Newswise — WASHINGTON —
The melting of the West Antarctic Ice Sheet is causing worry that it may reach a tipping point of irreversible retreat within a few decades if the global temperature increases by 1.5 to 2.0 degrees Celsius (2.7 to 3.8 degrees Fahrenheit) from preindustrial levels. Recent studies reveal that around 6,000 years ago, the ice sheet's grounded edge could have been up to 250 kilometers (160 miles) inland from its current position. This suggests that after the last ice age, the ice sheet retreated deeply into the continent and then re-advanced before its modern retreat began.
Ryan Venturelli, the lead author of the recent research and a paleoglaciologist at Colorado School of Mines, stated that "Before we began observing, the ice in certain areas of Antarctica withdrew and progressed over a significantly broader region than what we previously acknowledged." He added, "While the ongoing retreat of the Thwaites Glacier is quicker than any observed in the past, the geological record shows that ice can recuperate."
The research has been published in AGU Advances, a journal that publishes influential, openly accessible research and opinions in the fields of Earth and space sciences. This study provides the initial geological confirmation of the ice sheet's position and displacement since the end of the last ice age.
The point where a glacier or ice sheet departs from solid land and starts to float on water as an ice shelf is referred to as the grounding line. At present, the West Antarctic Ice Sheet's grounding line is located at the Ross Ice Shelf, stretching for hundreds of miles over the ocean. Due to the influence of ocean water washing up against the front of the ice, the grounding line can experience rapid melting.
According to Venturelli, "The reason why grounded ice loss is alarming is that it leads to an increase in sea level. As the grounding lines retreat towards the interior of the ice sheet, the thicker ice becomes increasingly exposed to the warming ocean, making the ice sheet more vulnerable."
Approximately 20,000 years ago, during the Last Glacial Maximum, the West Antarctic Ice Sheet was so extensive that it grounded on the ocean floor, beyond the continent's edge. Prior findings generally suggest that there has been a gradual retreat since that time, which has been expedited in the last century due to human-induced climate change.
The inquiry for Venturelli was to determine the extent of the ice sheet's retreat inland after the last ice age. Without this knowledge, it is difficult to estimate how vulnerable the Antarctic Ice Sheet is and how it will react to additional climate change.
To discover the answer, Venturelli and her team examined a lake, which was concealed under a kilometer (0.6 miles) of ice and almost twice the size of Manhattan. This lake was cut off from the contemporary atmosphere and provided valuable evidence. The team used a hot water "drill" to cautiously melt their way into the lake. After gaining access, they extracted samples of lake water and carbon-rich sediments from the bed of the lake. Through radiocarbon dating, they determined that the carbon in the sediments was approximately 6,000 years old.
Since radiocarbon (carbon-14) in these sediments must have originated from seawater, this discovery implies that the current lake, which is now 150 kilometers (93 miles) from the current ice edge, was once the ocean floor. When the ice sheet advanced, it covered the lake, preserving the carbon as part of the sediments at the bottom of the lake. Additionally, based on the radiocarbon levels found in water samples taken from the same lake, the grounding line might have been positioned an additional 100 kilometers (62 miles) further inland at that time.
Venturelli remarked, "When we embarked on the mission to collect samples from this lake, we were uncertain about what we would discover about the ice's history. However, the fact that deglaciation continued so far inland was not that surprising. The West Antarctic region is exceedingly flat, and there are no major topographical features that can slow the retreat of the grounding line."
The new evidence of Antarctic ice’s ability to make a comeback was welcome news for Venturelli.
Venturelli expressed, "Studying ice loss in Antarctica can be disheartening at times. However, the re-advance discovered in the geologic record, even if it happened over a span of several thousand years, gives me a glimmer of hope to consider studying the reversibility of this process."
The primary question for Venturelli and her fellow researchers is to determine the factors that facilitated the ice's re-advance. One possible explanation is that the rebound effect, caused by the release of the ice sheet's immense weight, elevated the land enough to hold back the ocean and permit the ice to regrow. Alternatively, minor alterations in climate conditions could have caused the ice sheet to transition from a state of retreat to one of advancement. It's possible that a combination of these factors played a role.
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Notes for journalists:
This study is published in AGU Advances, a fully open-access journal. View and download a pdf of the study here.
Additional media accompanying this story are available at request from the study author. The SALSA team also produced a documentary related to the study; video footage may be available on request from the SALSA team.
Paper title:
“Constraints on the Timing and Extent of Deglacial Grounding Line Retreat in West Antarctica”
Authors:
- Ryan A. Venturelli (corresponding author), Matthew R. Siegfried, Colorado School of Mines, Golden, CO, USA
- Brenna Boehman, Valier Galy, Woods Hole Oceanographic Institution, Woods Hole, MA, USA
- Christina Davis, Brent C. Christner, University of Florida, Gainesville, FL, USA
- Jon R. Hawkings, University of Pennsylvania, Philadelphia, PA, USA
- Sarah E. Johnston, University of Alaska Fairbanks, Fairbanks, AK, USA
- Chloe D. Gustafson, Cyrille Mosbeux, Helen A. Fricker, Scripps Institution of Oceanography, La Jolla, CA, USA
- Alexander B. Michaud, Bigelow Laboratory for Ocean Sciences, East Boothbay, ME, USA
- Trista J. Vick-Majors, Michigan Technological University, Houghton, MI, USA
- Robert G. M. Spencer, Florida State University, Tallahassee, FL, USA
- Sophie Warny, Louisiana State University, Baton Rouge, LA, USA
- David M. Harwood, University of Nebraska-Lincoln, Lincoln, NE, USA
- Amy Leventer, Colgate University, Hamilton, NY, USA
- John C. Priscu, Polar Oceans Research Group, Sheridan, MT, USA
- Brad E. Rosenheim, College of Marine Science, University of South Florida, St. Petersburg, FL, USA
- The SALSA Science Team
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