In the autumn of 1879, a canoe crept into a remote Alaskan bay. After days of rain, the weather had relented, revealing spectacle after spectacle: towering mountains, green waters and icebergs sparkling like 鈥渢he jewel-paved streets of the New Jerusalem鈥�. Most impressive of all were the glaciers, fed by distant heights and flowing far into the fjords of what would soon be called Glacier Bay. Winter was approaching and the expedition, led by American environmental icon John Muir, had to beat a hasty retreat. But Muir had come to an important conclusion: the bay hadn鈥檛 been there when his 100-year-old charts were made. So the glaciers had shrunk since then, and they were continuing to retreat at a prodigious rate.
BRUCE MOLNIA wants unambiguous proof that the world鈥檚 climate is changing, and he鈥檚 willing to get it the hard way 鈥� crashing through brush and scrambling over boulders in some of the most rugged parts of coastal Alaska. His goal is to identify the precise spots where 19th-century photographers first captured the splendours of the US鈥檚 northernmost territory, particularly its glaciers. When he finds one, he takes his own photos, carefully matched to the originals.
Many of the images centre on Glacier Bay, the dramatic fjord-and-glacier landscape into which self-taught glaciologist John Muir led his party in 1879, and to which he returned several times over the next 20 years. Muir himself didn鈥檛 take photos, but on the final expedition in 1899, Molnia says, 鈥渋t seems that almost everyone else had a camera鈥�.
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In his office at the national headquarters of the US Geological Survey in Reston, Virginia, Molnia has accumulated more than 1600 photos of Alaskan glaciers taken before 1920. Over the past five summers, he has pinpointed more than 200 of their locations.
Sometimes he gets lucky and finds an old cairn marking the spot where the picture was taken. Usually it鈥檚 much harder. The early visitors explored barren landscapes that had only recently emerged from their blankets of ice. But in the rainforest climate of coastal Alaska, vegetation grows nearly as fast as the glaciers recede. Molnia once tried to duplicate an image taken in 1899 from the summit of a rocky knob that is now covered with cottonwood trees. 鈥淥ne of my colleagues climbed a tree but still couldn鈥檛 get a photo.鈥�
Another time he thrashed through alder thickets (a favourite habitat of bears) to the top of a ridge, only to discover he was on the wrong ridge. When he reached the right one, his companions had to lie on top of the shrubs to bend them low enough for an unimpeded view. 鈥淭hat photo was one we never really did duplicate,鈥� says Molnia.
Finding the same locations is itself an art. In theory, it is merely a matter of matching mountains in the background to those in the photo, but that鈥檚 tricky if you鈥檙e not sure whether an old photo was shot with a wide-angle lens or a telephoto. Sometimes the old location proves to have been on a glacier that has since melted. And some photos were shot from the decks of steamboats in waters that see some of the world鈥檚 strongest tides. By the time Molnia鈥檚 team figures out that their own boat was in the right spot, it no longer is.
There are also technical difficulties. Several of the early photographers were enamoured of panoramic cameras that employed boom-mounted lenses that swung through a 180-degree arc, recording the exposure on a long strip of film. Modern lenses can capture the same field of view, but it is distorted by the fisheye effect. Worse, some of the old lens booms did not move at a uniform speed, creating an entirely different set of distortions. The solution is to stitch together a sequence of images and then to digitally stretch or compress portions of them until they match.
Molnia has no doubt that the results are worth all the effort. 鈥淚f a picture is worth a thousand words, what is the value of a pair of photos that span more than a century?鈥�
Matt Nolan, a hydrologist at the University of Alaska, Fairbanks, agrees. Nolan is making his own photographic assessment of glaciers in the Brooks Range, which lies in the state鈥檚 far-northern interior. 鈥淭here鈥檚 a lot of value to photos like this,鈥� he says. 鈥淵ou don鈥檛 need a background in science. This is something everyone can understand.鈥�
鈥淭he retreat was well under way when Muir first visited Glacier Bay鈥�
Nolan鈥檚 photos consistently show glaciers in wholesale retreat. In one particularly dramatic example, he duplicated a 98-year-old image of the Okpilak Glacier, near the range鈥檚 eastern end. In the older image, a broad tongue of ice extends far down the valley. Now the glacier has retreated almost out of sight and thinned dramatically, leaving a bathtub ring of light-coloured rocks where the ice used to be.
In the old photo, the glacier reaches the tops of the piles of debris, called moraines, pushed up at its sides and snout. That鈥檚 important, Nolan says, because it鈥檚 equivalent to seeing a river full to the brim. It means that a century ago, the glacier must have been at its maximum reach. The extent to which humans have caused its subsequent retreat and other changes is open to debate, he says. 鈥淏ut there is no debate about whether they occurred.鈥�
In Glacier Bay, the retreat was already well under way when Muir first visited. Canoeing through the bay鈥檚 dangerous waters, he knew he was in a young landscape, so young that his Native American guide, who had visited the region as a boy, was confused by topography that didn鈥檛 match his memories. 鈥淭hus, the domain of the sea鈥s being extended,鈥� Muir wrote in his journal. He was particularly intrigued by a rocky protrusion some 300 metres high near the glacier鈥檚 snout, which not long before must have been buried beneath a layer of ice twice that height. Now it was well on its way to becoming a mountainous island.
鈥淓merging from its icy sepulchre, it gives a most telling illustration,鈥� he wrote. 鈥淚n this instance, it is not the mountain, but the glacier, which is in labor, and the mountain itself is being brought forth.鈥�
Molnia has found that more than 99 per cent of the 2000-odd Alaskan glaciers that descend below 1500 metres are retreating. But a few have been advancing, even though the climate has warmed by between 2 and 3 掳C in the past 60 to 70 years. What these glaciers have in common, he says, is that they lie closer to the Pacific and have ice-accumulation zones extending well above 2000 metres.
This means that a warming climate is not necessarily synonymous with glacial retreat. Instead, a glacier鈥檚 fate can depend on regional and local factors. If a warming climate is accompanied by increasing precipitation, and the glacier鈥檚 accumulation zone is high enough for it to fall as snow, then climate change may be accompanied by glacial advance.
Nolan observed no such advances among his Brooks Range glaciers because the climate in that part of Alaska is not only warming but also becoming drier, dealing the glaciers a double whammy. What鈥檚 happening in Alaska is not a simple case of warmer climate, shrinking ice, says Molnia. 鈥淭here are a lot of regional complexities.鈥�
At lower latitudes, the picture is simpler: even the highest-elevation glaciers are in retreat. At a meeting of the American Geophysical Union last December, glaciologist Lonnie Thompson of Ohio State University pointed out that the ice of such tropical heights as Mount Kilimanjaro, Tanzania, and the Peruvian Andes has been melting at an alarming rate throughout the 20th century. 鈥淕laciers do not have a political agenda,鈥� he says. 鈥淭hey are the canaries in the coal mine for the Earth鈥檚 climate system.鈥�
Molnia鈥檚 next goal is to fill the gap between his Alaskan studies and Thompson鈥檚 work in the tropics by comparing old and new photos at mid-latitude in the continental US. About 18 national parks in the US have glaciers. 鈥淏y the time we are done, we will have at least a dozen of them with representative photo pairs to fill in what has happened over the past 100 years.鈥�
Meanwhile, he sees Muir鈥檚 pioneering work in Alaska as a benchmark for future studies. 鈥淗is observations were critical, not only in building an environmental consensus that led to the preservation of many of these places as national parks,鈥� Molnia says. 鈥淗e threw down the gauntlet to future scientists to use his observations as a starting point and document what is now a real issue about how the Earth is responding to climate change.鈥�
