In assessing the health of the Harding Ice Field, it helps to picture a savings account. Snow accumulation is like money deposited into the account. Melt drains it.
More snow deposited than mass withdrawn through melt would mean the account grows. More melt than snow results in a shrinking balance, while snow accumulation equal to melt creates stability.
Just like bank accounts, glaciers can benefit from factors that act like interest to boost the balance. For instance, having the bulk of a glacier at higher elevations, with a limited amount of slope exposed to warmer temperatures below, reduces melt. But bank fees and service charges — those pesky factors that exacerbate depletion — also have an impact. If a glacier terminates in a lake or at tidewater, its rate of loss will accelerate.
The Harding Ice Field, today covering more than 700 square miles of the Kenai Mountains, has weathered many booms and busts in ice-adding and melting conditions since it formed more than 23,000 years ago, during the Pleistocene Epoch. Yet its continued survival appears less and less a bankable certainty.
"The ice field essentially is thinning in almost all places," said professor Martin Truffer of the University of Alaska Fairbanks. In some spots, thinning has been measured at minus-3.5 meters annually.
"That's about 10 to 12 feet of elevation change per year — about the size of this room of elevation drop every year, averaged over the last 50 years. These are really large changes," Truffer told an audience assembled in the Seward Library Community Room late last month to hear an update of a project to measure the thickness of the Harding Ice Field.
Studies so far indicate a chilling future for the more than 30 glaciers radiating from the ice field along the eastern spine the Kenai Peninsula, but not necessarily the end of the field. There are still signs of health — primarily that each winter's snow on the top of the ice field doesn't all melt come the following fall.
"If you start seeing a lot of exposed ice on the ice field itself at higher elevations, then that's bad because that means it's going to thin and it's going to increase thinning over time. … If the ice field still has snow from last winter then, for the moment, the ice field is still OK," he said.
Truffer, a physics professor with the Glaciers Group at the Geophysical Institute at UAF, conducted a ground-based radio-wave survey of the ice field around Exit Glacier in 2010 as part of a project funded by the National Park Service, and is continuing annual laser aerial measurements with funding from NASA.
This is Truffer's first opportunity to study the Harding Ice Field, following his glacier research elsewhere in Alaska, in Greenland and Antarctica. The Harding is an interesting chunk of ice, though perhaps not at first glance. It's not the biggest ice field in Alaska — that would be the Bagley Ice Field in Southeast Alaska, at 127 miles long and six miles wide. But it is the largest ice field contained entirely within the U.S., as the larger fields in Southeast are shared with Canada. The Harding also is unique in that it lies entirely within federally protected lands. And it contains a variety of glaciers.
"Basically any type of glacier that exists in the world exists here in this ice field," he said.
Some end on land, such as Exit Glacier in Seward. Others end in lakes, including Skilak and Tustumena glaciers, while others end in tidewater, such as Bear and Aialik glaciers in the Kenai Fjords.
"We know that the biggest changes in glaciers always occur in the ones that end in water, in tidewater or lakes. You can get some really interesting behavior — some glaciers are advancing even at a time when the climate is quite warm, like now," Truffer said. Next door may be another glacier that's retreating.
The Harding Ice Field is a bit more boring in its uniformity, in that most of its glaciers are retreating, he said. Comparing U.S. Geologic Survey maps from the 1950s with a recent map compiled by NASA from a space shuttle radar topography survey establishes about a 50-year record of change, particularly noticeable in the glaciers and along the edges of the ice field. Some areas show ice now a mile back from where it used to be.
"That's fairly significant retreat over that time frame," Truffer said.
Other spots show even more, like Northwestern Glacier ending at tidewater in the Kenai Fjords southwest of Seward.
"That one in the last 50 years retreated six kilometers — so, about four miles. That's a really significant retreat. I grew up in Switzerland, and most glaciers there aren't even that long," Truffer said.
Several factors contribute to how fast a glacier retreats. Topography plays a big role. Melt happens the fastest along the exposed face of a glacier at lower elevations, so if the glacier is shaped in a way that its face is long or wide and exposed, it melts along that face.
All glaciers have an equilibrium line of altitude, or ELA — the point below which they melt, and above which they're stable or growing. Glaciers experiencing large amounts of melt at lower elevations will slough off the soggy spots, lessening the area of melt, and withdraw up to higher, cooler, more-stable elevations to maintain that equilibrium line.
