by Evan Koncewicz (St. Lawrence University) and Olivia Truax (Amherst College)
Over the past 50 years, glaciers around the world have been in sustained retreat in response to climatic warming. However, a small number of glaciers are advancing. At first glance this may seem like a contradiction, but this is because glacial dynamics are affected by an interplay of factors in addition to climate (glacial geometry, nature of the landscape they are in, among others). In 1946 JIRP was founded in part to study a particularly complex glacier in Southeast Alaska, the Taku Glacier, which was steadily advancing even as most other Alaskan Glaciers were in retreat. During the first week of JIRP 2016 we spent two days mapping the terminus of Taku Glacier with high precision GPS but, before we get in to what we found, a quick word on how glaciers work.
Glaciers, whether they are advancing or retreating, are always flowing downhill. Glaciers are formed when snow falls in the winter and doesn’t melt in the summer – the weight of this accumulation adds up each year and compacts the underlying snow into ice. This ice grows in mass, responds to gravity and begins to flow downhill. The area where the winter snow persists year round and adds to the overall mass of the glacier is called the accumulation zone. Downslope, where all of the snow and some of the underlying ice that has flowed down from a higher elevation melts in the summer, this section of the glacier is called the ablation zone. If the amount of melt lost in the summer in the ablation zone is equal to the amount of snow gained in the accumulation zone during the winter the glacier will remain the same size. If the amount of snow that falls in the accumulation zone is larger than the amount lost in the summer the glacier terminus will advance. On the other hand, if ice melt outpaces the accumulation and downhill flow of ice, the glacier will retreat.
The Taku is unique because it has an unusually large accumulation zone: most glaciers in equilibrium are comprised of about 60% accumulation zone and 40% ablation zone. By contrast, the Taku has about 80% accumulation zone and 20% ablation zone, helping to drive ongoing advance despite the warming climate However, conditions at the terminus of the Taku make rapid retreat a possibility in the near term. Because the base of the Taku is grounded below sea level, if the Taku begins to retreat, water may be able to slip between the glacial ice and the sediment. This would trigger a rapid retreat as the ice melted due to contact with the warmer water.
The Taku has advanced seven kilometers since 1850, but in the last few years it appears to be holding steady at its current position. We went out to survey the terminus of the Taku glacier to investigate if a retreat like this may begin in the near future. At the terminus we used GPS accurate to +/- one centimeter to map points along the glacier front. Wading through streams of meltwater, clambering over piles of sediment, and walking over sections of ice, we collected data points to mark where the exact end of the Taku glacier is this year. When processed, these points will give us a map of the terminus of the Taku Glacier in 2016. From past years we then have a sequence of maps of where the Taku has been year to year.
Our preliminary results suggest that parts of the Taku have retreated from its location in 2015 by roughly 10-20 feet (or approximately 5 meters) in certain locations. This, combined with the data from 2013-2015 that showed no change in the terminus extent after decades of advance of as much as 300 feet (approximately 100 meters) per year may indicate that the Taku is at the beginning stages of retreat. However, a year of melting at the terminus does not necessarily mean that the glacier is receding: these trends must be present for perhaps a decade before we can rely on them to make definitive conclusions.
Our survey data from the glacier helps contribute to JIRP’s long-term dataset of the Taku, which is one of the largest of its kind in the world. What makes our survey of the Taku special is that we very well could be witnesses to the beginning of the retreat of one of the last advancing glaciers in the world. Time will tell.