Past glacier movements offer clues to the future of ice melt
The West Antarctic ice sheet holds enough water to raise the world’s oceans an estimated 10 feet, and it’s shrinking.
Scientists analyzing the region have had a reasonable understanding of the ice sheet’s changes over the last three decades, thanks to satellite imagery and ground measurements. But solid data on how its glaciers behaved before that time is harder to come by.
University of Alaska, Fairbanks physics professor Martin Truffer, said glaciologists need historical data in order to predict glaciers’ future behavior. “If we just observe right now, we might not get the whole picture of how this process actually works,” he said.
A study published Wednesday by Truffer and 14 other researchers in the journal Nature offers some clues from the early 20th century. The team found the retreat of one of the largest glaciers in the region, the Pine Island Glacier, actually began with a pulse of warm, El Niño ocean water around 1940. Its retreat didn’t stop despite ocean temperatures in the region returning to normal quickly thereafter.
In December 2012 and January 2013, the team drilled completely through the ice sheet near a seafloor ridge where the glacier’s grounding line was believed to be in the 1940s. A grounding line is the barrier between where a glacier’s ice sits atop the ocean floor and where it becomes a suspended shelf in ocean waters.
After drilling through the ice to ocean water below, the researchers used special equipment to obtain cores of the sediment below. Oceanographers often use sediment cores in their research, said Truffer, but using them as a part of glacier research is relatively rare.
“What you find with the sediment cores is essentially a history of what has been going on at the bottom over the last couple of decades, because the sedimentation on the ocean floor is quite different once you have an ocean cavity from when you have ice actually touching the ground,” he said.
The research team looked at the content of the three sediment cores — two from the front side and one from the backside of the ridge — and analyzed differences in the grain size of sediment.
The ocean floor in front of a grounding line tends to have layers of coarse debris thrust upon it by a moving glacier. Any ocean bottom then exposed to open water by a retreating glacier begins to accumulate very fine sediments floating in the water column.
Dating the sediment layers in each sample by measuring the decaying isotope lead-210 showed the 1945 pulse of warm water opened a cavity behind the grounding line ridge. The water temperature returned to normal, but the cavity continued to grow. Over time, the thinning of the glacier from underneath caused it to detach from the ridge altogether. As a result, the grounding line of the Pine Island glacier has retreated roughly 30 miles in the last 70 years.
The study is a valuable contribution to glacial research, said Knut Christianson, a glaciologist at the University of Washington who has also studied the Pine Island Glacier but was not involved in the research.
“It has long been suspected that the trigger for starting retreat down a reverse slope at Pine Island Glacier was sufficient ocean-induced melt at the base of the glacier to thin the glacier to flotation and trigger instability via retreat into thicker ice,” he said. “That drives more flow, additional thinning and additional retreat.”
“To my knowledge, this is the first time sediment cores have been retrieved from under a very dynamic ice stream used to date grounding line retreat so precisely,” he added. “It will lead to advances in process understanding that will be useful elsewhere.”
The results indicate sudden climate forcing could cause rapid, irreversible melting in other glaciers.
One concern to Truffer is the Pine Island’s neighbor Thwaites Glacier, which holds about two feet of potential sea level rise. Like Pine Island, Thwaites also has ridges in it’s underwater ice basin, and warm pulses could cause spurts of rapid retreat into deeper cavities at a much quicker rate than expected.
“There’s an enormous amount of ice there,” said Truffler. “So, the question becomes, ‘Can you release that in 100 years or 1,000 years?’ I think the rate of retreat is where most of the uncertainty is right now.”