"If you get rid of that area where you melt a lot and retreat back up into your safe haven, then the glacier becomes stable," Truffer said. "Exit (Glacier), I wouldn't be surprised if we have well over 20 feet amount of melt this summer at lower elevations. That's a large amount."
That retreat doesn't always happen smoothly. Glaciers flow over and through various physical features and bedrock structures. An ice field, in fact, is differentiated from other ice structures in that many of the highest elevations are attained by the ice itself, rather than mountain peaks peeking through the ice — called nunataks.
"If you go to the highest point you're often standing on ice, not rock. There are mountains but they're under the ice. The topography of the ice cap is defined by ice itself, rather than by mountains," Truffer said.
Sometimes glaciers retreating back up a valley get hung up on the physical features over and through which they flow. Northwestern, for example, has been retreating in stages, with periods of stability, then a large retreat, followed by stability and another retreat.
Dramatic tidewater terminus, beautiful and problematic
Glaciers ending in water tend to shrink more dramatically. Glaciers ending in tidewater tend to extend all the way down below the water surface to tidewater, with chunks of ice calving off the face in dramatic fashion. Glaciers ending in lakes tend to float at their terminus, with icebergs more gently floating free when they break loose, Truffer said.
In either case, ending in water exacerbates a glacier's retreat, which poses a particular problem because glaciers tend to dig and erode, given the mass up high pushing them to flow out below.
"Often when glaciers retreat they leave lakes behind, and that can accelerate the retreat because then you can have ice calving into the water," Truffer said.
Glaciers and ice fields are maintained when the amount of new mass added from snow up high is balanced with the amount of loss happening through melt and calving down low. But a point can be reached where that equation no longer rules the equilibrium, if the thinning of the ice extends to such a point that it lowers its equilibrium line below the point where it is in balance with the melting and freezing.
"If the whole ice field up here starts thinning it will eventually get into a mode where it will just keep thinning. As it thins it gets to a lower average elevation, which increases thinning, which just makes it worse. The thinning then drags the elevation down, and it can drag it so far down that it is entirely below the ELA. Now you've just lost all area where there's mass gain," Truffer said.
Once that point is reached, an ice field would no longer be able to grow without a significant change in climate to provide much more ice and mass up high to raise the equilibrium line to a higher, more sustainable altitude. This has already happened in the Yakutat Ice Field in Southeast, Truffer said.
"(It) has essentially no snow whatsoever and it's getting thinner and getting worse every year. It's hard to imagine a situation now — you would have to have very significant cooling to rescue the Yakutat Ice Field. It's going to go away in the next 100 years almost entirely," Truffer said.
"The Harding Ice Field isn't in that same boat yet, but it's certainly something you want to watch out for. … The equilibrium lines right now are still below the large area of the ice field here, but should warming continue and the equilibrium line climbs up higher, at some point it will step out of these steep valleys onto this flat area. And if that happens then you start a process that becomes harder and harder to reverse. The thinner the ice field gets, the harder it is to actually make it grow back."
So far, though, surveys on the top of the Harding Ice Field show snow from a previous winter lasts until the next year's winter — a good indication that new mass is being added.
"You see that now if you flew out over the Harding Ice Field most of it is still white. At the higher elevations of the ice field, there's no exposed ice, it's snow from last winter. That's where you put money in the bank. If you go lower on the glaciers, that's where the winter snow melts and you have exposed ice. If you go on Exit Glacier right now you won't see any snow on the glacier, it's ice. What's melting now is ice, so you're taking money out of the bank."
The equilibrium line of the Harding is likely still at a point where the mass of the ice field is being maintained, but not so low that the ice field could regrow.
"The ice field is only there because it's there. If you took the Harding Ice Field away today there's good reason to believe that it wouldn't reform under today's climate. It would have had to be colder for the ice to form in the first place," Truffer said.
And the more the glaciers recede, the more the Harding inches toward the point where its mass as an ice field won't be maintained.
Deep ice as important ice extent
How long that might take depends on how much mass is there to melt. Truffer's project measuring ice depth is a step toward finding out.
Ice depth isn't just some idle measurement, though Truffer jokes of one reason to obtain this information.
