For decades, winter visitors to Apostle Islands National Lakeshore on Wisconsin's rugged Lake Superior coast have marveled at the artistry that happens when water, waves and subfreezing temperatures converge, creating natural ice sculptures as artful as glassworks.
The ephemeral event, when upstream rivulets flow into caves at the lake's edge and harden into blue-green stalactites anchored in a bed of clear Lake Superior ice, is so popular with tourists that the National Park Service maintains a telephone hotline to let people know when it's safe enough to make the 2-mile hike across the hardened lake to view what are called the sea caves.
On a sunny weekend in February, the Park Service phone line can receive hundreds of calls from visitors looking to experience the caves, said Bob Krumenaker, the Apostle Islands superintendent. "It's probably the most unique experience you can have up here."
It's also becoming one of the most rare. The ice hotline hasn't given an "all clear" for ice caving since 2009. That's because Lake Superior's winter isn't what it used to be. Higher temperatures -- both at the surface and in the water -- are conspiring to make what Krumenaker calls "an endangered park experience."
Indeed, climatological studies suggest Lake Superior is undergoing changes that could disrupt a broad range of economic and ecological activities, including maritime shipping, commercial and recreational fishing, and tourism at places like the Apostle Islands, considered one of the world's top destinations for sea kayaking and wild-island hopping.
Among the most worrying trends for scientists and policymakers is the loss of winter lake ice, a condition exacerbated by higher air and water temperatures, that has changed the way the gigantic lake and its micro-climate behave.
Lake Superior, while still the largest freshwater lake by surface area, holds more heat and less water than it has in a century, resulting in unusual shifts in behavior sometimes literally overnight, as evidenced recently by one of the most impressive ice blowouts ever witnessed at the Apostle Islands lakeshore.
Here today, gone tomorrow
In less than 24 hours on Feb. 6, the frozen expanse of Lake Superior that visitors walk across to see the ice caves transformed from rock-solid ice into a shattered quilt of icebergs. By Feb. 8, even the icebergs had disappeared as the lake reverted back to an open water environment.
Krumenaker, who was poised to open the ice caves Feb. 7, believes strong winds created a powerful upwelling in the lake, enough to crack the ice sheet into hundreds of pieces. As the ice loosened, warm air and water made quick work of the bobbing bergs.
"The remarkable thing is that in a year like this when we've got good ice, there should be ample opportunity for people to walk out and see those caves," Krumenaker said in a recent telephone interview from Bayfield, Wis., the headquarters and main entry point to the Apostle Islands lakeshore. "We never say it's completely safe. But when we stop saying it's unsafe, that's usually a good indication that the ice is sticking around for a while."
But such assumptions, which have been solid for most of the past century, are getting thinner each year, he said.
Data collected by Jay Austin, a University of Minnesota-Duluth physicist and scholar on Lake Superior warming, suggest the lake's open-water summer temperatures warmed by 3.5 degrees Celsius between 1906 and 2005. Yet most of that warming has occurred since 1980, according to research published by Austin and colleague Steve Colman in 2008.
According to Austin, Lake Superior's warming is caused not only by higher air temperatures, which are occurring throughout the Great Lakes region, but by marked declines in winter ice that used to hold the lake in a cold embrace well into early summer. The icy cold conditions delayed the lake's stratification, a normal summer condition whereby a layer of buoyant warmer water settles atop a deeper reservoir of cold water.
Once stratified, surface water becomes more vulnerable to the sun's thermal and evaporative powers, resulting in a warmer, shallower Lake Superior. And while Upper Midwest winters can still bring periods of brutal cold, the lake rarely freezes the way it used to, and when ice accumulations do occur they are more vulnerable to rapid changes in wind or currents, experts say.
"There is plenty of variability from year to year -- it's a complex system!" Austin notes on a website detailing his research. "The point is that on average, things are getting warmer, overturns [from colder to warmer surface water] are becoming earlier."
'Downward trend' heightens risks
And when ice is thinner, it does a poorer job reflecting sunlight, allowing the sun's energy to penetrate the lake more easily and earlier in the year, creating what some experts call a "negative loop cycle" of less ice and more heat at the lake's surface.
