In 1974 four of my colleagues at the US Geological Survey and I spent the summer studying modern glaciers in Alaska. One of the glaciers we visited was the Mendenhall Glacier, about 20km from Juneau. The Mendenhall is a lake-terminating, calving glacier. That’s “geology speak” for the glacier terminating (ending) in a lake where large chunks of ice calve (break off) and float off as icebergs.
When people talk in geologic time, a generation (about 25 years) is a blink of an eye. Sometimes a lot can happen in that blink.
My parents were born in 1913. The world was just coming out of the Little Ice Age, a cold climate period from the early 14th century through the mid-19th century when mountain glaciers around the world, including Alaska, grew tremendously. From about 1750 until just before my parents’ birthdays, the Mendenhall appeared to stand still. This was because the forward movement of the ice front (due to the accumulation of snow and ice in the Juneau Ice Field) had been about the same as the ablation (sublimation, calving and melting of ice) at the front of the glacier. The advancing ice acted like a giant conveyor belt, carrying large amounts of dirt to the front of the ice. As the front of the ice melted, the dirt the ice was carrying was deposited as gigantic piles of boulders, gravel, sand, silt and clay called a moraine complex.
{{image2-a:r-w:200}}The Mendenhall Glacier began to retreat during the warmer climate after my parents’ birthdays. The moraine complex dammed meltwater from the glacier, forming Lake Mendenhall. As the glacier continued to retreat, the lake became longer and longer.
From the time my parents were born to when my wife Pam and I were born (circa 1947) the ice retreated 1.9km, or about 56m per year. From our birthdays to the birth of Kimberly and Rob (our daughter and son-in-law, circa 1970), the ice front retreated about 0.5km; a much slower rate of about 19m per year.
Then the ice began melting faster. By the start of the next generation (Donovan and Delaney, circa 2004) the glacier had retreated about 1km, or about 29m per year. Since then the rate of retreat has continued to accelerate, and by 2014 the ice front retreated another 0.8km, or 80m per year. Overall, the glacier has retreated 4.2km in 100 years – or about 42m per year.
I have projected where the ice front might be if and when Donovan or Delaney have kids. Maybe they will take them to the Mendenhall Glacier to see what can happen in the blink of an eye.
Author’s note: If you ever get to Juneau, you should stop by the Mendenhall Glacier Visitor Centre. You can plan your trip by visiting www.mendenhallglacier.net.