By Paul Homewood
For nearly four decades, Mauri Pelto has been studying the advance and retreat of glaciers around the globe. He has watched them succumb, one-by-one, to rising temperatures. Of 250 glaciers that he has watched, all had retreated (or shortened) except one: Taku Glacier.
Now a new analysis shows that Taku has lost mass and joined the rest of the retreating glaciers. “This is a big deal for me because I had this one glacier I could hold on to,” said Pelto, a glaciologist at Nichols College. “But not anymore. This makes the score climate change: 250 and alpine glaciers: 0.”
Taku stands north of Juneau, Alaska, and is one of 19 notable glaciers in the Juneau Icefield. (The area also includes the famous Mendenhall Glacier, which has experienced an unusually fast retreat—about one third of a mile in the past decade.) Taku is extremely thick: In fact, it is one of the thickest known alpine glaciers in the world, measuring 4,860 feet (1,480 meters) from surface to bed. It is also the largest glacier in the Juneau Icefield.
Using satellite imagery, aerial photography, and GPS field mapping, glaciologists with the Juneau Icefield Research Program (JIRP) have been tracking the thickness of Taku’s annual snow layer since 1946. Pelto has personally observed the glacier over a span of three decades, and even spent six months living on the glacier in huts and tents with JIRP in the 1980s.
In his latest research, he used Landsat imagery to look at changes in the transient snowline—the boundary where snow transitions to bare glacier ice. At the end of the summer, the height of the snowline represents the point where the glacier experienced an equal amount of melting and snow accumulation. If a glacier experiences more melting than snow accumulation in a season, the glacier’s snowline migrates to higher altitudes. Researchers can calculate net changes in glacier mass by tracking the shift of the snow line.
For half a century, the Taku Glacier was the only glacier in the Juneau Icefield that did not experience a net loss in mass. In fact, Pelto and colleagues found that the glacier was advancing and gaining mass at around 0.42 meters (1.4 feet) per year from 1946-1988. But by 1989, glacier thickening slowed down significantly; eventually, the researchers noticed some thinning. The terminus also slowed its advance and then stalled
As Mr Pelto should know, Taku is a tidewater glacier, and this is what tidewater glaciers do – advance and retreat. Here is a good explanation by another expert:
To understand the dynamics of Taku Glacier, we have to know the story of the tidewater glacier cycle. Here is a summary derived from a lecture delivered to JIRP students by Martin Truffer earlier this summer at Camp 17. As the end of a tidewater glacier, known as the terminus, rests in a fjord, the elevation of the glacier’s bed is below sea level. As a result, the melt water beneath the terminus of the glacier becomes pressurized so that it can still flow into the ocean despite the weight of the seawater column. The terminus is quickly eroded as big chunks of ice peel away during calving events and as warm sea water circulates against the terminus. Consequently, the glacier is driven into a rapid retreat, and it recoils up its valley until it reaches a resting point above sea level. There, the glacier is able to stabilize and to eventually begin an advance by pushing its dirty, icy terminus forward on a terminal moraine (a pile of sediment collected by the glacier at its terminus as it grinds forward). By advancing a homemade mound of sediment ahead of itself, the glacier can rest above the deep water of the fjord and the subglacial hydraulics are less pressurized, so the glacier is protected from the intense melting and erosion that previously drove it back. As it continues to bulge onward, the glacier eventually reaches a state where its surface balance nears zero, which means that its accumulation and ablation (melting) are equal. At this point, the glacier can reenter a rapid retreat as the tidewater glacier cycle continues.
And this is a good illustration of how it works:
llustration by Meghan Murphy
This illustration shows a tidewater glacier slowly advancing on a sediment pile. The melting glacier terminus eventually begins to erode the sediment pile. That undercuts the ice’s support and triggers a rapid collapse, in a process explained by a new model developed by a University of Alaska Fairbanks researcher.
Studies have established that there have been at least five major cycles of growth and recession in the last three thousand years on the Taku glacier. In the most recent era, the Taku reached its fullest extent around 1750. Although it has been growing since 1890, prior to then there had been a rapid retreat way back up the valley. Neither that retreat or the subsequent advance had anything to do with climate change:
There is one more chart to show you, which illustrates how the glacier has seesawed over the years:
The idea that mankind can have any influence on these mammoth natural forces is farcical.
via NOT A LOT OF PEOPLE KNOW THAT
March 28, 2021 at 10:12AM