Recent research based on satellite imagery from 1973 to 2010, and published by the NOAA Great Lakes Environmental Research Laboratory in Ann Arbor, Mich., found "a significant downward trend in ice coverage from 1973 to the present for all of the lakes," with Lake Ontario bearing the largest loss at 88 percent, while Lake Superior ice coverage declined by 79 percent.
The researchers also found that Great Lakes ice cover "experiences large variability in response to predominant natural climate forcing and has poor predictability." In other words, today's seemingly impenetrable ice sheet could be tomorrow's open water, with very little in the way of warning.
Year-round locals say the nearby Apostle Islands offer living proof of the research findings, whether in the diminished visitor experience at the Park Service's ice caves or in the shortened winter driving season between Bayfield and the year-round island community of La Pointe, on Madeline Island, separated from the mainland by about 2 miles of Lake Superior open water.
In winters past, the ice road offered islanders as much as six weeks of free, four-wheel access to the mainland for car repairs, provisioning trips or just joy-riding across a frozen expanse of the South Channel, which separates the island from the mainland. For nondrivers -- like schoolchildren and the sick or elderly -- a passenger van operated by the La Pointe Chamber of Commerce shuttled passengers five times daily across the ice highway to attend classes or make doctor's appointments.
But nobody plies the Madeline Island Ice Road the way they used to, and certainly not in anything heavier than a passenger car or lightly loaded pickup. In most years now, if the road opens at all, it does so days or weeks at a time because lake conditions change so frequently that the ice sheet can crack and separate overnight.
"We don't have the ice we used to have," said Arnie Nelson, whose Madeline Island construction company helps build and maintain the ice road every year. Forty years ago, when he began monitoring the ice with his father, Nelson said the channel between La Pointe and Bayfield would see ice accumulations of 3 or more feet. Today, he said, the channel is lucky to see 12 inches of ice, which happens to be 1 inch thicker than the minimum required for driving.
Death of a 'safety fanatic'
And last year, he said, "the ice was so vulnerable that it would shrink at night, then the sun would come out the next day and it would just explode. You'd get cracks that would rise up 3 or 4 feet into the air."
Robin Trinko Russell, a year-round islander and vice president of the Madeline Island Ferry Line, which operates four auto ferries between La Pointe and Bayfield during open-water season, said the lake's diminished winter ice has resulted in much longer seasons for her company's boats, two of which have V-hulls capable of breaking ice up to 6 inches thick.
"We prefer not to have to use them that way," Russell said in a telephone interview. But when the ice isn't thick enough for driving, "we don't have much choice but to keep operating."
Russell said she remembers two winters, 1998-99 and last year, when the ferries never stopped running because the South Channel failed to freeze over. Many more winters of late have seen thick ice for only weeks at a time. And this year, the ferries ran until Jan. 31 before giving way to a wind-sled passenger and courier service that offers crossings in mid-ice conditions that are suitable for neither boats nor cars.
As with the Apostle Island sea caves, officials provide daily updates on the ice road's status via a telephone hotline. And notwithstanding an occasional reckless driver, the ice road has not experienced a major "break-through" since 1977, when a truck attempting to haul a two-story house across the ice in early March strayed off the thickest ice. The trailer broke through first, then the seven-room home heaved toward the water before eventually sinking just off the Madeline Island shore.
Today, Nelson said, "People are very respectful of the road. They know if someone breaks it up it by hauling something heavy, they're putting everyone else at risk."
But off the road, where monitoring is more difficult and conditions change fast, thin ice has proved deadly for drivers of other vehicles, namely snowmobilers for whom Lake Superior's near-shore ice has long provided a tabletop speedway.
This winter alone, three deaths in the Bayfield area have been attributed to snowmobilers crashing through thin ice. All the victims were locals in their 30s, including a well-known fishing guide and former Bayfield police officer who had helped countless others navigate the Apostle Islands seashore in both summer and winter, the Park Service's Krumenaker said.
"He was a tremendous safety fanatic. He taught everybody how to fish in the summer and snowmobile on the lake in the winter. For reasons we'll never know, he went out on some very unsafe ice," he said.
Correction: Lake Superior is the largest freshwater lake by surface area, not by volume, as stated in an earlier version of this story.