Glacier melting underwater up to 100 times faster than thought
US researchers directly measure below the waterline for the first time.
Underwater melting of tidewater glaciers is occurring much faster than previously thought, according to a US study in Alaska.
The findings are based on a new method developed by the researchers Rutgers University and the University of Oregon that directly measures below the waterline.
"Tidewater glaciers around the globe – in Greenland, Alaska, Antarctica and beyond – are retreating and raising sea levels globally," says Rutger oceanographer Rebecca Jackson.
"Submarine melting has been implicated as a trigger for this glacier retreat, but we have had no direct measurements of melting, let alone how it might vary in time. In our study, we show that the prevailing theory for melt significantly underestimates melt rates.”
Jacksons says the results suggest a stronger coupling between the ocean and glaciers than previously expected.
For this research, team of oceanographers and glaciologists studied the underwater melting of the conveniently accessible LeConte Glacier from 2016 to 2018.
They used sonar to scan the glacier's underwater face; downstream measurements of currents, temperature and salinity to estimate the meltwater flow; radar to measure the glacier's speed above water; time-lapse photography to detect iceberg calving; and weather station data to measure the surface melt from the glacier.
They then looked for changes in melt patterns between the August and May measurements.
"We found that melt rates are significantly higher than expected across the whole underwater face of the glacier: in some places 100 times higher than theory would predict," Jackson says.
"We also find, as expected but never shown, that melt rates are higher in summer than in spring, and that variations in melt rates across the terminus cause overcutting and undercutting."
Jackson says the existing theory about what controls underwater melting has proved to be “wildly inaccurate” for the only glacier where a robust comparison has been made between theory and observations, suggesting that “we need to revisit our basic assumptions”.
"Our results also align with several recent studies of other glaciers that have indirectly suggested that theory under-predicts melting,” she says.
The findings are published in the journal Science.