A forest revealed under glacial ice


Icy view of Mendenhall Glacier in wintertime. Frozen Mendenhall Lake in foreground. / (Creative Commons Attribution-Share Alike 3.0 Unported license (no author listed))

Laura Nielsen for Frontier Scientists

Alaska’s Mendenhall Glacier is shrinking, and its retreating ice has bared the remains of an ancient forest. Preserved stumps and trunks, many still rooted and even bearing bark, sit in a gravelly mix of stone churned up by the glacier. The trees are being exposed to open air for the first time in over two thousand years.

Cataloging melting glaciers

The Jakobshavn glacier imaged July 7, 2001, with overlayed lines displaying historic calving fronts. Historic locations in gray, 1850 through 1964, compiled by Anker Weidick and Ole Bennike. Newer locations in color, 2001 through 2006, derived from satellite imagery. / Courtesy National Aeronautics and Space Administration

Glaciers are rivers of ice created when years of snowfall accumulate in high elevations and slowly compact into ice. The ice, summoned by gravity due to its own colossal weight, slides in agonized slow motion downslope over the course of many years. Glaciers which accumulate enough new snowfall to more than balance the water they lose (due to melting) have a positive mass balance; the snow compacts into new glacial ice and the glacier advances downslope. Most of Alaska's glaciers are retreating. Factors like warm summer temperatures, low winter precipitation, and high pressure systems that encourage sunny weather inflict a trend of glacier ice melting faster than it collects new ice.

GLIMS, the Global Land Ice Measurements from Space, is working to inventory the world’s approximately 160,000 glaciers. The project database combines information gathered by satellite, by airborne analyzing tools, and by on-the-ground measurements. GLIMS aims to provide detailed numeric information and picture records of the changing nature of our globe’s glaciers through time. The project was developed jointly by the United States Geological Survey (USGS) and the National Snow and Ice Data Center (NSIDC). Their work will help us understand how glaciers are responding to a warming climate, which is important because glacial runoff both contributes to sea level rise and regulates regional fresh water supplies.

Forests at the edge of the ice

Alaska’s Juneau ice-field is home to 32 glaciers. Only one of them is currently increasing in size: the Taku Glacier. Compared to its neighbors, Taku begins at a higher elevation, a place where modern-day snowfall accumulates with more certainty. Today, the Taku Glacier is sealing cottonwood trees in ice just as the Mendenhall once descended on an ancient boreal forest. As Taku’s ice grinds slowly downhill, it crushes rock into smaller pieces. Spring and summer heat releases meltwater from the advancing edge of the glacier along with stony detritus. Before the ice ever reaches the trees, their trunks become buried in layers of pebbles. When the ice arrives it snaps off the tops of the trees. Meltwater carries the branches and leaves downstream toward the ocean, but the trunks remain rooted below.

Ice cave under Mendenhall Glacier, 1991. / Photographer Commander John Bortniak, National Oceanic and Atmospheric Administration

As Taku engulfs a forest today, it gives researchers a good idea of what happened millennia ago at the site of Mendenhall Glacier. The ancient trees are likely spruce, and University of Alaska Southeast Environmental Science Program coordinator Professor Cathy Connor called the place “The tomb of the king spruce tree.” Spruce trees and northern pine have a long history of existing at the edge of the ice. Our planet has experienced a series of repeated ice ages, in which glaciers and ice sheets grew and then retreated. Just 18,000 years ago, most of North America was covered in the Laurentide Ice Sheet, and living vegetation could only find fertile ground in the south. As the glaciers slowly retreated, boreal forests followed the southern edge of the ice northward. It was not until about 6,000 years ago that the continental ice sheets were melted, allowing forests to resemble their current incarnations.

Our climate, then and now

Understanding that glaciers have advanced and retreated historically is not a reason to set aside concern about anthropogenic (human-caused) climate change today. We have a history of ice ages, and small alterations in the Earth’s orbit do, over the span of tens or hundreds of thousands of years, slightly alter the energy balance by changing how much sunlight reaches different latitudes, but those are slow processes. The changes happening now are occurring at lightning speed compared to climate shifts of the past. Warming is escalating: Earth has warmed twice as fast in the last 50 years as in the 50 years prior.

Temperature and CO2 levels are historically closely tied, and we have a record of CO2 levels spanning the last 450,000 years. During that time, CO2 never rose over 280 parts per million. Then in 1950 CO2 levels began spiking; by 2009 they had reached 388 parts per million. Industrialization, fossil fuel dependence, and altered land use policies -human endeavors- have all increased greenhouse gases like CO2 which keep heat trapped in Earth’s atmosphere. Human actions interact with the natural processes of our incredibly complex climate system, and we are still working to understand all the implications.

Kings of the boreal forest

Much like the layers glimpsed in ice cores and ocean sediment can help scientists understand our climate past, the tree rings in the stumps preserved below Mendenhall Glacier can give us information about past climates, such as how much precipitation fell and what the atmosphere resembled. They are a record from the past which can help us analyze our own future.

You can view beautiful pictures of the forest remains coming to light under Mendenhall's ice by visiting the feature article at LiveScience.

Mendenhall Glacier in the Tongass National Forest outside of Juneau, Alaska, 2011. / (Creative Commons CC0 1.0 Universal Public Domain Dedication)

Frontier Scientists: presenting scientific discovery in the Arctic and beyond

References:

  • 'Ancient trees emerge from frozen forest 'tomb'' Mary Catharine Martin, Juneau Empire (2013)

    http://juneauempire.com/outdoors/2013-09-13/ancient-trees-emerge-frozen-forest-tomb#.UkOQ7IY3uA9

  • 'Ancient Forest Thaws From Melting Glacial Tomb' Laura Poppick, Live Science (2013)

    http://www.livescience.com/39819-ancient-forest-thaws.html

  • 'Sizing Up the Earth's Glaciers' Evelyne Yohe, National Aeronautics and Space Administration NASA Earth Observatory (2004)

    http://earthobservatory.nasa.gov/Features/GLIMS/

  • 'The Migrating Boreal Forest' Rebecca Lindsey, National Aeronautics and Space Administration NASA Earth Observatory (2002)

    http://earthobservatory.nasa.gov/Features/BorealMigration/boreal_migration.php

Leave a Reply


− four = 0