In northwest Greenland, a pair of glaciers, Tracy and Heilprin, flow side-by-side into Inglefield Gulf, and yet, they're melting at dramatically different rates.

Now, scientists know why. New data from NASA's Oceans Melting Greenland survey suggests an especially warm plume of ocean water is melting Tracy from beneath.

Since they were first surveyed by explorers in 1892, Heilprin has retreated 2.5 miles. Over the last 125 years, Tracy has receded by 9.5 miles.

Despite the fact that the neighbors experience the same weather and ocean conditions, Tracy is losing ice nearly four times faster than Heilprin.

As part of the OMG campaign, NASA scientists have been surveying water and ice conditions along the Greenland coast since 2015. The project's goal is to help scientists better understand the loss of ice experienced by the Greenland Ice Sheet and its many glaciers.

Previous observations made by NASA's Operation IceBridge hinted at a possible explanation for the duo's divergent melt rates. The survey's ice-penetrating radar showed Tracy's base, where ice meets bedrock, is situated 2,000 feet below sea level. Heilprin's base lies just 1,100 feet below sea level.

But until now, scientists had not been able to accurately map the ocean floor and temperature dynamics inside Inglefield Gulf. New data collected by the OMG survey showed the deeper layer of warm water found in Inglefield begins some 600 feet below the ocean surface. The water gets warmer as it gets deeper.

Tracy's greater exposure to the deep-lying warm water explains the glacier's accelerated melting rate. The glacier's greater melting outflows also produce a negative feedback loop.

As the meltwater flows into the fjiord, it plunges down before rising back to the surface. Freshwater is more buoyant than saltwater. As it rises, the freshwater pulls warm subsurface water to the surface, further exposing Tracy's face to a plume of warm water.

"Most of the melting happens as the water rises up Tracy's face," Josh Willis of NASA's Jet Propulsion Laboratory said in a news release. "It eats away at a huge chunk of the glacier."

Heilprin experiences the same phenomena but to a much smaller degree.

Scientists detailed the melting dynamics affecting the Tracy and Heilprin glaciers in a new paper published this week in the journal Oceanography.

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