Most glaciers around the world and on the Juneau Icefield are receding, but as a tidewater glacier, the Taku is different. Since around 1890, the glacier has advanced about four and a half miles, changing the landscape around it.
The Juneau Icefield Research Program’s measurements in the last two years, however, show that the Taku’s overall advance has stopped — at least for now.
That could mean a few different things.
The most likely possibility, said 2015 JIRP academic council member Matt Beedle, is the stagnation is “a short-lived standstill as (the glacier) continues to push forward slowly.”
“We don’t know at this point,” Beedle said. “Having (just) two years of stagnation.”
Alternately, the Taku could be entering into a period where its terminus stays more or less the same. That phase could last “for quite some time,” Beedle said.
It could also be the beginning of the glacier’s retreat, though predicting that today would be premature, Beedle said. When the Taku Glacier does start to retreat, however, it could lose those four and a half miles faster than they were gained. And glaciologists know for sure it will retreat — that’s part of every tidewater glacier’s cycle.
Not so long ago, the Taku Glacier calved icebergs that floated past the downtown Juneau docks. When the glacier retreats, the sediment the glacier’s advance has pushed up will wash away, creating a fjord, and icebergs will float past downtown Juneau once more, Beedle said.
How they measure
U.S. Fish and Wildlife Service cartographer Scott McGee has long been involved with JIRP, a scientist-led program started by Dr. Maynard Malcolm “Mal” Miller and William O. Field that has brought high school, college and graduate students to the Juneau Icefield each summer since 1946.
McGee has led the almost-annual GPS surveying efforts at the Taku Glacier terminus since they began in 2001.
“Up until 2013, we saw advance every year,” McGee said. “It was plainly obvious in the data.”
Since 2013, however, “the majority of the terminus has been pretty much unchanged,” he said.
Satellite imagery, which isn’t as exact, also doesn’t show any significant advance, he said.
JIRP students take their readings in late June or early July, not at the end of the melt season, when those measurements would usually be taken. Regardless, Beedle said the data still shows the general trends, and they’re now showing a net stagnation.
The terminus of the Taku is about 9 kilometers long and “doing different things in different places,” Beedle said. “It’s not advancing in every single location or retreating in every single location,” he said.
The advance varies depending on the year and the location of the measurement, Beedle said, but since 2001, it has ranged between 10 and 30 meters per year. Now, some portions have advanced up to 15 meters, and others have lost up to 20 per year, Beedle said.
The students that participate in JIRP each year measure between 50 and 80 percent of the terminus using high precision GPS, Beedle said. The net terminus gain or loss is an average of those readings.
In short, on average, the Taku Glacier is no longer advancing according to JIRP’s GPS data — and definitely not as much as in past years.
Part of the reason the Taku has been advancing is that over much of the 69-year record of mass balance — the overall mass of a glacier — JIRP has recorded the mass balance of the Taku has been positive. Over the last decade, however, that’s changed — the glacier has been losing mass overall.
“Obviously, there had to be a huge amount of accumulation up on the ice field to drive that advance for such a long period of time,” McGee said, referring to the glacier’s four and a half mile gain since around 1890. “It appears that there’s just not as much accumulation up on the main part of the ice field anymore to provide that driving force to continue its advance.”
The icefield overall has been thinning significantly as well, McGee said. The Taku Glacier is now around five meters lower than it was in 2001 — more than a meter of which was lost just in the last year.
On the Canadian side, where the weather is less rainy and warmer, the changes are much more drastic. The Llewellyn Glacier is 17 meters lower than it was in 2001.
In such a large glacier, those changes — and the slow-down in momentum that a negative mass balance trend creates — take a while to become apparent. Beedle compares it to a Prius versus a large truck slamming to a stop. Because of inertia, the smaller vehicle is able to stop more quickly than the larger.
“It takes a while for the changes we see higher up at the glacier to impact what’s happening at the terminus,” he said.
The Taku has also spread out as it has advanced, McGee said. The terminus has left the valley that formerly constrained it and spread out into a “huge lobe” in Taku Inlet. This also affects the speed at which it advances.
“There’s more area for it to spread out into,” he said.
The changes are “very exciting scientifically,” Beedle said.
“It helps us to understand what’s going on in other places around the world,” he said.
“We’re moving into a climate where the Taku is going to be losing mass from the surface,” Beedle said. “Eventually, that will result in the terminus pulling back, but when that will happen — it probably won’t happen in the near future, would be my guess. The glacier is so massive, the response time can be many decades.”
• Contact Outdoors writer Mary Catharine Martin at firstname.lastname@example.org.