If you've ever wanted to take an Alaskan cruise to see glaciers, do it sooner rather than later for the best views. According to the Swiss-based World Glacier Monitoring Service, glaciers worldwide have been shrinking faster than they have been growing. The service labeled losses in some recent years as extreme.

But you don’t have to run right out and book today. Glacier melting — or retreating to use glacier terminology — is a slow process. Still, scientists predict that up to a quarter of the world’s mountain glacier mass could disappear by 2050 and up to half by 2100.

The glaciers you’ll see on an Alaskan cruise originate in mountains, when one years’s snow does not melt away before the next year’s arrives. And while these glaciers may look to be just frozen masses, as inanimate as the mountains from which they flow, there’s actually much more to one than a casual look might suggest.

Like all mountain glaciers, Alaska’s flow along the path of least resistance, generally through valleys. Hence, they’re called valley glaciers, but they’ve also picked up the nickname “rivers of ice.” If a valley glacier reaches the sea, it earns the name tidewater glacier. On a cruise, obviously, most of the glaciers you’ll see will be tidewaters.

Cruise lines sailing Alaskan waters typically visit glaciers located in four areas, moving from south to north: Juneau, Glacier Bay, Yakutat Bay and Prince William Sound.


Juneau’s Glacier

Thirteen miles north of downtown Juneau, lies Alaska’s most visited glacier — Mendenhall. Among your cruise ship’s shore excursions for its Juneau port call you’ll undoubtedly find a bus trip to Mendenhall Glacier.

The 12-mile-long Mendenhall is a valley glacier. But since it flows into Mendenhall Lake not the sea, it’s not a tidewater.

Still, the Mendenhall will sometimes show you a typical feature of tidewater glaciers — calving, or ice breaking off its 100-foot-high, 1.5-mile-wide face.

The Mendenhall Glacier is retreating and has been doing so since 1750. As a result, a half-mile of Mendenhall Lake lies between its face and the glacier’s visitor center.

The visitor center can give you some insight into glaciers — it has a model on display that helps explain how glaciers form and behave.

One glacier behavior is movement. Glaciers may appear to be stationary, but they are actually moving, in two distinct ways.

To begin with, a glacier can advance or retreat depending on the amount of snow it accumulates. If more snow falls and turns to ice than is lost by melting, evaporating or calving, a glacier mass advances. If more ice is lost than is accumulated, a glacier shrinks or retreats.

If ice gain and loss are equal, a glacier neither advances nor retreats — yet it’s still moving in another way. Ice within it and at its surface is always moving forward, pulled by gravity and pushed by the weight of accumulating snow and ice.

Upper ice in a glacier, however, moves faster than the lower ice. The average Alaska valley glacier’s ice travels about 12 inches a day near its surface, but it grinds along its valley floor at just four inches a day.


Shrinking Ice in Glacier Bay

Glacier Bay’s tidewater glaciers flow down valleys from the St. Elias and Fairweather ranges.

For up-close sightseeing, cruise ships typically visit the tidewater glaciers in the bay’s Johns Hopkins and Tarr inlets, especially the Johns Hopkins Glacier and the Tarr’s Margerie Glacier.

You’re sure to spot a black stripe running lengthwise down the middle of some tidewater rivers of ice. As these masses of ice move forward, they rip, wrench and grate a valley’s floor and walls. Some of this rock and debris becomes frozen into the sides of glaciers. When two mountain valleys merge to become one, so do the glaciers filling them. The black stripe you see, which glaciologists call a moraine, is the rocky debris that was frozen into the sides of the two glaciers before they merged.

Glacier Bay serves as proof that glacier mass has been shrinking. In the early 1790s, British Royal Navy Captain George Vancouver was exploring the Pacific in search of the Northwest Passage. When Vancouver sailed past what today is Glacier Bay, he saw only a small bay indented in a wall of ice. Naturalist John Muir visited Glacier Bay in 1879. He found a much larger bay than did Vancouver — the wall of ice had retreated in just over 80 years more than 40 miles.

Glacier Bay’s ice has continued to dwindle. Muir, for example, never saw the Johns Hopkins and Tarr inlets. In his day they were buried beneath ice.

Most researchers attribute ice loss to warming trends in the climate. In addition, there’s a vicious circle — when glacier ice melts, the newly exposed land and water surfaces retain heat, which leads to even more melting.


Yakutat Bay’s River of Ice

A few cruise ships pull into Yakutat Bay so passengers can view one of North America’s longest rivers of ice — the Hubbard Glacier, running some 75 miles in length. An international glacier, the Hubbard begins its run to the Gulf of Alaska from Mount Logan, Canada’s tallest mountain.

Not only is this tidewater glacier unique because of its length, it’s also slowly advancing.

When looking at the face of a glacier you can generally determine if it is advancing or retreating. An advancing glacier tends to have a steep face, a retreating one a sloping face.

The face of some glacier’s may also show you their annual accumulations of ice through black ribbons of dirt separating the yearly layers.


Icebergs Roam Prince William Sound

Ships cruising Prince William Sound typically visit Columbia Glacier and some of the glaciers in College Fjord. The fjord’s glaciers include: Barnard, Bryn Mawr, Dartmouth, Harvard, Wellesley, Vassar, Williams and Yale.

Obviously, College Fjord’s rivers of ice were not arbitrarily named. An expedition sponsored by railroad magnate Edward H. Harriman explored Prince William Sound in 1899. The names of the glaciers reflect the Harriman Expedition’s Ivy League funding and the institutions with which its scientists were associated.

College Fjord is home to the most tightly packed cluster of glaciers in Alaska. Most of these valley and tidewater glaciers are retreating. Several, however, are advancing, including the tidewater Harvard Glacier.

One Prince William Sound tidewater glacier has achieved notoriety. The miles-wide face of the Columbia Glacier calves icebergs. These huge ice masses then drift into the shipping lanes used by tankers entering and leaving the sound’s Valdez oil port. In 1989, the outward-bound Exxon Valdez altered course to avoid ice calved from the Columbia. The tanker then slammed into Bligh Reef, spilling almost 11 million gallons of crude oil into Prince William Sound.

As the decades pass, though, Prince William Sound’s shipping lanes should become safer — unfortunately at the Columbia’s expense. As a part of the world’s disappearing glacier mass, some authorities think the Columbia Glacier will retreat some 20 miles by the mid-21st century.