Third in a three-part series.
KANGERLUSSUAQ, Greenland -- In her knitted ski hat, parka and hiking boots, Åsa Rennermalm doesn't look like an accountant -- or a plumber.
But the Rutgers University hydrologist draws on both disciplines as she works at the southwestern edge of Greenland's vast ice sheet. She's spent the past four years trying to answer a deceptively simple question: When Greenland's ice melts, where does the water go?
"Satellites show the surface of the ice is melting and the volume of ice is decreasing," Rennermalm says. "But how much meltwater is leaving and reaching the ocean? That will influence future sea level rise."
She and colleagues from the University of California, Los Angeles, are studying the runoff from one chunk of the Greenland ice sheet -- Russell Glacier, which sits 16 miles outside of this small town.
Because it rests entirely on land, it isn't vulnerable to the influxes of warming ocean water that cause outlet glaciers to calve. The main way Russell loses ice is surface melting. Some of the resulting water runs off the glacier into nearby lakes and streams. Some drains into cracks and channels in the ice, where it can refreeze or flow to subglacial streams and lakes that feed into the sea.
The scientists are trying to track the water that runs off Russell Glacier and trace it back to its source.
A piece of a global puzzle
"We want to understand what happens to meltwater," Rennermalm says, showing visitors around her team's camp on a sunny July day. "What we're doing here is providing a piece of the puzzle, because we're looking at the margins of the ice sheet."
A few days into the group's two-week field season, it's clear there are still kinks to be worked out.
Rennermalm and her assistant, Colin Gleason, are crouched at the edge of the glacial stream that runs next to their camp. They've run into trouble with the equipment they use to measure the water's speed and depth, and it's time to double-check the wiring.
But a few miles up the rocky dirt road, the UCLA group is having more success. They're dipping another set of the same instruments -- this time attached to a long silver pole -- into the silty water rushing under a small plank bridge.
A few miles away, in Kangerlussuaq proper, temperatures are mild, in the low 70s. But here, closer to the glacier, a strong chill wind blows as geologist Larry Smith and doctoral student Vena Chu go about their work.
Just after 4 p.m., the current is strong and the water is rising. That's not unusual. The temperature on Russell Glacier peaks at around 1 p.m. each day, and the water level responds a few hours later, hitting its daily high.
"This morning, the stream was 2 feet lower," Chu says. "But last night around 6 or 7 p.m., it was hitting the bottom of the bridge."
Smith lowers the instrument pole into the water. The bottom of the pole rests on the streambed, and markings along its length reveal the water's depth. A silver pinwheel that sits a few inches from the bottom of the pole will reveal the stream's speed.
Every five spins of the pinwheel, an attached "squawk box" beeps. The scientists can convert the number of beeps over a certain period of time to the speed of the current in meters per second. Chu's job today is to monitor the output from the squawk box.
The researchers will take that information about the water's speed and depth and combine it with measurements of stream width to estimate how much glacial meltwater reaches the three different locations the team monitors.
"All the water here is coming from the ice sheet," Rennermalm says, but each site gives different information about the ice sheet's inner workings.
Measuring water rushing to the sea
The place Smith and Chu are working today is fed by a stream that flows underneath Russell Glacier. The area where Rennermalm and Gleason are stationed is fed by overflow from an ice-dammed lake that collects meltwater from the glacier's surface. A third site receives water from both sources.
But the data they're collecting are limited. The researchers want to know how glacial runoff changes from season to season and year to year, but they're only able to visit Greenland for a few weeks each summer. To solve that problem, they've installed pressure gauges at the three stream sites that record the water's height every 15 minutes, year-round.
By comparing their summer field measurements with pressure gauge data from the same periods, the scientists know which stream heights correspond to different water speeds. That allows them to estimate glacial discharge at each of the sites throughout the year. It's a good system, but not a perfect one.
"We have some data that suggest melt events are occurring in the winter," Rennermalm says. "But we're very uncertain about those data. Our instrument measures something, but what does it mean?"
Her team is confident, however, that they understand what's happening during the summer melt season. They hope to use that knowledge, and the data they're collecting, to model how water drains from the glacier -- and how much of that water reaches the ocean, where it can raise sea level.
Rennermalm is also working with researchers at the University of Alaska, Anchorage, to track the chemical composition of glacial runoff.
"What we hope to do is fingerprint where the water is coming from," she said, "whether it's from the surface of the Greenland ice sheet or subglacial lakes. We want to understand, how did the water get in the stream?"