Antarctica permafrost melt spurred by solar radiation boost
Accelerated melting of buried ice in a dry valley of Antarctica may be a harbinger for widespread thawing of permafrost at Earth’s far latitudes as worldwide climate patterns change, according to a new study.
The tenfold increase from ancient melt rates evident in a dry valley near McMurdo Bay over little more than a decade comes despite a local two-decade cooling trend.
Cliff-face measurements of the buried ice in the four-mile-long Garwood Valley revealed melt rates that shifted from a creeping annual rate of about 40,000 cubic feet per year over six milleniums, to more than 402,000 cubic feet last year alone, according to the report published Wednesday in the journal Nature Scientific Reports. (That’s a leap from the capacity of about eight standard railroad boxcars to 77.)
“We think what we’re seeing here is sort of a crystal ball of what coastal Antarctica is going to experience,” said geologist Joseph Levy, of the University of Texas, lead author of the study. “When you start warming buried ice and other permafrost in the dry valley, it’s going to start to melt and it’s going to start melting in a style that’s consistent with permafrost thaw in the Arctic.”
The data come from measurements of a thermokarst area, an irregular formation of marshy subsiding ground often found in thawing permafrost areas at Earth’s other extreme latitudes, the Arctic, and in high alpine environments.
Researchers wanted to know if the thawing of the debris-covered ice was static or dynamic, and how it compared with long-term patterns since the last ice age some 26,000 years ago. They set up laser-guided monitoring devices, time-lapse photography and a weather station, measuring a period from 2001 to 2010, then taking two more annual readings.
They found that changing patterns of soil erosion altered the amount of solar radiation absorbed in the area, known as the albedo effect, adding about two watts of energy per square meter. (That’s about the power of a candle style lightbulb radiating on less than a third of a sheet of plywood.)
“It doesn’t sound like much to crank up two watts at at time but when you add this up over the last several decades, it’s enough to tip the balance in Garwood from having buried ice in equilibrium to having accelerated melting,” Levy said. “It’s serious because when you think about permafrost thaw in the Arctic and Antarctic peninsula, usually people think about air temperature. But the dry valley has been in a cooling trend that started about 20 years ago.”
Adding a small rise in temperature, as predicted by climate models, would cause a wide swath of the valley to melt, Levy warned.
“At the end of the day, ground ice doesn’t care if it’s being warmed up by sunshine or by warm-air temperatures,” he said. “It’s still going to melt.”
That type of accelerated melt could become typical at the fringes of much larger areas, such as Antarctica’s ice sheets and the land ice in Greenland. Unlike the dry valley thaw, those melts would contribute significantly to rising sea levels.
Numerous studies have shown an acceleration in ice melt in Antarctica’s ice sheets. A report in the review Nature Geoscience last week suggested that satellite measurement of volume loss on broader Antarctic ice sheets has been going on for too short a period to enable researchers to distinguish between a long-term acceleration and short-term trend.
Garwood Valley lies at the 164th meridian, between Australia and New Zealand. It contains a remnant of the Ross Sea ice shelf, deposited about 26,000 years ago, that now is covered by more than six feet of glacial till, the uppermost eight inches of which thaw seasonally.
Researchers focused on the 1,300-foot-wide Garwood Valley ice cliff, where a slab of about 32-49 feet of the ice sheet is exposed and has been retreating southward at an annual rate of 32-180 feet in the past decade.
They calculated the volume of ice missing since it was deposited some 6,300 years ago, arriving at a melt rate of about 40,000 cubic feet per year. From 2001 to 2010, that rate accelerated to 190,000 cubic feet, then jumped to 247,000 in 2011, and about 403,000 last year.
The team included Levy, then at Oregon State University, and researchers from Portland State University, Brown University, Boston University and UNAVCO, a multiuniversity geoscience consortium.
Copyright © 2013, Los Angeles Times