"I assume that the (park) rangers get asked that question a lot, so at least they can say something," Truffer said. "(But) we want to do a little bit better than that. There are scientifically sound reasons why you would want to know the ice depth."
Knowing ice depth helps scientists inch further toward the more-difficult estimate of the ice field's volume. That, along with knowledge of the rate of ice melt, helps predict what could happen as glaciers shrink.
"To look globally, what's going to happen when the glacier retreats? How is the landscape going to look? Are you going to have a big lake? Is it going to be a mountainside? Those kinds of questions will affect changes in hydrology — how are the outlet rivers going to develop? If you have lakes, for example, then you have a trap for sediments. If you don't then that sediment ends up in the ocean. That's interesting to people who think about sediment and nutrient supply into the ocean.
So there's a lot of interesting global questions that you can go after if you assume that the glaciers are going to keep retreating, which is what most people expect," Truffer said.
Ultimately, such research would help predict how much sea level would rise as ice cover melts.
"We don't really have a good answer for that because it's difficult to measure. It's relatively straightforward to measure how much ice is melting, but it's much harder to figure out how much ice is actually there because you can't see the bottom of the ice," he said.
Truffer and his team tried to gauge ice depth using radio waves.
A team of five skied across the top of Exit Glacier and a portion of the ice field in that area, dragging a radio wave transmitter in one sled and a receiver in another about 30 meters behind. The radio waves travel through the ice, bounce off bedrock at the bottom and come back up, detected by the receiver, which can estimate the time it took for the wave to return and, thus, the thickness of the ice through which it traveled.
It's a reliable method that generates accurate results, though it's not ideal because of the effort and safety precautions required to hike up onto a glacier and haul heavy transmitters and batteries around on skis.
"That's all great fun, but it's slow. If you want to cover the entire ice field, it's going to take you a long, long time," Truffer said.
And it's too dangerous to cover crevassed areas along the face of a glacier. So part of the study was adapting the technology to be used with snowmachines — which hasn't yet happened on the Harding, but has on other ice fields — and via planes.
But that's not a foolproof method, either. Though aerial radar-depth surveys are more efficient than ground surveys, they aren't as useful in narrow valleys. For one thing, planes can't safely fly back and forth across a steep valley if it's narrow. For another, the radar waves, if shot from far above, start to bounce off the valley walls, rather than the bottom to the glacier.
"As more information is gathered the ice depth data can be combined with other measurements, such as the rate of snow melt, the amount of snow accumulation and the rate and extent to which glaciers are "flowing" and receding, to model the future of the Harding Ice Field and its glaciers.
Data already obtained answers some of those questions.
Exit Glacier at lower elevations is fairly shallow, with ice up to about 400 feet, deep. Ice depth higher up on the glacier is around 900 feet. The deepest ice Truffer recorded around Exit was at a ridge in the ice field above the glacier, about 1,500 feet, deep.
Ice field will recede as snow melts faster than it falls
So what's that mean for the future? It will take more study to know. The middle of the ice field appears to be in good shape, and even shows some thickening — though those estimations are based in part on old USGS survey maps.
"Some of this might be real thickening, that might have to do with additional precipitation, it just gets more snowfall in the winter. But a lot of what's blue (indicating growth in thickness on a map generated from depth findings) has to be taken with a bit of a grain of salt, and it's only continued measurements that will show the true behavior," Truffer said.
It does not appear that the Harding Ice Field could regrow itself without climate conditions cooling, and its glaciers are expected to continue to recede as the amount of snow no longer keeps up with warmer temperatures.
"Globally, statistically, most glaciers react mostly to temperature. That's the No. 1 thing," Truffer said. "In a lot of glaciers, for example in the Interior, it's almost all driven by temperature. We had a really snow rich winter this year in the Alaska Range, but one heat wave in June got rid of most of that really quickly."
But the Kenai Peninsula isn't going to shed its icy cap immediately.
"Frankly, to get rid of 2,000 feet of ice is not something that happens in our lifetime. Even when it goes rapidly, that takes a long time, on the order of 100 or more years," he said. "This will always be an interesting landscape. Some of these mountains are quite high, so they will probably retain remnant glaciers for years to come. Those kind of glaciers will be here for a long time."
Reprinted with permission from The Redoubt Reporter.