November 27, 2012
Alaskan Glacier Predicted To Stop Its Retreat by 2020
April Flowers for redOrbit.com - Your Universe Online
The Columbia Glacier is a multi-branched glacier located in south-central Alaska that covers 425 square miles. The glacier flows mostly south out of the Chugach Mountains to its Prince William Sound tidewater terminus.
The iceberg calving rate, whereby large pieces of ice detach from the glacier and float into the ocean, has increased for the Columbia Glacier triggered by warming air temperatures.
"Presently, the Columbia Glacier is calving about 2 cubic miles of icebergs into the ocean each year -- that is over five times more freshwater than the entire state of Alaska uses annually," William Colgan of the CU-Boulder headquartered Cooperative Institute for Research in Environmental Sciences (CIRES) said. "It is astounding to watch."
The finish of the recession, or retreat, of the front of the glacier has surprised scientists and highlights the difficulties of trying to estimate future rates of sea level rise.
"Many people are comfortable thinking of the glacier contribution to sea level rise as this nice predictable curve into the future, where every year there is a little more sea level rise, and we can model it out for 100 or 200 years," Colgan said.
The findings of this study demonstrate otherwise. Over a couple years, a single glacier's contribution to sea level rise can "turn on" and "turn off" quite rapidly with the precise timing of the life cycle being difficult to forecast. The global population of glaciers presently contributes the majority of sea level rise, and many of these glaciers are just starting to retreat. Some will soon cease to retreat.
"The variable nature and speed of the life cycle among glaciers highlights difficulties in trying to accurately predict the amount of sea level rise that will occur in the decades to come," Colgan said.
First documented in 1794, the Columbia Glacier appeared to be stable with a length of 41 miles. It began a rapid retreat in the 1980s, by 1995 it was only about 36 miles long, and in 2000 it had retreated to 34 miles long.
This loss of a massive area of the tongue of the glacier has generated an enormous number of icebergs since the 1980s. Significant resources were invested to understand its iceberg production, after the Exxon Valdez ran aground trying to avoid a Columbia Glacier iceberg in 1989. The Columbia Glacier became one of the most well-documented tidewater glaciers in the world as a result. This intense study provided a bank of observational data for scientists trying to understand how a tidewater glacier reacts to a warming climate.
Colgan became curious as to how long the glacier would continue to retreat after being motivated by the compelling imagery of the glaciers retreat documented by James Balog's collection of time-lapse photography of disappearing glaciers around the world, called the Extreme Ice Survey.
Founded in 2007, the Extreme Ice Survey (EIS) is a an ambitious project that marries art and science to "give a visual voice to the planet's changing ecosystems." It is a long-term photography project that will provide a unique baseline for revealing how climate change and human activity impacts the planet.
The team of researchers created a flexible model of the Columbia Glacier to reproduce different criteria — such as ice thickness and terminus extent — to answer this question. They compared thousands of outputs from the computer model, each from a different set of assumptions with the wealth of data available for the Columbia Glacier.
After identifying the batch of outputs that most accurately reproduced the history of retreat, the team ran these outputs into the future to predict the changes the Columbia Glacier will most likely experience until the year 2100. They found that by 2020, the terminus of the glacier will retreat into water that is sufficiently shallow to provide a stable position through 2100 by slowing the rate of iceberg calving production.
Colgan says that the glacier's rapid retreat is due to the unique nature of tidewater glaciers. A land glacier only loses its mass by run-off when warming temperatures melt the surface of a land glacier. Tidewater glaciers, however, have changes in ice thickness resulting from surface melt, which can create striking changes in ice flow. This triggers an additional dynamic process for retreat.
The Columbia Glacier's dynamic response to the surface melt will continue until the glacier reaches its new stable position at roughly 26 miles long.
"Once the dynamic trigger had been pulled, it probably wouldn't have mattered too much what happened to the surface melt -- it was just going to continue retreating through the bedrock depression upstream of the pre-1980s terminus," Colgan said.
Colgan intends to continue this research by attempting to use similar models to predict when Greenland glaciers, which are currently the major contributors to sea level rise, will "turn off" and complete their retreats. The Columbia Glacier's future looks bleak, however.
"I think the hope was that once we saw climate change happening, we could act to prevent some irreversible consequences," Colgan said, "but now we are only about eight years out from this retreat finishing -- it is really sad. There is virtually no chance of the Columbia Glacier recovering its pre-retreat dimensions on human time-scales."
The results of this study were published in The Cryosphere.