Northern Canada is speckled with lakes. And those lakes tell stories, according to Tamlin Pavelsky, an associate professor at the University of North Carolina at Chapel Hill.
Pavelsky and other researchers are using data from a series of flights flown for NASA’s Arctic-Boreal Vulnerability Experiment (ABoVE) to understand how permafrost—which rests beneath roughly 50% of Canada and 80% of Alaska—affects the lakes that lie above it.
The goal is to understand “whether there’s a relationship to water levels in lakes and whether or not there’s permafrost underneath them,” said Pavelsky. He presented the team’s research methods in a poster session on 12 December at the American Geophysical Union’s 2017 Fall Meeting in New Orleans, La.
Where’s the Permafrost?
Permafrost is a chilled layer of earth—rocks, soil, organic material, and ice—below the surface that stays below 0°C for at least 2 years. Some 20% of land throughout the Northern Hemisphere is composed of varying amounts of permafrost.
Flux towers can measure gases released over a given area, which could give clues to hot spots of permafrost melt, but such towers are sparsely distributed over the northern region. Field work could also paint a fuller picture, but fieldwork is difficult in these remote locations, not to mention expensive and time-consuming. And there’s no way fieldwork alone could get the spatial coverage needed to assess the fate of permafrost across the north.
Remote sensing could get the coverage needed, but what would airborne surveys or satellite passes be looking for? What’s needed is a “tell,” some signature in the remotely sensed data that indicates the health of permafrost over a given area, explained Pavelsky.
Pavelsky and the research team are eagerly looking for that tell. Their current hypothesis involves the water levels and elevations of lakes perched on permafrost.
When a network of lakes is connected to the same groundwater source, like an aquifer, the lakes are drained and replenished at the same rate. When lakes are underlain by permafrost, they rise and fall independently, largely disconnected from any central system.
Thus, monitoring lake levels over time by air or by satellite, tracking which ones started out being independent and then shifted toward fluctuating in concert with neighboring lakes, may help decipher whether permafrost underlying a given lake is degrading, Pavelsky noted.
Drifters on the Surface, Flights Overhead
To test their ideas, researchers used the Jet Propulsion Laboratory’s Air Surface Water and Ocean Topography (AirSWOT) platform, a suite of Earth science instruments mounted on a King Air B200 aircraft.
A small radar sensor within this bundle measures the elevation of the surface of the water in lakes above sea level. “Doing so allows us to compare changes in elevation not just for individual lakes over time but also between lakes,” Pavelsky explained. “If, for example, we observe two lakes connected by a short river, we can see how the slope of that river changes over time.”
By measuring the changes in elevation of lakes in Canada and Alaska and then conducting field surveys to assess the health of permafrost underneath the lakes, Pavelsky says that the team can understand how changes to permafrost beneath a lake affect the water above. Once a robust relationship is established, the team can apply the technique broadly, say, across North America.
The researchers are currently in the process of establishing relationships between permafrost and lake levels. This past summer, the team’s aerial surveys flew two swaths over Canada and Alaska, one in July and one in August.
As AirSWOT soared above the tundra, researchers hopped from lake to lake via seaplane, fighting off mosquitoes as they launched floating GPS units called drifters into each lake. These drifters then collected exact measurements of lake elevations relative to sea level, data that are now being compared with AirSWOT readings. Through this comparison, the drifters will be “validating the map reflected by the AirSWOT plane flying above,” explained Sarah Cooley, a Ph.D. student at Brown University who participated in the project.
In total, drifters were deployed in 49 lakes on a path stretching from Saskatoon through Yellowknife to Yukon Flats in Alaska.
Looking to the Future
The field work is over, and the data analysis has begun, Pavelsky said. And more than just tracking the fate of Arctic lakes is at stake.
The AirSWOT project is a precursor to another project, the Surface Water and Ocean Topography (SWOT) satellite mission. Scheduled to launch in 2021, the satellite takes a more global view in monitoring and measuring lakes around the world and can capture data every 11 days.
“We’ll be able to measure the water surface elevation and inundation in every lake and river around the globe,” said Pavelsky. With that data, more scientists can search for more tells that directly map how climate change affects the environment.
Correction, 20 December 2017: The image with ice wedge polygons now has the correct photographer’s credit line.