal place to study climate change.
"I would suggest that (the icefield's location) makes it a rather turbulent place to study because it's not a consistent weather pattern - it's always switching back and forth," he said.
Despite the complexities of glacier movement and climatology, or maybe because of those complexities, the Juneau Icefield likely will continue to draw scientists for years to come.
"I think the more we know, the better we are," said Motyka. "It's just the basic curiosity in how things work. ... For me, just learning is an end in itself."
Christine Schmid can be reached at email@example.com.